High efficiency hydrofoil and swim fin designs

BACKGROUND-FIELD OF INVENTION

This invention relates to hydrofoils, specifically to such devices which are used to create directional movement relative to a fluid medium, and this invention also relates to swimming aids, specifically to such devices which attach to the feet of a swimmer and create propulsion from a kicking motion.

BACKGROUND-DESCRIPTION OF PRIOR ART

One of the major disadvantages which plague prior fin designs is excessive drag. This causes painful muscle fatigue and cramps within the swimmer's feet, ankles, and legs. In the popular sports of snorkeling and SCUBA diving, this problem severely reduces stamina, potential swimming distances, and the ability to swim against strong currents. Leg cramps often occur suddenly and can become so painful that the swimmer is unable to kick, thereby rendering the swimmer immobile in the water. Even when leg cramps are not occurring, the energy used to combat high levels of drag accelerates air consumption and reduces overall dive time for SCUBA divers. In addition, higher levels of exertion have been shown to increase the risk of attaining decompression sickness for SCUBA divers. Excessive drag also increases the difficulty of kicking the swim fins in a fast manner to quickly accelerate away from a dangerous situation. Attempts to do so, place excessive levels of strain upon the ankles and legs, while only a small increase in speed is accomplished. This level of exertion is difficult to maintain for more than a short distance. For these reasons scuba divers use slow and long kicking stokes while using conventional scuba fins. This slow kicking motion combines with low levels of propulsion to create significantly slow forward progress.

Much of the drag created is due to the formation of turbulence around the blade portion of the fin. This turbulence occurs because prior fin designs do not adequately address the problem of flow separation and induced drag while lift attempting to generate lift. This destroys efficiency and severely reduces lift. On an airplane wing for instance, Bernoulli's principle explains that the air flowing over the convexly curved upper surface must travel over a greater distance than the air flowing underneath the lower surface of the wing. As a result, the air flowing over the upper surface must travel faster than the air flowing underneath the wing in order to make up for the increase in distance. Because of this, the air pressure along the upper surface of the wing decreases while the air pressure underneath the lower surface of the wing remains comparatively higher. This difference in pressure between the upper and lower surfaces of the wing causes “lift” to occur in the direction from the lower surface towards the upper surface. Because of this pressure difference, the lower surface on an airfoil is called the high pressure surface, while the upper surface is called the low pressure surface.

Another way of creating lift is to very the angle of attack. This is the relative angle that exists between the actual alignment of the oncoming flow and the lengthwise alignment of the foil (or chord line). When this angle is small, the foil is at a low angle of attack. When this angle is high, the foil is at a high angle of attack. As the angle of attack increases, the flow collides with the foil's high pressure surface (also called the attacking surface) at a greater angle. This increases fluid pressure against this surface. While this occurs, the fluid curves around the opposite surface, and therefore must flow over an increased distance. As a result, the fluid flows at an increased rate over this opposite surface in order to keep pace with the fluid flowing across the attacking surface. This lowers the fluid pressure over this opposite surface while the fluid pressure along the attacking surface is comparatively higher. Because of this pressure difference, the attacking surface is the high pressure surface and the opposite surface is called the low pressure surface or lee surface.

The increase in pressure along the high pressure surface combines with the decrease in pressure along the low pressure surface to create a lifting force upon the foil. This lifting force is substantially directed from the high pressure surface towards the low pressure surface. Varying the foil's angle of attack in this manner is important in swim fin designs because it enables lift to be generated on both the upstroke and the down stroke of the kicking cycle.

Although this method of generating lift is commonly used on prior swim fin designs, many problems occur that significantly reduce performance. One problem is that prior designs place the propulsion foil at excessively high angles of attack. In this situation, the flow begins to separate, or detach itself from the low pressure surface of the foil. When this occurs, the foil begins to stall. The separated flow forms an eddy which rotates around a substantially transverse axis above the low pressure surface. This eddy causes the fluid just above the low pressure surface to flow in a backward direction from the trailing edge towards the leading edge. This separation decreases lift since it reduces the amount of smooth flow occurring over the low pressure surface. This is a serious problem because smooth flow must exist in order for lift to be generated efficiently.

When the angle of attack becomes too high, the foil stalls completely and the flow along the low pressure surface separates into chaotic turbulence. This destroys lift by preventing a strong low pressure zone from forming over the low pressure surface, or lee surface. As a result, only a small difference in pressure exists between the opposing surfaces of the foil. Many prior fin designs suffer from this problem because they employ a horizontally aligned blade which is kicked vertically through the water. In this situation, the angle of attack is substantially close to 90 degrees, and therefore the blade is completely stalled out. This causes the blade to act more like an oar blade or paddle blade rather than a wing.

As well as destroying lift, stall conditions also cause high levels of drag. When areas of laminar flow (a flow condition where fluid passes over an object in a series of undisturbed layers) are abruptly converted into chaotic turbulent flow, a high drag condition known as transitional flow occurs. Because prior swim fin designs create stall conditions and chaotic turbulence along their low pressure surfaces, they generate high levels of drag from transitional flow.

Another problem that occurs at higher angles of attack is the formation of vortices along the outer side edges of the blade which cause induced drag. The difference in pressure existing between the attacking surface and the low pressure surface causes the fluid existing along the blade's attacking surface to flow outward toward the side edges of the blade, and then curl around the outer side edges toward the low pressure surface. As this happens, the swirling motion creates a streamwise tornado-like vortex along each side edge of the blade just above the blade's low pressure surface. As the water curls around the side edges of the blade, these vortices carry the water in an inward direction along the low pressure surface. After this happens, the vortices curl the water in a downward direction against the blade's low pressure surface. As this water is forced downward against the low pressure surface, it is moving in the opposite direction of desired lift thereby further reducing lift. This downward moving flow deflects the fluid leaving the trailing edge at an undesirable angle that is oppositely directed to the direction of desired lift. Because the direction of lift is perpendicular to the direction of flow, this downward deflected flow (called downwash) causes the direction of lift to tilt in a backward direction. Consequently, a significant component of this lifting force is pulling backward upon the blade in the opposite direction of blade's movement through the water. This backward force is called induced drag. Induced drag becomes greater as the blade's angle of attack is increased. Because prior designs typically use extremely high angles of attack, they experience high levels of induced drag.

In addition to increased drag, the downward deflected flow (downwash) behind the trailing edge significantly decreases the blade 's effective angle of attack which further reduces lift. As the flow behind the trailing edge is deflected downward (in the opposite direction of the lifting force) the angle of attack existing between the blade and this downward deflected flow (called the induced angle of attack) is less than the angle of attack existing between the blade and the oncoming flow (called the actual angle of attack). This reduces the blade's ability to create a significant difference in pressure between its opposing surfaces for a given angle of attack. This creates a significant decrease in lift on the blade.

The induced drag vortex also decreases performance by further decreasing the pressure difference between the opposing surfaces of the blade. As the water escapes sideways around the side edges of the blade, it expands in a spanwise direction along the blade's attacking surface. This decreases pressure along this surface, thereby decreasing lift. Also, because a substantial portion of the water flowing along the attacking surface is traveling in a more sideways direction and less of a lengthwise direction, this water is less able to assist in creating forward propulsion.

In addition, the high speed rotation of the vortex creates centrifugal force which evacuates fluid away from the center of each vortex (the vortex core). This creates a large decrease in pressure within the vortex core. The decreased pressure within this core is lower than the low pressure zone originally created along the low pressure surface by the foil's angle of attack. As a result, this new low pressure zone increases the rate at which water flows around the side edges away from the high pressure surface and toward the low pressure surface. This further decreases the pressure within the high pressure zone existing along the attacking surface. Because this reduces the overall pressure difference occurring across the blade, lift is significantly reduced.

As the vortex forces this outwardly escaping fluid down upon the blade's low pressure surface, fluid pressure is increased along this surface. This decreases lift by decreasing the difference in pressure occurring between the opposing surfaces of the blade. The swirling motion of each vortex also prevents water from flowing smoothly over a significant portion of the blade's low pressure surface. This decreases lift by preventing the blade from forming a strong low pressure center along a substantial portion of its low pressure surface. In addition, this disturbance within the flow over the low pressure surface (created by the induced drag vortex) can cause the blade to stall prematurely.

The problems associated with induced drag vortex formation increase as the blade's aspect ratio decreases. Aspect ratio can be described as the ratio of the blade's overall spanwise dimensions to its lengthwise dimensions. A blade that has an overall spanwise dimension that is relatively small in comparison to its overall lengthwise dimension, is considered to have a low aspect ratio. Low aspect ratio foils tend to produce stronger induced drag vortices, and are therefore highly inefficient.

Low aspect ratio blades are commonly found in prior swim fins which are used separately by each foot in a scissor-like kicking motion. The spanwise dimensions are limited in these designs in order to prevent the blade on one foot from colliding with the blade on the other foot during use. In this situation, the only way to increase the blade's surface area is to further increase the blade's lengthwise dimensions. This further reduces the blade's aspect ratio and increases induced drag.

Prior fin designs do not provide effective methods for reducing induced drag type vortices. Many designs use vertical ridge-like members which run substantially parallel to the lengthwise fin's center axis, and extend perpendicularly from at least one surface of the blade. The purpose is to encourage aftward flow, reduce spanwise flow, and stiffen the blade. However, these devices do not adequately reduce spanwise flow or induced drag type vortices. Moreover, these devices create additional drag of there own.

Another problem with prior fin designs is that they exhibit severe performance problems when they are used for swimming across the surface of the water. While kicking the fins at the water's surface, they break through the surface on the up stroke, and then on the down stroke they “catch” on the surface as they re-enter the water. Before the fin re-enters the water, it moves freely through the air and gains considerable speed. As the fin re-enters the water, a majority of the blade's attacking surface is oriented parallel to the water's surface. As a result, the blade slaps the surface of the water and its downward movement is abruptly stopped. This instantaneous deceleration creates high levels of strain for the user's ankles and lower leg muscles. Because downward movement ceases upon impact with the water, the strong downward momentum generated while the swim fin moves through the air (above the surface) is wasted and is not converted into forward propulsion after re-entering the water.

After this impact with the water's surface has occurred, the fin is slow to regain movement under water because of severe drag. This lag in time that occurs on the down stroke prevents the user from attaining fully productive kicking strokes. Before the downward moving fin is able to regain enough speed to begin effectively assisting with propulsion, it must be lifted out of the water again because the other fin (which is on its upstroke) has already broken the water's surface and is ready to begin its down stroke. Because it is difficult to kick both feet in an unsynchronized manner, this situation is awkward, strenuous, irritating, and highly inefficient. Over large distances, this problem can create substantial fatigue. This is particularly a problem for skin divers, body surfers, and body board surfers who spend most of their time kicking their fins along the water's surface. It is also a problem for SCUBA divers who swim along the surface to and from a dive site in an attempt to conserve their supply of compressed air. Fatigue and muscle strain to SCUBA divers during surface swims is particularly high because prior SCUBA type fins have significantly long lengthwise dimensions. This causes increased levels of torque to be applied to the diver's ankles and lower legs as the blade slaps the surface of the water. Because such longer fins create high levels of drag from a decreased aspect ratio, prior SCUBA type fins are significantly slow to re-gaining downward movement after catching on the water's surface. Even below the surface, such prior fins offer poor propulsion and high levels of drag which severely detract from overall diving pleasure.

Both U.S. Pat. Nos. 169,396 to Ahlstrom (1875), and 783,012 to Biedermann and Howald (1906) use two parallel propulsion blades which are mounted beneath the soul of the foot. The design is intended to be used with forward and backward kicking strokes along a horizontal plane. This stroke is awkward and extremely inefficient. Each of the parallel blades pivot along a lengthwise axis that extends parallel to the sole of the swimmers foot. The blades swing closed to a zero degree angle of attack on the forward stroke, and then swing open to about a 90 degree angle of attack on the backward, or propulsion stroke. This fin design attempts to gain propulsion from a pushing motion rather that a kicking motion. Both designs produce high levels of drag on the propulsion stroke and are not appropriate for use with contemporary vertical kicking strokes.

U.S. Pat. No. 2,950,487 to Woods (1954) uses a horizontal blade mounted on the upper surface of the foot which rotates around a transverse axis to achieve a reduced angle of attack on both the upstroke and the down stroke. The blade has a deep V-shaped cut down the center of the blade which divides the blade into a left half and a right half. These two sections are connected by a narrow strip of blade section running between them at the apex of the V-shaped cut out. Both left and right blade halves are fixed to each other within the same plane and no system is used to encourage any portion of these halves to flex, twist, or rotate in a way that can significantly reduce induced drag. The use of vertical ridges to encourage aftward flow does not significantly reduce outwardly directed spanwise flow and adds considerable drag.

U.S. Pat. No. 3,084,355 to Ciccotelli (1963) uses several narrow hydrofoils which rotate along a transverse axis and are mounted parallel to each other in a direction that is perpendicular to the direction of swimming. Although each hydrofoil has a substantially high aspect ratio, no system is used to adequately reduce induced drag.

U.S. Pat. No. 3,411,165 to Murdoch (1966) displays a fin which uses a narrow stiffening member that is located along each side of the blade, and a third stiffening member that is located along the central axis of the blade. Between the three members is a thin flexible web that is baggy so that when the blade is moved through the water, the web fills to form two belly shaped pockets along the length of the blade. These pockets increase in depth towards the trailing edge. Other ramifications include the use of a solitary pocket, as well as a plurality of such pockets.

A major problem with these designs is that the angle of attack is high and significant back pressure develops within each pocket. Although it is intended that the water is to be channeled towards the trailing edge, this does not efficiently occur. Because the water is striking the blade's webbing at a substantially high angle of attack (close to 90 degrees), the water resists making a sharp change in direction and is not efficiently accelerated toward the trailing edge. Consequently, the relatively large volume of water attempting to enter the pocket soon backs up and spills around the side edges of the pocket like an overfilled cup. This outwardly directed spanwise flow strengthens induced drag type vortices which further drain water from the pocket. Only a small amount of water is discharged aftward and propulsion is poor. No method is utilized to significantly decrease lee surface flow separation and induced drag.

French patent 1,501,208 to Barnoin (1967) employs two side by side blades which are oriented within a horizontal plane and extend from the toe of the foot compartment. The two blades are separated by a space between them. A vertically oriented blade is mounted to the front portion of the foot compartment and is located within the space existing between the two blades. This vertical blade is relatively thin and extends above and below the plane of the horizontal blades as well as a significant distance in front of the toe.

This vertical blade does not significantly contribute toward propulsion. It also adds drag and blocks water from flowing between the horizontal blades. Its extension below both the blades and the foot compartment make the fin difficult to walk on across land or stand up while in the water.

The most significant problem with this design is that the structure of each horizontally aligned blade prevents it from significantly twisting about an axis that is substantially parallel with its length. No structure is offered to encourage such twisting to occur in an efficient manner. In addition, no mention is given to suggest a need for such twisting. As a result, the blades stall through the water during use.

Although each blade is made of flexible material, its structure creates stresses within the blades' material which prevent the blades from achieving a substantially twisted shape along their lengths during kicking strokes. If any twisting forces are applied to the blades during use, significantly high levels of torsional stress forces occur in the form of tension and compression within the blades' material. These stress forces occur diagonally across the entire length of each blade. As a result, a large volume of each blade's material must succumb to these forces before any twisting can occur. A simple bending motion across each flexible blade places a much smaller volume of each blade's material under the influence of tension and compression forces than that would occur during a twisting motion. Consequently, the exertion of water pressure causes the blade to bend backwards around a substantially transverse axis under the exertion of water pressure created during use before it can begin to attain a twisted shape around a substantially lengthwise axis.

Although Barnoin's end view drawing shows that the blades taper in a sideways direction from the outer side edge toward the inner side edge, the blades remain highly resistant to twisting around a lengthwise axis. Barnoin does not state that the inner side edges of each blade should be more flexible than the outer side edge. However, even if it is assumed that the tapered inner side edge is more flexible, only a significantly small amount of flexing occurs because each blade tapers in a uniform manner from its outer side edge to its inner side edge. Such uniform tapering causes the resistive forces of tension and compression to be exerted over an increased volume of material within each blade. This is because the cross sectional thickness of the blade is significantly thick over most of its span. This substantially increases each blade's resistance to bending around a lengthwise axis. Also, as each blade bends back under water pressure around a transverse axis, each blade becomes arched across its length. This makes each blade even more resistant to bending around a lengthwise axis.

These torsional stress forces existing within each blade that inhibit twisting occupy a significantly large portion of each blade's material, and no adequate system or structure is used to control these stress forces in a manner that permits the blades to twist around a significantly lengthwise axis. In Barnoin's design, these stress forces are strongest on an area of each blade that exists behind (toward the foot pocket) an imaginary line which originates substantially from the root portion of each blade's inner side edge near the foot pocket and extends to a point on each blade's outer side edge that is about half way between the root and the trailing edge. The imaginary line actually originates at a position along the inner side edge that is approximately one third of the way between the foot pocket and the trailing edge. This is because the tapered spanwise cross sectional shape of each blade transfers anti-bending stress forces from the thicker outer side edge to the thinner inner side edge, thereby artificially stiffening the inner side edge of each blade. This imaginary line then extends approximately to the mid-way portion of each blade's outer side edge because the outer half of each blade is shown and described as tapering significantly along its length and becoming highly flexible about half way between the root and the trailing tip. Between this transversely directed imaginary line and the foot pocket, each blade is plagued with high levels of stress forces which prevent this area from twisting during kicks. This causes flow separation and stall conditions to occur along the low pressure surface of these blade portions.

The areas of each blade which are forward (toward the trailing edge and away form the foot pocket) of this imaginary line are much less effected by these stress forces. If each blade is made from a highly flexible material, then each blade bends around this transversely directed imaginary line. This causes the portions of each blade between this imaginary line and the trailing edge to deform to a reduced angle of attack by bending around a substantially transverse axis which is substantially parallel to the imaginary line. Because this axis is slightly swept back, the outer portions of each blade bend in a slightly anhedral manner. However, this anhedral angle is not sufficiently anhedral enough to create any significant reductions in lee surface flow separation, induced drag, or outward spanwise cross flow conditions. This is because the blades are bending around a highly transversely directed axis. In addition, when highly flexible materials are used in this design, the outer half of each blade collapses to a zero, or near zero angle of attack. This creates high levels of lost motion between strokes and does not permit significant levels of lift to be generated.

Another problem not anticipated by Barnoin is that if the two separate blades are permitted to deform in a slightly anhedral manner, a small amount of water can be deflected toward the space between the blades. This inwardly defected flow creates an equal and oppositely directed force against each blade which pushes outward on each blade in a spanwise direction. As a result, the portions of each blade existing between the imaginary line and the trailing edge spread apart a significantly large distance from each other and collapse to an excessively low angle of attack. Barnoin does not mention that he is aware of any such outward spanwise deformation of the blades and does not describe a method or structure that is capable of effectively controlling this undesirable occurrence.

As each blade pair spreads apart from each other on each of the users feet, the overall span of each swim fin increases substantially. This can cause the swim fin on one foot collide with the swim fin on the other foot as the swim fins pass each other during use in a scissor-like kicking stroke. In addition, much of the energy created by the kicking motion is wasted because it is used to spread the blades apart rather than propel the swimmer in a forward direction. Significantly high levels of lost motion also occur during the time that the blades are spreading apart at the beginning of each stroke, as well as when they are coming back together at the end of each stroke. This combines with the lost motion occurring as each blade bends backward around a transverse axis. The stress on each blade created by this spreading motion also causes each blade to collapse to an excessively low angle of attack that is incapable of producing significant levels of lift.

Because no structural solution to these problems are mentioned, the only way that this spreading motion can be controlled within the confines of Barnoin's design is to make the blades out of a more rigid material. This only further increases each blade's resistance to twisting or flexing around a lengthwise axis. Consequently, using a more rigid blade causes a larger portion of each blade's surface area to suffer from stall conditions, induced drag vortex formation, and inadequate lift generation just as making the blades out of a more flexible material causes a larger portion of each blade to bend backward around a transverse axis to an excessively low angle of attack which is incapable of generating significant levels of lift. Either way, serious problems result which destroy performance.

If Barnoin's design is made with sufficiently rigid enough blades to avoid excessive levels of lost motion and spanwise spreading, the spanwise tapering of the blades causes the anti-bending stress forces at the outer side edges of the blades to be transferred to the inner side edges of the blades. This stiffens the inner side edges of each blade and prevents them from deforming significantly under water pressure. As a result, a significant difference in rigidity does not exist between the outer side edges and inner side edges of the blades. This prevents the blades from bending around a significantly lengthwise axis.

If any flexing occurs during use on such rigid blades, it can occur only on an insignificantly small portion of each blade's inner side edge. Because the cross sectional shape of this design transfers anti-bending stress forces from the outer side edge to the inner side edge of each blade, the majority of each blade's spanwise alignment remains at excessively high angles of attack. This permits high levels of flow separation to occur as water spills around the outer side edges of each blade. This stalls the blades and produces high levels of drag from induced drag vortices and transitional flow. In addition, the transference of this stiffening effect to the inner side edge of each blade causes the inner side edge of each blade to also be at an excessively high angle of attack. This causes high levels of flow separation to occur at this location. As a result, significantly strong induced drag vortices form along the inner side edge and outer side edge of each blade's lee surface. This creates high levels of drag and inadequate levels of lift.

German patent 259,353 to Braunkohlen (1987) suffers from many of the same problems and structural inadequacies as Barnoin's fin discussed above. Braunkohlen uses a wedge like incision along the fin's center axis which leads from the trailing edge of the fin to a small circular recess near the toe area of the foot pocket. This incision divides the blade region into left and right blade halves. Each blade half decreases in thickness from its outer side edge to its inner side edge (the incision side of each blade half) to make the blade continuously weaker toward the incision. The tapering reaches a uniform thickness along the incision side of the blade.

Gradation markings in the drawing show that each blade also decreases in thickness and strength from the base of the blade (near the foot pocket) towards its trailing edge which is extreme end of each blade located in front of the foot pocket. These gradation markings show that a significantly large portion of each blade's trailing portion is as thin and structurally weak as the inner edge of each blade bordering the incision. This causes a significantly large portion of each blade's surface area to be highly vulnerable to excessive deformation around a transversely aligned axis. This type bending creates an arched contour around this a transverse axis which significantly increases each blade's resistance to twisting around a significantly lengthwise axis. No adequate structure is offered by Braunkohlen to compensate for this occurrence.

Because Braunkohlen's blades are highly vulnerable to bending around a transverse axis, a substantially large portion of each blade's surface area can bend to a zero or near zero angle of attack during use. At such low angles of attack, the blades are inefficient at generating significant levels of lift. High levels of lost motion occur as the blades “flop” loosely back and forth at the inversion point of each alternating stroke. As a result, much of the energy used to kick the blades through the water is used up deforming the blades to inefficient orientations rather than being converted into propulsion.

Because no adequate structure is shown to significantly reduce this problem, the only way to reduce lost motion is to make the blades out of a sufficiently rigid enough material to prevent excessive levels of bending around a transverse axis from occurring during strokes. By making the blades out of a stiffer material, high levels of stress forces are allowed to build up within each blade's material. Because the blades taper in a uniform manner from outer side edge to inner side edge, these stress are transferred to the weaker portions of the blade bordering the incision. This significantly stiffens the inner side edge of each blade and prevents a significant portion of each blade near the incision from flexing when water pressure is applied during strokes. This prevents each blade from bending or twisting about an axis that is substantially parallel to the lengthwise alignment of each blade. This stiffening effect causes a significantly large portion of each blade's outer side edges to remain at an excessively high angle of attack during use. This causes high levels of separation to occur as the water passes around each blade's outer side edge. In addition, the transference of this stiffening effect to the inner edge of each blade bordering the incision causes the inner side edge of each blade to also be at an excessively high angle of attack. This causes high levels of flow separation to occur at this location. As a result, significantly strong induced drag vortices form along the inner side edge and outer side edge of each blade's lee surface. This creates high levels of drag and inadequate levels of lift.

Also, Braunkohlen does not anticipate that any significant amount of deformation along the inner side edge of each blade half deflects water toward the incision and thus creates an outward spanwise force on each blade half. If the blades are flexible enough to permit significant deformation to occur near the incision, this outward force causes the blade halves to spread apart from each other during use. Braunkohlen does not mention a method for effectively countering this outward force and no adequate structural system is provided for controlling or reducing such spanwise spreading. As a result, this design is vulnerable to high levels of lost motion as the blade halves spread apart from each other at the beginning of each stroke and coming back together at the end of each stroke. Also, the energy expended in deforming the blades in a spanwise direction is wasted since it is not converted into propulsion.

Another problem with this design is that while the blades are spreading apart from each other, each blade buckles under stress and bends around a substantially transverse axis. This is largely because the trailing portions of each blade are much weaker and more flexible than the leading portions of each blade. This causes a significantly large portion of each blade to bend to an excessively low angle of attack which is inefficient at generating lift.

Because no structural features are used to efficiently overcome these problems and exert control over each blade's shape, any attempt to merely change each blade's flexibility cannot not significantly improve performance. While an increase in rigidity causes more of the fin's surface area to remain at an excessively high angle of attack, an increase in flexibility only increases the tendency for each blade to bend backward around a transverse axis and spread apart from each other in a spanwise direction. In either situation, flow separation is high and lift is low.

The circular recess at the base of the incision is shown to be relatively small and only slightly larger than the narrow incision. Braunkohlen states that it's purpose is to prevent the base of the incision from tearing during use. Also, the span of the circular recess is proportionally too small for it to have any other benefit to performance. The elevated section behind the recess is also used only to reinforce the base of the incision so that the fin is less likely to tear along the center axis.

French patent 1,501,208 to Barnoin (1967) also displays a differently configured alternate embodiment which uses four blades attached to one foot compartment. An end view drawing from the tips of the blades illustrates that the four blades are arranged in a cross sectional configuration that is substantially X-shaped. This orientation places the four blades within two diagonal planes which cross each other at the fin's center axis. The blades are spaced apart from each other to form a gap at the middle of the X-configuration. The drawing reveals that each blade tapers in thickness towards this gap to form a sharp inner side edge and a thicker outer side edge.

The X-configuration of the blades is highly inefficient and causes excessive drag while kicking because the trailing blades on each stroke prevent the leading blades from efficiently generating lift. When the fin is kicked upward, the upper pair of blades are the leading blades and the lower pair of blades are the trailing blades. When the fin is kicked down, the opposite occurs. Although in both situations the leading blades are angled in anhedral manner to offer a reduced angle of attack, the trailing blades are always angled in a dihedral manner that prevents the leading blades from generating lift. Because the trailing blades are positioned at an extremely high angle of attack relative to the water curving around the outboard edges of the leading blades, the path of water traveling along the low pressure surfaces of the leading blades becomes blocked by the orientation of the trailing blades. This prevents the water curving around the lee surface of the leading blades from efficiently joining the water that is leaving the attacking side of the leading blades at the inner side edge of the leading blades. This prevents the formation of a significantly strong a low pressure zone along the lee pressure surface of the leading blades, and therefore prevents significant levels of lift from being generated.

The high angle of attack of the trailing blades also increases induced drag vortex formation around the outer side edges of the leading blades by creating a pocket on each side of the fin between the leading and trailing blades. The induced drag vortex becomes trapped, protected, and amplified within this pocket. The separation created by this vortex completely stalls each leading blade. This creates high levels of drag and destroys lift. In addition, the swirling eddy-like motion of this trapped induced drag vortex causes the water flowing along the lee surface of the attacking blades to flow backward from the inner side edge toward the outer side edge. This backward directed flow created by this eddy-like swirling motion is highly undesirable since it occurs in the opposite direction of what is needed to generate lift on the leading blades.

This undesirable eddy also reverses the direction of expected flow along the attacking surface of the trailing blades so that water along these surfaces flow from the outer side edge toward the inner side edge on each blade. This prevents lift from being generated by the trailing blades as well.

Other problems of this design occur as the flexible blades deform in an uneven manner during kicking strokes. When water pressure is exerted against the leading pair of blades, the flexibility of these blades enable them to bend backward around a transverse axis and press against the trailing blades. Because the trailing pair of blades are not exposed to the oncoming flow, they remain relatively straight while the leading blades push against them. As the inner side edges of the leading blades contact the inner side edges of the trailing blades, the path of water traveling along the low pressure surfaces of the leading blades becomes completely blocked so that it cannot merge with the water leaving the attacking side of the leading blades at the inner side edge of the leading blades. This prevents a low pressure zone from forming along the low pressure surface of the leading blades, and therefore prevents lift from being generated.

Although the leading pair of blades are anhedrally oriented in a manner that can encourage water to flow toward the void existing between the two leading blades, no method or structure is discussed for countering the spanwise directed outward forces exerted upon each blade by such inward flowing water. Because the blades are flexible and vulnerable to this outward force, they spread apart from each other in a transverse direction. This wastes energy, creates lost motion, and produces awkward blade orientations that inhibit performance.

In addition to offering poor levels of performance, this arrangement of four blades increases production costs through increased materials, parts, and steps of assembly. Also, both the added weight and bulk increase the cost of packaging, shipping, and storage. Such added weight and bulk inconveniences the user as well.

U.S. Pat. No. 3,934,290 to Le Vasseur (1976) uses a single fin which receives both feet of the user for use in dolphin style kicking strokes. Because no system is used to reduce outwardly directed spanwise flow along the attacking surface of the blade near the tips, this design is subject to high levels of induced drag.

Le Vasseur uses a series of vents which are aligned in a spanwise direction. The passage ways of these vents extend from above the toe of the foot pockets diagonally through the blade to a line near the trailing edge on the underside of the blade. This orientation only permits the vents to be used on the down stroke. These vents do not significantly reduce the creation of induced drag.

U.S. Pat. No. 4,007,506 to Rasmussen (1977) uses a series of rib-like stiffeners arranged in a lengthwise manner along the blade of a swim fin. The ribs are intended to cause the blade to deform around a transverse axis so that the trailing portions of the blade curl in the direction of the kicking stroke. The blade employs no method for adequately decreasing induced drag. The blade's high angle of attack stalls the blade and prevents smooth flow from occurring along its low pressure surface.

The ribs are not intended to encourage the blade to twist or bend in a manner that decreases separation along the low pressure surface of the blade. Instead, the ribs prevent the blade from bending to a lower angle of attack. Rasmussen's uses ribs in an attempt to increase the angle of attack existing at the outer portions of the blade.

U.S. Pat. No. 4,025,977 to Cronin (1977) shows a fin in which the blade is aligned with the swimmers lower leg. This design is highly inefficient on the upstroke. No system is used to reduce the presence of induced drag.

U.S. Pat. No. 4,521,220 to Schoofs (1985) uses a fin designed for use by breast stroke swimmers. It employs a horizontal blade with a transversely directed asymmetric hydrofoil shape. The design is stated to be stiff enough to hold its shape during swimming. This prevents the fin from being effective when used in a conventional up and down scissor-like kicking stroke. This is because the hydrofoil shape is perpendicular to the direction of such strokes. This causes the blade to stall. Even during breaststroke kicking styles, no system is employed to significantly reduce induced drag.

U.S. Pat. No. 4,541,810 to Wenzel (1985) employs a single fin designed to be used by both feet in a dolphin style kicking motion. The design uses a stiff, load bearing Y-shaped frame member, and a highly resilient webbing secured between the forks of the frame The web is intended to cup the flowing water by arching its surface as the forks flex inward in response to the water pressure placed on the web during strokes.

This method of creating a cup to channel water toward the center of the fin and out the trailing edge is highly inefficient since it quickly builds up excessive back pressure within the webbing's pocket. This back pressure reverts flow back over the outboard side edges of the fin like an over filled cup. This increases the formation of induced drag vortices along the low pressure surface along these side edges. These vortices create drag, decrease lift and quickly drain the high pressure center occurring in the arched pocket. Because a significantly large portion of the water flowing along the attacking surface spills sideways around the outer side edges of the hydrofoil, forward propulsion is poor and drag is high.

Another problem is that as the webbing bows under water pressure, it forms a parabolic shape in which the outer side edges of the webbing experiences the least amount of curvature and the center regions of the webbing experience the greatest amount of curvature. This type of parabolic shape occurs whenever an evenly distributed load is applied to a material that is suspended across a surrounding frame. This parabolic shape cause the outer edges of the webbing near the frame member to remain at an excessively high angle of attack relative to the oncoming water. The high angles of attack exhibited by the leading and side edges of the blade also create separation and stall conditions along the low pressure surface of the blade which further reduce lift and increase drag.

Although some of Wensel's embodiments show a deep V-shaped cut-out section along the trailing edge, no system is used to control the shape of these trailing portions as they deform. The cut-out along the trailing edge consists of two concavely curved outer portions existing near the tips, as well as two convexly curved inner portions which meet at the center of the webbing to form a small and narrow V-cut which ends in a sharp point. An imaginary straight line extending from a tangent of each concave outer portion to the sharp point of the V-cut at the center of the trailing edge, is the rearward limit (toward the trailing edge) of the spanwise tension forces which occur across the resilient webbing. The region of the webbing existing between this imaginary line and the forked frame are highly resistant to twisting around a lengthwise axis. This is because this region is plagued with anti-twisting stress forces of compression and expansion. On the other hand, the portions of the webbing which exist between this imaginary line and the trailing edge are structurally weaker than the rest of the webbing because this area is significantly less affected by the tension forces occurring across the resilient webbing which are created while bowing under water pressure. As a result, the convex portions of the trailing edge region tend to fold substantially along this imaginary line to a significantly lower angle of attack than the rest of the webbing during use. This creates an abrupt change in the webbing's contour and causes significant drag and loss of lift. Wenzel uses no system to support this zone. Because his webbing is highly resilient and easily deformable, it is especially vulnerable to this problem. The use of a more rigid material for the webbing only further inhibits the webbing's ability to bow under water pressure.

Another problem with his design is that the forked ends of the stiff load bearing frame member will not adequately flex inward enough to create significant results. If the forked portions of the frame member are made strong enough to substantially maintain its lengthwise alignment during strokes and not bend excessively backward around a transverse axis under the exertion of water pressure, it will not be flexible enough to permit significant flexing to occur in an inward spanwise direction. This is primarily because the spanwise tension across the webbing, which is responsible for causing the forked ends of the frame to flex inward, is significantly less than the force created by drag which pushes backward against the forks in a direction that is opposite to the direction in which the fin is kicked through the water. This problem is further increased because the forks have a spanwise hydrofoil shape that causes each fork act like a sideways I-beam which is significantly more resistant to horizontal flexing (spanwise flexing) than to vertical flexing (backward bending around a transverse axis). If the forks are flexible enough to bend sufficiently inward to form a pocket in the webbing, they will not be rigid enough to avoid excessive backward bending (opposite to the fin's direction of stroke) around a transverse axis to an excessively low angle of attack during use.

The structure of the forks also prevents them from experiencing significant levels of twisting during use. When twisting forces are applied to the forks, high levels of torsional stress forces build up within the fork's material. In order for twisting to occur, the material must succumb to these stress forces and undergo significantly large amounts of expansion and compression across a majority of its length and volume. Since a significantly large portion of the fork's material is forced to experience relatively high levels of compression and expansion, resistance to such twisting is significantly high. In comparison, a simple bending motion around a transverse axis permits significantly reduced levels of compression and expansion to occur over a significantly smaller portion of the fork's material. As a result, solid objects many times less resistance to bending along the length than to twisting about their length. Because of this, the forks will not adequately twist during use in an amount sufficient to significantly reduce stall conditions and flow separation along the edges of the hydrofoil. This causes the hydrofoil shaped forks to remain at an excessively high angle of attack during use, thus creating further drag and loss of lift.

If the forks are made from a sufficiently resilient material to permit a significant amount of twisting to occur, it will bend backward and collapse around a transverse axis because the comparative resistance to such deformation is many times lower than that created during a twisting motion. In addition, the forces which attempt to twist the forks along their length (created from tension across the webbing), are significantly weaker than the forces created by drag on the hydrofoil which attempt to bend the forks backward in the opposite direction of the blade's motion through the water.

If the forks are rigid enough to withstand the force of drag on the fin without excessive deformation, than they are not flexible enough to twist significantly along their length. Because of this, the spanwise hydrofoil shape of each fork remains at a high angle of attack during use. This creates high levels of flow separation along the lee surface of the fork during use. This increases induced drag vortex formation, stall conditions, and transitional flow. Because the leading edge portions of the fork also remain at an excessively high angle of attack, the leading edge of the hydrofoil stalls as well. As a result, drag is high and lift is poor.

U.S. Pat. No. 4,738,645 to Garofalo (1988) employs a single blade which deforms under water pressure to form a concave channel for directing water toward the trailing edge. The blade uses two narrow and lengthwise directed strips of flexible membrane located near the stiffening rails on each side edge of the blade. Between the two narrow strips of flexible membrane is a stiff and centrally located blade portion which is attached to the inner side edges of the two membrane strips. When the fin is kicked, water pressure pushes against the stiff central blade portion which applies tension to the flexible strips. As this occurs, a loose fold within each flexible strip elongates, thereby enabling the central blade portion to drop so that fin forms a scoop like channel.

Although this shape is intended to reduce flow around the sides of the blade and increase aftward flow, it does not do so efficiently and suffers from high levels of drag. Because the blade's central portion is at a significantly high angle of attack, the water's inertia resists a quick change in flow direction as it strikes the blade's central portion. This creates a significant amount of back pressure within the channel. Because this design lacks a method for reducing such back pressure, the water backs up within the channel and spills sideways around the side edges of the blade like an overflowing cup. As this happens, the flow separates from the blade's low pressure surface. This increases induced drag and destroys lift. The vertical ridges along the side edges of the blade do not efficiently reduce this problem and only add extra drag of their own.

Another problem is that the portion of the blade that lies between the side rails and the flexible strip is relatively wide and has significant torsional stress forces within it which prevent it from twisting significantly along its length during strokes. As a result, this portion always remains at a high angle of attack which increases the strength of induced drag vortices. Both the central and side portions of the blade remain at a high angle of attack which stalls the fin. This depletes lift and further increases drag.

U.S. Pat. No. 4,781,637 to Caires (1988) shows a single fin designed to be used by both feet in a dolphin style kicking motion. It uses a transversely aligned hydrofoil that extends from both sides of a centrally located foot pocket. The hydrofoil is made of a flexible material which has a stiffening rod located within it that runs parallel with the hydrofoil's leading edge. The flexible material is loosely disposed around the stiffening rod to permit rotation. A plate-like member is located within the central portion of the hydrofoil to prevent the blade from rotating around the stiffening rod at this location.

Although the tips are intended to twist about the rod to a reduced angle of attack while the center region remains at a high angle of attack, the centrally located plate-like member introduces stress forces within the hydrofoil's flexible material that strongly oppose such twisting. When water pressure applies a twisting force against the hydrofoil, torsional stresses of compression and tension build up within the flexible material in directions that are diagonal to the axis of rotation. While compression forces exist along one diagonal direction, tension forces exist along another direction that is substantially perpendicular to the direction of compression. This creates a complex network of stress forces within the flexible material between the plate-like member and the outer tips of the stiffening rod. Resistance to twisting is high because these forces are exerted across significant distances, and therefore large volumes of the flexible material must experience significant amounts of expansion and compression before twisting can occur. Because no adequate method is used to reduce these stress forces within the blade's material, the blade demonstrates high levels of resistance to any twisting forces created by water pressure.

This is a major problem since the twisting force created by water pressure during strokes is significantly small. If the hydrofoil cannot twist quickly and substantially under conditions of significantly light pressure, the blade remains at an excessively high angle of attack which causes flow separation to occur along the lee surface thereby stalling the hydrofoil. When the flow quickly separates from the low pressure surface in this manner, the twisting force created by the water pressure drops off dramatically. Because the resistance to twisting is at a high, and the twisting force provided by water pressure is significantly low, the blade remains at a high angle of attack. This destroys lift and creates high levels of drag. Caires does not mention that he recognizes these problems created by torsional stress forces and offers no solution for controlling them.

Another problem with this design is that a much of the hydrofoil's flexible material is poorly supported by the stiffening system. This makes the foil vulnerable to bending forces which can adversely deform the foil's shape during use. The areas that are most vulnerable to such bending forces are located aft (towards the trailing edge) of an imaginary line which extends from each outboard tip of the stiffening rod, to the trailing portion of the centrally located stiffening plate. The areas between this imaginary line and the trailing edge bend abruptly to a reduced angle of attack. This bending occurs along an axis that is substantially parallel to this imaginary line.

This abrupt change in contour creates an undesirable cross sectional hydrofoil shape that causes the low pressure surface to become concavely curved, and causes the attacking surface to become convexly curved. According to Bernoulli's principle, this shape reduces lift because it decreases the distance that the water must travel along the low pressure surface, while it simultaneously increases the distance that the water must travel along the high pressure surface (attacking surface). This reduces the overall difference in pressure existing between the low pressure surface and the attacking surface. In addition, the concavely curved low pressure surface formed during strokes also encourages the flow to separate from this surface. This further decreases lift and increases drag. While the trailing portions of the foil bend in this manner during use, the leading portions of the foil existing between the imaginary line and the leading edge remain at a high angle of attack because of the anti-twisting stress forces which exist in this region. This is highly inefficient because it stalls the leading portion of the blade.

Because of the structural inadequacies of this design, any attempts to merely change the resiliency of the blade can not significantly improve performance. If highly flexible materials are used to make the hydrofoil blade, the portions of the blade existing aft of the imaginary line collapse completely to a zero, or near zero angle of attack. This dramatically reduces leverage on the hydrofoil, and therefore reduces the twisting force created by the water pressure. Thus, even with highly flexible materials, the entire leading edge remains in a stall position during strokes. This destroys lift and creates drag.

Although the use of stiffer materials can reduce the abruptness and degree of this bending tendency, it also causes a larger portion of the blade to remain at an excessively high angle of attack. This is because less flexible materials permit the stiffening effect of the anti-twisting stress forces (present in the leading portions of the foil) to extend farther out towards the trailing edge. A major dilemma thus results: if the flexible material within the hydrofoil is resilient enough to twist under extremely light pressure its trailing portions collapse to an excessively low angle of attack during use; however, if the flexible material is sturdy enough to prevent the inadequately supported trailing portions from bending excessively, the material is no longer resilient enough to twist sufficiently under significantly light pressure. As a result, this design is highly inefficient.

Another problem displayed by the drawings is that the stiffening system within the leading edge of the hydrofoil does not extend far enough toward the outer tips of the hydrofoil. This permits the highly resilient material at the tips to flex in an uncontrolled and undesirable manner when the fin is kicked through the water. Significantly large areas of improperly supported resilient material are able to bend to an orientation that produces significant turbulence and drag. This is especially a problem at the outer side edges because the outboard flow conditions produced by induced drag vortices force the unsupported tips to bend dihedrally, along a chordwise axis. This encourages outwardly directed flow and therefore increases the strength of induced drag vortices. No method is employed to adequately reduce the formation of induced drag vortices.

The same problem is seen in the design which places the blades in a slightly swept back configuration. Lack of adequate support along the outer edges of the tips, permit the flexible material, which extends aft of the ends of the stiffening rod, to bend along a transverse axis. At the same time, dihedral bending occurs at the outboard ends of the flexible material because the span of the stiffening rod is significantly smaller than the span of the hydrofoil.

In the swept back version of his design, the blade-halves are not swept back enough to encourage a significant inward directed flow from occurring along the attacking surface of each blade-half. Although the extreme outer edges of the blade are significantly swept, these highly swept portions of the blade are not properly supported and therefore encourage outward spanwise directed flow to occur along the attacking surface near the tips of each blade-half.

Another problem with this design is that the significantly high aspect ratios that Caires uses causes the spanwise dimensions to be significantly wide. This greatly reduces the ability of the swimmer to use this design in confined areas such as narrow passageways, arches, ravines, caves, kelp forests, and ship wrecks. Such wide spanwise dimensions also prevent this design from being used on separate fins for each foot for use in a scissor-like kicking stroke since the fin on one foot can collide with the fin on the other foot during use.

An alternate embodiment shows a cross sectional view of a hydrofoil having a chordwise linkage member suspended within a hollow hydrofoil made from a resilient plastic skin. The leading portion of this member is pivotally linked to a transverse stiffening member located within the leading edge of the hydrofoil. The trailing portion of the linkage member extends rearward and attaches to the inside of the trailing edge of the hollow hydrofoil. The only connection between the linkage member and the hollow skin is at the trailing edge. All other portions of the skin are free from the linkage member.

The sole purpose of this linkage member is to create a variance in skin tension between the upper and lower surfaces of the hollow hydrofoil so that an asymmetrical shape is created during use. The chordwise linkage members are not used, or intended to be used in a manner that can relieve or control anti-twisting stress forces that are created within the blade's material during use. This prevents the hydrofoil from achieving a smooth and efficient contour when twisting forces are applied to the blade.

Because of the structure of this design, the loosely disposed skin tends to buckle and wrinkle when anti-twisting stress forces of compression and tension build up within it during use. Because these stress forces are created diagonally across the span of the skin, diagonally directed wrinkles form across the upper and lower slices of the hydrofoil. These wrinkles can be observed forming when one end of a hollow object such as a water bottle (semi-filled with either water or air) is twisted while the opposite end is held stationary. Because the skin on the upper and lower surfaces is loosely disposed above and below each linkage member within the hydrofoil, this buckling tendency cannot be controlled by the linkage members. The greater the degree of spanwise twisting, the greater the degree of buckling and wrinkling within the skin. The resulting wrinkles create turbulence and separation. This destroys lift and creates high levels of drag. Also, because two separate skins are used (upper surface and lower surface) twice as much resistance to twisting (from tension forces) results than if only a single membrane is used.

U.S. Pat. No. 4,857,024 to Evans (1989) shows a fin which has a flexible blade with a V-shaped cut along the trailing edge. The blade does not form an anhedrally oriented channel along the attacking surface of the blade during strokes. The V-shaped cut along the trailing edge only extends a relatively small distance in from the trailing tips and does not cover a significant length of the blade. Because of this, the V-shaped cut is not in a position for significantly preventing excessive back pressure within the fluid existing along the center regions of the blade.

The blade is thickest and most rigid along its center axis. The blade decreases in thickness on either side of this center axis toward its side edges for increased flexibility near these edges. The center axis of the blade lies in the same horizontal plane as the foot pocket, while the portions on either side of the center axis angle upward toward the outer side edges. These angled portions form a convex up V-shaped valley. When this upper surface is kicked forward the outer portions start out in an anhedral orientation relative to the direction of movement. However, as soon as water pressure is applied against these upwardly angled outer portions, these portions flex back into alignment with the horizontal plane of the center axis, and then continue to flex beyond this point to assume a dihedral orientation during this upwardly directed kicking stroke. At this point, the stiffer central portion of the blade arches back around a transverse axis to an excessively reduced angle of attack where the blade then slashes back at the end of the stroke in a snapping motion to propel the swimmer forward.

This snapping motion acts more like a paddle than a wing, rather than creating lift like a wing, this design snaps backward at such a high angle of attack that no smooth flow can occur along the lee surface of the blade. Consequently, this snapping motion attempts to push the swimmer forward by applying the stored energy within the backward bent blade against the drag that the blade creates within the water. This design creates significantly high levels of drag during use and causes significant levels of ankle fatigue. Also, the excessive backward deformation of the blade creates significant levels of lost motion during strokes.

On the opposite stroke where the lower surface of blade is the attacking surface, the angled outer ends are oriented at a dihedral angle relative to the direction of travel. The water pressure created during this stroke only increases this dihedral angle. This orientation directs water away from the center of the blade and toward the outer side edges. This increases induced drag and decreases lift. No system is used to create smooth flow conditions along the low pressure surface of the blade.

This design is especially difficult to use while swimming along the surface. Since the swimmer is usually face down in the water, the anhedrally oriented upper surface is also face down in the water. Because no system is used to reduce back pressure along the attacking surface of the blade, the anhedral blade acts like a parachute when re-entering the water. This brings the fin to an immediate stop as the blade strikes the surface. This transfers significant levels of strain to the user's ankles and lower legs. The energy initially built up on the doffs stroke is wasted and new energy must be applied in order to regain movement.

U.S. Pat. No. 4,934,971 to Picken (1990) shows a fin which uses a blade that pivots around a transverse axis in order to achieve a decreased angle of attack on each stroke. Because the distance between the pivoting axis and the trailing edge is significantly large, the trailing edge sweeps up and down over a considerable distance between strokes until it switches over to its new position. During this movement, lost motion occurs since little of the swimmer's kicking motion is permitted to assist with propulsion. The greater the reduction in the angle of attack occurring on each stroke, the greater this problem becomes. If the blade is allowed to pivot to a low enough angle of attack to prevent the blade from stalling, high levels of last motion render the blade highly inefficient.

Picken uses an elliptical shaped blade design in an effort to decrease induced drag. Because of its low aspect ratio and the significantly high angles of attack used during strokes, this design does not effectively reduce induced drag. In addition, no adequate method is offered for effectively discouraging outward flow along the side edges of the blade.

U.S. Pat. No. 4,940,437 to Piatt (1990) uses a swim fin blade that has a stiffening rod within the blade which runs along its center axis. This stiffening rod is not used in a manner that effectively reduces induced drag. No twisting motion is encouraged within the blade along a lengthwise axis.

Many of the same problems that exist with prior swim fin designs also exist in prior flexible propulsion blade designs that oscillate back and forth to generate propulsion. All such designs lack an efficient method for reducing induced drag and stall conditions. Designs that are intended to flex do not include an effective method for controlling or reducing undesirable stress forces within the blade that cause the blade to deform in an undesirable manner.

U.S. Pat. No. 144,538 to Harsen (1873) uses a series of pendulous arms driven by a rotating worm shaft to produce a wriggling or worm-like action. The system is dependent on a rotating worm shaft to provide shape. No system is used to reduce induced drag vortex formation along the submerged bottom edge of the blade system.

A book reference found in the United States Patent and Trademark Office in class 115/subclass 28 labeled “3302 of 1880” shows a horizontally aligned reciprocating propulsion blade. The planar blade has a narrow void existing along the center axis of the blade which divides the blade into two side-by-side blade halves. This void originates at the trailing edge of the blade and ends near the base of the blade. No system is used to encourage the blades to twist along a substantially lengthwise axis, and no system is used to encourage water to flow away from the outer side edges of each blade half. The blades only flex backward around a transverse axis in response to water pressure. Consequently, the blade stalls through the water and produces high levels of drag and poor propulsion.

Spanish patent 17,033 to Gibert (1890) shows a vertically aligned flexible oscillating propeller blade that has a triangular shaped void along its center axis that divides the blade into two blade-halves. The void is widest at the trailing edge and converges to a point at the base of the blade. No system is used to encourage the blade to twist or bend around a lengthwise axis. The blade-halves stall through the water and produce high levels of drag and poor levels of lift.

U.S. Pat. No. 787,291 to Michiels shows a vertically aligned oscillating propulsion system which has two blades with a space existing between them. Both blades lie within the same vertical plane. No system is used to permit the blades to twist along a lengthwise (chordwise) axis, and no system is used to reduce stalling or induced drag.

U.S. Pat. No. 871,059 to Douse (1907) shows a vertically aligned oscillating propeller which has a caudal shaped frame with a flexible membrane stretched between it. No adequate system is offered for reducing back pressure within the flexible membrane. As a result, outward spanwise cross flow conditions are created which decrease propulsion and increase induced drag. No system is used to reduce the membrane's tendency to form a parabolic pocket when water pressure is applied. This parabolic shape causes the leading and side edges of the membrane to remain at a high angle of attack while the center region of the pocket becomes bowed. Consequently, the blade stalls and produces high levels of induced drag. In addition, the wide structure of the rigid frame member causes additional flow separation and drag.

U.S. Pat. No. 1,324,722 to Bergin shows a flexible oscillating propeller that has a narrow void along its center axis that divides the blade into two blade-halves. The void originates at the trailing edge and ends at a point near the base of the blade. The blade is made of a resilient material and is reinforced with a series of chordwise stiffening members which are joined to a transversely aligned stiffener a significant distance from the base of the blade. Because a significantly large portion of flexible blade material is unsupported along the outer side edges of the blade, these side portions deform in a dihedral manner under the exertion of water pressure. This increases outward spanwise flow conditions along the attacking surface of the blade. The stiffening members are not arranged in a manner that encourage the blade to deform in a manner that reduces such stall conditions and induced drag.

British patent 234,305 to Bovey (1924) uses propeller blades that have a fixed leading portion and a hinged trailing portion that swings freely along a substantially transverse axis. Because the trailing portion swings freely its inclination is uncontrollable. This allows this portion of the blade to bend backward under water pressure to an excessively low angle of attack. Consequently, sharp changes in contour can destroy efficiency and create drag. No system is used to effectively reduce induced drag.

U.S. Pat. No. 2,241,305 to Hill (1941) shows a vertically aligned propelling blade that uses a rigid frame which is shaped like the lower half of a caudal fin. A resilient membrane is stretched between the frame members. No system is used to reduce the membrane's tendency to bow in a parabolic manner. Consequently, the edges of the membrane bordering the frame members remain at an excessively high angle of attack during use. This causes the blade to stall and produce high levels of induced drag.

U.S. Pat. No. 3,086,492 to Holley shows a vertically aligned oscillating propulsion blade that is made of a flexible material. The blade's center axis has a V-shaped recess which divides the trailing portion of the blade into upper and lower halves. Paired stiffening ribs extend from both sides of the vertical blade in three locations. These blade pairs do not extend fully from the trailing edge to the base of the blade. Instead, a significantly large area of the blade's flexible material exists between the leading ends of the ribs and the base of the blade. This lack of support renders the blade vulnerable to collapse around a spanwise axis.

The positioning of the rib pairs are also poorly organized. Although two of the rib pairs run parallel to the outer side edges of the blade, a significant distance exists between these rib pairs and the outer side edges of the blade. Consequently, a substantially large portion of the blade's side edges are unsupported. This causes these edges to deform in a dihedral manner during use. This increases stall conditions as well as induced drag. The rib pair existing along the blade's center axis only adds extra leverage to the bending forces which allow the blade to bend around a spanwise axis. This spanwise axis exists substantially along an imaginary line connecting the leading ends of each rib pair. The ribs are not arranged in a manner that encourages the blade to bend or twist around a substantially lengthwise axis. As a result, the blade stalls through the water and delivers poor performance.

U.S. Pat. No. 3,453,981 to Gause (1969) uses a series of horizontally aligned propulsion blades that are intended to convert wave energy into forward motion on a boat. Each blade has a space along its center axis that divides it into a left and right blade half. The most significant problem with this blade design is that it has no system for controlling the undesirable stress forces created within the blade's flexible material during use. As a result, these stress forces prevent the blade from deforming in a desirable manner, and performance is poor.

Each blade has a rigid leading edge portion that is rounded and tapers gradually to a relatively resilient trailing portion. Although a dotted line in the diagram at first appears to represent a junction between these two areas of the blade, the description states that these two portions “merge smoothly into one another without any abrupt change in characteristic.” Such a smooth transition and gradual tapering transfers anti-flexing stress forces aft on the blade (toward the trailing edge). Thus, the rigidity of the leading edge portion is extended a significant distance toward the more resilient portions of the blade. This prevents the more resilient blade portions from flexing significantly near the leading and side edges of the blade. Consequently, these leading and side edges remain at an excessively high angle of attack during use which causes the blade to stall. Strong induced drag vortices are permitted to form along the outer side edges and performance is poor.

Another problem with the structure of this design is that stress forces of compression and tension are permitted to build-up within the blade's material during use. This prevents each blade half from adequately twisting along its length. These stress forces are strongest forward (toward the leading edge) of an imaginary line on each blade half that extends from the outer side edge of the extreme tip of the blade half to the most forward point of the trailing edge existing at the blade's center axis. The strength of the anti-twisting stress forces prevent this portion of the blade from twisting along its length. This is because these stress forces are significantly strong in comparison to the water pressure applied during use. As a result, the leading portions of the blade to remain at an excessively high angle of attack which stalls the blade and increases induced drag.

The portion of each blade half that exists between this imaginary line and the trailing edge are less affected by these stress forces. Consequently, this portion of each blade half bends around an axis that is substantially parallel to this imaginary line. However, because the blade tapers gradually from the rigid leading portion to the more flexible trailing portion, the stress forces existing forward of this imaginary line are extended aft of the imaginary line. As a result, the blade deforms around an axis that is significantly aft (toward the trailing edge) of this imaginary line. Thus, only a small portion of the blade bends under water pressure. If the blade's trailing portions are made from a significantly flexible material, the portions aft of the imaginary line collapse sharply under water pressure. In any case, the areas forward of this line remain in a stall condition which severely reduces lift.

Another problem occurs when the portions aft of the imaginary line bend back-ward from water pressure during use. As this happens, the swept alignment of each blade half causes some of the water traveling aft of this imaginary line along the attacking surface to be deflected toward the blade's center axis. This inward deflection of water creates an outward spanwise force against each blade half. This causes the blade halves to spread apart from one another in a spanwise direction during each stroke. This destroys efficiency by creating high levels of lost motion and lost energy.

Gause does not anticipate this problem of spanwise spreading and offers no solution for avoiding it. Although he states that the leading portions of the foil are to be significantly rigid, he does not mention that it should be rigid enough to prevent this problem. If his design is made rigid enough to avoid this problem, the gradual tapering in the blade's cross section extends this rigidity significantly toward the blade's trailing portions. This causes the entire blade to be much too rigid to flex in a significant manner. Because no method is employed to control these problems, this design is highly inefficient.

U.S. Pat. No. 3,773,011 to Gronier (1973) shows a horizontally aligned propulsion blade having a forked frame and a flexible membrane stretched between the forks. The most significant problem with this design is that no system is used to reduce the occurrence of back pressure within the membrane's attacking surface. As a result, back pressure causes the water along the attacking surface to spill in an outward spanwise direction around the side edges of the hydrofoil. This increases induced drag and severely inhibits propulsion.

Also, no method is used to control the membrane's natural tendency to attain a parabolic shape as it bows out under water pressure. As a result, the greatest degree of bowing occurs near the center of the membrane near the trailing edge, while the leading and side portions of the membrane located near the forks experience only a minimal defection from the horizontal plane. This causes the water flowing around the leading and side edges of the hydrofoil to separate from the low pressure surface of the membrane. This stalls the blade, creates drag, and destroys lift.

Although Gronier shows a spanwise cross sectional drawing that depicts his membrane as being capable bowing in a substantially elliptical manner, this is not what actually occurs. It is well known that when an evenly distributed load is placed on a flexible material that is suspended across a frame, a parabolic shape results across the material. Even if the membrane is able to bow out a significantly large degree during use, the parabolic shape still causes the greatest amount of bulging to occur along the membrane's center axis. This takes curvature away from the leading and side portions of the membrane and places them in a stall condition. Increased bowing also creates increased lost motion since a greater portion of each stroke is use to merely deform the membrane.

U.S. Pat. No. 4,193,371 to Baulard-Caugan (1980) shows a swimming apparatus that uses a vertically aligned caudal-shaped propulsion blade together with a caudal-shaped hydrofoil for reducing drift during use. Both the Propulsion blade and the “anti-drift member” are rigid and lack a system for reducing stall conditions and induced drag.

Japanese patents 61-6097 (A) to Fujita (1986) and 62-134395 (A), also to Fujita (1987) show a caudal-shaped propulsion blade which has a thin flexible membrane stretched across a forked frame. No system is used to relieve back pressure within the attacking side of the membrane and no system is used to reduce the membrane's tendency to form a parabolic shape as it bows out during use. As a result, this design produces high levels of drag and low levels of lift.

My own U.S. patent application Ser. No. 08276407 to McCarthy filed Jul. 18, 1994 describes several methods for reducing induced drag on foil type devices. However, the designs shown which are capable of being used in reciprocating motion situations (where the angle of attack reverses itself) require the use of complex control devices to invert the foil's shape. No system is shown that permits this inversion process to occur automatically and repeatedly in resilient swim fin applications and resilient propulsion blade applications.

OBJECTS AND ADVANTAGES

Accordingly, several objects and advantages of the present invention are:

(a) to provide hydrofoil designs that significantly reduce the occurrence of flow separation their low pressure surfaces (or lee surfaces) during use;

(b) to provide swim fin designs which significantly reduce the occurrence of ankle and leg fatigue;

(c) to provide swim fin designs which offer increased safety and enjoyment by significantly reducing a swimmer's chances of becoming inconvenienced or temporarily immobilized by leg, ankle, or foot cramps during use;

(d) to provide swim fin designs which are as easy to use for beginners as they are for advanced swimmers;

(e) to provide swim fin designs which do not require significant strength or athletic ability to use;

(f) to provide swim fin designs which can be kicked across the water's surface without catching or stopping abruptly on the water's surface as they re-enter the water after having been raised above the surface;

(g) to provide swim fin designs which provide high levels of propulsion and low levels of drag when used at the surface as well as below the surface;

(h) to provide swim fin designs which provide high levels of propulsion and low levels of drag even when significantly short and gentle kicking strokes are used;

(i) to provide resilient hydrofoil designs which offer significantly less resistance to twisting about their length than to bending across their length;

(j) to provide methods for substantially reducing the formation of induced drag type vortices along the side edges of hydrofoils;

(k) to provide hydrofoil designs which significantly reduce outward directed spanwise flow conditions along their attacking surface;

(l) to provide hydrofoil designs which efficiently encourage the fluid medium along their attacking surface to flow away from their outer side edges and toward their center axis so that fluid pressure is increased along their attacking surface;

(m) to provide methods for significantly reducing back pressure along the attacking surface a hydrofoil in a manner that significantly reduces the occurrence of outward directed spanwise cross flow conditions near the outer side edge portions of the hydrofoil;

(n) to provide methods for significantly reducing separation along the lee surface of reciprocating motion foils which are used at significantly high angles of attack, and

(o) to provide methods for controlling the torsional stress forces of tension and compression within the material of a flexible hydrofoil so that the material exhibits significantly reduced levels of resistance to twisting along its length.

Still further objects and objectives will become apparent from a consideration of the ensuing description and drawings.

DRAWING FIGURES

FIG. 1

shows a perspective view of a simplified version an improved swim fin.

FIG. 2

shows a cross sectional view taken along the line

2

—

2

of

FIG. 1

while is water flowing around the swim fin.

FIG. 3

shows a cross sectional view taken along the line

3

—

3

of

FIG. 1

while water is flowing around the swim fin.

FIG. 4

shows the same view shown in

FIG. 3

except that the water is flowing in the opposite direction around the swim fin.

FIG. 5

shows a perspective view of a swim fin which has two highly swept blades that are spaced apart and mounted at an angled orientation to each other.

FIG. 6

shows a cross sectional view taken along the line

6

—

6

from

FIG. 5

as streamlines are flowing by the blades during use.

FIG. 7

shows the same view shown in

FIG. 6

except that the blades are being kicked in the opposite direction.

FIG. 8

shows an end view of a prior art swim fin with streamlines displaying the undesirable flow conditions it creates.

FIG. 9

shows a perspective view of an improved swim fin having two side by side flexible blade halves.

FIG. 10

shows a cross sectional view taken along the line

9

—

9

from FIG.

9

.

FIG. 11

shows a comparative cross sectional view of a prior art swim fin having side by side blades that taper evenly toward each other.

FIG. 12

shows a top perspective view of the spreading apart effect exhibited during use by prior art fin designs that have the cross sectional shape displayed in FIG.

11

.

FIG. 13

shows a perspective side view of the prior art swim fin shown in

FIG. 12

as it collapses around a substantially transverse axis.

FIG. 14

shows a perspective cut-away view which displays the right half of the same swim fin shown in FIG.

9

.

FIG. 15

shows a cross sectional view taken along the line

15

—

15

from FIG.

14

.

FIG. 16

shows a cross sectional view taken along the line

16

—

16

from FIG.

14

.

FIG. 17

shows a cut-away perspective view of the same swim fin shown in

FIG. 14

except that in

FIG. 17

, a transverse recess is added to the right blade half near the foot pocket.

FIG. 18

shows the same view of the same swim fin shown in

FIG. 14

except that in

FIG. 18

, a total of three transverse recesses are added which separate the right blade half into a leading panel, an intermediate panel, and a trailing panel.

FIG. 19

shows a perspective view of the complete swim fin shown in

FIG. 18

while it is being kicked through the water.

FIG. 20

shows a cut-away perspective view displaying the right half of the same swim fin shown in

FIGS. 18 and 19

except that in

FIG. 20

, the transverse recesses extend further toward the swim fin's outside edge, and a series of flexible membranes are added to bridge the spaces created by the transverse recesses.

FIG. 21

shows a perspective side view of the embodiment shown in

FIG. 20

while it is being kicked through the water.

FIG. 22

shows a cut-away perspective view displaying the right half of the same swim fin shown in

FIGS. 20 and 21

except that in

FIG. 22

, a longitudinal recess is added to the outer edge of the right blade half to separate the leading panel, intermediate panel, and trailing panel from the stiffening member, and a narrow strip of flexible membrane is added to fill in the longitudinal recess and connect the leading panel, intermediate panel, and trailing panel to the stiffening member.

FIG. 23

shows a cross sectional view taken along the line

23

—

23

from FIG.

22

.

FIG. 24

shows a front perspective view of another embodiment of a swim fin which has a pre-formed lengthwise channel with a recess existing along the center axis of the swim fin.

FIG. 25

shows a side perspective view of the same swim fin while it is kicked upward.

FIG. 26

shows a side perspective view of the same swim fin while its channel-like blade portions invert themselves during a downward kicking motion.

FIG. 27

shows the same swim fin except that it has a vented central membrane stretched across the center recess.

FIG. 28

shows a cut-away perspective view displaying the right half of a symmetrical swim fin having a flexible membrane that is structurally supported by an outer stiffening member and two separately positioned rib pairs.

FIG. 29

shows a cross sectional view taken along the line

29

—

29

from

FIG. 28

as the swim fin deforms during use.

FIG. 30

shows a cross sectional view taken along the line

30

—

30

from

FIG. 28

as the swim fin deforms during use.

REFERENCE NUMERALS IN DRAWINGS

70

foot pocket

72

blade

74

trailing tip

76

right edge

78

left edge

80

upper surface

82

oncoming flow

84

lower surface

85

oncoming flow

86

lift vector

88

vertical component

90

horizontal component

92

oncoming flow

94

lift vector

96

vertical component

98

horizontal component

100

foot pocket

102

platform member

104

right blade

106

left blade

108

outer edge

110

inner edge

112

upper surface

114

trailing tip

116

outer edge

118

inner edge

120

upper surface

122

trailing tip

124

root

126

root

128

reinforcement member

130

oncoming flow

132

lower surface

134

lower surface

136

lift vector

138

vertical component

140

horizontal component

142

lift vector

144

vertical component

146

horizontal component

148

oncoming flow

150

lift vector

152

vertical component

154

horizontal component

156

lift vector

158

vertical component

160

horizontal component

162

foot pocket

164

oncoming flow

166

right upper blade

168

right lower blade

170

left upper blade

172

left lower blade

174

vertical blade

180

foot pocket

182

right blade half

184

left blade half

186

flexible blade portion

188

right stiffening member

190

outer edge

192

inner edge

194

outer edge

195

trailing tip

196

trailing edge

196

′ trailing edge

198

inner edge

199

upper surface

200

flexible blade portion

202

left stiffening member

204

outer edge

206

inner edge

208

outer edge

210

trailing edge

212

inner edge

214

upper surface

216

trailing tip

218

lower surface

220

lower surface

222

oncoming flow

224

lift vector

226

lift vector

228

vertical component

230

horizontal component

232

vertical component

234

horizontal component

236

oncoming flow

238

bending zone

240

oncoming flow

242

neutral position

244

semi-flexed position

246

highly-flexed position

248

zone of separation

249

oncoming flow

250

zone of separation

251

lift vector

252

transverse recess

254

bending zone

256

forward transverse recess

258

intermediate transverse recess

260

trailing transverse recess

262

outer bending zone

264

intermediate bending zone

266

inner bending zone

267

root portion

268

forward panel

270

intermediate panel

272

trailing panel

274

forward transverse recess

276

intermediate transverse recess

278

trailing transverse recess

280

forward panel

282

intermediate panel

284

trailing panel

286

forward transverse recess

288

intermediate transverse recess

290

trailing transverse recess

291

root portion

292

forward panel

294

intermediate panel

296

trailing panel

298

forward transverse flexible membrane

300

intermediate transverse flexible membrane

302

trailing transverse flexible membrane

304

bending zone

306

forward panel

308

intermediate panel

310

trailing panel

312

forward transverse flexible membrane

314

intermediate transverse flexible membrane

316

trailing transverse flexible membrane

318

lengthwise flexible membrane

319

root portion

320

leading panel

322

intermediate panel

324

trailing panel

326

oncoming flow

328

lift vector

348

foot pocket

350

foot platform

352

right stiffening member

354

left stiffening member

356

channeled blade portion

358

right flexible membrane

360

right blade member

362

intermediate flexible membrane

364

left flexible membrane

366

left blade member

368

center recess

370

vented central membrane

372

venting system

374

foot pocket

376

foot platform

378

right stiffening member

380

flexible blade portion

382

flexible membrane

384

forward rib pair

386

trailing rib pair

388

initial bending zone

390

trailing tip

392

inner edge

394

modified bending zone

396

oncoming flow

398

lift vector

400

oncoming flow

402

lift vector

Description—

FIGS. 1

to

4

In

FIG. 1

, a perspective view shows a simplified swim fin. At the leading portion of the swim fin is a foot pocket

70

for holding the user's foot. Foot pocket

70

is preferably molded out of a substantially resilient thermoplastic to comfortably adapt to the characteristics of the user's foot. However, foot pocket

70

can occur in any desirable form of a foot attachment mechanism such as a single strap (thick, thin, wide, narrow, adjustable, or padded), a network or series of straps, a harness, a partial boot, a full boot, a shoe member, a single foot cavity, a dual foot cavity for enclosing both feet of the user for kicking in a porpoise-like swimming stroke, or any other suitable method for attaching to a foot or the feet of a user. Extending from foot pocket

70

is a blade

72

which extends toward a trailing tip

74

. It is preferred that blade

72

is made of a significantly rigid thermoplastic, and that blade

72

is attached to foot pocket

70

in any suitable manner that is able to provide an adequately strong connection. A right edge

76

of blade

72

is located on right side of the user. A left edge

78

of blade

72

is located on the left side of the user. An upper surface

80

is seen between right edge

76

and left edge

78

. Blade

72

twists along its length from a substantially horizontal spanwise alignment near foot pocket

70

, to an angled alignment near trailing tip

74

. Preferably, this transition in alignment occurs in a smooth manner, however, it can also occur in a series of steps or in an abrupt manner.

FIG. 2

shows a cross sectional view taken at the line

2

—

2

from FIG.

1

. An oncoming flow

82

is created as the fin is kicked forward so that upper surface

80

is the attacking surface. Oncoming flow

82

is illustrated by a series of streamlines which display the direction of flow around this portion of blade

72

when blade

72

is kicked upward. A lower surface

84

is visible from this view.

FIG. 3

shows a cross sectional view taken at the line

3

—

3

from FIG.

1

. This view shows the angled orientation of blade

72

near trailing tip

74

. An oncoming flow

85

is seen approaching and flowing around blade

72

in an angled manner. Oncoming flow

85

is created by the same kicking stroke that produces oncoming flow

82

shown in FIG.

2

. In

FIG. 3

, the flow conditions displayed by the streamlines of oncoming flow

85

create a lift vector

86

which is illustrated by an arrow that points away from lower surface

84

. Lift vector

86

is perpendicular to the direction of the streamline flowing along lower surface

84

. A vertical component

88

of lift vector

86

is displayed by a vertical arrow aiming downward. A horizontal component

90

of lift vector

86

is displayed by a horizontal arrow aiming sideways and away from lower surface

84

.

FIG. 4

shows the same cross sectional view as seen in

FIG. 3

, however, the fin is now being kicked in the opposite direction so that lower surface

84

is now the attacking surface. An oncoming flow

92

is displayed by two streamlines flowing smoothly around blade

72

. Oncoming flow

92

is illustrated by an arrow that points away from upper surface

80

. A lift vector

94

is perpendicular to the streamline flowing along upper surface

80

. A vertical component

96

of lift vector

94

is displayed by a vertical arrow pointing away from upper surface

80

. A horizontal component

98

of lift vector

94

is displayed by a horizontal arrow point sideways and away from upper surface

80

.

Operation—

FIGS. 1

to

4

FIG. 1

shows a simplified version of an improved swim fin. Blade

72

twists along its length so that a significant portion of blade

72

is inclined at a reduced angle of attack during use. By giving blade

72

this twisted form, separation is greatly reduced along the low pressure surface of a given stroke. This reduces drag and increases lift on blade

72

.

In

FIG. 2

, blade

72

is being kicked forward so that upper surface

80

is the attacking surface and lower surface

84

is the low pressure surface on this stroke. Because this portion of blade

72

is at a high angle of attack relative to oncoming flow

82

, the streamlines separate from lower surface

84

after passing around right edge

76

and left edge

78

. Many prior art designs have these flow conditions along the entire length of their working surface areas.

On the opposite stroke of that shown in

FIG. 2

, the same flow patterns exist except that they are inverted. In this situation, the water approaches from the other side of blade

72

so that lower surface

84

is the attacking surface and upper surface

80

is the low pressure surface.

FIG. 3

shows the angled orientation of

72

taken at line

3

—

3

of FIG.

1

. Relative to the direction of oncoming flow

85

, right edge

76

is seen to be the leading edge from this view while left edge

78

is the trailing edge. The cross sectional shape of this embodiment is shown to be symmetrically tapered at right edge

76

and left edge

78

. This enables this embodiment to generate efficient levels of lift when the direction of flow reverses around blade

72

on reciprocating strokes. However, this embodiment can also employ an asymmetrical hydrofoil shape that works most effectively during one particular stroke. For example, a symmetrical or asymmetrical tear drop cross sectional shape can be used.

From the view shown in

FIG. 3

, it can be seen that this segment of blade

72

is at a significantly reduced angle of attack relative to oncoming flow

85

. The streamline next to lower surface

84

is flowing smoothly in an attached manner. This attached flow condition shows that separation is greatly reduced along the low pressure surface of blade

72

. This significantly reduces drag and increases lift. It is preferred that blade

72

is twisted over a substantial portion of its length so that a significant portion of blade

72

is oriented at a significantly reduced angle of attack.

Because this reduced angle of attack increases attached flow along the low pressure surface, a strong low pressure field is forms along lower surface

84

as water curves around this surface. Efficiency is high because the flow of water around the lower surface

84

(the low pressure surface or lee surface) is not blocked or restricted. While this low pressure field forms, a high pressure field forms along upper surface

80

as water pushes against this surface. The pressure difference existing between these two pressure fields creates lift vector

86

, which is perpendicular to the direction of the streamline flowing along lower surface

84

. Because the streamlines of oncoming flow

85

are able to meet each other in a constructive manner at left edge

78

, lift is efficiently generated.

Because lift vector

86

is at an angle, it is composed of vertical component

88

and horizontal component

90

. Vertical component

88

of lift vector

86

pushes against blade

72

in the opposite direction of the swim fin's movement through the water. This force offers forward propulsion for the user. Horizontal component

90

of lift vector

86

pushes sideways on blade

72

toward the user's right side (toward right edge

76

). It is preferred that blade

72

be made from a sufficiently rigid enough material to substantially maintain its shape during use while horizontal component

90

of lift vector

86

pushes sideways against it. Examples of rigid materials can include fiber reinforced thermoplastics.

To increase such resistance to sideways deformation in alternate embodiments, a stiffening member, beam, strut, or network of such members can be used to reinforce blade

72

and provide added rigidity. Such stiffeners can be connected internally or externally to blade

72

in any suitable manner. An alternate embodiment can also use a horizontally aligned planar shaped stiffener within blade

72

to resist sideways forces while still permitting blade

72

to bend around a horizontally aligned transverse axis. Blade

72

can also be made significantly thicker to increase its rigidity. The use of a more rounded upper surface

80

and lower surface

84

can also further improve attached flow conditions and lift generation along the lee surface of blade

72

.

FIG. 4

shows the same view as seen in

FIG. 3

except that blade

72

is being kicked in the opposite direction as that shown in FIG.

3

. In

FIG. 4

, oncoming flow

92

approaches lower surface

84

, and therefore lower surface

84

is the attacking surface while upper surface

80

is the low pressure surface. Relative to oncoming flow

92

, left edge

78

is seen to be the leading edge and right edge

76

is seen to be the trailing edge. Because the streamline next to upper surface

80

is flowing smoothly, a strong low pressure field forms as the water flowing along the low pressure surface is forced to travel over a greater distance than the water flowing along the attacking surface. This combines with the formation of a high pressure field along lower surface

84

to create lift vector

94

which is perpendicular to the streamline flowing next to upper surface

80

. Lift vector

94

is composed of vertical component

96

and horizontal component

98

. Vertical component

96

offers propulsion by providing a force to push off of during strokes. Horizontal component

98

pushes sideways against blade

72

toward the user's left side. Again, it is preferred that blade

72

is sufficiently rigid enough to avoid substantial sideways deformation during use.

This design offers improved performance near the surface of the water in comparison to prior designs. If blade

72

breaks the surface of the water during strokes and then attempts to reenter the water, it does not slap the water and stop abruptly on impact. Because a significant portion of blade

72

is oriented at a reduced angle of attack, the blade slices easily through the surface like a knife and therefore maintains its downward momentum. As a result, this momentum is easily converted into forward propulsion. Because a majority of blade

72

has significantly reduced levels of separation and induced drag vortex formation, blade

72

continues to slice through the water with low substantially reduced levels of drag. This makes the swim fin easy to use and greatly improves stamina.

Another benefit to this design is that the twisted form of blade

72

encourages water to flow aftward. Because blade

72

is twisted along its length, the angle of attack of blade

72

decreases along its length. This causes the high pressure field along the length of a particular attacking surface to decrease in intensity from the leading portions of blade

72

toward trailing tip

74

. This lengthwise decrease in the intensity of the high pressure field causes water to flow in a substantially lengthwise manner across the attacking surface of blade

72

toward trailing tip

74

. This increases forward propulsion.

Other embodiments can place the trailing portions of blade

72

at a higher or lower angle of attack than is shown in

FIGS. 3 and 4

. Also, blade

72

can be angled along its entire length. In this situation, it can maintain a constant angle or twist from a relatively higher angle of attack to a relatively lower angle of attack. Blade

72

can also begin near foot pocket

70

with an angled orientation in one direction and then reverse its angle of attack farther toward tip

74

. This can create two opposing sideways components of lift on blade

72

which neutralize each other so that a net zero horizontal force results. These sideways forces can be arranged to either partially or completely neutralize each other.

Description—

FIGS. 5

to

8

FIG. 5

shows a perspective view of an improved swim fin. A foot pocket

100

receives the user's foot and is preferably made from a substantially resilient thermoplastic to provide comfort to the user. Foot pocket

100

is attached in any suitable manner to a platform member

102

. Platform

102

is preferably made of a significantly rigid material such as a fiber reinforced thermoplastic. Platform

102

is attached in any suitable manner to a right blade

104

located to the right of the user, and to a left blade

106

located to the left of the user. Right blade

104

has an outer edge

108

and an inner edge

110

. An upper surface

112

is seen located between outer edge

108

and inner edge

110

. Outer edge

108

and inner edge

110

converge at trailing tip

114

. Left blade

106

has an outer edge

116

and an inner edge

118

. An upper surface

120

is seen located between outer edge

116

and inner edge

118

. Outer edge

116

and inner edge

118

converge at a trailing tip

122

. At the leading portion of right blade

104

is a root

124

. At the leading portion of left blade

106

is a root

126

. Between root

124

, root

126

, and platform

102

is a reinforcement member

128

which is attached to root

124

, root

126

, and platform

102

in any suitable manner. Member

128

is used to maintain the set inclination of each blade. In this embodiment, member

128

is shaped like a panel in order to reduce turbulence around root

124

and root

126

during use. This design may also be used without member

128

.

It is preferred that platform

102

, member

128

, right blade

104

, and left blade

106

are all molded from a significantly rigid material such as a fiber reinforced thermoplastic. However, any suitably rigid material may be used.

FIG. 6

shows a cross sectional view taken along the line

6

—

6

in FIG.

5

. An oncoming flow

130

is illustrated by a series of streamlines flowing over right blade

104

and left blade

106

. A lower surface

132

of right blade

104

and a lower surface

134

of left blade

106

are both visible from this view. These flow conditions result when right blade

104

and left blade

106

are kicked upward so that upper surface

112

and upper surface

120

are both the attacking surfaces. Next to right blade

104

, a lift vector

136

is displayed by an arrow extending away from lower surface

132

. Lift vector

136

is composed of a vertical component

138

and a horizontal component

140

. Next to left blade

106

, a lift vector

142

is displayed by an arrow extending way from lower surface

134

. Lift vector

142

is composed of a vertical component

144

and a horizontal component

146

.

FIG. 7

shows the same cross sectional view shown in

FIG. 6

except that the swim fin is being kicked in the opposite direction. This causes an oncoming flow

148

to approach right blade

104

and left blade

106

from the opposite direction as oncoming flow

130

shown in FIG.

6

. In

FIG. 7

, oncoming flow

148

is displayed by a series of streamlines flowing around right blade

104

and left blade

106

. lower surface

132

and lower surface

134

are seen to be the attacking surfaces on this stroke. Next to right blade

104

, a lift vector

150

extends away from upper surface

112

. Lift vector

150

is composed of a vertical component

152

and a horizontal component

154

. Next to left blade

106

, a lift vector

156

extends away from upper surface

120

. Lift vector

156

is composed of a vertical component

158

and a horizontal component

160

.

FIG. 8

shows a prior art comparison to the embodiments shown in

FIGS. 5

to

7

.

FIG. 8

shows an end view of a swim fin design having four blades which is displayed in French patent 1,501,208 to Barnoin (1967). Although the many problems of this prior art reference are already discussed in the prior art section of this specification, the illustration shown in

FIG. 8

enables the highly undesirable flow conditions it creates during use to be visualized.

In

FIG. 8

, the trailing portions of the swim fin (located in front of the toe region of the foot pocket) are facing the viewer. At the top of the swim fin is the upper portion of a foot pocket

162

. An oncoming flow

164

is illustrated by a series of streamlines flowing toward the upper portion of the swim fin. These streamlines then flow around the swim fin to illustrate the areas where flow separation and induced drag vortex formation occurs. The swim fin has a right upper blade

166

and a right lower blade

168

on the right side of the swim fin. A left upper blade

170

and a left lower blade

172

is on the left side of the swim fin. Each blade tapers in thickness toward the fin's center axis. At this center axis is a vertical blade

174

. The streamlines flowing toward the swim fin's right side are labeled a, b, c, and d. Because the swim fin is symmetrical, the streamlines flowing toward the swim fin's left hand side behave similarly, and therefore they are not labeled and described. The streamlines show the flow conditions created when the swim fin is kicked upward through the water. Because the blade configuration is symmetrical, the same type of flow conditions occur when the fin is kicked in the opposite direction, except that the flow conditions are inverted.

Operation—

FIGS. 5

to

8

In

FIG. 5

, both upper surface

112

and upper surface

120

are seen to slope down toward the space between right blade

104

and left blade

106

. When the swim fin is kicked upward so that upper surface

112

and upper surface

120

are the attacking surfaces, the sloped orientation of upper surface

112

and upper surface

120

creates a valley shaped channel along the length of the swim fin that encourages water to flow away from outer edge

108

and toward inner edge

110

on right blade half

104

, as well as flow away from and outer edge

106

and toward inner edge

118

on left blade half

106

. This significantly increases performance during this stroke by significantly reducing outward spanwise cross flow conditions along the attacking surfaces as well as reducing induced drag vortex formation around the outside of outer edge

108

and outer edge

106

. Because a space exists between inner edge

110

and inner edge

118

, excess pressure can escape though this space in the bottom of the channel when upper surface

112

and upper surface

120

are the attacking surfaces. By significantly reducing back pressure within this channel during such a stroke, this design prevents water from backing up and flowing in an outward direction along upper surface

112

and upper surface

120

toward outer edge

108

and outer edge

116

, respectively.

In

FIG. 6

, the streamlines from oncoming flow

130

display that when the swim fin is kicked upward, water is able to flow through the space between inner edge

110

and inner edge

118

. As the water converges toward this space, a strong high pressure field is created within the water between upper surface

112

and upper surface

120

. At the same time, the streamlines traveling along lower surface

132

of right blade

104

, and lower surface

134

of left blade

106

are seen to flow smoothly in an attached manner. This permits a strong low pressure field to form along lower surface

132

of right blade

104

as well as lower surface

134

of left blade

106

.

The creation of a strong high pressure field along upper surface

112

and upper surface

120

combines with the creation of a strong low pressure field along lower surface

132

and lower surface

134

to enable the swim fin to efficiently generate high levels of lift. Next to right blade

104

is lift vector

136

which is perpendicular to the streamline flowing along lower surface

132

. Vertical component

138

of lift vector

136

provides forward propulsion for the swimmer while horizontal component

140

of lift vector

136

applies a sideways force to right blade

104

. Next to left blade

106

is lift vector

142

which is perpendicular to the streamline flowing around lower surface

134

. Vertical component

144

of lift vector

142

provides forward propulsion while horizontal component

146

of lift vector

142

applies a sideways force against left blade

106

. In this embodiment, it is intended that both right blade

104

and left blade

106

are made of a sufficiently rigid enough material to substantially maintain their lengthwise alignment during use and avoid excessive sideways deformation from horizontal component

140

and horizontal component

146

, respectively. Because horizontal components

140

and

146

are oppositely directed, they counteract each other and no net horizontal force is applied to the user's foot.

Because both separation and induced drag vortex formation are greatly reduced, the swim fins create less drag and are easier to use than prior designs. The attached flow conditions created along the low pressure surfaces permit high levels of lift to be generated during use which are efficiently converted into forward propulsion. Because most swimmers who use swim fins tend to swim face down in the water, the benefits of the forward kicking stroke shown in

FIG. 6

are highly beneficial in the swimmers down stroke (upper surface

112

and upper surface

120

are the attacking surfaces and are facing down in the water). This is the more powerful of the two possible stroke directions.

If this fin is used while swimming along the water's surface, it works exceptionally well when it breaks the water's surface during kicks. As the fin re-enters the water and strikes the surface, the angled orientation of right blade

104

and left blade

106

permit them to easily slice through the surface like two knives and the swim fin does not “catch” like prior swim fins. As the swim fin is undergoing re-entry, water immediately begins flowing in a smooth manner around lower surface

132

and lower surface

124

to quickly form lift generating low pressure fields which efficiently propel the swimmer forward. Because separation and induced drag vortices are reduced, the swim fin does not suddenly decelerate from high levels of drag. Instead, the momentum of the down stroke is maintained re-entering the water. As a result, the energy possessed by this momentum is efficiently converted into forward propulsion.

FIG. 7

shows the same cross sectional view shown in

FIG. 6

except that

FIG. 7

illustrates what the flow conditions are like when the swim fin is kicked downward through the water relative to the orientation shown in FIG.

5

. In

FIG. 7

, oncoming flow

148

flows toward lower surface

132

and lower surface

134

. As oncoming flow

148

collides with lower surface

132

and lower surface

134

, a high pressure field is formed along these two surfaces. The streamlines shown flowing through the space between inner edge

110

and inner edge

118

spread apart and flow smoothly along upper surface

112

and upper surface

120

in an attached manner. As this happens, a low pressure field forms along upper surface

112

and upper surface

120

.

Because both high pressure fields and low pressure fields are formed, these pressure fields combine to create significantly strong lifting forces on right blade

104

and left blade

106

. Vertical component

152

and vertical component

158

provide propulsion for the user. Horizontal component

154

and horizontal component

160

apply a sideways force on right blade

104

and left blade

106

, respectively. It is preferred that right blade

104

and left blade

106

are rigid enough to prevent them from flexing substantially toward each other under the forces of horizontal component

154

and horizontal component

160

. Because horizontal component

154

and horizontal component

160

are oppositely directed, they counteract each other so that no net horizontal force is applied to the user's foot.

In

FIG. 7

, the space between inner edge

110

and inner edge

118

permits water to flow around the “lee” portion of each blade in an attached manner. Because the streamlines which split apart at the leading edge of each blade are able to meet again at the trailing edge of each blade, the water traveling a greater distance around the lee surface of each blade must travel farther, and therefore faster than the water flowing around the attacking surface of each blade. Because this design significantly decreases separation along the lee surface of each blade, drag is reduced and lift is increased.

Many variations of this design are possible. For instance, the angled inclination of each blade can be reversed so that upper surface

112

and upper surface

120

are at a dihedral orientation to each other when the swim fin is kicked upward (relative to the view in FIG.

5

), and lower surface

132

and lower surface

134

are at an anhedral orientation when the swim fin is kicked downward.

Other embodiments can include using one single swim fin for both feet in a dolphin style kicking stroke. In such cases, the spanwise dimensions (as well as overall dimensions) can be increased significantly. In one of many such embodiments, blades

104

and

106

can be further separated from one another and mounted to either end of a transversely mounted wing-like hydrofoil. The angled inclination of blades

104

and

106

can significantly reduce induced drag vortex formation at the outer ends of the transverse hydrofoil. In addition, the lift vectors produced by blades

104

and

106

can significantly increase the total lift produced by the swim fin. If desired, blades

104

and

106

can be molded onto the transverse hydrofoil so that a smoothly contoured streamlined shape results. The lengthwise dimensions of blades

104

and

106

can also be decreased if desired.

Alternate embodiments of the design shown in

FIGS. 5 through 7

can also include having right blade

104

and left blade

106

pivotally attached to foot pocket

100

. In this embodiment, blades

104

and

106

are pivotally attached so that they may pivot around a substantially lengthwise axis in order to vary their angle of attack. Any suitable manner of pivotally attaching blades

104

and

106

to foot pocket

100

may be used. In this situation, reinforcement member

128

is either not needed at all, or it may be made of a highly resilient material which permits right blade

104

and left blade

106

to rotate and invert their orientations on reciprocating strokes. In such cases, member

128

can serve to stop rotation once a predetermined reduced angle of attack has been reached on each stroke.

One such way of pivotally attaching blades

104

and

106

to foot pocket

100

is to have two rod-like members extending from either side of foot pocket

100

and, or platform

102

in a direction that is substantially parallel to outer edge

108

and outer edge

116

. These rod-like members can then be inserted into a corresponding longitudinal cavity located substantially within outer side edge of each blade. This permits each blade to pivot around a lengthwise axis located near its outer side edge. Consequently, outer edges

108

and

116

are leading edges on both reciprocating strokes. As a result, outer edges

108

and

116

may be made rounded while inner edges

110

and

118

may be made relatively sharp so that each blade tapers in an inward direction to form a tear dropped cross sectional shape. This creates an improved hydrofoil shape which further increases lift and decreases drag.

Such a longitudinal cavity within each blade may be secured to each rod-like member in any suitable manner that permits both secured attachment and rotation. For instance, a flange or protrusion within each rod-like member can extend into a groove within each longitudinal cavity, or vice versa. Such a mating arrangement between flange and groove can be designed to permit relative movement in the direction of desired pivoting while preventing the blade from sliding off the rod-like member in a lengthwise direction.

For embodiments not using any type of member

128

, the range of pivotal motion within each blade can be limited in any suitable manner. For instance, a flange-like structure may extend from a portion of each rod-like member into a recess located within the corresponding cavity of each blade. This recess may be made larger than the size of the flange to permit the flange to pivot back and forth within the recess over a predetermined range. When the flange pivots into contact with the boundaries of this recess, pivoting stops and the blade reaches a maximum reduced angle of attack.

Pivotal range can also be limited by securing a flexible or semi-flexible strip, cord, flange, or member between inner edge

110

and inner edge

118

which has a predetermined degree of slack or looseness within it. This member expands as the blades rotate to a reduced angle of attack. When the member becomes fully expanded, pivoting is brought to a stop. The looseness built into such a member can also be made adjustable to suit the user's tastes. Other methods can include securing such a member between the inner edge portion of each blade's root to foot pocket

100

and, or platform

102

. Any suitable method of limiting the range of motion in a permanent or variable manner may be used.

Another way of pivotally connecting the blades to foot pocket

100

is to have a rod-like member extend out from the root of each blade which is inserted into a corresponding cavity within foot pocket

100

and, or platform

102

. The rod-like member can be secured in any suitable manner that permits rotation while preventing it from sliding out of its corresponding cavity during use. Such a rod-like member and its corresponding blade may be molded in one piece from any desirable material that is preferably rigid and durable such as a fiber reinforced thermoplastic, or composite material. A removability feature can permit damaged blades to be replaced as well as different shaped blades to be substituted for one another.

Still other embodiments can employ any desirable number of such rotating blades arranged in any desirable manner. For instance, a plurality of narrow and highly swept rotating blades may be used instead of two wider swept rotating blades. A plurality of fixed blades may be used as well.

FIG. 8

shows an end view of a prior art swim fin which is displayed in French patent 1,501,208 to Barnoin (1967). This drawing permits the undesirable flow conditions of a prior art example to be compared with the highly efficient flow conditions of the present invention displayed in

FIGS. 1

to

7

. In the illustration shown in

FIG. 8

, the prior art swim fin is kicked forward so that oncoming flow

164

is approaching the upper portion of the swim fin. The streamlines a, b, c, and d of oncoming flow

164

display the undesirable flow conditions existing in this design.

As the outer streamline a begins to curve around the outer edge of lower blade

168

, it separates from the lower surface of lower blade

168

. This is because lower blade

168

is oriented at an undesirable angle of attack relative to oncoming flow

164

. The resultant separation stalls lower blade

168

and prevents a low pressure field from forming along the lower surface (low pressure surface on this stroke) of lower blade

168

. This prevents lift from being created and creates high levels of drag from transitional flow. After streamline a separates from the lower surface of

168

, it forms a large induced drag type vortex below the lower surface of

168

. This further destroys lift and creates significantly large levels of induced drag.

As streamline b tries to curve around the outer end of upper blade

166

, it is blocked by the upper surface (attacking surface) of lower blade

168

. This causes streamline b to curl back around toward the lower surface (lee pressure surface) of upper blade

166

and form a rotating eddy in the space between upper blade

166

and lower blade

168

. Because the dihedral orientation of lower blade

168

blocks water flowing around the outer end of blade

166

, this water cannot merge in a constructive manner with the water exiting the attacking surface of blade

166

at its inner side edge (near vertical blade

174

). In addition, the eddy formed between blade

166

and blade

168

causes the water to flow backward along the lower surface (lee surface) of upper blade

166

. This flow is oriented in the opposite direction needed to generate lift. Consequently, The dihedral orientation of lower blade

168

prevents attached flow conditions from occurring along the lower surface of upper blade

166

. Furthermore, the dihedral orientation of lower blade

168

creates highly undesirable turbulence patterns which stalls upper blade

166

and prevents it from generating lift.

Just as a stalled airplane wing can prevent an airplane from generating the needed lift to get off the ground, the severely stalled blades in this swim fin prevent them from generating adequate levels of lift. As a result, propulsion is poor and drag is exceedingly high. When considering that the presence of one or two stalled blades on other prior art swim fins create excessive levels of drag which often cause painful muscle cramps, the drag created by the four completely stalled blades in Barnoin's swim fin can be unbearable. The combination of this swim fin's propensity to generate high levels of induced drag and transitional flow on all four blades, places drag generation at unusable levels.

The eddy created between upper blade

166

and lower blade

168

forms into a powerful induced drag vortex that further destroys lift and increases drag. This induced drag vortex creates an outward flow condition along the upper surface of upper blade

166

near the outer edge of upper blade

166

. As a result, streamline c is deflected outward and drawn toward the vortex existing between upper blade

166

and lower blade

168

. Although streamline d is able to flow inward along the upper surface of upper blade

166

, the lower surface of upper blade

166

is completely stalled out. This prevents upper blade

166

from generating a substantial pressure difference between its opposing surfaces.

Description—

FIGS. 9

to

13

FIG. 9

shows a perspective view of an improved swim fin which has a recess along the swim fin's center axis. This recess extends from the trailing portion of the swim fin to a predetermined distance (in this case a significantly short distance) from the toe portion of a foot pocket

180

. However, any desirable distance may be used. The recess divides the swim fin into a right blade half

182

and a left blade half

184

. Right blade half

182

is made up of a flexible blade portion

186

and a right stiffening member

188

. An outer edge

190

of flexible portion

186

is connected to an inner edge

192

of stiffening member

188

in any suitable manner. For instance, flexible portion

186

and stiffening member may be molded as one piece out of the same material. An outer edge

194

of stiffening member

188

is located opposite from inner edge

192

. Stiffening member

188

tapers in thickness toward a trailing tip

195

. Flexible portion

186

is seen to have a trailing edge

196

, an inner edge

198

, and an upper surface

199

.

Left blade half

184

is constructed in the same manner as right blade half

182

. Left blade half

184

has a flexible blade portion

200

and a left stiffening member

202

. An outer edge

204

of flexible portion

200

is attached to an inner edge

206

of stiffening member

202

in any suitable manner. Opposite from inner edge

206

is and outer edge

208

of stiffening member

202

. Flexible portion

200

is seen to have a trailing edge

210

, an inner edge

212

, and an upper surface

214

. Stiffening member

202

tapers in thickness toward a trailing tip

216

.

Between the forward portion of the recess and foot pocket

180

, flexible portion

186

and flexible portion

200

merge together. Foot pocket

180

is connected to this portion of flexible portion

186

and flexible portion

200

in any suitable manner. It is preferred that this area of flexible portion

186

and flexible portion

200

extend below foot pocket

180

to form a sole that is thick enough to prevent excessive wear while walking across land. To achieve this, it is preferred that the thickness of this portion of flexible portion

186

and flexible portion

200

become substantially thicker beneath foot pocket

180

. It is also preferred that the sole of foot pocket

180

is made sufficiently rigid enough to provide rigid support for stiffening member

188

and stiffening member

202

. Other embodiments can use a separate, more rigid material beneath foot pocket

180

if desired.

FIG. 10

shows a cross sectional view taken along the line

10

—

10

of FIG.

9

. In

FIG. 10

, stiffening member

188

and stiffening member

202

are both seen to have a hydrofoil shape. Both outer edge

194

and outer edge

208

are rounded while both inner edge

192

and inner edge

206

are tapered and relatively narrow. Flexible portion

186

and flexible portion

200

are seen to be generally planar in form and are significantly thinner than stiffening member

188

or stiffening member

202

. Inner edge

198

and inner edge

212

are relatively sharpened. The majority of tapering across right blade half

182

and left blade half

184

is seen to occur along stiffening member

188

and stiffening member

202

, respectively. On flexible portion

186

, a lower surface

218

is seen opposite from upper surface

199

. On flexible potion

200

, a lower surface

220

is opposite from upper surface

214

.

This view shows how right blade half

182

and left blade half

184

deform during use. An oncoming flow

222

is displayed by a series of streamlines flowing around right blade half

182

and left blade half

184

. Flexible portion

186

and flexible portion

200

are deflected downward because the swim fin is being kicked upward so that upper surface

199

and upper surface

214

are the attacking surfaces. The horizontal broken lines indicate the positions of flexible portion

186

and flexible portion

200

while they are at rest. The upwardly deflected broken lines indicate the position of flexible portion

186

and flexible portion

200

when the stroke is reversed and the swim fin is kicked downward so that lower surface

218

and lower surface

220

are the attacking surfaces.

The streamlines traveling next to lower surface

218

and lower surface

220

are flowing in a smooth and attached manner. This generates a lift vector

224

on left blade half

184

, and generates a lift vector a

226

on right blade half

182

. Lift vector

224

has a vertical component

228

and a horizontal component

230

. Lift vector

226

has a vertical component

232

and a horizontal component

234

.

FIG. 11

shows a comparative cross sectional view of the tapered prior art blade-halves used in both German patent 259,353 to Braunkohlen (1987) and French patent 1,501,208 to Barnoin (1967). Although the many problems of these designs are discussed previously in the Background—Description of Prior Art section of this specification,

FIG. 11

offers the ability to visualize the undesirable flow conditions which they create. Because the blades of these prior art designs have similar cross sectional shape,

FIG. 11

is able to show the problems inherent to both designs. For comparative purposes, the prior art sectional view in

FIG. 11

is taken from a similar orientation as the sectional view shown in

FIG. 10

which is taken along the line

10

—

10

from FIG.

9

.

In

FIG. 11

, the prior art blades are seen to flex differently than those shown in FIG.

10

. In

FIG. 11

, an oncoming flow

236

is displayed by a series of streamlines which identify undesirable flow conditions around the flow the prior art blade halves.

FIGS. 12 and 13

show perspective views of the deformation problems encountered by a swim fin having the structural inadequacies of the prior art blade halves shown in

FIG. 11

when such blade halves are highly flexible. Although Braunkohlen's prior art design is intended to be used by both feet in one fin with a dolphin type kicking stroke, the main problems with his design lie within the structural inadequacies existing within his blade designs, and not with the foot attachment apparatus. Such structural inadequacies in blade designs are shared by both Braunkohlen's and Barnoin's blade designs. For this reason, the same severe structural inadequacies shared by both designs are displayed in

FIGS. 12 and 13

as one simplified embodiment.

FIG. 12

shows a top perspective view of such a prior art swim fin spreading apart in a spanwise manner during use.

FIG. 13

shows a side perspective view of the same swim fin shown in

FIG. 12

except that its blades are seen to bend backward around a substantially transverse axis during use. Just as

FIG. 11

shows the problems created when the prior art blades are made of a significantly rigid material,

FIGS. 12 and 13

show the problems the same prior art design creates when the blades are made out a highly flexible material.

Operation—

FIGS. 9

to

13

The embodiment shown in

FIGS. 9 and 10

is designed to permit right blade half

182

and left blade half

184

to twist along a substantially lengthwise axis. This embodiment uses the same fundamental methods for generating lift that are described in

FIGS. 5

to

7

except that in

FIGS. 9 and 10

, the blades are able to twist so that they can achieve an anhedral orientation during each reciprocating stroke.

The structure of this embodiment permits right blade half

182

and left blade half

184

to bend efficiently around a substantially lengthwise axis during use so that they can attain a twisted form. Right blade half

182

and left blade half

184

are preferably made of a material that can be relatively rigid when it is substantially thick, and relatively flexible when it substantially thin. This allows stiffening members

188

and

202

to be substantially rigid while portions

186

and

200

are substantially flexible. For instance, a fiber reinforced thermoplastic having an appropriate variance in thickness may be used. Any suitable material or combinations of materials may be used as well in any suitable arrangement to produce such desired results. The rapid decrease in thickness near the outer side edges of each blade half enables flexible portion

186

and flexible portion

200

to deform significantly near these outer side edges. This is because such rapid tapering substantially reduces anti-bending stress forces along outer edge

190

of flexible portion

186

, as well as along outer edge

204

of flexible portion

200

. Since deformation can occur substantially close to the outer side edges of each blade half, separation is significantly reduced along the low pressure surface of each blade. This significantly increases lift and decreases drag. Preferably, flexible portion

186

and flexible portion

200

are made sufficiently flexible to bend to a significantly lowered angle of attack during relatively gentle kicking strokes. Experiments show that such high levels of flexibility are necessary to reduce stall conditions and generate lift.

The rapid change in thickness near the outer side edges of each blade half also permits stiffening members

188

and

202

to remain substantially thick and rigid while flexible portions

186

and

200

are made significantly thin and highly resilient. In alternate embodiments, outer edges

190

and

204

can be thinner that the rest of flexible portions

186

and

200

, respectively. This can further increase flexibility by further reducing the volume of material that must succumb to bending stresses near stiffening members

188

and

202

.

In

FIG. 9

, stiffening members

188

and

202

are seen to taper in thickness along their lengths toward trailing tips

195

and

216

, respectively. This permits the trailing portions of each blade half to experience increased flexibility so that a whip-like action is created during use. As the trailing portions of each blade arch backward, lift vectors

224

and

226

can become tilted slightly forward toward the swimmer's intended direction of travel. The flexibility of these trailing portions should not be so great as to significantly reduce the lengthwise twisting moment within each blade, nor should it create undesirable levels of lost motion or spanwise spreading. Sufficient levels of rigidity should be maintained along the entire length of stiffening members

188

and

202

to prevent excessive levels of deformation from occurring. The tapered shape of stiffening members

188

and

202

also reduces separation near the trailing portions of each blade half by providing a more streamlined hydrofoil shape near these trailing portions.

Many variations of this embodiment are possible. Stiffening members

188

and

202

can maintain constant thickness and, or rigidity along their lengths. If any tapering or change in rigidity is used, it may occur in a series of steps along the length of each blade. A small zone of decreased thickness may be created near foot pocket

180

to permit the base of stiffening members

188

and

202

to achieve some degree of backward bending capability around a transverse axis near foot pocket

180

.

Other alternate embodiments can include the use of multiple materials within each blade half. Flexible portion

186

and stiffening member

188

can be made of two different materials joined together with a mechanical and, or chemical bond. The same situation can apply for flexible portion

200

and stiffening member

202

. By using more rigid materials for stiffening members

188

and

202

, their thickness can be reduced to improve the efficiency of the hydrofoil shape. This allows the change in each blade's cross sectional shape to be reduced without decreasing the change in flexibility between stiffening member

188

and flexible portion

186

, as well as between stiffening member

202

and flexible portion

200

. Also, stiffening members

188

and

202

may be made of a group of materials. This can include the use of reinforcement members, beams, struts, wires, rods, tubes, ribs, and fibers.

In

FIG. 9

, stiffening members

188

and

202

are seen to be highly swept and diverge away from each other along their length. The degree of sweep used in the alignments of stiffening members

188

and

202

may be varied according to desire. If less sweep is desired, members

188

and

202

may diverge away from each other at an increased rate. If each fin is intended to be used independently by each of the user's feet and members

188

and

202

are intended to be highly divergent, the length of each blade half can be reduced to decrease the span of each swim fin so that the fins do not collide with one another during use. In this situation, it is preferred (but not required) that the outer portions of stiffening members

188

and

202

become highly swept. It is also preferred that at least the outer portions of stiffening members

188

and

202

are sufficiently swept back enough for the blade halves to twist anhedrally in an amount effective to significantly reduce the occurrence of outward directed spanwise cross flow conditions along the attacking surface of the blade halves.

Other alternate embodiments can include using both of the user's feet within one swim fin for use in a porpoise-like kicking motion. This type of use enables the span (and overall dimensions) to be significantly increased if desired. This is because collisions with another fin is avoided by using a solitary fin. In such a situation, right blade half

182

and left blade half

184

can be located on the outer ends of a substantially transversely aligned wing-like hydrofoil. This would form two highly swept trailing tips on each end of the transverse hydrofoil. The streamwise length of the blade halves can be varied according desire on different embodiments. The anhedral orientations achieved by blade halves

182

and

184

as they twist around a lengthwise axis during use can significantly reduce induced drag vortex formation on either side of such a transverse hydrofoil. The lift vectors produce by the reduced angle of attack achieved by blade halves

182

and

184

can also significantly increase the lift generated by the transverse hydrofoil. The transverse hydrofoil can also be swept back to any desired degree. Any desired spanwise dimensions or aspect ratios can be used.

FIG. 10

shows a sectional view taken along the line

10

—

10

from FIG.

9

. The view show in

FIG. 10

illustrates that the blades are able to twist around a substantially lengthwise axis to a significantly reduced angle of attack while the positions of stiffening members

188

and

202

remain significantly stable during a kicking stroke. Such twisting is seen to occur significantly close to the outer side edge of blade halves

182

and

184

. This is possible because a significantly large change in thickness on blade halves

182

and

184

occurs significantly close to outer edges

194

and

208

. This rapid change in thickness permits a rapid change in flexibility to also occur near these locations. As a result, a significantly high degree of flexibility occurs at the junction of flexible blade portion

186

and stiffening member

188

, as well as at the junction of flexible blade portion

200

and stiffening member

202

. Because the spanwise dimensions of blade portions

186

and

200

are significantly large in comparison to the spanwise dimensions of blade halves

182

and

184

, respectively, blade portions

186

and

200

are able to exert a significantly large amount of leverage upon their junction to stiffening members

188

and

202

, respectively.

Similarly, the rapid increase in thickness occurring between inner edge

192

and outer edge

194

of stiffening member

188

, as well as between inner edge

206

and outer edge

208

of stiffening member

202

, permits a large increase in rigidity to occur within stiffening members

188

and

202

. Some flexibility may be permitted to exist within stiffening members

188

and

292

so long as such flexibility does not cause substantially large levels of lost motion to occur which significantly reduce performance. It is preferred that stiffening members

188

and

202

are sufficiently rigid enough to prevent blade halves

182

and

188

from deforming excessively during use. It is also intended that any deformation exhibited during use along the lengths of stiffening members

188

and

202

does not occur in an amount or manner which may significantly inhibit flexible blade portions

186

and

200

from efficiently deforming in an anhedral manner.

Preferably, the degree of rigidity should be selected to significantly reduce the tendency for blade half

182

and

184

to bend backward around a substantially transverse axis during use under the exertion of vertical component

232

of lift vector

226

, and under the exertion of vertical component

228

of lift vector

224

, respectively. It is also preferred that the degree of rigidity should be selected to significantly reduce the tendency for blade half

182

and

184

to spread apart from each other in a substantially sideways manner during use under the exertion of horizontal component

234

of lift vector

226

and horizontal component

230

of lift vector

224

, respectively. This significantly reduces the degree of lost motion existing between strokes. It also enables each blade half to substantially maintain orientations that efficiently generate significantly high levels of lift. Furthermore, such rigidity enables the lift generated by blade half

182

and blade half

184

to be efficiently transferred onto foot pocket

180

which in turn pushes forward upon the swimmer's foot for propulsion.

In

FIG. 10

, oncoming flow

222

is illustrated by a series of streamlines flowing around blade halves

182

and

184

. The streamlines curving around stiffening members

188

and

202

toward lower surfaces

218

and

220

, flow in a smooth and attached manner. This permits high levels of lift to be efficiently generated on blade halves

182

and

184

. Also, the streamlines flowing along upper surfaces

199

and

214

flow in an inward direction toward the recess between the blades. This illustrates that outward directed spanwise cross flow conditions have been significantly reduced. Because the streamlines above and below blade halves

182

and

184

are able to merge in a constructive manner, lift is efficiently generated. This is because such a merging causes the water flowing a greater distance around the lee surface of each blade half to flow at a faster rate in order to keep up with the water flowing a shorter distance across the attacking surfaces of the blades. This increase in flow speed along the lee surfaces causes the water flowing across these surfaces to experience a decrease in pressure. It is this decrease in pressure which creates lift on the blades.

The presence of inward flowing streamlines above upper surfaces

199

and

214

demonstrate that fluid pressure is increasing above these surfaces. This combines with the low pressure field generated below lower surfaces

218

and

220

to further increase lift by increasing the overall difference in pressure existing between the attacking surfaces and the lee surfaces of the blades. Some of the streamlines are seen to pass through the recess existing between inner edges

198

and

212

. Such movement through this recess permits flow exiting the attacking surfaces to merge with the flow exiting the lee surfaces, thereby making lift generation possible according to Bernoulli's principle. In addition, this passage of water through the recess also permits excess back pressure along the attacking surfaces to be vented through this recess. This prevents such back pressure from building up to levels which cause the flow along the attacking surfaces to back up and expand in an outward spanwise direction.

Because outward spanwise cross flow conditions are significantly reduced, or even eliminated along the attacking surfaces, the water flowing across these surfaces is efficiently jettisoned in a focused manner toward the trailing edges of the blades. This significantly increases forward propulsion when combined with lift generating attached flow conditions along the lee surfaces of the blades. The streamlines shown in

FIG. 10

which are flowing in an inward direction along upper surfaces

199

and

214

, are also flowing at a significantly fast rate toward the trailing edges of the blades (out of the plane of the paper toward the viewer). The ratio of inward spanwise directed flow to aftward directed flow can be varied according to desire.

Wind tunnel tests of smoke trails flowing around blade designs using the flow control methods of the present invention demonstrate significantly reduced levels of outward spanwise cross flow conditions along the attacking surfaces of the blades. In addition, these tests demonstrate that substantially high levels of attached flow conditions occur along the lee surfaces of the blades. Comparative smoke trail tests of many prior art blade designs show that significantly high levels of outward spanwise flow conditions occur along their attacking surfaces. Such comparative tests of prior art designs also show that significantly high amounts of flow separation and induced drag vortex formation along their lee surfaces.

Wind tunnel tests of models employing the flow controlling methods of the present invention show that many variations can be created within both the spanwise cross flow conditions and the aftward directed flow conditions that exist along the attacking surfaces of the blades. By manipulating various variables each of these flow conditions and their ratio to each other can be varied. For instance, a controlled reduction in the size of the recess that exists during use can cause the streamlines flowing along the attacking surfaces to flow straight in an aftward direction toward the trailing edges of the blades without experiencing either inward cross flow conditions toward the recess, or outward cross flow conditions toward the outer side edges of the blades. In this situation, the orientation of the blades and the size of the recess are trimmed to permit high levels of aftward flow to occur across the attacking surfaces without the presence of noticeable cross flow conditions. The size of recess is trimmed to drain back pressure out of the center region between the blades in an amount effective to prevent outward directed spanwise cross flow conditions from occurring. By increasing the size of the recess that exists during use (this can be achieved by allowing the blades to twist to a more anhedral orientation), the streamlines can be made to converge toward the recess with inward directed spanwise cross flow conditions. This can increase the potential speed with which the blades can be moved through the water since an increase in the recess's flow capacity permits the maximum back pressure the recess can handle is also increased. This is beneficial because an increase in flow speed creates a corresponding increase in lift generated along the low pressure surfaces of the blades.

Many variables contribute to a particular ratio of spanwise cross flow conditions to aftward directed flow conditions. These include the lengthwise angle of attack of the blades (controlled by the lengthwise alignment of stiffening members

188

and

202

), the transverse angle of attack of the blades (substantially controlled by the ease of pivoting around a transverse axis as well as by the overall range of motion that is achievable during use), the overall shape, contour, width, and length of the recess existing both at rest and during use, the speed and direction of the blade moving through the water (substantially controlled by the strength and direction of the blade through the water), and the strength of the lifting force generated by the blades (substantially controlled by the quality and orientation of attached flow conditions along the lee surfaces of the blades, as well as the shape, contour, texture, degree of sweep, and size of the blades).

In alternate embodiments, many of these variables and their controlling factors can be manipulated and changed according to desire and combined in any manner. If desired, some or all of these variables can be made continuously adjustable to enable the user to make fine tune adjustments or dramatic changes according to their individual preferences. The lengthwise angle of attack exhibited by the blades is substantially controlled by the lengthwise alignment of stiffening members

188

and

202

. Alternate embodiments can have stiffening members

188

and

202

pivotally attached to foot pocket

180

in a manner that permits them to pivot around a transverse axis relative to foot pocket

180

through a predetermined range of motion. This would enable stiffening members

188

and

202

to pivot along their length to create a lengthwise reduced angle of attack during use. This pivotal action is often observed in marine mammals and fish. In order to minimize lost motion during this pivoting, the range of motion can be limited to significantly small levels. For instance, the amount of time used during each stoke to vary the lengthwise angle of attack can be arranged to coincide with the time the blades take to pivot to a transverse reduced angle of attack around a lengthwise axis (anhedral pivoting). Once stiffening members

188

and

202

have pivoted to their desired range limit, a suitable stopping device may be used to halt all other movement (either gradually or immediately). It is intended that such a stopping device have sufficient strength and rigidity to permit the blades to maintain orientations effective in generating lift while efficiently transferring such lift from the blades to foot pocket

180

so that propulsion is maximized. Also some degree of resistance or spring-like tension can occur within a given range of motion as stiffening members

188

and

202

experience lengthwise pivoting. This allows advantageous flow conditions to occur while stiffening members

188

and

202

are pivoting through their limited range of motion. Such spring-like tension can also serve to snap stiffening members

188

and

202

back to a neutral orientation at the end of a stroke.

Wind tunnel tests of blade designs employing the methods of the present invention which show significant reductions in outward spanwise flow conditions also show that flow conditions beyond the fin's trailing edges are also significantly improved over the prior art. In tests with prior art designs, any streamlines that are able to flow past the trailing edge are quickly redirected with the direction of the surrounding flow.

However, in tests with designs using the flow control methods of the present invention, almost all of the smoke trails flowing above the attacking surface are deflected in a direction that is substantially parallel to the lengthwise alignment of the blades. These smoke trails are then projected a significantly farther distance into the free stream than that achieved by prior art designs before becoming re-aligned with the-downstream movement of the surrounding flow. This shows a substantial increase in flow velocity and momentum within the fluid ejected from the trailing edges of blade designs of the present invention in comparison to the prior art.

Because the methods of the present invention permit advantageous cross flow conditions to be created along the attacking surfaces of the blades while attached flow conditions are permitted to form along the lee surfaces of the blades, significantly high levels of propulsion can be attained. While advantageous flow conditions along the attacking surfaces can improve performance, test models of working swim fins show that the main factor affecting overall propulsion is the degree of flow separation along the lee surfaces. As lee surface separation and induced drag vortex formation is replaced by attached flow conditions, propulsion is significantly increased. Test models with swim fins having blades that exhibit stall conditions offer little or no propulsion, while test models of the present invention having blades with attached flow conditions along their lee surfaces offer significantly high levels of propulsion. The methods of the present invention succeeds in achieving significant reductions in lee surface flow separation and induced drag formation while where prior designs fail to do so.

FIGS. 11

to

13

show several problems of prior art dual blade designs which are solved by the present invention.

FIG. 11

shows the substantially limited anhedral bending capabilities exhibited by evenly tapered blade halves. The evenly tapered blades made from a single type of material permit only a gradual change in flexibility to occur. Because this change in flexibility occurs over a significantly large distance, bending tends to occur a significantly long distance from the outer side edge of each blade half. The significantly large volume of material used within a gradually tapering cross sectional shape substantially increases the material's resistance to bending. This is because it increases the amount of material that must succumb to the stress forces of compression and tension before any such bending can occur.

Because of these disadvantages, the evenly tapered cross sectional shape of each blade half shown in

FIG. 11

is highly inefficient at bending around a significantly lengthwise axis. If the blade halves are made rigid enough to avoid excessive backward bending around a transverse axis under the pressure of oncoming flow

236

during use, the blades are too rigid to experience significant bending around a lengthwise axis. As a result, only a small portion of each blade half is seen to deform in an anhedral manner around a lengthwise axis under water pressure generated during use. The broken lines show the resting position of each blade half. Because a majority of each blade half remains at an excessively high angle of attack relative to oncoming flow

236

, the blades stall during use. This prevents lift from being generated.

The streamlines of oncoming flow

236

shown in

FIG. 11

display the undesirable flow conditions existing around the prior art blade halves. Although a small amount of water is channeled toward the space between the blade halves, the high angle of attack existing across a majority of the each blade's span prevents water from being efficiently focused away from the outer side edge of each blade half. This causes water pressure to quickly back up along the attacking surfaces (the upper surfaces in this view) and spill sideways around the outer side edges of the blades. As the streamlines curve around these outer side edges, the flow is seen to separate from the lee surfaces (the lower surfaces in this view) of the blades. This forms a significantly large induced drag vortex below the lee surface of each blade half. The induced drag vortices draw water away from the attacking surface at an increased rate. The separation destroys lift and creates high levels of drag. In addition, the induced drag vortices are seen to curl the water so that it flows back toward the lee surfaces of each blade half. This curling water pushes against the lee surfaces of the blade halves in the opposite direction of desired lift. Experiments with test models show that substantially rigid blades having the structural inadequacies shown in

FIG. 11

suffer from significantly high levels of drag and do not offer significant levels of propulsion.

FIG. 12

shows a top view of a swim fin during use which suffers from the same structural problems of the prior art discussed in

FIG. 11

, except that the blades shown in

FIG. 12

are made from a more flexible material than the blades shown in FIG.

11

. When the blade halves shown in

FIG. 11

are made more flexible so that they are more able to deform in an anhedral manner around a lengthwise axis, the blade halves become highly vulnerable to the type of deformation illustrated in FIG.

12

.

In

FIG. 12

, the broken lines show the position of the prior art type blades while they are at rest. The solid lines show that the blades deform significantly in a spanwise manner during use. From this top view, the swim fin is being kicked toward the viewer. The curved arrows show each blade's direction of movement as the swim fin is kicked after being at rest.

The spread apart orientation illustrated in

FIG. 12

results because increasing the flexibility of each blade half reduces the ability for each blade to resist the outward force created by the inward flowing water near the space between the blades. Also, Because such an increase in flexibility permits the blades to experience more anhedral deformation during use, more water is deflected in an inward direction toward the space between the blades. This in turn significantly increases the force with which this inward moving water pushes in an outward spanwise direction upon the blade halves. As a result, the greater the degree of anhedral deformation, the greater the degree to which the blade halves spread apart from each other during use. If each blade is made flexible enough to permit significant levels of anhedral bending around a lengthwise axis, it is not rigid enough to avoid destructive spanwise deformation. As discussed in the Background—Description of Prior Art section of this specification, such spanwise spreading destroys the efficiency of the swim fin.

FIG. 13

shows a perspective side view of the same swim fin shown in

FIG. 12

as it is kicked upward during use. While

FIG. 12

shows the blades spreading outward, the view in

FIG. 13

shows that the blades also tend to simultaneously bend backward around a transverse axis during use. The broken lines show the position of the blades at rest. The arrow above the user's foot shows the direction of the kicking stroke. The curved arrows show each blade's direction of movement as the swim fin is kicked forward after being at rest. Such backward bending occurs because the structure of each blade is highly vulnerable to bending around a transverse axis when it is made flexible enough to experience significant anhedral deformation along its length.

Experiments with test models having the structural inadequacies shown in

FIGS. 12 and 13

demonstrate that such dramatic levels of undesirable deformation occur commonly when highly resilient materials are used. Such experiments show that propulsion is poor for blades having these deformation problems. Experiments also show that merely increasing the rigidity of the material used for each blade, only causes a larger portion of each blade to remain at an excessively high angle of attack which causes stall conditions that destroy lift and generate high levels of drag. These problems render such prior art designs unusable.

Looking back to the embodiment of the present invention shown in

FIGS. 9 and 10

, it can be seen that the combination of significantly rigid stiffening members

188

and

202

with highly resilient flexible blade portions

186

and

200

, respectively, efficiently solve the performance debilitating structural problems inherent to the prior art. Unlike the prior art, the methods of the present invention provide the blades with sufficient flexibility to twist in an anhedral manner around a significantly lengthwise axis while providing sufficient rigidity to permit the blades to substantially maintain their orientations during use. This permits drag producing stall conditions to be replaced by lift generating attached flow conditions on each blade. In addition, the blades have enough structural integrity to efficiently transfer their newly derived lift to foot pocket

180

so that the swimmer is propelled forward. By significantly reducing the occurrence of spanwise spreading and backward bending during use, the methods of the present invention permit lost motion to be significantly reduced as well.

Not only did Barnoin and Braunkohlen not offer methods for establishing lift generating attached flow conditions along the lee surfaces of their blade designs, they did not mention that they were aware that this is necessary, nor did they mention that they were aware that their blades create high levels of drag from high levels of stall conditions and induced drag vortex formation. Not only did Barnoin and Braunkohlen not offer any methods for preventing their blades from spreading apart in a spanwise direction, neither of them mentioned that they were aware that such a problem existed with their designs. They also did not mention that they were aware that the use of highly resilient and deformable materials renders their blades highly vulnerable to excessive levels of lost motion due to backward bending around a transverse axis.

Description—

FIGS. 14

to

23

FIG. 14

shows a cut-away perspective view displaying the right half of the same swim fin shown in FIG.

9

. Because both blade halves of this embodiment function in the same manner,

FIG. 14

solely describes the right half. Also, the cut-away view in

FIG. 14

allows one to see the significantly thick portion of flexible portion

186

that extends below foot pocket

180

to form the sole of foot pocket

180

(discussed previously in FIG.

9

). Another reason why only the right blade half is shown is because this design may also be used with only one blade half and no other companion blades or blade halves. Such an embodiment is similar to that shown in

FIGS. 1-4

except that a flexible blade is provided in the figures below to permit the angle of attack to be changed on each reciprocating stroke. Alternate embodiments may employ any desirable number of additional blades in any desirable arrangement or configuration. How ever, the preferred embodiment will employ two substantially symmetrical blade halves.

In

FIG. 14

, a broken line shows the presence of a bending zone

238

along flexible portion

186

which extends from the base of the center recess near foot pocket

180

to trailing edge

196

near trailing tip

195

.

FIG. 15

shows a cross sectional view taken along the line

15

—

15

from FIG.

14

. In

FIG. 15

, bending zone

238

is displayed by a vertically oriented broken line extending above and below the plane of

186

. Bending zone

238

is shown in this manner so that its position on flexible portion

186

may be seen from this cross sectional view. An oncoming flow

240

is displayed by a series of streamlines flowing toward and around right blade half

182

. A neutral position

242

of flexible portion

186

is displayed by horizontally aligned broken lines. A semi-flexed position

244

of flexible portion

186

is displayed by downward angled solid lines. A highly flexed position

246

of flexible portion

186

is displayed by downward angled broken lines. The deformation of blade half

182

to flexed positions

242

and

246

occur as the swim fin is kicked upward through the water with upper surface

199

being the attacking surface. It can be seen that the deformation of flexible portion

186

from neutral position

242

to either semi-flexed position

244

or highly flexed position

246

occurs between bending zone

238

and inner edge

198

. The portion of flexible portion

186

existing between bending zone

238

and stiffening member

188

remains substantially stationary relative to the orientation of stiffening member

188

under the exertion of oncoming flow

240

. As the streamlines of oncoming flow

240

pass around the outside of stiffening member

188

when flexible portion

186

is deformed to position

244

, a zone of separation

248

is formed along the low pressure surface of right blade half

182

.

FIG. 16

shows a cross sectional view taken along the line

16

—

16

from FIG.

14

. This sectional view taken at line

16

—

16

from

FIG. 14

occurs closer to trailing edge

196

than the sectional view taken along the line

15

—

15

from

FIG. 14

, and also occurs closer to foot pocket

180

than the sectional view taken along the line

10

—

10

from FIG.

9

. In

FIG. 16

, an oncoming flow

249

is displayed by two streamlines flowing toward and around right blade half

182

as the swim fin is kicked through the water during the same upward stroke as that occurring in FIG.

15

. Thus, oncoming flow

249

in

FIG. 16

is produced by the same kicking motion used to form oncoming flow

240

shown in FIG.

15

. In

FIG. 16

, positions

242

,

244

, and

246

of flexible portion

186

are the same as those shown in

FIG. 15

, except that in

FIG. 16

these positions are taken along the line

16

—

16

from FIG.

14

. In

FIG. 16

, position

242

of flexible portion

186

is displayed by horizontally broken lines. Position

244

of flexible portion

186

is displayed by downward angled solid lines. Position

246

of flexible portion

186

is displayed by downward angled broken lines. Again, bending zone

238

is displayed by a vertically aligned broken line so that the position of bending zone

238

on flexible portion

186

can be seen from this view. Because bending zone

238

is substantially close to stiffening member

188

, an increased portion of flexible portion

186

is able to deform to either position

244

or position

246

during use.

As the streamlines of

249

flow around the outside of stiffening member

188

, a separation zone

250

is formed along the low pressure surface of right blade half

182

. Separation

250

is significantly smaller than separation

248

shown in FIG.

15

. As a result, the streamline flowing around the outside of stiffening member

188

in

FIG. 16

is able to flow substantially parallel to the alignment of semi-flexed position

244

of flexible portion

186

. A lift vector

251

is exerted on right blade half

182

.

FIG. 17

shows a cut-away perspective view of the same swim fin shown in

FIG. 14

except that in

FIG. 17

, a transverse recess

252

is cut out of flexible portion

186

near foot pocket

180

, and also a trailing edge

196

′ is seen to be more swept than trailing edge

196

shown in FIG.

14

. In

FIG. 17

, transverse recess

252

extends in a substantially chordwise direction from inner edge

198

toward stiffening member

188

and terminates before reaching stiffening member

188

. A bending zone

254

is represented by a broken line along flexible portion

186

which extends from the outside end of recess

252

to trailing edge

196

′ near trailing tip

195

.

FIG. 18

shows a cut-away perspective view of the same swim fin shown in

FIG. 14

, except that the embodiment shown in

FIG. 18

has a forward transverse recess

256

, an intermediate transverse recess

258

, and a trailing transverse recess

260

cut out of flexible portion

186

at various intervals along inner edge

198

. An outer bending zone

262

is displayed by a broken line along flexible portion

186

which extends from the outside end of recess

256

to trailing edge

196

′ near tip

195

. An intermediate bending zone

264

is displayed by a broken line along portion

186

which extends from the outside end of recess

258

to trailing edge

196

′ near tip

195

. An inner bending zone

266

is displayed by a broken line along portion

186

which extends from the outside end of recess

260

to trailing edge

196

′ near tip

195

. Recess

256

, recess

258

, and recess

260

separate portion

186

into a root portion

267

, a forward panel

268

, an intermediate panel

270

, and a trailing panel

272

.

FIG. 19

shows a perspective view of the same swim fin shown in

FIG. 18

except that in

FIG. 19

, both halves of the swim fin are shown deforming during use. Because left blade half

184

is now visible from this view, a forward transverse recess

274

, an intermediate transverse recess

276

, and a trailing transverse recess

278

are seen to exist along flexible portion

200

. Recess

274

, recess

276

, and recess

278

are seen to separate flexible portion

200

into a root portion

267

, a forward panel

280

, an intermediate panel

282

, and a trailing panel

284

.

The upwardly inclined arrow located above foot pocket

180

shows that the swim fin is being kicked upward through the water so that the upper surface of each blade half is the attacking surface. During use, forward panels

268

and

280

are seen to deform to an anhedral orientation relative to each other. Intermediate panels

270

and

282

are deformed in an increased anhedral orientation. Trailing panels

272

and

284

are deformed in the most anhedral orientation. As this happens, it can be seen that each transverse recess widens in a divergent manner to form a substantially triangular shaped void. From this view, the highly anhedral orientation of trailing panel

284

causes lower surface

220

of portion

200

to be visible along left blade half

284

. Stiffening members

188

and

202

are seen to flex backward under water pressure near tips

195

and

216

, respectively.

FIG. 20

shows a perspective side view of the same swim fin shown in

FIGS. 18 and 19

except that in

FIG. 20

, a forward transverse recess

286

, an intermediate transverse recess

288

, and a trailing transverse recess

290

are substituted for recesses

256

,

258

, and

260

showy in

FIGS. 18 and 19

. When comparing

FIG. 20

to

FIGS. 18 and 19

, recesses

286

,

288

, and

290

in

FIG. 20

are seen to extend closer to stiffening member

188

than recesses

256

,

258

, and

260

shown in

FIGS. 18 and 19

. In

FIG. 20

, recesses

286

,

288

, and

290

separate portion

186

into a root portion

291

, a forward panel

292

, an intermediate panel

294

, and a trailing panel

296

. Panels

292

,

294

, and

296

are seen to be significantly larger than panels

268

,

270

, and

272

shown in

FIGS. 18 and 19

.

Another difference existing between FIG.

20

and

FIGS. 18 and 19

is that in

FIG. 20

, significantly flexible chordwise membranes are added to fill the chordwise voids in portion

186

created by recesses

286

,

288

, and

290

. In

FIG. 20

, a forward transverse flexible membrane

298

, an intermediate transverse flexible membrane

300

, and a trailing transverse flexible membrane

302

are loosely suspended across recesses

286

,

288

, and

290

, respectively. The outside edges of each flexible membrane is attached to the inside edges of its respective recess in any suitable manner. A mechanical and, or chemical bond may be used to secure these edges together. Examples of mechanical bonds may include a system of small mating protrusions and orifices existing within the joining edges. Such mating features can include holes, grooves, ridges, teeth, wedges, and other similar gripping shapes. Suitable adhesives and, or welds may be used to provide a chemical bond instead of, or in addition to a mechanical bond.

In this embodiment, it is preferred that membranes

298

,

300

, and

302

are significantly more flexible than portion

186

. Membranes

298

,

300

, and

302

may be made of a highly resilient thermoplastic, however, any flexible material may be used as well. Examples of such flexible materials may include fabric, silicone rubber, silicone thermoplastics, neoprene, rubber or plastic impregnated fabric, fiber reinforced thermoplastics, and fabric reinforced thermoplastics.

The view shown in

FIG. 20

shows the position of this embodiment at rest. Each flexible membrane is seen to have a loose fold from extra material. The transversely aligned dotted line extending from the outside end of each membrane to inner edge

198

displays that the amount of extra material used in each membrane increases toward inner edge

198

. A bending zone

304

is represented by a broken line along portion

186

that extends from the outside end of recess

286

to trailing edge

196

′ near tip

195

. In this embodiment, the outside ends of both recess

288

and recess

290

terminate at positions along portion

186

that are in alignment with bending zone

304

.

FIG. 21

shows a perspective side view of the complete embodiment shown in

FIG. 20

while it is kicked through the water during use. The arrow pointing downward beneath foot pocket

180

displays that the swim fin is being kicked downward. Left blade half

184

is closer to the viewer than right blade half

182

.

On right blade half

182

, lower surface

218

of portion

186

is most visible on panel

296

while being less visible on panel

294

and least visible on panel

292

. Membrane

300

is seen to have stretched out to achieve a substantially triangular shape between panels

292

and

294

. Membrane

302

has also stretched out to a triangular shape between panels

294

and

296

.

Left blade half

184

deforms similarly to right blade half

182

under water pressure. Upper surface

214

of portion

200

is most visible along a trailing panel

310

, less visible along an intermediate panel

308

, and least visible along a forward panel

306

. Between foot pocket

180

and panel

306

is a forward transverse flexible membrane

312

which is barely visible from this view. An intermediate transverse flexible membrane

314

is seen to be stretched to a triangular shape between panel

306

and panel

308

. Similarly, a trailing transverse flexible membrane

316

is stretched to a triangular shape between panel

308

and panel

310

.

FIG. 22

shows a cut-away perspective view of the same swim fin shown in

FIGS. 20 and 21

, except that in

FIG. 22

a lengthwise flexible membrane

318

is added.

FIG. 22

shows that Membrane

318

is a narrow strip of resilient material that separates stiffening member

188

from portion

186

. Membrane

318

is seen to merge with membranes

298

,

300

, and

302

. As a result, portion

186

is completely divided into a root portion

319

, a leading panel

320

, an intermediate panel

322

, and a trailing panel

324

. The outer edge of membrane

318

(closest to stiffening member

188

) is preferably attached to inner edge

192

of stiffening member

188

with a mechanical and, or chemical bond. The inner side edge of membrane

318

(furthest from stiffening member

188

) is attached to the outer side edges of panels

320

,

322

, and

324

in a similar manner.

This embodiment may be injection molded to minimize production time. For example: stiffening member

188

, root portion

319

, panel

320

, panel

322

, and panel

324

may be molded first out of one material and then arranged so that foot pocket

180

, membrane

298

, membrane

300

, membrane

302

, and membrane

318

can be molded out of a more resilient material into (or onto) their respective parts in a final step of assembly. Any suitable method of construction may be used.

In alternate embodiments, membrane

318

can be separate from one or more of the transverse membranes. In addition, any number of transversely aligned membranes can be used to create any number of segmented panels.

FIG. 23

shows a cross sectional view taken along the line

23

—

23

from FIG.

22

. In

FIG. 23

, the horizontally aligned broken lines show the position of trailing panel

324

while the swim fin is at rest. An oncoming flow

326

is created as the swim fin shown in

FIG. 22

is kicked upward. In

FIG. 23

, oncoming flow

326

is displayed by two streamlines flowing toward and around right blade half

182

. The pressure exerted by oncoming flow

326

causes membrane

318

to deform so that panel

324

becomes inclined to a reduced angle of attack relative to oncoming flow

326

. As the two streamlines flow around right blade half

182

, a lift vector

328

is formed.

This cross sectional view displays that the outer edge of membrane

318

(closest to stiffening member

188

) extends into inner edge

192

of stiffening member

188

. Also, the inner edge of membrane

318

(farthest from stiffening member

188

) is seen to extend into the outer side edge of

324

. This only one example of how such edges may be joined. To strengthen the bond, any suitable arrangement of holes or perforations may be added to one or more of the joining edges of stiffening member

188

and panel

324

so that when membrane

318

is injection molded into them, the material used for membrane

318

fills into such holes or around such perforations to provide a secure grip. Chemical bonds may used as well.

Operation—

FIGS. 14

to

23

FIG. 14

shows a cut-away perspective view of the right half of the same swim fin shown in FIG.

9

. The cut-away view in

FIG. 14

shows that portion

186

increases in thickness below foot pocket

180

. As stated previously, it is preferred that this portion of portion

186

is rigidly attached to stiffening member

188

. The thickened portion of portion

186

increases the rigidity of the swim fin beneath foot pocket

180

and provides structural support for stiffening member

188

. As a result, the kicking motion applied to the swimmer's foot is transmitted to stiffening member

188

in an efficient manner. In alternate embodiments, foot pocket

180

can be made more rigid while portion

186

below foot pocket

180

is made more resilient. In still other embodiments, portion

186

below foot pocket

180

can be flexible while the user's foot inserted within foot pocket

180

stiffens foot pocket

180

in an amount effective to permit the kicking motion to be transferred to stiffening member

188

in an efficient manner. In this situation, the material within foot pocket

180

is made sufficiently strong enough to resist stretching out of shape, and therefore foot pocket

180

is able to stabilize the position of stiffening member

188

during use. It is still preferred, however, that portion

186

becomes substantially more rigid beneath foot pocket

180

as shown in

FIG. 14

so that energy is transferred with increased efficiency from stiffening member

188

to the foot of the user.

Since stiffening member

188

makes the outer side edge of right blade half

182

significantly rigid while the thickened area of portion

186

below foot pocket

180

makes the base of right blade half significantly rigid, the more flexible areas of portion

186

existing between bending zone

238

, stiffening member

188

, and foot pocket

180

are significantly resistant to deforming during use. This is because this triangular shaped region of portion

186

is supported by two rigid structures that provide support in two different dimensions. Because the areas of portion

186

existing between bending zone

238

, trailing edge

196

, and inner edge

198

are less supported by the swim fin's more rigid structures, these regions of portion

186

are significantly more able to deform under water pressure. Bending zone

238

is therefore an imaginary line that marks a border which separates the more deformable areas of portion

186

from the less deformable areas of portion

186

.

Because stiffening member

188

is sufficiently rigid enough to avoid substantial deformation during use, bending zone

238

on portion

186

extends all the way to trailing edge

196

near tip

195

. This allows bending zone

238

to have a substantially lengthwise alignment across right blade half

182

. Consequently, the rigidity of stiffening member

188

permits portion

186

to bend around a substantially lengthwise axis so that water along the attacking surface is directed away from stiffening member

188

and toward inner edge

198

during use.

Because the rigidity of stiffening member

188

enables bending zone

238

to extend to tip

195

, blade half

182

has increased resistance to spanwise or sideways directed bending during use. This is because bending zone

238

marks a zone of tension created within portion

186

. When an outward directed force is applied to blade half

182

as portion

186

twists to a reduced angle of attack during use, the outward force tries to stretch the area of portion

186

existing between bending zone

238

, foot pocket

180

, and stiffening member

188

. Because this area contains a substantially large amount of material, resistance to such stretching is relatively high and outward spanwise bending is significantly reduced. Also, because the alignment of bending zone

238

is at an angle to the alignment of stiffening member

188

, tension within portion

186

long bending zone

238

is applied at an angle to stiffening member

188

. This provides a moment arm which further increases resistance to spanwise bending of stiffening member

188

. Also, because bending zone

238

extends all the way to tip

195

, the entire length of blade half

182

(including the tip region) has significant resistance to sideways bending. As a result, stiffening member

188

can be made to possess a significant level of flexibility along its length if desired while remaining sufficiently rigid enough to prevent excessive levels of sideways bending from occurring.

FIG. 15

shows a cross sectional view taken along the line

15

—

15

in FIG.

14

. In

FIG. 15

, it can be seen that portion

186

is significantly more deformable between bending zone

238

and inner edge

198

than it is between bending zone

238

and stiffening member

188

. Position

242

shows the orientation of portion

186

when the swim fin is at rest. Position

242

can also occur during use if the material used to make portion

186

is not sufficiently resilient enough to deform significantly under the water pressure generated during use. Position

244

shows the orientation of portion

186

during use when the material used to make portion

186

is significantly flexible. Position

246

shows the orientation of portion

186

during use if the material used to make portion

186

is too flexible.

In this embodiment, position

244

is a more preferable flexed orientation during use than either position

242

or position

246

. This is because position

244

achieves a reduced angle of attack without creating an abrupt change in contour across portion

186

. Position

246

is undesirable since an abrupt change in contour is created within portion

186

as it bends to an excessively low angle of attack. Consequently, portion

186

is preferably made of an appropriate material and thickness to provide sufficient flexibility so that it can deform to an orientation between the range of position

242

and position

246

when the swim fin is kicked through the water. Preferably, the angle of such orientation is substantially similar to position

244

. However, the reduced angle of attack achieved during use can occur at any desirable angle which is capable of offering improvements in performance.

Position

246

is shown in this example to illustrate that the structural characteristics of the swim fin prevent portion

186

from flexing between bending zone

238

and stiffening member

188

even if portion

186

is made of a highly resilient material. It is important to visualize how the position of bending zone

238

influences the deforming characteristics of portion

186

. This permits the further improvements described ahead in the specification to be more fully understood and appreciated.

FIG. 16

shows a cross sectional view taken along the line

16

—

16

from FIG.

14

. In

FIG. 16

, the same positions

242

,

244

, and

246

shown in

FIG. 15

are viewed from another region of portion

186

, when comparing

FIG. 16

to

FIG. 15

, it can be seen that in

FIG. 16

bending zone

238

is significantly closer to stiffening member

188

than it is in FIG.

15

. Consequently, separation

250

shown in

FIG. 16

is substantially smaller than separation

248

shown in FIG.

15

. This is because in

FIG. 16

, the region of portion

186

existing between bending zone

238

and stiffening member

188

is significantly smaller than it is in FIG.

15

. As a result, the streamline of oncoming flow

249

that is flowing around the outside of stiffening member

188

in

FIG. 16

is able to become re-attached to the low pressure surface (or lee surface) of portion

186

. The rotational direction of separation

250

also assists in creating attached flow conditions along the low pressure surface of portion

186

. This enables this region of right blade half

182

to generate lift vector

251

during use. Consequently, the trailing portions of right blade half

182

are highly efficient at generating lift. This efficiency increases with proximity to tip

195

.

Alternate embodiments can create limited flow separation such as shown by separation

250

in

FIG. 16

as a method for creating re-attached flow conditions along portions of a blade that are at significantly high angles of attack. This is similar to the intentional formation of leading edge vortices by leading edge vortex flaps on delta wing fighter jets. Vortex generators in the form of ridges can be used to form leading edge vortices in a manner that enables flow to become re-attached further downstream on the foil's low pressure surface. As long as substantially attached flow conditions occur downstream on the foil, lift can be generated efficiently enough to significantly increase propulsion. It is preferred that any separation created along the low pressure surface of blade half

182

is kept within levels that permit attached flow conditions to be created in an amount effective to significantly increase the propulsion created by the blade and to prevent the blades from stalling during use.

In other alternate embodiments, stiffening member

188

can originate near the toe region of foot pocket

180

near the base of the recess and extend forward from the toe in a swept direction that is substantially parallel to bending zone

238

. This enables the alignment of stiffening member

188

to be closer to the alignment of bending zone

238

so that the surface area of portion

186

existing between stiffening member

188

and bending zone

238

is significantly reduced. This can significantly reduce the occurrence of flow separation along the low pressure surface of blade half

182

by reducing the surface area of portion

186

that remains at a high angle of attack during use. This decreases drag and increases lift. In this type of alternate embodiment, it is preferred that stiffening member

188

is made from a highly rigid material because such an orientation between stiffening member

188

and bending zone

238

causes tension the created within portion

186

during twisting to be significantly reduced.

FIG. 17

shows a cut-away perspective view of the same swim fin shown in

FIG. 14

except that in

FIG. 17

recess

252

is cut out of portion

186

near foot pocket

180

. Because recess

252

extends a significant distance toward stiffening member

188

, bending zone

254

is substantially close to stiffening member

188

along its entire length. Consequently, a greater area of portion

186

is allowed to bend to a reduced angle of attack during use. This allows a greater region of portion

186

to participate in generating lift. Because the size of the area of portion

186

existing between bending zone

254

and stiffening member

188

is reduced, separation along the low pressure surface of right blade half

182

is significantly reduced during use. The combination of these situations permit this embodiment to offer increased propulsion and reduced drag over the embodiment shown in FIG.

14

. In

FIG. 17

, it is preferred that the material used for portion

186

is sufficiently flexible to deform during use to a reduced angle of attack that efficiently generates lift with low levels of drag.

Trailing edge

196

′ shown in

FIG. 17

is significantly more swept than trailing edge

196

shown in

FIG. 14

in order to further reduce drag. The more swept trailing edge

196

′ shown in

FIG. 17

permits a smoother transition to occur between trailing edge

196

′ and inner edge

198

. By making this corner more obtuse in form, less turbulence is created at this corner and efficiency is increased. In alternate embodiments, the radius of curvature in this convexly curved corner can be increased to provide a smoother transition between trailing edge

196

′ and inner edge

198

. A significantly larger radius of curvature at this transition between trailing edge

196

′ and inner edge

198

may be used to further reduce drag and increase efficiency. In other embodiments, trailing edge

196

′ can be made concavely curved near trailing tip

195

, and convexly curved near inner edge

198

.

FIG. 18

shows a cut-away perspective view of the same swim fin shown if

FIG. 17

except that the embodiment shown in

FIG. 18

has recesses

256

,

258

, and

260

cut out of to

186

at various intervals along inner edge

198

. Recess

256

in

FIG. 18

is seen to extend slightly closer to stiffening member

188

than recess

252

shown in FIG.

17

. This causes bending zone

262

in

FIG. 18

to be closer to stiffening member

188

than bending zone

254

shown in FIG.

17

. In

FIG. 18

, recess

258

creates bending zone

264

and recess

260

creates bending zone

266

. Consequently, panels

268

,

270

, and

272

all bend around bending zone

262

during use. Similarly, panels

270

and

272

both bend around bending zone

264

, and panel

272

bends around bending zone

266

during use. This permits panel

268

to deform to a reduced angle of attack while panel

270

to deforms to a further reduced angle of attack and panel

272

deforms to the most reduced angle of attack.

In alternate embodiments, one or more of the transverse recesses can have a substantially lengthwise recess located at its outer side end. Such a lengthwise recess can extend forward and, or backward from the base of the transverse recess. This can cause the transverse recess to be substantially L-shaped or substantially T-shaped. Using these shapes to form a transverse recess can further reduce an adjacent panel's resistance to bending around a substantially lengthwise axis. If the lengthwise recess at the base of the transverse recess extends backward (toward foot pocket

180

) into a panel, that panel behind the transverse recess can pivot forward around a transverse axis to a reduced angle of attack as it simultaneously twists around the lengthwise bending zone created by that transverse recess. This can improve efficiency by improving attached flow conditions along the low pressure surface of that panel. In other embodiments, any transverse recesses can have a significantly swept alignment.

FIG. 19

shows a perspective view of the same swim fin shown in

FIG. 18

except that in

FIG. 19

, both halves of the swim fin are shown deforming during use. Both right blade half

182

and left blade half

184

are seen to twist along their lengths to a reduced angle of attack. As water pressure applies a twisting force to right blade half

182

and left blade half

184

, the voids created by the transverse recesses significantly reduce the formation of anti-twisting stress forces within portion

186

and portion

200

. Because each transverse recess is able to widen during use, portions

186

and

200

are permitted to expand under water pressure and the total quantity of material within portion

186

and portion

200

that must succumb to the torsional stress forces of expansion and compression is significantly reduced. Consequently, recesses

256

,

258

,

260

,

274

,

276

, and

278

provide expansion zones for portions

186

and

200

. This enables portion

186

and portion

200

to exhibit significantly decreased levels of resistance to twisting around a substantially lengthwise axis.

Without such expansion zones, the material within portions

186

and

200

would have to stretch an amount similar to that displayed by the expanded transverse recesses shown in FIG.

19

. However, a material which lacks such transverse recesses and is capable of stretching such a significantly large amount under a substantially light kicking stroke is structurally weak and highly vulnerable to collapsing to a zero, or near zero angle of attack around a bending zone such as bending Line

238

shown in FIG.

14

. In

FIG. 19

, it can be seen that the use of transverse recesses

256

,

258

,

260

,

274

,

276

, and

278

permit sufficiently large amounts of expansion to occur across portions

186

and

200

so that substantial twisting results even under relatively light kicking strokes. This permits portions

186

and

200

to be made from a less resilient material that has sufficient structural integrity to not collapse to excessively low angles of attack during such strokes. Thus the strategic placement of expansion zones within portions

186

and

200

permits significantly high levels of twisting to occur under conditions of relatively light pressure with more structurally rugged materials.

As blade halves

182

and

184

twist to reduced angles of attack, the rigidity of stiffening members

188

and

202

reduces the tendency for each blade half to bend backward around a transverse axis or spread apart from each other during use. Consequently, each blade half is able to efficiently twist around a substantially lengthwise axis during use without deforming excessively around a substantially transverse axis and without experiencing excessive levels of spanwise spreading.

In the embodiment shown in

FIG. 19

, stiffening members

188

and

202

are seen to increase in flexibility near tips

195

and

216

, respectively. This is seen as stiffening members

188

and

202

arch backward in a controlled manner under water pressure exerted during use. This allows the direction of lift on panel

272

and panel

284

to become more aligned with the swimmer's direction of travel. Such increased flexibility also produces a whip-like snapping motion to occur near the tips of each blade half as the kicking direction is reversed between strokes. It is preferred that such an increase in flexibility is sufficiently limited to prevent the tip regions of each blade half from experiencing excessive levels of lost motion or sideways spreading. It is also preferred that stiffening members

188

and

202

remain sufficiently rigid enough across their entire length to create a significantly strong twisting moment during use within portions

186

and

200

, respectively. It is also intended that stiffening members

188

and

202

are sufficiently rigid enough to permit blade halves

182

and

184

to substantially maintain orientations that are effective in generating significantly high levels of lift as such a lifting force is transferred from stiffening members

188

and

202

to foot pocket

180

during use.

Each blade half's resistance to twisting can be changed by either increasing or decreasing the transverse dimensions of each transverse recess. On right blade half

182

for instance, if the transverse dimensions of each recess is decreased, portion

186

becomes less able to attain a twisted shape during use. This is because the area of portion

186

existing between the outside end of each transverse recess and stiffening member

188

is unable to expand in a sufficient manner to permit this region of portion

186

to twist around a substantially lengthwise axis. However, if the outside end of each transverse recess is extended further toward stiffening member

188

, portion

186

becomes less resistant to achieving a twisted shape during use. Because this decreases the amount of portion

186

that exists between the outer end of each recess and stiffening member

188

, the total volume of material within portion

186

that must succumb to anti-twisting stress forces is also reduced. Consequently, the longer the transverse dimension of each transverse recess, the lower the resistance of portion

186

to attaining a twisted shape during use. Preferably, the orientation, location, and transverse dimension of each transverse recess on each blade half is selected to provide desirable levels of twist during use. Numerous transverse recesses of differing transverse lengths can be used to provide a wide variety of twisted shapes, forms, and contours in alternate embodiments.

As one or more transverse recesses on each blade half are extended closer to their corresponding stiffening member (member

188

or

202

), the rigidity of stiffening members

188

and

202

must be increased. This is because each blade half becomes more vulnerable to spanwise spreading as the transverse dimensions of each recess is increased. This is because the bending zone created by that transverse recess is moved closer to its corresponding stiffening member. This decreases the moment arm of tension within portion

186

and decreases the amount of material existing between the outer end of each recess and the corresponding stiffening member on each blade half. This decreases spanwise tension within portion

186

on blade half

182

, and within portion

200

on blade half

184

. By decreasing such spanwise tension, each blade half becomes more vulnerable to spanwise spreading during use. This is also due to the increased spanwise direction of lift produced as each blade half is able to twist to a more reduced angle of attack. In such situations, the rigidity of stiffening members

188

and

202

must be increased in an amount effective to significantly reduce the occurrence of spanwise spreading during use. This reduces lost motion and increases the amount of lift transferred from each blade half to foot pocket

180

. Stiffening members

188

and

202

can be made more rigid by increasing their thickness, changing their cross sectional shape, by substituting more rigid materials, or by adding reinforcement structures such as fibers, beads, beams, wires, rods, tubes, filaments, woven materials and meshes, or other similarly reinforcing members.

FIG. 20

shows the same swim fin shown in

FIGS. 18 and 19

except that in

FIG. 20

, recesses

286

,

288

, and

290

are substituted for recesses

256

,

258

, and

260

shown in

FIGS. 18 and 19

. In

FIG. 20

, it can be seen that recesses

286

,

288

, and

290

all extend significantly close to stiffening member

188

and terminate on bending zone

304

. In alternate embodiments, one or more of the transverse recesses can extend all the way to stiffening member

188

so that at least two adjacent panels of portion

186

are completely separated from one another. In

FIG. 20

, membranes

298

,

300

, and

302

are seen to bridge the gap formed by recesses

286

,

288

, and

290

, respectively. Because membranes

298

,

200

, and

302

each have a loose fold within them while the swim fin is at rest, panels

292

,

294

, and

296

are able deform in a manner that creates a twisted shape across portion

186

during use. This can occur because the loose fold existing in membranes

298

,

300

, and

302

permits each transverse recess to widen when water pressure deforms each panel on portion

186

. Membranes

298

,

300

, and

302

provide expansion zones within portion

186

that have a continuous material across such zones so that water does not flow through recesses

286

,

288

, and

290

.

In alternate embodiments, a smooth continuous strip can be secured to inner edge

198

. A groove can exist within inner edge

198

that has holes, recesses, orifices, or the like within the groove so that when the smooth strip is molded to inner edge

198

, it fills into the groove and the corresponding recesses to form a strong mechanical bond. Membranes

298

,

300

, and

302

can be attached to this smooth strip so that membranes

298

,

300

, and

302

are molded integrally with this smooth strip. This strip can be used to provide a more secure bond as well as to control differences in shrinkage tendencies existing between membranes

298

,

300

, and

302

and portion

186

. Such a smooth strip can also extend around the entire length of trailing edge

196

′ and inner edge

198

if desired.

FIG. 21

shows a perspective side view displaying both halves of the embodiment shown in

FIG. 20

during use. In

FIG. 21

, the swim fin is being kicked in a downward direction indicated by the arrow existing below foot pocket

180

. It can be seen that as the blade halves deform during use, each transverse recess is permitted to widen as its corresponding transverse flexible membrane expands into a substantially triangular shape. When each transverse membrane becomes fully expanded during use, tension is created within its material. This tension within a given transverse membrane causes its corresponding transverse recess to stop expanding. Thus, the degree of looseness designed into each transverse membrane while the swim fin is at rest substantially determines the amount of deformation that can occur along each blade half during use when a membrane is fully expanded it prevents the recess between adjacent panels from spreading further apart. This benefit can be used to enable portion

186

to twist only to a desired maximum level. Such a restraining system can prevent the blade halves from experiencing excessive levels of deformation during hard kicking strokes, or while the swim fins are used in highly turbulent waters such as large surf or strong currents.

Another benefit to the use of a transverse membrane across each transverse recess is that it creates a more continuous blade shape and reduces turbulence between each segmented panel. In addition, the effective surface area of each blade half is increased. In alternate embodiments, any number of transverse recesses can be used with transverse membranes disposed within them. The more of these systems that are used the smoother the resulting contour that is created as a twisted shape is formed. As more membranes are used, the amount of looseness designed into each transverse membrane may be reduced to make the twisted contour smoother and more gradual during use. If desired, each transverse membrane can be designed without any significant levels of looseness built into it while the swim fin is at rest. The level of looseness within each transverse membrane can also vary between adjacent panels to permit a wide variety of contours to be achieved within the deformed blade halves.

The general purpose of the flexible membrane is to create a strategically placed flexing zone that permits each blade half to twist with reduced levels of resistance during use. The directional alignment, shape, orientation, and placement of such flexing zones may be varied in any desirable manner that significantly reduces each blade halfs resistance to twisting during use.

FIG. 22

shows a cut-away perspective view of the same swim fin shown in

FIGS. 20 and 21

except that in

FIG. 22

lengthwise flexible membrane

318

is added. Membrane

318

separates the newly formed panels

320

,

322

, and

324

from stiffening member

188

with a highly flexible material. This significantly increases the ability of panels

320

,

322

, and

324

to pivot relative to stiffening member

188

when water pressure is applied during use. The material used to make membrane

318

is preferable more flexible than the material used to make panels

320

,

322

,

324

. Consequently, membrane

318

offers less resistance to deformation and increases the efficient movement of panels

320

,

322

, and

324

to a reduced angle of attack during use. This combines with the high degree of looseness in membrane

298

to permit panel

320

to pivot a significant distance below root portion

319

during use. Because this allows panel

320

to pivot to a substantially decreased angle of attack, significantly high levels of attached flow conditions may be created along an increased region of the low pressure surfaces on blade half

182

.

FIG. 23

shows a cross sectional view taken along the line

23

—

23

from FIG.

22

. In

FIG. 23

, trailing panel

324

deforms during use to a significantly reduced angle of attack. Membrane

318

is seen to extend into inner edge

192

of stiffening member

188

as well as into panel

324

. The highly resilient nature of membrane

318

permits it to curve around a significantly small bending radius. This increases the streamlined shape of right blade half

182

.

The significantly reduced angle of attack shown by panel

324

in this embodiment significantly reduces separation and increases attached flow along the low pressure surface of right blade half

182

. Because the streamline of oncoming flow

326

which passes around the outside of stiffening member

188

is able to flow in a well attached manner, lift vector

328

is efficiently produced. Although the angle of attack of panel

324

is shown to be significantly reduced in

FIG. 23

, panel

324

may be designed to deform to any desirable angle of attack and contour during use.

In alternate embodiments, each transverse recess and its corresponding transverse membrane does not have to be connected to lengthwise membrane

318

. Instead one or more of the transverse recesses and their corresponding membranes can exist separately from membrane

318

so that the two panels adjacent to that transverse recess and membrane are connected near lengthwise membrane

318

. Any combination of lengths of membranes and degrees of connectedness between transverse membranes and lengthwise membrane

318

may be used. Any number of such transverse membranes may be used. Also, any number of additional lengthwise membranes may be used as well. In still other embodiments, all or some membranes may be made of the same material as the panels and, or stiffening member

188

. In such situations, these membranes are molded at the same time as the rest of the blade, however, they are made much thinner than the rest of the blade. In still other embodiments, panels

320

,

322

, and

324

can be made out of significantly rigid materials so that all deformation is created by membranes

318

,

298

,

300

, and

302

.

Experiments with flexible test model swim fins having the various design characteristics displayed in

FIGS. 14 through 23

show dramatic improvements in performance over test model swim fins having the structural inadequacies of the prior art. When the improved swim fin designs of the present invention are designed to permit significant twisting to occur around a substantially streamwise axis while the stiffening members provide sufficient rigidity to maintain efficient lift generating orientations during use, swimming speeds are vastly increased while strain to the leg, ankle, and foot is dramatically reduced. While prior art fin designs (including some of the most popular fin designs currently available) offered cruising speeds (gentle to moderate strength kicking strokes) of approximately 0.75 miles an hour, properly designed swim fins of the present invention offered speeds substantially exceeding 2 miles an hour with the same or even gentler kicking strokes. Many of the swim fin designs of the present invention permit swimming speeds to be achieved that easily exceed 2 miles an hour even if only the swimmer's ankles are kick to and zero leg motion is used. A similar kicking stroke on prior art fins creates high levels of ankle strain and almost zero forward movement.

In addition to increasing propulsion, the swim fin designs of the present invention also offer a dramatic reduction in drag and kicking resistance over the prior art. While the prior art test models create significantly high levels of leg, ankle, and/or foot fatigue within a time period ranging from 1 to 20 minutes of gentle kicking strokes, the properly designed swim fins of the present invention permit hours of continuous use without incurring significant levels of fatigue to the legs or ankles of the swimmer. When significant twisting is allowed to occur around a substantially lengthwise axis during use, drag levels are so low that the swimmer feels that the swim fins moves through the water with about the same ease as a bare foot. This allows the muscles in the user's legs, ankles, and feet to relax completely during gentle kicking strokes so that the possibility of fatiguing and cramping is almost completely eliminated. After several hours of continuous use, the swimmer is more exerted by the general act of swimming than by any strain to legs, ankles, or feet. This is a significant improvement over prior art designs in which drag on the blades cause the swimmer's legs, ankles, or feet to fatigue prematurely.

These results contradict conventional swim fin design principles that are hold the belief that the more resistance a swim fin has to moving through the water, the more propulsion it offers. This belief is especially strong within the realm of SCUBA type swim fin designs in which stiff and unyielding fins are considered to be most efficient.

Description—

FIGS. 24

to

27

FIG. 24

shows a front perspective view of an alternate embodiment swim fin which has a pre-formed channel within the blade portion. A foot pocket

348

receives the swimmer's foot and a foot platform

350

exists below foot pocket

348

. Foot pocket

348

is preferably attached to platform

350

with a mechanical and, or chemical bond. On the right side of platform

350

is a right stiffening member

352

and on the left side of platform

350

is a left stiffening member

354

. Both member

352

and member

354

are attached to platform

350

in any suitable manner. For instance, platform

350

, member

352

, and member

354

can be molded in one piece from a substantially rigid material. Examples of materials may include corrosion resistant metals, metallic fiber reinforced thermoplastics, and other fiber reinforced thermoplastics. A combination of materials can also be used to offer desired levels of rigidity.

Between platform

350

, member

352

, and member

354

is a channeled blade portion

356

which hangs loosely below the plane formed by platform

350

, member

352

, and member

354

. In this embodiment, portion

356

has a right flexible membrane

358

, a right blade member

360

, an intermediate flexible membrane

362

, a left flexible membrane

364

, and a left blade member

366

. Membrane

358

is stretched between stiffening member

352

and blade member

360

. Membrane

358

is preferably made from a highly resilient material, while blade member

360

is preferably made from a material that is substantially more rigid that used to make membrane

358

. Membrane

358

is connected to stiffening member

352

and blade member

360

in any suitable manner. Membrane

364

is connected in a similar manner to stiffening member

354

and blade member

366

. Between blade member

366

and blade member

360

is a center recess

368

. Membrane

362

is connected to platform

350

, membrane

358

, blade member

360

, membrane

364

, and blade member

366

in any suitable manner that permits relative movement thereof Membrane

362

is preferably made of a highly resilient material such as that used to make membranes

358

and

364

.

This embodiment may be made in as little as two steps and two materials. First, platform

350

, stiffening member

352

, stiffening member

354

, blade member

360

, and blade member

366

may be molded from a substantially rigid thermoplastic. Second, foot pocket

348

, membrane

362

, membrane

358

, membrane

364

are molded from a highly resilient thermoplastic so that it fills into appropriately placed orifices, grooves, or recesses in platform

350

, stiffening member

352

, stiffening member

354

, blade member

360

, and blade member

366

. In alternate embodiments, membrane

362

can be made of a rigid or semi-rigid material that is pivotally connected in any suitable manner to platform

350

, membrane

358

, blade member

360

, membrane

364

, and blade member

366

.

In this embodiment, it is preferred that members

358

, blade member

360

, membrane

362

, membrane

364

, and member

366

are connected and arranged in a manner that produces a pre-formed lengthwise channel when the swim fin is at rest. The depth, span, length, shape, alignment, and contour of this channel can be varied according to desire.

FIG. 25

shows a perspective side view of the same swim fin during use. The arrow above foot pocket

348

shows the direction that the swim fin is being kicked.

FIG. 26

shows a perspective side view of the same swim fin kicked in the opposite direction. The arrow below foot pocket

348

shows the direction of the kicking motion. The shape of portion

356

is seen to be inverted on this stroke.

FIG. 27

shows a front perspective view of the same swim fin except that a vented central membrane

370

is added to fill the gap created by center recess

368

. Vented membrane

370

is connected to blade member

360

, membrane

362

, and blade member

366

in any suitable manner such as a mechanical and, or chemical bond. Vented membrane

370

is seen to have a venting system

372

arranged in a lengthwise orientation. In this embodiment, venting system

372

uses four substantially rectangular vents, however, the vents can be of any shape, size, number, and arrangement. For instance, venting system

372

can have larger vents or even one large vent so that vented membrane

370

is made out of only a substantially small amount of material. In this situation, vented membrane

370

can actually be as little as a narrow flexible strip, string, cable, or chord stretched transversely across center recess

368

to connect blade member

360

to blade member

366

.

Preferably, vented membrane

370

is made out of a highly flexible material. If it is desired, vented membrane

370

may be made from the same material that is used to make membrane

358

, membrane

362

, and membrane

364

. In alternate embodiments, vented membrane

370

can be made out of a more rigid material as long as it is pivotally mounted to blade member

360

, membrane

362

, and blade member

366

in any suitable manner that permits movement thereof.

Operation—

FIGS. 24

to

27

In

FIG. 24

, portion

356

is seen to form a pre-formed lengthwise channel while the swim fin is at rest. It is preferred that membrane

358

, membrane

362

, and membrane

364

are sufficiently flexible enough to permit portion

358

to form this shape without the need for significant levels of water pressure to be applied. Such flexibility also permits portion

356

to quickly and efficiently invert its shape when the direction of kick is reversed.

It is preferred that portion

356

is pre-shaped in such a manner that membrane

358

and membrane

364

are automatically oriented at a more reduced angle of attack relative to the oncoming flow than blade member

360

and blade member

366

, respectively. As a result, the greatest change in curvature within portion

356

occurs substantially near its outer side edges. Thus, a parabolic shape is avoided across the span of the channel. This offers an improved hydrofoil shape by forming a concave attacking surface and a convex low pressure surface between membrane

358

and blade member

360

, as well as between membrane

364

and blade member

366

.

Such a pre-formed hydrofoil shape is made possible by the use of membrane

362

. The side edges of membrane

362

are seen from this view to have an angled orientation to create an improved hydrofoil shape on each blade half. In alternate embodiments, these same methods can be used to create more sophisticated hydrofoil shapes with greater degrees of curvature through the use of more blade segments, flexible membranes, and pivotal connections. In all situations, center recess

368

is used to reduce the level of back pressure created within the channel during use.

FIG. 25

shows a side perspective view of the same swim fin during use. Membrane

362

is seen to be sloped in a manner that promotes movement of water into the channel as well as toward the trailing portions of the swim fin.

FIG. 26

shows that the shape of portion

356

becomes inverted as the direction of kick is reversed. This is possible because the joining edges of membrane

358

, blade member

360

, membrane

362

, membrane

364

, and blade member

366

are attached to each other, as well as to the joining portions of platform

350

, stiffening member

352

, and stiffening member

354

, in a manner that permits flexing, bending, or pivoting thereof. Only platform

350

, stiffening member

352

, and stiffening member

354

are rigidly attached to each other to in a manner that resists such movement. The rigidity of platform

350

, stiffening member

352

, and stiffening member

354

allow the shape of portion

356

to be controlled in a desirable manner.

Because the channel is pre-formed, resistance to deformation is reduced. This permits the swim fin to be at its optimum orientation over a greater portion of each stroke. This is because the minimum water pressure needed to create such an orientation is significantly reduced. This allows a greater portion of the energy and time normally expended to create optimum deformation to be efficiently converted into propulsion.

In

FIG. 27

, vented membrane

370

is added to fill the gap created by center recess

368

. Because vented membrane

370

is made of a flexible material, it can easily fold in upon itself as blade members

360

and

366

swing toward each other at the inversion point of each stroke. This allows the channel to quickly invert its shape without jamming as it passes between stiffening members

352

and

354

.

One of the benefits of vented membrane

370

is that it permits increased control to be achieved over the angled orientation of blade members

360

and

366

. Vented membrane

370

can be used to prevent center recess

368

from widening to undesirable levels during use. This permits the reduction in angle of attack existing near the trailing portions of blade member

360

and blade member

366

to be limited so that they do not exceed a desired maximum level. This can prevent the trailing portions of blade members

360

and

366

from twisting to an excessively low angle of attack during hard kicking strokes.

Venting system

372

is used to reduce back pressure within the attacking side of the channel during use. Because the sides of the channel slope inward to direct water into the channel along the attacking side of portion

356

, venting system

372

permits excess levels of back pressure created by inward moving water to be vented out the bottom of the channel. This permits inward moving flow to continue flowing toward the center of the channel in an unobstructed manner. Consequently, the channel is less vulnerable to “overflow conditions” which can cause water to reverse its flow direction and spill outward around the side edges of the swim fin. Because this problem is avoided, the formation of destructive induced drag type vortices are significantly reduced along these outside edges.

Since venting system

372

encourages water to continually flow in an inward direction from each side of portion

356

, water pressure is increased along the attacking surfaces as this inward flowing water collides along the swim fin's center axis. Also, as some of the water which flows along the attacking surfaces of portion

356

passes through venting system

372

, it is able to rejoin the water flowing around the low pressure surfaces (lee surfaces) of portion

356

. This causes the water along the low pressure surfaces to flow at a faster rate and generate lift in accordance with Bernoulli's principle. These factors dramatically reduce drag and increase propulsion. These benefits offer a major improvement over prior art swim fins that attempt to gain propulsion by using a lengthwise channel.

In alternate embodiments, venting system

372

can appear in any desirable form. The size of the vents can be made larger to increase the volume of flow through them. The leading and trailing portions of vented membrane

370

which exist around each vent can be made more hydrofoil shaped to improve efficiency and further reduce drag. Venting system

372

can also have less total vents that are larger in size to improve efficiency. Venting system

372

can also have a series of longitudinal vents that are parallel to each other and spaced apart in a side by side manner instead of a series of rectangular vents as shown. Such longitudinal vents can spread across the entire span of the swim fin if desired. The blade portions existing between such vents can have a substantially spanwise tear drop hydrofoil shape to increase lift.

Other embodiments can have membrane

370

made from a rigid material that does not flex, but is connected to blade member

360

, blade member

366

and membrane

362

in any suitable manner that permits pivotal movement thereof. Also, membrane

370

can be eliminated entirely. In this situation, blade members

360

and

366

can be molded as one piece to form a central blade portion, and a series of vents can be cut out of this central blade portion for reducing back pressure along the blade's attacking surface. For similar performance on opposing strokes the central blade portion can be made substantially planar in form. The concave channel can be produced solely by membranes

358

and

364

, which can be made sufficiently loose enough to permit the central blade portion to deform into a concave channel on both reciprocating strokes. This still permits a significant improvement in performance to exist over the prior art because back pressure is reduced within the channel while the outer edge portions of the channel exhibit the greatest degree of anhedral deformation. The centrally located vents also help stabilize the movement of the fin through the water and significantly decreases its tendency to wobble side to side like a falling leaf as it is kicked vertically. The decrease in back pressure also decreases the drag created by the fin as it is kicked through the water and makes the fin less fatiguing to use. The reduced back pressure within the channel also makes the fin easier to use on at the water's surface since it reduces the fin's tendency to catch on the surface as it re-enters the water during a kicking stroke.

Description—

FIGS. 28

to

30

FIG. 28

shows a cut-away perspective view of the right half of a substantially symmetrical swim fin. A foot pocket

374

receives a swimmer's foot and is attached to a foot platform

376

in any suitable manner such as a mechanical and, or chemical bond. The outside edge of foot platform

376

is attached to a right stiffening member

378

in any suitable manner. For instance, platform

376

and stiffening member

378

can be molded in one piece from the same material. It is preferred that platform

376

and stiffening member

378

are made of a significantly rigid material so that they do not deform excessively during use.

Suspended between the front of platform

376

(near the toe of foot pocket

378

) and the inner edge of stiffening member

378

is a flexible blade portion

380

, which is composed of a flexible membrane

382

, a forward rib pair

384

, and a trailing rib pair

386

. Membrane

382

is preferably made of a highly resilient material which deforms easily under significantly low levels of water pressure. Membrane

382

may be attached to platform

376

and stiffening member

378

in any suitable manner such as a mechanical and, or chemical bond. Preferably, membrane

382

recedes into a groove along the inside edge of stiffening member

378

as well as along the front of platform

376

. These groves can have a series of holes, recesses, or orifices into which membrane

382

fills during the molding process. From this view, membrane

378

is seen to recede into a groove along the front edge of foot platform

376

.

In this embodiment, rib pair

384

is preferably made from two narrow strips of a significantly rigid material. One of these strips is attached to the upper surface of membrane

382

while the other strip is attached to the lower surface of membrane

382

. These strips can be attached to membrane

382

in any suitable manner. For instance, the two strips of rib pair

384

can “sandwich” membrane

382

while being attached to each other with suitable mechanical protrusions passing through openings, recesses, or holes within membrane

382

. Mechanical and, or chemical bonds may be used to secure the two strips of rib pair

384

to each other as well as to membrane

382

. Similarly, trailing rib pair

386

is secured to membrane

382

in any suitable manner.

In alternate embodiments, a single rib can extend from one side of membrane

382

while the other side of membrane

382

remains smooth. Rib pair

384

can also be a thickened portion of membrane

382

created during the molding process that extends above and, or below the plane of membrane

382

so that fewer parts and steps of assembly are needed. A rigid member can also be used within the interior of membrane

382

so that both the upper and lower surface of membrane

382

remain substantially smooth. In this situation, membrane

382

is molded onto and around such a member.

An initial bending zone

388

is represented by a broken line along membrane

382

that originates from a position on membrane

382

near a trailing tip

390

and extends to the base of an inner edge

392

of membrane

382

near foot platform

376

. A modified bending zone

394

is represented by a broken line along membrane

382

that is seen to first originate from a position on membrane

382

near trailing tip

390

and extends to the outer side end of rib pair

386

, then extends to the outside end of rib pair

384

, and finally extends to the base of inner edge

392

near foot platform

376

. Because the outside ends of rib pair

384

and rib pair

386

are spaced a relatively small distance from the inside edge of stiffening member

378

, modified bending zone

394

is also spaced this same relatively small distance from the inside edge of stiffening member

378

. Bending zone

394

is seen to exist significantly closer to stiffening member

378

than initial bending zone

388

.

FIG. 29

shows a cross sectional view taken along the line

29

—

29

from-

FIG. 28

as membrane

382

deforms during use. In

FIG. 29

, an oncoming flow

396

is displayed by two streamlines flowing toward and around stiffening member

378

, membrane

382

, and rib pair

384

. The horizontally broken lines show the position of rib pair

384

and membrane

382

at rest while the solid lines show the position of rib pair

384

and membrane

382

when membrane

382

deforms under the pressure of oncoming flow

396

during use. The streamlines of oncoming flow

396

flow smoothly and generate a lift vector

398

.

FIG. 30

shows a cross sectional view taken along the line

30

—

30

from

FIG. 28

as membrane

382

deforms during use. In

FIG. 30

, the horizontally aligned broken lines display the position of rib pair

386

and membrane

382

while the swim fin is at rest. The solid lines show the position of rib pair

386

and membrane

382

during use when an oncoming flow

400

causes membrane

382

to deform. The cross sectional view having solid lines shows rib pair

386

extending from both sides of membrane

382

. Oncoming flow

400

is displayed by two streamlines approaching and flowing smoothly around stiffening member

378

, membrane

382

, and rib pair

386

. The smooth flow conditions efficiently generate a lift vector

402

. Oncoming flow

400

is created during the same kicking stroke that creates oncoming flow

396

shown in FIG.

29

.

Operation—

FIGS. 28

to

30

Because membrane

382

in

FIG. 28

is highly resilient, it deforms easily under significantly low levels of water pressure. Consequently, if rib pair

384

and rib pair

386

are not used to provide structural support in this design, the portions of membrane

382

existing between initial bending zone

388

and inner edge

392

are vulnerable to collapse and bend around bending zone

388

to a zero or near zero angle of attack. Such excessive levels of deformation can be seen when looking back to

FIG. 15

or

16

and observing position

246

. Thus, to prevent such an undesirable form of deformation from occurring in

FIG. 28

, rib pair

384

and rib pair

386

are used to prevent membrane

382

from bending abruptly around bending zone

388

. Because rib pairs

384

and

386

are substantially rigid, membrane

382

cannot bend around bending zone

388

and modified bending zone

394

is created along membrane

382

.

Although the portions of membrane

382

existing between bending zone

388

and stiffening member

378

exhibit significantly higher resistance to twisting around a substantially lengthwise axis than the portions of membrane

382

existing between bending zone

388

and inner edge

392

, the presence of rib pair

384

and rib pair

386

permit a greater portion of membrane

382

to deform in a desired manner.

Because the portions of membrane

382

existing between bending zone

388

and inner edge

392

are able to deform easily under water pressure, a twisting moment is exerted on rib pair

384

and rib pair

386

with bending zone

388

behaving substantially as the axis of rotation. This causes the portions of rib pair

384

and rib pair

386

existing between bending zone

388

and inner edge

392

to pivot away from the applied water pressure. At the same time, the portions of rib pair

384

and rib pair

386

existing between bending zone

388

and stiffening member

378

try to pivot in the direction toward the oncoming water pressure. However, because the outside ends of rib pair

384

and rib pair

386

terminate on membrane

382

at a significantly close distance to stiffening member

378

, tension is created within the material of membrane

382

between stiffening member

378

and the outer side ends of rib pairs

384

and

386

. This tension prevents the outer ends of rib pairs

382

and

386

from rotating significantly above the horizontal plane occupied by stiffening member

378

. The rigidity of stiffening member

378

prevents further maximizes this tension that restricts the movement of the outer side ends of rib pairs

384

and

386

during use. As a result, the twisting moments created on rib pairs

384

and

386

during use apply leverage onto the portions of membrane

382

existing between bending zone

388

and bending zone

394

and cause them to pivot to a reduced angle of attack. Because membrane

382

is made out of a highly resilient material, adequate levels of deformation can be achieved even under conditions of significantly low water pressure. Consequently, the portions of membrane

382

existing between bending zone

394

and inner edge

392

are able to quickly pivot around bending zone

394

to a reduced angle of attack in a substantially even and efficient manner even when the swimmer is using relatively light kicking strokes.

Because the portions of membrane

382

existing between bending zone

388

and bending zone

394

offer resistance to such deformation, the degree of pivoting is controlled by this resistance. This permits the majority of membrane

382

to deform to a desirable reduced angle of attack during use without collapsing to a zero, or near zero angle of attack. Thus, the resistance provided by these more resistant portions of membrane

382

now becomes an advantage by permitting a desired level of control to be achieved over the actual angles of attack exhibited during use. Some of the variables that affect the degree of deformation include the actual resiliency of membrane

382

, the tension (or lack of tension) existing across membrane

382

between platform

376

and stiffening member

378

while the swim fin is at rest, the degree of rigidity/flexibility built into stiffening member

378

, and the degree of rigidity/flexibility built into rib pair

384

and rib pair

386

. One or more of these variables can be altered to create desired amounts of deformation during use.

Another advantage to this embodiment is that the total area of membrane

382

that Ad remains at a high angle of attack during use is substantially reduced. The only portions of membrane

382

that remain at a high angle of attack exist between bending zone

394

and stiffening member

378

. This is a significantly smaller area than which exists between bending zone

388

and stiffening member

378

. Because bending zone

394

is closer to stiffening member

378

, smoother flow is achieved along the low pressure surface of membrane

382

. Also, a greater volume of water is channeled away from stiffening member

378

and toward inner edge

392

. This significantly increases efficiency and propulsion.

When comparing the cross sectional views shown in

FIGS. 29 and 30

, it can be seen that membrane

382

and rib pair

386

in

FIG. 30

are inclined at a more reduced angle of attack than membrane

382

and rib pair

384

shown in FIG.

29

. This shows that membrane

382

assumes a twisted orientation along its length during use.

Rib pair

386

in

FIG. 30

is able to pivot to a more reduced angle of attack than rib pair

384

in

FIG. 29

because rib pair

386

in

FIG. 30

is less affected anti-twisting stress forces within

382

. Looking back to

FIG. 28

, it can be seen that a majority of the length of rib pair

386

exists between bending zone

388

and inner edge

392

, while only a substantially small portion of membrane

386

exists between bending zone

388

and bending zone

394

. Consequently, only a substantially small portion of rib pair

386

exists on a portion of membrane

382

that resists twisting (between bending zone

388

and bending zone

394

. When looking at rib pair

384

in

FIG. 28

, it can be seen that a substantially larger portion of its length exists between bending zone

388

and bending zone

394

(where tension within membrane

382

is significantly higher). This difference in resistive forces permits rib pair

386

to pivot to a significantly lower angle of attack than rib pair

348

since rib pair

386

encounters less resistance to twisting than rib pair

384

. Because the angle of attack of membrane

382

decreases toward the trailing portions of the blade, water is encouraged to flow toward the these trailing portions at an accelerated rate. This significantly increases propulsion.

The cross sectional views shown in

FIGS. 29 and 30

, rib pair

384

and rib pair

386

demonstrate their ability to cause membrane

382

to deform substantially close to stiffening member

378

. Efficient lift generating flow conditions are created while flow separation and drag are significantly reduced. It is intended that membrane

382

is able to deform in a similar manner when the direction of kicking is reversed on the opposite stroke.

Summary, Ramifications, and Scope

Accordingly, the reader will see that the swim fin designs, flow control methods, and stress controlling methods of the present invention can be used to efficiently generate improved levels of lift by increasing the difference in pressure occurring between the opposing surfaces of the blade. The reader will also see that the present invention can be used to significantly reduce the drag on the blade created during swimming strokes. Furthermore, the designs and methods of the present invention offer additional advantages in that they

(a) provide a flexible hydrofoil design that significantly reduces flow separation around its low pressure surface during use;

(b) provide a swim fin which significantly reduces the occurrence of ankle and leg fatigue;

(c) provide a swim fin which offers increased safety and enjoyment by significantly reducing a swimmer's chances of becoming inconvenienced or immobilized by leg, ankle, or foot cramps during use;

(d) provide swim fin designs which are as easy to use for beginners as they are for advanced swimmers;

(e) provide swim fin designs which do not require significant strength or athletic ability to use;

(f provide swim fin designs which can be kicked across the water's surface without catching or stopping abruptly on the water's surface as they re-enter the water from above the surface on the down stroke;

(g) provide swim fin designs that offer high levels of propulsion and low levels of drag when used at the surface as wall as below the surface.

(h) provide swim fin designs that provide high levels of propulsion and low levels of drag even when significantly short and gentle kicking strokes are used;

(i) provide methods for substantially reducing the formation of induced drag type vortices along the side edges of a hydrofoil;

(j) provide hydrofoil designs which significantly reduce outward directed spanwise flow conditions along their attacking surfaces;

(k) provide hydrofoil designs which efficiently focus a fluid medium traveling along the attacking surface away from their outer side edges and toward their center axis so that fluid pressure is increased along their attacking surface;

(l) provide hydrofoil designs in which the outer side portions of the hydrofoils are sufficiently anhedral enough to encourage a significant portion of the aftward flow to have a large enough inward spanwise component to significantly reduce the formation of induced drag vortices along the outer side edges of the hydrofoils;

(m) provide fin designs which offer improved lift by significantly reducing stall conditions along their low pressure surfaces;

(n) provide methods for significantly reducing separation along the lee surface of reciprocating motion foils which are used at significantly high angles of attack;

(o) provide a highly swept leading edge portion and, or an outer side edge portion of a flexible hydrofoil with a stiffening member which is sufficiently rigid enough to permit the flexible hydrofoil to maintain orientations that are effective in generating a significantly strong lifting force during use while the hydrofoil is oriented at a substantially spanwise directed reduced angle of attack;

(p) provide a low aspect ratio hydrofoil design which offers significantly reduced levels of induced drag;

(q) provide a method for a rigid propulsion hydrofoil to efficiently generate lift on both opposing strokes of a reciprocating motion cycle;

(r) provide a method for enabling a reciprocating motion propulsion hydrofoil to generate high levels of lift and low levels of drag on at least one stroke of the reciprocating cycle;

(s) provide methods for controlling and reducing the build-up the torsional stress forces of tension and compression within the material of a flexible blade in an amount effective to permit the material within the flexible blade to exhibit significantly less resistance to twisting around its length to a reduced angle of attack than it does to bending along its length;

(t) provide methods for controlling and reducing the build-up the torsional stress forces of tension and compression within the material of a flexible blade in an amount effective to permit the material within the flexible blade to deform efficiently and easily to a predetermined reduced angle of attack that is capable of efficiently generating significantly high levels of lift, and such deformation is able to occur under the influence of water pressure created during a significantly gentle kicking stroke;

(u) provide methods for controlling and reducing the build-up the torsional stress forces of tension and compression within the leading edge portions and, or outer side edge portions of a flexible hydrofoil in an amount effective to permit such leading edge portions and, or outer side edge portions to deform efficiently and easily to a predetermined reduced angle of attack that is capable of efficiently generating significantly high levels of lift along the lee surfaces of such leading edge portions and, or outer side edge portions, and such deformation is able to occur under the influence of water pressure created during a significantly gentle kicking stroke; and

(v) provide the highly swept leading edge portion of a flexible blade with a stiffening member that is arranged to create a sufficiently strong twisting moment around a substantially streamwise axis within the flexible material to permit the flexible material to deform to a significantly reduced angle of attack in reference to its spanwise alignment under water pressure exerted during use, while simultaneously providing methods for permitting such deformation to occur sufficiently close to the highly swept leading edge to reduce separation around the lee surface of the blade in an amount effective to significantly increase lift and reduce drag.

Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, instead of having two blade halves that are symmetrical, the two blade halves can be asymmetrical in respect to the swim fin's center axis. In such embodiments each blade half can differ in length, width, thickness, degree of sweep, degree of flexibility, change in flexibility, degree of rigidity, degree of twist, overall shape, topographic shape, aspect ratio, contour, and cross sectional shape in comparison to the other blade half.

Other variations can include using only one of the flexible blade halves without its counterpart. In this situation, the size of this blade can be substantially increased to make up for the space previously occupied by the other blade half. This blade can twist back and forth with each reciprocating stroke in a similar manner as the elongated single blade tail of a nurse shark or thresher shark.

Also, any number of blades may be used rather than just one or two. When more that two blades are used, any orientation, arrangement, alignment, and configuration of blades may be used. For instance, blades can branch out from other blades in a wide variety of patterns. Also, a series of narrow highly swept blades may extend from the foot pocket in a substantially parallel manner or in a substantially radiating manner.

When two side by side highly swept and flexible blade halves are used, they do not necessarily have to twist to form an anhedral channel along the attacking side of the swim fin on each stroke. Instead, they can twist in the opposite direction to a dihedral orientation on each stroke. In this case, the stiffening members exist along the inside edge of each blade half. Between these two stiffening members is the recess between the blades. Consequently, water flowing along the attacking surface of the blade halves is focused away from the center recess and toward the outer side edges of each blade half. Because water is able to flow through the recess, attached flow is created along the low pressure surface of each blade half. It is intended that the stiffening members on each blade half are sufficiently rigid enough to prevent them from bending significantly toward each other during strokes. This enables the center recess to remain open and between the blades so that attached flow is maintained along the low pressure surface of each blade half. If desired, one or more transversely aligned beams can be secured between the two stiffening members to bridge the recess and prevent the stiffening members from bending toward each other during use.

Another alternate embodiment can include using a single twisting flexible foil which attaches to other parts of the user's body than the feet. The root portion of the foil can attach in any suitable manner to any desirable region of the swimmer's body and extend outward and away from the body in a manner that enables the user to create additional propulsion and, or directional stability. Such fins can have a suitable system for attaching to the user's lower legs, upper legs, hips, waist, back, torso, diving equipment, shoulders, arms, wrists, or hands. Multiple fins may be used simultaneously in any desirable combination or arrangement. Preferably, such foils are highly swept at least along their outer portions, and such outer portions are arranged to twist around a substantially streamwise axis. However, the methods used in the present invention which significantly increase the ease to which a flexible hydrofoil can achieve a twisted shape may also be used on hydrofoils which are only slightly swept back, not swept back at all, or even swept forward (either in part or entirely).

Alternate embodiments which have a blade member attached to a stiffening member may use any suitable method for providing a pivotal type of attachment thereof. For example the blade member may have a series of hoop-like structures attached to its outer side edge portions and, or leading edge portions, and the stiffening member is inserted through such hoop-like structures to provide a connection that permits pivotal motion of the blade member around the stiffening member. A looped piece of material may also be used in a similar manner.

Flexible foils equipped with systems for controlling anti-twisting stress forces may also be used for purposes other than swimming aids. Such improved flexible foils may be used as improved hydrofoils, hydroplanes, rudders, skegs, directional stabilizers, keels, flexible propeller blades, flexible impeller blades, nacelles, oars, paddles, propulsion foils, oscillating propulsion foils, and other similar foil-type devices. These may be used on power boats, sailboats, submersibles, semi-submersibles, recreational water craft, human powered water craft, sailboards, surfboards, water skis, aerodynamic and hydrodynamic toys, and personal propulsion devices.

In addition, any of the embodiments and individual variations discussed in the above description may be interchanged and combined with one another in any desirable order, amount, arrangement, and configuration.

Accordingly, the scope of the invention should not be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

Number	Name	Date	Kind
5384	Heathorn	Dec 1847	A
144538	Harsen	Nov 1873	A
169396	Ahlstrom	Nov 1875	A
234305	Bovey	May 1880	A
476092	Valente	Dec 1892	A
783012	Biedermann et al.	Feb 1905	A
787291	Michiels	May 1905	A
871059	Douse	Nov 1907	A
998146	Alfier et al.	Jul 1911	A
1061264	Bys	May 1913	A
1324722	Bergin	Dec 1919	A
1607857	Zukal	May 1926	A
1684714	Perry	Aug 1928	A
2241305	Hill, Jr.	Feb 1941	A
2321009	Churchill	Sep 1943	A
2343468	Messinger	Mar 1944	A
2423571	Wilen	Jul 1947	A
2588363	Corlieu	Mar 1952	A
2850487	D'Alello	Sep 1958	A
2865033	Jayet	Dec 1958	A
2889563	Lamb et al.	Jun 1959	A
2903719	Wozencraft	Sep 1959	A
2950487	Woods	Aug 1960	A
3019458	De Barbieri et al.	Feb 1962	A
3055025	Ferraro et al.	Sep 1962	A
3082442	Cousteau et al.	Mar 1963	A
3084355	Ciccotelli	Apr 1963	A
3086492	Holley	Apr 1963	A
3109495	Lang	Nov 1963	A
3112503	Girden	Dec 1963	A
3171142	Auzols	Mar 1965	A
3239857	Gwynne	Mar 1966	A
3256540	Novelli	Jun 1966	A
3411165	Murdoch	Nov 1968	A
3422470	Mares	Jan 1969	A
3453981	Gause	Jul 1969	A
3491997	Winters	Jan 1970	A
3665535	Picken	May 1972	A
3773011	Gronier	Nov 1973	A
3810269	Tabata et al.	May 1974	A
3922741	Semeia	Dec 1975	A
3934290	Le Vasseur	Jan 1976	A
3952351	Gisbert	Apr 1976	A
4007506	Rasmussen	Feb 1977	A
4025977	Cronin	May 1977	A
4083071	Forjot	Apr 1978	A
4193371	Baulard-Caugan	Mar 1980	A
4197869	Moncrieff-Yeates	Apr 1980	A
4209866	Loeffler	Jul 1980	A
4342558	Wilson	Aug 1982	A
D278165	Evans	Mar 1985	S
4521220	Schoofs	Jun 1985	A
4541810	Wenzel	Sep 1985	A
4657515	Ciccotelli	Apr 1987	A
4689029	Ciccotelli	Aug 1987	A
4738645	Garofalo	Apr 1988	A
4752259	Tackett et al.	Jun 1988	A
4773885	Ciccotelli	Sep 1988	A
4775343	Lamont et al.	Oct 1988	A
4781637	Caires	Nov 1988	A
4820218	Van de Pol	Apr 1989	A
D302999	McCredie	Aug 1989	S
4857024	Evans	Aug 1989	A
4869696	Ciccotelli	Sep 1989	A
4895537	Ciccotelli	Jan 1990	A
4929206	Evans	May 1990	A
4934971	Picken	Jun 1990	A
4940437	Piatt	Jul 1990	A
4944703	Mosier	Jul 1990	A
4954111	Cressi	Sep 1990	A
4954112	Negrini et al.	Sep 1990	A
D327933	Evans	Jul 1992	S
D327934	Evans	Jul 1992	S
D327935	Evans	Jul 1992	S
D328118	Evans	Jul 1992	S
5163859	Beltrani et al.	Nov 1992	A
D340272	Evans	Oct 1993	S
5304081	Takizawa	Apr 1994	A
5324219	Beltrani et al.	Jun 1994	A
5356323	Evans	Oct 1994	A
5358439	Paolo	Oct 1994	A
D353181	Evans	Dec 1994	S
5374210	Sardella et al.	Dec 1994	A
5387145	Wagner	Feb 1995	A
5417599	Evans	May 1995	A
5429536	Evans	Jul 1995	A
5435764	Testa et al.	Jul 1995	A
5443593	Garofalo	Aug 1995	A
5522748	Cressi	Jun 1996	A
5527197	Evans	Jun 1996	A
5533918	Sanders	Jul 1996	A
5595518	Ours	Jan 1997	A
5597336	Evans	Jan 1997	A
D393684	Lin	Apr 1998	S
5746631	McCarthy	May 1998	A
D396897	Evans	Aug 1998	S
5810629	Parr	Sep 1998	A
D404456	Evans	Jan 1999	S
6050868	McCarthy	Apr 2000	A
6095879	McCarthy	Aug 2000	A
6126503	Viale et al.	Oct 2000	A
6146224	McCarthy	Nov 2000	A

Number	Date	Country
1075 997	Feb 1960	DE
3438808	Apr 1986	DE
259 353	Aug 1988	DE
0 308 998	Mar 1989	EP
787291	Jul 1935	FR
1208636	Sep 1959	FR
1245395	Sep 1960	FR
1.245.395	Sep 1960	FR
1501208	Nov 1967	FR
2 058 941	May 1971	FR
2 213 072	Aug 1974	FR
2 490 498	Mar 1982	FR
2 543 841	Apr 1983	FR
2574 748	Jun 1986	FR
17033	Oct 1890	GB
234305	Jul 1924	GB
1284765	Aug 1972	GB
553307	Dec 1956	IT
625377	Sep 1961	IT
61-6097	Jan 1986	JP
62 134395	Jun 1987	JP
1323 463	Mar 1986	SU

	Number	Date	Country
Parent	09/776495	Feb 2001	US
Child	10/039001		US
Parent	08/713110	Nov 2000	US
Child	09/776495		US
Parent	09/313673	May 1999	US
Child	08/713110		US
Parent	09/021105	Feb 1998	US
Child	09/313673		US
Parent	08/583973	Jan 1996	US
Child	09/021105		US

High efficiency hydrofoil and swim fin designs

Information

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Abstract

Description

Claims

RELATED APPLICATIONS

US Referenced Citations (102)

Foreign Referenced Citations (22)

Non-Patent Literature Citations (14)

Continuations (5)

Entry
Rossier, Robert N., entitled, “Moving Forward—Getting your best kick in the water can come from a combination of design, efficiency and hydrodynamics in fins”. Alert Diver, Nov.-Dec. 1997, pp. 26-28.
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“3302 of 1880” (Picture From Book-Found in US PTO).
“Test Dive” Article in Jan./Feb. 1996 issue of Sport Diver Magazine.
“Oceanic V-Drive Fins” May/Jun. 1996 Issue of Sport Diver Magazine.
Walton and Katz “Application of Leading-Edge Vortex Manipulations to Reduce Wing Rock Amplitudes” Journal of Aircraft vol. 30, No. 4, pp. 555-559.
Traub and Nurick “Effects of Wing-Tip Vortex Flaps” Journal of Aircraft vol. 30, No. 4, pp. 557-559.
Grantz and Marchmann III“Trailing Edge Flap Influence on Leading Edge Vortex Flap Aerodynamics”, Journal of Aircraft vol. 20, No. 2, pp. 165-169.
Rao “An Exploratory Study of Area-Efficient Vortex Flap Concepts” Journal of Aircraft, vol. 20, No. 12, pp. 1062-1067.
Lamar and Campbell “Vortex Flaps-Advanced Control Devices for Supercruise Fighters” Jan. 1984, Aerospace America.
“BZ” bodysurfing fin (Picture), 1st production in 1990.
“Blade Pro” bodysurfing fin (Picture), 1st production 1991.
“Wave Rebel” bodysurfing fin (Picture), 1st production 1994.
Art. 1400 Pinna “Professional” Fin (1989).