Surfboards may generally be described as water planing devices used to ride waves. The term “surfboard” may include boogie boards, wind surfing boards, and other hulled craft which are maneuvered by shitting the weight borne by the craft relative to the craft's center of gravity. Surfboards may be constructed of various lengths, width, shapes and thicknesses. Initially, surfboards had a single vertical fin located along the centerline of the surfboard at the rear that provided directional stability. Later designs added additional smaller fins of various sizes and shapes along the sides of the surfboard to improve the stability and maneuverability of the surfboards.
In terms of its design, a surfboard fin is analogous to a “fixed wing.” The surface curvature of conventional fins reflects architecture similar to that of a “fixed wing” aircraft. When the physics are applied to an aqueous environment, the “fixed wing” is termed a “hydrofoil” or sometimes simply a “foil.”
Holding all other variables constant and for a constant hydrofoil speed, the velocity of water passing over the top surface of the hydrofoil wing is greater than that which passes below the flat bottom surface. With an increase in water velocity over the top of the hydrofoil wing, the pressure of the water over this surface is reduced when compared to the pressure below the hydrofoil wing. This difference in pressure tends to push the hydrofoil wing toward the side of lowest pressure of the pressure gradient. The velocity of the water over the top surface of the wing is forced to rise with increased velocity because (a) water is essentially incompressible and (b) the distance a water molecule must travel for a given linear distance is longer due to the curvature of the wing over the hydrofoil surface as compared to the straight line distance enjoyed by water molecules passing below the hydrofoil wing. The upward force generated through relative motion between the wing and the environment through which it travels, in this case water, is proportional to the velocity of the water over the wing surfaces and the surface area of the wing's bottom surface. The velocity of a surfboard is largely a function of wave height, wave velocity, and gravity.
A reduction in lift may be expected from “turbulence” or “cavitation” that could theoretically develop over the wing surface. For example, turbulence may be created by high surface friction coefficients and disrupt desired laminar flow. Cavitation is a phenomenon whereby gasses actually come out of solution in the surrounding water mass due to significant pressure reduction. Similar to opening a carbonated beverage, and under the appropriate temperature and pressure conditions, air bubbles may form in the low pressure zones over the wing's surface. Cavitation may disrupt laminar flow and will also produce aquatic sounds as the bubbles collapse and return to solution.
The single vertical fin is generally speaking, a type of hydrofoil, whose function is to provide lift or some other force to the surfboard in reaction to its motion through the water. The single vertical fin is usually a symmetrical foil (a “50/50” foil) with both sides convex, which provides for even water flow on both sides of the fin such that a single vertical fin promotes stability and control. If the hydrofoil has a convex, top surface and a flat bottom surface, the velocity of water flow over the top surface of the hydrofoil is increased, thus creating a water pressure differential between the bottom and top surfaces and producing Sift or thrust in the direction of the pressure differential toward the area of lowest pressure.
The performance of a surfboard is affected by the design, placement, and number of fins affixed to the surfboard. For example, a fin may be defined by its dimensions: its base, its depth, its sweep, its flex, and its cant, and changes in these dimensions affect the performance characteristics of the surfboard. As for the hydrofoil effects, the fin may be the aforementioned symmetrical foil or a flat foil having one flat side and one convex side, which promotes maneuverability and fast transitions between turns. Other fins may combine the characteristics of a symmetrical foil and a flat foil in various proportions dependent on the desired performance characteristics of the surfboard. For optimum foil performance, flexibility in design characteristics is necessary, as well as the ability to modify these design characteristics as surfing conditions change.
Therefore, as competitive surfers attempt more challenging maneuvers and ride bigger waves, there is need for improved fin designs that improve the lift, maneuverability, and other performance characteristics of the surfboards.