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High efficiency hydrofoil and swim fin designsRelated Patent Categories: Buoys, Rafts, And Aquatic Devices, Swimming Aid To Increase Stroke Efficiency, Foot Attached, FlipperHigh efficiency hydrofoil and swim fin designs description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070173143, High efficiency hydrofoil and swim fin designs. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] 1. Field of Invention [0002] 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. [0003] 2. Description of Prior Art [0004] 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. [0005] 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 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. [0006] 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. [0007] 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 toward 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. [0008] 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. [0009] 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. [0010] 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. [0011] 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. [0012] In addition to increased drag, the downward deflected flow (downwash) behind the trailing edge significantly decreases tile 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. [0013] 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. [0014] In addition, the high speed rotation of the vortex creates centrifugal force which evacuates fluid away from the center of each vortex (tile 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. [0015] 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. [0016] 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. [0017] 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. [0018] 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. [0019] 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. [0020] 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. [0021] Both U.S. Pat. Nos. 169,396 to Ahistroin (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 swimmer's 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. [0022] 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. [0023] 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. Continue reading about High efficiency hydrofoil and swim fin designs... 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