Patent Grant
9944372
The present invention relates to water propellers, and more particularly to modular propellers used on boats that provide near equal reverse thrust and forward thrust.
Sterndrive and outboard props do not work very well in reverse. The reason is that conventional props have been designed to provide optimum thrust going forward. But reverse thrust is the principal way boat skippers use to slow down and stop their boats quickly, especial when moving too fast or when the boat is very heavy and inertia continues to propel the boat after power is cut.
Heavy, high windage houseboats, pontoon boats, and barges are especially challenged in slowing down because going into reverse is about the only way to stop a boat faster than just allowing it to drift to a stop. That was the big advantage in big paddlewheel steamboats that were in common usage in the 19th Century Mississippi riverboats.
A few patented devices have been marketed commercially that added ridges on the aft surfaces of a prop blade to help to improve reverse thrust. The “solution” was to streamline the cupping by fairing it into the underside of the leading edge. This is said to create an aft tapered cup in the top of the trailing edge. These two cups on opposite sides of the blade are supposed to “grip” the water for better control in both directions. On the forward side of the blade, a parallel ridge is formed 8% to 12% back from the edge. The ridge profile rises 3/16″ above the blade surface, in effect, creating a “double-cup”. This is claimed to be the single most important element in improving the reverse thrust. The “leading” edge (in forward direction) of each blade is thus a bit thicker than conventional props, with the desirable side-effect of enhancing durability. One Inventor, Charles S. Powers, claims only a slight loss of forward speed. Today, all modern blades incorporate localized cupping on the (forward direction) trailing edges of the blades.
Such now conventional cupping does not change the basic hydrofoil shape looked at in cross-section, it is no more than a localized bending of the leading or trailing edges up or down. Some just add a “special feature” to one edge of their propellers. Conventional propellers all use a “standard” propeller hydrofoil shape. Such do not change the “mean camber line” except maybe very locally at the leading and trailing edges.
A modular plastic marine propeller and hub assembly is described by the present inventor, Brad Stahl, in U.S. Pat. No. 4,930,987, issued Jun. 5, 1990. Three plug-in blade “roots” are slipped into an interlocking matching hub between front and rear end caps. The main parts are made of injection-molded high-strength fiber-reinforced plastic but the design is not limited to just composite materials. The costs of manufacturing such propellers are significantly less than conventional metal propellers. Nine to eighteen inch diameter three-blade propellers intended for 9-350 horsepower motors are typical.
Briefly, an efficient reverse-thrusting modular propeller embodiment of the present invention comprises a center hub and a set of three replaceable blades. Each such blade has a symmetric hydrofoil cross-section but disposed along a full wave (360°) sinusoidal mean camber line. The identical blades all have a thicker symmetric hydrofoil cross-section at the propeller root which widens with a longer wavelength and thins with a diminishing amplitude moving outward, and then the blades narrow again in shorter wavelength and thin more in lessening amplitudes progressing toward the distal tip. Localized cupping on the leading or trailing edges is ruled out as undesirable and counterproductive.
The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.
Efficient reverse-thrusting modular propeller embodiments of the present invention comprise a set of identical, replaceable blades with tapered root bases that exactly fit into corresponding interlocking and tapered slots in a center hub. The center hub includes an integrated front cap and a solid aluminum core encapsulated with fiber-reinforced composite polymer resin. The blades together are seized in place between fore and aft caps. Each blade at every radius station has a symmetric hydrofoil cross-section disposed along a substantially sinusoidal mean camber line. The blades begin at their roots with thicker symmetric hydrofoil cross-sections that then widen and thin at greater radii outward, and then narrows but continues thinning more as the distal tip is approached. No localized cupping on the leading or trailing edges is desirable or productive.
The mean camber line of the hydrofoil shapes remain the same from the root to the tip as the thickness decreases. A symmetrical hydrofoil is superimposed onto a 360° sinusoidal shape. In embodiments of the present invention, the symmetrical hydrofoil varies from about a 3% thickness-to-chord-ratio to about a 20% thickness-to-chord-ratio.
The upper and lower surface contours (the difference being the thickness) can be expressed mathematically, for example at the 75% radius station:
Y=0.575x3−0.966x2+0.393x+0.008; upper surface:
Y=0.575x3−0.759x2+0.185x−0.010; and lower surface:
Y=A sin x, where 0.02≤A≤0.08. mean camber line:
Each replaceable propeller blade 102, 103, 104 includes a blade fin 106 and a root 107. A rear cap 108 seizes the replaceable propeller blade 102, 103, 104 within the center hub 105. The assembled modular propeller 100 typically mounts on a splined shaft 110 of an outboard marine engine. A threaded portion 111 is used to fasten the whole assembly with a machine nut 112. The replaceable propeller blade 102, 103, 104 are all identical, and commercial implementations allow users to choose a variety of sizes and blade pitches. As few as two and as many as six evenly distributed blades can be used in various embodiments of the present invention. Typical applications are 8-18 inch diameter three blade propellers intended for 6-350 horsepower motors but are not limited in any way with respect to horsepower, pitch, diameter, number of blades in open or ducted configurations.
In non-symmetrical hydrofoils, their mean camber lines lay only to one side of the straight chord line. In symmetrical hydrofoils, the mean camber lines and the chord line are coincident. For symmetrical hydrofoils with a sinusoidal mean camber, as shown in
In a typical embodiment, the “wavelength” of the sinusoidal chord line is about 4.5″ at the neck next to the root as in
Each blade's many radius stations have the same basic shape, only the thickness-to-chord-ratio changes. The sinewave remains the same, albeit the blade section gets thinner progressing from the root of the blade to the tip. The sinusoidal mean camber line always completes 360-degrees of angle, from leading edge to trailing edge, along each same radius station across the width of the blade. The sinusoids individual wavelengths vary, but each still nevertheless completes 360-degrees. This holds true moving out from the root to the more distal radius stations.
The upper and lower surface equations given herein merely describe the actual coordinates of one particular section of an exemplary blade at its 75% radius station. The mean camber line equation can describe many sinewave shapes, wavelengths, and amplitudes. The amplitude is one variable chosen to fit each instance.
The upper and lower surface hydrofoil shapes can be described in general by modifying the upper and lower surface equations such that they were become a function of the thickness-to-chord-ratio, thus describing all the actual sections of the blades from their roots to their tips. Alternatively, starting with the set of equations herein that describe the 75% radius station, thickness to chord ratios of 0.20 to 0.03 can be created and superimposed on a sinusoidal mean camber line with varying wavelengths to fit the spans available.
Pitch is typically measured at 75% radius station and is constructed by passing an imaginary line through the points of the leading and trailing edges. One revolution of the propeller will theoretically move it forward the pitch amount. For example a 15-inch propeller turning one revolution in water will move 15-inches. In actual practice, no propeller is 100% efficient and such 15-inch pitch will yield something less, like 14.5-inches.
Pitch is a measure understood by end users to help them select among of similarly performing propellers. The range of pitches included in embodiments of the present invention is about 5.0 to 50.0 inches.
There is a group-staggering that exists between raised ridges 301-303 and raised ridges 303-306 on the opposite surface. The small curved surfaces control the water flow over the blades which enhances the performance of the entire propeller system.
A prototype of the reverse thrust blades and conventional blades for comparison were tested on the same boat. Here is what we found. A gain of one mile-per-hour (MPH) was observed in the 2000 to 3200 revolutions per minute (RPM) range as compared to Piranha HYDROBYTE™ blades. The reverse thrust blades expressed a forward top speed of 8-mph at wide-open-throttle (WOT) and revolution limiting to 3200-RPM. The stopping power was the most notable change, a full boat length (55′) shorter. A basic rule for prop selection is the engine should be running within its specified RPM range at WOT. Most owner's manuals usually specify 5000-5500 RPM for an outboard, and 4200-5000 RPM for a sterndrive. If the engine is not able to do this in operation, it may be severely overworked by the propeller. A prime cause of premature engine failure. Over-revving is bad as well, it can cause severe damage. The wrong propeller will blow, ventilate, or otherwise suck air excessively when turning or accelerating. If the RPM is too high, a propeller with a longer pitch should be substituted. If the RPM is too low, a propeller with a shorter pitch should be exchanged.
A heavy houseboat was used for testing, a 2014 Sunstar™ 15′ by 55′ with a single engine Volvo PENTA A-200-V6 with an SX-A 1.97 gear drive, and that is used in rental service. Its original blades ventilated at 2100-RPM in reverse, and the reverse thrust held to 2500-RPM.
A Piranha Propeller was earlier tested on a 1989 Ski Pro with a 260 Mercury inboard/outboard tied to a dock with an electronic load measuring device. A standard 15×15 Piranha Propeller operated at:
A competing conventional 15×15 “high thrust” propeller at:
A Piranha 15×15 propeller (in an embodiment of the present invention) running at:
Although particular embodiments of the present invention have been described and illustrated, such is not intended to limit the invention. Modifications and changes will no doubt become apparent to those skilled in the art, and it is intended that the invention only be limited by the scope of the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4930987 | Stahl | Jun 1990 | A |
| 5158434 | Weetman | Oct 1992 | A |
| 6164919 | Vanmoor | Dec 2000 | A |
| 6699016 | Dean | Mar 2004 | B1 |
| 7056092 | Stahl | Jun 2006 | B2 |
| 7637722 | Koepsel | Dec 2009 | B1 |
| 8636469 | Powers | Jan 2014 | B2 |
