1. Field of Invention
This invention refers to devices which increase the swimming speed when attached to the feet or legs, flapping in tandem (fins) or in unison (monofins), of swimmers.
2. Prior Art
Swim devices, whether fins or monofins, are fastened to the feet of the swimmer, who imposes on them an oscillating motion (flapping), usually in the sagittal plane. In general terms, the swim device consists of a “blade” which is fastened to the feet of the swimmer and may be provided with “pockets”, “shoes” or similar devices which accommodate the feet of the swimmer, to which it is attached. The blade and the feet are fastened to each-other in a variety of ways, sometimes in a fixed relationship, other times in a manner which allows a relative movement or re-orientation. The strength of the thrust produced is function of the speed and amplitude of the oscillatory movement of the legs and size, shape and orientation of the swim device.
The shapes of most of the blades vary between two extremes. On one side, the shape of the blade is modeled after the flukes of cetaceans, mainly dolphins, in terms of shape, size, cross section, elasticity. A few typical devices, out of the numerous patents belonging to this class are: U.S. Pat. No. 5,429,536 (Evans), U.S. Pat. No. 6,086,440 (Fechtner), U.S. Pat. No. 6,183,327 (Meyer), U.S. Pat. No. 7,510,453 (Nguyen). One of the advanced designs of this group of swim devices is the Lunocet [Adventure, September 2008, p. 21]. The design was based on detailed measurements of the shape, profile, texture, of the cetacean flukes as well as of advanced methods of mathematical modeling. The result is a swim monofin having a span of approx. 1 m, provided with the means to be fastened to the feet of the swimmer. The legs of the swimmer are close and parallel to each-other. Swimmers using the Lunocet claim that they can achieve a speed by 50% higher than when using regular (one on each foot) fins. On the other extreme, the blade is shaped as a hydrofoil, with a more or less straight (un-swept) leading edge, incorporating the features which will confer to it high lift and low drag. The blade has hydrodynamic cross section (hydrofoil) and high aspect ratio, such as in U.S. Pat. No. 4,781,637 (Caires), U.S. Pat. No. 6,881,113 (2005) to Smith, U.S. Pat. No. 7,988,508 (2011) to Langenfeld et al. Such monofins are attached to the feet of the swimmer, by means of a pair of shoes, foot pockets, provided with straps, buckles or similar devices. Hundreds of patents for fins and monofins have been issued over the years, by the US Patent Office. Only a few typical patents which are somewhat relevant to the present invention, will be reviewed.
U.S. Pat. No. 4,689,029 (1987), U.S. Pat. No. 4,767,368 (1988), U.S. Pat. No. 4,869,696 (1989), all to Ciccotelli, refer to swim fins which basically consist of a foot pocket to which a rigid blade is pivotally attached by means of beams, such that a free space is provided between the foot pocket and the blade.
The exterior of the foot pocket has basically a rectangular plane form. The slim beams connecting the foot pocket to the blade will reduce somewhat the resistance to movement through the water. Still, the shape of the foot pocket will produce a high drag.
U.S. Pat. No. 3,665,535 (1972) and U.S. Pat. No. 4,934,971 (1990) both to Picken, refer to swim fins in which a web, acting as the blade is attached to the shoe assembly by means of a rigid supporting frame, provided with means for limiting the amplitude of the pivotal movement of the web.
The design addresses only the shape pf the blade. No attention is given to the drag and power loss caused by the shape of the shoe assembly.
In U.S. Pat. No. 5,421,758 (1995) to Watson and Sigler, a scuba fin is described which consists of a foot pocket and a fin section, connected by a shaft of minimal thickness, so that it encounters a minimal resistance when moving through water. The cross-section of the shaft is rhombic in shape, with straight or rounded sides. However, no attempt is made to reduce the drag encountered by the foot pocket when moving through water. Also, the blade of the fin is tilted relative to the bottom surface of the foot plate, by an angle of less than 45 degrees, which is claimed to be beneficial for walking with the fins on.
The imposing of such an angle is not conducive to good swimming power efficiency. Although the shaft is intended to reduce drag, no other component of the swim fin is suitable to this objective. The drag generated due to the not hydrodynamic shapes of the foot pocket and of the swimmer's foot will result in wasting a significant fraction of the energy spent for flapping.
U.S. Pat. No. 5,460,557 (1995) to Arnold, has a fin blade positioned so that it “always has a positive angle of attack” which is connected by means of two struts having a flattened profile, to the shoe on the foot of the swimmer and to a cuff placed above and including the ankle. The blade may oscillate so that it is always positioned in the “undertow” of the flapping foot. The shoe has a “form of low flow resistance” in order to improve the “flow conditions” around the swimmer's foot. This is important “because the profile is in the undertow of the foot”. A second profile, may be placed downstream of the first profile, to which it is “elastically” connected.
The approach taken in this patent for reducing the drag encountered during flapping is to allow the blade to swing between two extreme positions, at the beginning of the up- or down-movements of the feet. The presence of the lateral struts will disturb the flow up-stream of the blade and lower the power efficiency of the device. Significant drag will result due to the position, of the feet which interferes with the water flow produced by the fin.
In U.S. Pat. No. 5,906,525 (1999) to Melius et al., describes a swim fin of modular constructions, inspired after aquatic mammals or fishes, consisting of a “heel”, a “boot” and a “wing-like” blade, mimicking the head, body and tail respectively, of a fish. The boot and the blade are connected to each-other by a “flexible caudal shaft” and extend in the plane of the swimmer's foot sole beyond the toes. The device is claimed to produce propulsion by using only “power strokes” which generate “lift” and produce a deformation of the blade and shaft, while releasing vortices from the tips of the fin. The “recoil” to the original shape is claimed to provide additional thrust.
The swim fin mimics the contour of a flattened the body of a fish, the axis of which forms a large angle with the sheens of the swimmer. This will reduce the efficiency of thrust generation.
In U.S. Pat. No. 6,183,327 (2001) to Meyer, the swim fin is made of an “attachment portion” “removably secured” to a swimmer's foot and consists of a two symmetrical halves, which basically define the blade, shaped like a dolphin fluke, which has its leading edge right at the tip of the swimmer's toes. In this design no attempt is made to reduce the drag produced by the foot attachment portion of the fin. In U.S. Pat. No. 6,375,531 (2002) to Melius, a “flat blade” attached with its narrow end to the foot of the swimmer is continued, with a “dolphin tail” connected at the center of the wider end of the flat blade. When a “proper angle of attack” is achieved, the water displaced by the flat blade is claimed to be interacting with the dolphin-tail, to produce lift and additional thrust. The subject device is claimed to efficiently generate thrust by moving water, by means of a tail shaped after the flukes of dolphins. Only a small amount of the water moved by the flat blade will follow the shown paths needed for generating thrust. Most of the moved water will not produce thrust, but will generate eddies and drag. According to the same patent, blades mimicking the shape of dolphin tails may be attached to the hands of the swimmer and used for increasing the propulsion through water.
U.S. Pat. No. 6,893,307 (2005) to Melius, covers an “ergonomic swim fin apparatus” consisting of a foot-pocket, provided laterally with two “channeling scoops” and a “flexible blade” which runs beyond the toes, parallel with the swimmer's sole, to a trailing edge′ to which a wing shaped tail fin is secured. The channeling scoops are claimed to preferentially direct water over the blade and tail fin, thus enhancing lift and thrust.
This is a modification of U.S. Pat. No. 6,375,531. The area of the foot-pocket is extended on both sides of each foot by means of “scoops” intended to increase the amount of thrust-producing water displaced during flapping. Actually, the increase of the area of the foot pocket (transverse to the direction of the flapping movement) will increase the power consumed by the swimmer, more than the amount of thrust produced. If an improvement is seen in the swimming speed, it is gained on the expense of disproportionately large increase of power expenditure.
A similar approach is taken in U.S. Pat. No. 7,614,928 (2009) to Grivna, which installs to both sides of each foot pocket two “lateral fins” for increasing the volume of the water moved by the flapping feet. The extra amount of water moved by the lateral fins will be ejected in directions which are not favorable for producing useful thrust, but a substantial drag increase will result.
U.S. Pat. No. 7,040,942 (2006) to Houck, describes a fin consisting of a sleeve which surrounds the lower leg (or the arm) of the swimmer, to which two fins are attached. The fins are extended laterally and form surfaces parallel to the legs and normal to the plane in which the flapping is performed. The fins as described will displace more water than the flapping legs alone. Being tied to the shins of the swimmer, only a small fraction of the water displaced will be pushed in the direction parallel to the axis of the body, and generate thrust. Their efficiency is diminished by the feet which “stick out” from between the fins and will disturb the direction of propelling streams.
In U.S. Pat. No. 7,083,485 (2006) to Melius, the swim fin is a blade shaped as a flat projection of a fish, (including pectoral fins, caudal fins and tail), on top of which the foot pocket is attached. The distal portion of the swim fin has the shape of a fish tail, claimed to produce lift/thrust, which propels the swimmer forward.
This is a further expansion of the U.S. Pat. Nos. 6,375,531 and 6,893,307. The soles of both feet of the swimmer's are attached to the flat blade. The sheens stick out of the plane of the blade at a large angle. The drag produced by the planar blade and the poor hydrodynamic profile of the foot pocket represent an important power waste. Also, a large fraction of the produced thrust will not be parallel to the axis of the swimmer's body and thus will not contribute to the movement in the desired direction. Several US patents, such as U.S. Pat. No. 6,146,224 (2000) to McCarthy, U.S. Pat. No. 6,379,203 (2002) to Kuo. U.S. Pat. No. 7,115,011 (2006) to Chen, U.S. Pat. No. 7,753,749 (2010) to Mun, describe fin blades which are provided with a variety of openings, through which the water can flow from one side to the other side of the blade. While the formation of such water streams is claimed to reduce the drag, it for sure reduces the efficiency of the conversion into thrust of the power used by the swimmer. Indeed, a portion of the water which was moved by the blade moves from the high-pressure side of the blade to the low-pressure side without producing thrust.
One of the advanced designs in the class of monofins is the PowerSwim device, developed by DEKA [Popular Mechanics, November 2007, p. 22], U.S. Pat. No. 7,988,508 (2011) to Langenfeld et al., which is based on the early concept of U.S. Pat. No. 3,122,759 (Gongwer). This device consists of two hydrofoils. One of them has a high aspect ratio (approx. 10), with a span of 1.80 m, which oscillates at about the midriff, in the front of the swimmer. The blade is connected to a lever which is fastened between the ankles and the knees of the swimmer. The second hydrofoil, of a smaller aspect ratio and span is fastened to, and oscillates right above the ankles of the swimmer. It is claimed that a swimmer using the PowerSwim device can achieve 150% higher velocity than with regular fins.
The device of U.S. Pat. No. 6,881,113 (2005) to Smith uses as blade a rigid hydrofoil, connected to the feet of the swimmer in a flexible manner. While the movement of the blade relative to the foot pockets is controlled, no effort is made to control the direction of the water streams produced by the flapping movement, around the “support structure” the feet or the rigid blade in order to increase the thrust or reduce the drag.
The main deficiencies of the fins of the prior art may be summarized as follows.
In order to generate propulsion which results in velocity, human swimmers as well as fishes or aquatic mammals need to spend power (energy per unit time). For the present discussion only the power consumed by the swimmer for moving the legs will be considered.
Cetaceans and fishes of the tuna or marlin families produce propulsion by the oscillating movement of the rear ⅓rd part of the body (carangiform and thunniform movement), while the front part of the body does not oscillate practically at all. Human swimmers can use the same swimming style. However, the movement of the body of human swimmers using swim fins and practicing the “dolphin kick” style is closer to the “anguiliform”, where the propulsion is produced by a wave-like movement of the body. The body bends in a progressive manner as if a wave passes through it, from the head to the end of the toes (and the end of the swim fin). In the animal realm, the anguiliform style is less efficient and results in lower velocities than the carangiform or thunniform movement.
The mechanism by which flapping tails or fins generate propulsion is as follows: at each up- or down-stroke, or half cycle of the flapping of the tail or fin (by a cetacean or by a human swimming face down or on the back), a certain amount of water is pushed backwards and as reaction to it, the body of the swimmer is propelled forwards. The water is pushed as a backwards-directed stream (jet). The force (thrust) by which the swimmer is propelled forward at each stroke of the fin depends on the size, shape and orientation of the blade during the stroke, the amplitude and frequency of the flapping movement and on the flow conditions (water streams) produced as result of the flapping movement in the space close to the fin and to the body of the swimmer.
Despite the ability to closely copy the shape, size, elasticity, texture and movement of the flukes of cetaceans [References: Bose, N. et al. Proc. R. Soc. London B 242 (No. 1305) pp 163-173(1999), Fish, F. E. et al., Bioinsp. Biomim. 1 (2006) pp 42-48] and of tails of fast swimming fishes, the swimming performance of the man-made swim fins is not matching that of the model animals. The efficiency of converting the power consumed by the swimmer into propulsion is much lower for humans than for the aquatic animals used as models for designing the swim fins. For the same power consumed, a human swimmer will generate less propulsion than a dolphin of the same weight and height.
The power consumed or used by the animal or human swimmer can be perceived as consisting of several components:
Pt (total power consumed by swimmer)=Pp (power used for generating thrust)+Pd (power used for overcoming the body drag)+Pw (power wasted during the flapping of the legs and swim fin).
For a given swimmer and conditions, Pt is given. In order to maximize Pp, one needs to minimize Pd and Pw. Only small reduction of Pd has been achieved by using body suits, such as Spandex. The swim devices covered here address the reduction of Pw.
The power wasted Pw, can be seen as having two components:
Pw=Ppf (power wasted by producing parasitic, i.e. non-thrust producing water streams)+Pdf (power wasted to overcome the drag of the flapping feet)
The designs of the swimming devices of prior art have no methods for reducing the parasitic (called here also “shunt” streams or currents) streams, which do not produce thrust, or for reducing the power wasted as drag, during the flapping of the legs.
a, 2b, 2c, show the main outlines and movements of a blade connected to the end of a flapping rod (2a), cylinder (2b) and cone (2c).
a, 3b. Show the water streams set in motion by a swim fin of this invention, shown as a ghost side view in two positions of a flapping foot and a ghost plan view of a foot wearing a swim fin.
There are two main ways in which a portion of the power expended by the swimmer for moving the flapping legs/fins is wasted (Pw).
An important fraction of the expended power is wasted by the blade setting in motion a mass of water which will not be part of the thrust-producing jet (Ppf), as discussed below.
Here, the convention is made to call “in front” or “ahead” a position facing, or streams flowing in the same sense as the instantaneous movement of the fin, or adjacent to the “front” side of the fin. Positions or streams flowing in the opposite sense of the instantaneous movement of the fin are called “behind”, as they are adjacent to the “back” side of the fin. The front and back of the fin switch at every up- and down-stroke of the fin.
For a swimmer in “ventral” position, the flapping action is divided into a “dorsal” or “up-stroke” and a “ventral” or “down-stroke”.
During both the up- and down-movements of the swim fin, a zone of transient, high dynamic pressure (caused by the movement of the fin) forms in front of the fin. Simultaneously, a zone of lowered dynamic pressure is produced behind the fin. The push exerted by the fin produces powerful stream of water (propulsion-generating or propelling jet) directed backwards, away from the body, along a path which is generally parallel to the axis of the swimmer's body. The size and direction of these water jets is controlled by the pattern in which the blade moves (flapping amplitude, orientation) and by the size and design (shape, profile) of the blade. Propulsion of the swimmer's body results as reaction to these propelling jets of water.
Not all the water corresponding to the volume swept by the fin during a half stroke will be part of the propelling jets. Observations show that portions of the water mass pushed by the dorsal or ventral sides of the swim fin (during the dorsal or ventral movement, respectively) separate from the direction of the main streams and turn around the borders of the swim fin to flow “backwards”, into the space which during that half of the flapping cycle is “behind” (i.e. ventral or dorsal respectively) the moving fin. These streams do not produce propulsion. Thus, the power which was spent for accelerating the corresponding water mass is wasted.
The shunt streams flow from the high pressure zone produced ahead of the plate, during its movement, around the borders of the plate, into the low-pressure zone generated behind the plate.
In the drawing, lever 2 is at the maximum amplitude (measured by angle V), during the oscillating (flapping) movement about the neutral position N. As it moves downwards, plate 1 exerts in front of it (in the direction of the movement), a push on the water, manifested as a dynamic pressure increase in the zone marked “in front”, which forms a jet, moving as indicated by the arrows 8. As result of the movement of plate 1, a zone of lowered pressure is formed on the opposite side of the blade, in the zone marked “behind”. In order to balance the pressure difference between the two sides of plate 1, a portion of jet 8 will leave the high-pressure zone, will turn around the borders of plate 1 and will enter the low-pressure zone, as parasitic, or shunt streams 9. The water contained in shunt streams 9 is subtracted from the water which was initially part of jet 8 and thus, reduces the size of the propelling jet. As result, the power consumed by the swimmer for accelerating the water ending as shunt streams 9 is wasted.
In none of the swim devices of the prior art are there attempts made to reduce the amount of wasted power, by reducing the streams which flow from the front to the back of the flapping blade and diminish (shunt) the flow of water which generates propulsion.
A significant portion of the power consumed by the swimmer is lost for overcoming the drag associated to the flapping of the legs. In order to advance, a human swimmer will flap the feet/legs in the sagittal plane of the body, dorsally and ventrally (if the body is in prone position, this will correspond to up and down). Hydrodynamic studies have shown that cylinders (and by analogy, the sheens and legs of the swimmer) moving in water produce significant drag if the direction of the movement is normal to the axes of the cylinders (cross-flow). (Reference: S. F. Hoerner, “Fluid Dynamic Drag” 1965. Library of Congress No. 64, 1966). The drag is proportional to the “frontal area” and to the square of the velocity of the water in cross-flow over the cylinders. The frontal area is defined as the largest cross-section of the submerged body, normal to the direction of the water flow. The drag is also strongly dependent on the shape of the submerged body. A body with a hydrodynamic cross section e.g. elliptical, with the long axis parallel to the direction of flow, and the short axis perpendicular to it, with the ratio of the long axis A to the short axis B of A/B=2 will experience only approx. 70% and for A/B=4, only 50% of the drag of a circular cylinder with the diameter equal to the axis B of the ellipsoid.
None of the swim fin designs of prior art, addresses in specific manner the reduction of the drag generated by the flapping movement of the legs of the swimmer, by giving a hydrodynamic shape to the foot pocket. On the contrary, many of the fins of the prior art increase the frontal area of the foot pocket in the attempt to increase the volume of the water pumped at each stroke. This results in a moderate increase of the thrust but in a much higher increase of the drag.
The existence of an interaction between the blade and the flapping feet/legs of the swimmer was investigated by means of simplified models. A blade mimicking a dolphin fluke was built at a reduced scale of 1/10 and was attached by means of a hinge to an elongated body, simulating the feet/legs of a swimmer which was oscillated to an angle “V” of about 35 degrees up and down, around a pivoting axis “PA”, perpendicular to the longitudinal axis “LA” of the elongated body. (
The oscillation of the blade attached to the rod of
For the case of the blade connected to the cylindrical body (
The blade connected to the apex of the oscillating conical body (
A mechanism by which the cross-over currents lead to increase efficiency of the conversion of the energy expended for flapping into thrust will be discussed below. (
Consequently, this invention addresses swim devices which are shaped with the objectives of improving the overall efficiency of converting the power exerted by a swimmer into thrust, by favoring the formation of producing of currents which will cross over the long axis of the flapping feet and balance the dynamic pressure difference between the two sides of the flapping blade, thereby resulting in less wasted energy by the shunt currents, as well as in less the drag experienced by the flapping legs and feet.
The swim devices of this invention achieve these objectives and comprise:
The drawings of
In
A further embodiment of this invention is shown in
Another embodiment of this invention is a monofin where both feet are fastened within a single containment, called cowling, depicted in
In a variation of this embodiment, the cowling can extend from the region of the ankles up, to the region of the knees, as marked by 37. The transversal cross-section of extended cowling 37 has a hydrodynamic, low drag shape.
A still further embodiment of this invention is a monofin where both feet are fastened within the cowling, depicted in
In
The foot pockets cover individually each foot of the swimmer up to the region of the ankles and confer to them a hydrodynamic shape which generates low drag, during the flapping movement. The foot pockets may be attached each to one blade, as in regular swim fins, or both to one single blade, resulting in a monofin.
The cowling covers the joint feet and a predetermined length of the flapping legs. The cross-section of the cowling is designed with a hydrodynamic shape, for minimizing the drag encountered during the flapping movement.
More or less of the length of the legs may be covered by the cowling, as illustrated in
In a further embodiment, in order to reduce the frontal area, of the flapping cowling, the swimmer may assume a posture with feet crossed. (
When assuming the crossed feet position, either foot can be placed above (ventrally) the other one, which is placed below (dorsally).
Inside the foot pockets 11 (
The cross-sections M, N, P and R of
As depicted in
With reference to
As seen in the frontal sections in
Blades (12, 22, 32, 32 of
The aspect ratio AR=(L̂2)/S (fin span L, squared divided by the plane area S, of the fin) of the preferred fin is larger than 1. For fins using hydrofoils, the aspect ratio is preferably AR=2-6. The blade may be rigid or may posses a certain span-wise and chord-wise flexibility. The materials of which blades may be made include synthetic rubber silicon rubber (both of which may be reinforced by adequate spars or spines), polyurethanes, polyesters, etc. By incorporation in the structure of the blade of reinforcing ribs it is possible to control the elasticity, resilience and degree of deformation during flapping, especially for larger AR values. The materials of the ribs include metals, especially wires or bars of metal, plastics, battens made of glass-reinforced plastics, or of carbon fibers-reinforced plastics, which will confer to the blade the desired elasticity and sturdiness etc.
Swim fins or monofins of this invention provide excellent propulsion for blade spans of 0.3-0.5 m, which is in the same range as the fluke spans of cetaceans with body weights and body-lengths similar to those of human swimmers. [References: Bose, N. et al. Proc. R. Soc. London B 242 (No. 1305) pp 163-173(1999), Fish, F. E. et al., Bioinsp. Biomim. 1 (2006) pp 42-48],
With reference to
The blade may be attached to the lever by a hinge means which may be a rigid or elastic connection, as known in the art. Spiral springs, torsion bars or slit pipe springs may be used to this effect, as well as rubber or plastic parts which possess the desired properties. The blade follows the flapping movement of the foot pocket or cowling and, as reaction to the dynamic pressure it exerts on the water, bends or pivots around said hinge means or in its absence, around a pivot point in the necking, to limiting angles schematically depicted in
For blades mimicking the tail of tuna (
Other methods for connecting the blade to the feet of the swimmer, can be practiced so as to fit specific material or design peculiarities. For example, by building the foot pocket and the cowling including the taper suitably sturdy, it is possible to connect the blade directly to the necking at the distal end of the taper of the foot pocket or cowling, to which the swimmer's feet are attached. This is a suitable procedure for achieving a functional connection between the feet of the swimmer and the blade of the swim devices covered here.
The swim fin or the monofin are preferably built in a manner which allows them to be rapidly attached to the feet/legs of the swimmer, and rapidly disconnected from them. According to a preferred construction, the foot pocket or the cowling consist of two or more parts, which can be attached to each-other along corresponding matching sides. In a more preferred construction, the foot pocket and the cowling are made of two parts which have matching sides along the legs of the swimmer, in the coronal plane. The foot pocket or the cowling are thus divided into a dorsal and a ventral part. Both parts may accommodate devices to attach the parts to each other, as well as to the feet or legs of the swimmer, and to the footplate, lever and blade itself. Said devices, will allow for a rapid assembly and a rapid attachment and fastening of the foot pocket and cowling, inclusive blade to the legs of the swimmer and a rapid detaching and removal thereof. Preferred devices may be fasteners, flexible and/or elastic tapes which may be provided with the possibility to adjust the fastening tension, such as clamps, Velcro closures, buckles fasteners, etc., as known in the art. For the case of monofins, it is preferred that the feet be attached to the foot plate in the cowling, one at the time. Inside the cowling, at several convenient levels, supports may be provided for means of further fastening if so needed, the legs of the swimmer to each-other, to the cowling, and to the foot plate.
In a preferred embodiment, within the taper, supports (not shown) are provided for attaching and guiding the foot plate and the lever in the desired orientation for suiting the individual preference of the swimmer.
The details of the design of the foot pocket and of cowling and the materials of construction can vary. The thickness of the shell is function of the material used in its manufacture. The foot pocket and the cowling have to be sufficiently sturdy to maintain their shape during the flapping movement. Adequate materials for making the shell are: wood, metals (aluminum etc.), plastic materials (such as PVC, polyethylene, polypropylene, ABS, polyamides, polyurethanes, polyesters, etc.) glass-reinforced or carbon fibers-reinforced plastics, etc.
The proximal rims of the foot pockets or cowling may be protected by a soft and cushioning material which will prevent injuring the legs/feet of the swimmer and will ensure a smooth (low drag) transition for the water flowing along the legs, to the outer surface of the cowling.
The space not occupied by the feet and the legs within the foot pocket or cowling is preferably filled with materials to ensure a tight connection between the feet and the cowling, without however, unduly cramping the feet and legs. Preferred materials are molded foams including rubber, polyethylene, poly-propylene, poly-styrene, poly urethanes, etc. bags or pouches which may be inflated with gas or filled with liquid, to achieve a suitable tight fit (no play) between the foot pocket or the cowling and the foot/feet/legs of the swimmer. The overall specific gravity of the foot pocket and of the cowling as installed, including the legs, may be lower, equal or higher than of water. A specific gravity close to that of water is preferred, in order to confer neutral buoyancy to the body of the swimmer. Added materials (preferably inside the foot pocket or the cowling) can be used in order to obtain the desired buoyancy.
In a further embodiment, the foot pocket and the cowling can be built as massive entities (block, monolith) of the desired outside shape, without a distinct shell, and provided with internal cavities for accommodating the feet (and legs) of the swimmer, as well as the means for connecting the legs to the blade. The block may be made up of several shaped parts which are assembled together and allow for the rapid fastening to the legs of the swimmer, and removal there-from of the swim fins or monofin. This type of foot pocket and of cowling may be made of a convenient plastic material provided with voids (foam) in order to have the desired overall specific gravity and be sufficiently sturdy, as required for maintaining the prescribed shape when undergoing the flapping movement.
The fabrication of the parts which make the fin and the monofin is not a problem for someone skilled in the operations of molding and casting plastic materials. The foot pocket and cowling may be fabricated by operations including vacuum forming, hot pressing or similar techniques. The details of the operation depend on the properties of the material selected. The taper is best fabricated as integral part of the foot pocket and of the cowling, although other options are possible. The blade fabrication includes methods such as casting (of silicon rubber or similar), injection molding, forming by sculpturing or by bonding together pre-formed sub-components. The other components, such as fasteners, tying tapes, foot plate, lever, etc. can be either purchased on the market or fabricated by using broadly available materials and skills.
The taper at the distal end of the foot pocket and of the cowling is of essence for improving the power efficiency of propulsion generation by the flapping swim fin and monofin described here. The simplified typical water flow patterns produced by the fin and monofin of this invention are sketched in
With reference to
While the direction of the flows across the taper and necking alternates for each half-stroke, the resulting propelling jets have the same general direction. The stronger jets will produce more thrust and higher power efficiency for the swimmer than other devices where balancing the dynamic pressure difference around the two sides of the blade is performed by shunting streams flowing around the borders of the blade or through openings practiced in the blade itself.
In conclusion, the swim devices covered here are built with a hydrodynamic and tapered shape which, at each half-stroke of the flapping feet, will cause that water from the higher dynamic pressure area, in front and up-stream of the foot pocket or cowling, moves across the necking of the taper, into the area of lowered dynamic pressure, behind the flapping blade, thereby improving the efficiency of converting the power expended by a swimmer into thrust, and reducing the drag encountered during flapping, compared to the devices of prior art. The swim device, whether fin or monofin of this invention consist of the following parts: