Science Applied to the Art of Surfing and Surfboard Fins:

            Surfboard fin science has progressed relatively little over the past several decades, and has drawn little from advancements that were made during that time in aerodynamics and hydrodynamics. Aerodynamics and hydrodynamics have much in common because both disciplines involve the study of the movement of a fluid, air or water, past a structure.  Surfing, and other water sports such as sailing, power boating, windsurfing, kite surfing, wakeboarding and water skiing, for example, thus share some common aspects derived not only from aerodynamic principles, but also from hydrodynamic principles.  

            The sport of surfing involves a complex interaction between surfboard, surfboard rider, and waves.  As in the sports of skiing and snowboarding, and unlike other board-riding sports such as windsurfing, kite surfing, water skiing, and water boarding, surfers while surfing are propelled by the effects of gravity pulling the surfer down wave faces.  Unlike other board sports, surfers after riding a wave toward shore typically must propel themselves back to the spot where they can catch the next wave.

Surfboard Speed and Maneuverability -- Goals to be Achieved With High-Lift, Low-Drag Surfboard Fins:

            But surfing requires more than simply sliding uncontrolled down a wave face; good surfers are able to control both surfboard speed and surfboard direction.  Similarly, good surfboards are those that are capable of high speeds if the surfer so desires, are otherwise easily maneuverable, are easy to paddle, and are quick to catch waves.  Surfboard speed and maneuverability depend on a variety of characteristics of the surfboard itself, and its fins.  

            Within certain limits, surfboard speed typically is accomplished by adjusting the pitch of the board in relationship to the wave face.  Pitch is the longitudinal angle the surfboard makes from the horizontal.  Surfboard pitch is controlled by the surfer moving or shifting weight forward toward the nose of the board, or backward toward the tail of the board, and thus adjusting the center of gravity of the surfer and surfboard system in relation to the center of buoyancy such that the board slides down the wave face with the correct inclination so that it either planes on the water or stalls in the water partially or totally, to speed up or to slow down and even to stop the board.  Generally speaking and to certain limits, surfboard speed is increased by moving forward on the board to a planing attitude and decreased by moving backward on the board to a higher-drag or partially stalled attitude.  All else being equal, a surfboardsurfingincluding its finssurfingwith less drag will move faster through the water because drag is the force of resistance to forward motion.  Thus with less resistance, a board with less drag can move faster through the water.  Surfboard speed is thus is inversely related to the surfboard’s drag.  

            Likewise, surfboards are easier to paddle where they have less drag, again because drag is the force of resistance to the surfboard’s forward motion.  Thus a surfer paddling a surfboard that has less drag can do so more easily and for a longer period of time before exhaustion. Or the surfer can paddle faster, a feature that is sometime important on those big-wave days when a monster wave suddenly appears further outside, and you realize you're in the crash zone unless you get the heck outta there.

            Surfboards typically cannot catch waves by lying idle in the surf.  Rather, the surfer at the lineup must await the approach of a suitable wave, then turn to the wave’s direction of travel, and quickly accelerate by paddling to an appropriate velocity to catch the coming wave.  Surfboards that have less drag will of course catch more waves more easily because the surfer can expend less energy to accelerate the board to wave-catching velocity, can accelerate more quickly, or can better place himself or herself in a spot to catch the waves, thus having greater range per unit of time. Paddling, wave-catching and maneuverability also are better on surfboards with less drag.  Surfboard acceleration is this inversely related to the surfboard’s drag.  How many great waves have you missed, just barely?  Imagine what just a little more speed, a little more acceleration could do for your surfing.

            Drag is a function of the surfboard’s shape, surface area, attitude in the water, as well as a function of the design of the surfboard fins.  Minimizing surfboard drag and surfboard-fin drag is particularly important because unlike other ski or board sports, after surfers have successfully ridden a wave, they must propel themselves by paddling back through the surf, or back to the lineup to catch another wave, which is increasingly tiresome or exhausting with increasing drag.  In order to catch waves, drag is likewise important to keep that a minimum so that surfers may paddle quickly to catch the wave, and may optimally position themselves with the greater range per unit of time allowed by boards with less drag, both of which are is increasingly difficult to do with increasing drag.   Decreased drag thus enables surfers to surf longer and to catch more waves.

            Turning of a surfboard involves a complex interaction between a surfer adjusting the roll angle of the board, by adjusting the pitch of the board, and by surfboard and surfboard-fin design. Generally speaking, surfboards having more rocker, the curved shape of a banana with surfboard tip and tail elevated from the horizontal, have a natural tendency when placed on edge to turn consistent with the rocker shape.  But increased rocker also increases drag, compromising speed, compromising the ability to accelerate to catch waves, and increasing the difficulty of paddling the surfboard.  Surfboard edges, or rails, can be anything from circular or rounded in shape, known as soft rails, to flat or hard rails in which the flat surfboard bottom turns sharply upwardly to meet the surfboard’s top deck.  Surfboards with hard edges tend to turn more quickly or more sharply than those with softer rails.  Surfboards, as opposed to surfboard fins, have undergone a significant and largely empirical design evolution applying these concepts since the beginning of the modern sport in approximately the 1950s and 1960s. 

            But surfboard-fin design has evolved relatively little over the past several decades of the modern sport of surfing.  Surfboard fins assist turning of a surfboard much as rudders and ailerons help boats or airplanes turn or maneuver, by providing largely lateral resistance and lift, with some vertical lift component depending on the orientation to the vertical of the fin in the water.  Without any fins, a surfboard in a turning maneuver would tend to spin out, whereas with one or more fins, a surfboard rider can use his or her weight to control the yaw angle of the fin while riding a wave, and can use the attached fin or fins as a lever against which to turn the board to optimally position the board, and to enjoy a good ride.  As in surfboard design, minimizing drag in designing fins is an important objective because doing so increases speed, increases acceleration capabilities, minimizes necessary paddling effort, and increases paddling range per unit of time. 

            But minimizing drag of the fins is not enough; else the fins could of course be infinitesimally small to the point of nonexistence.  To the contrary, surfboard-fin design must minimize drag while maximizing lift, because lift is the force that makes the surfboard turn, just as the force of lift allows sailboats to sail toward the wind, airplanes to fly, and both to turn.  But neither can lift alone be maximized; else the best fin would be infinitely large.  The trick is to find the proper balance between the two, to create a design using the behaviors of fluids as taught by aerodynamics and hydrodynamics to come up with a high-lift, low drag fin that provides excellent control with minimum draft for typical surfers and surfing conditions.  

            A highly efficient fin design thus has features to decrease drag while increasing lift.  Fins that produce lots of lift can thus be smaller in size, have less surface area, and thus pay a smaller penalty in drag.  Or they can have the same drag as with older fins, while having more lift and thus more maneuverability and power than older fin designs, or something in between.

            Surfboard fins today resemble the high-sweepback fin keels of the 1960s and 1970s.  Surfboards fins have not evolved much since then, just look at those boards that pass by on the tops of cars on their way to or from the beach--all the fins look pretty much the same.  In the meantime, sailboat keels have evolved rapidly, spurred on by Ben Lexcen's landmark 1983 design for the America's Cup boat, Australia II.  That keel was not only upside down, but had winglets.  Constrained by the 12-meter rule that applied to America's Cup boats at that time, Lexcen's keel was very low aspect, but was nonetheless such an improvement over other designs, that all other competitive boats soon adopted wings or winglets. Thereafter, the rules changed allowing the outdated 1930s-vintage 12-meter rule to be replaced by the current International America's Cup Class rule, which allows more efficient high-aspect ratio planforms for keels.  These highly efficient keels still have winglets, for reasons discussed below.

Surfboard-Fin Science Applied to Achieve Design Goals:

            Hydrodynamics teaches that interference drag is caused by the intersection of a watercraft hull and appendage such as a keel.  Designers have attempted to minimize interference drag by shortening the length of the keel-to-hull intersection by means of a cut away at the trailing edge of the keel.  Although helpful, the cut-away trailing edge tends to be less effective at reducing interference drag than a forwardly upwardly protecting root at the leading edge.  Some surfboard fins, as in some old sailboat keel designs, decrease the fin root length by means of a cutaway or a scallop where the fin meets the board at the fin’s trailing edge.  But hydrodynamic principles teach that a cutaway at the trailing edge, while helpful to decreasing drag, is less effective at minimizing drag than a forward-projecting, foil-shaped blended keel or fin section, much like bulbs on the bows of freighters and the other ocean-going ships actually decrease drag by projecting forward of the ship’s hull.  The cutaway need not be large, because the area of disturbed water flow, is comparatively small, and can approximated to the fin width.

            Hydrodynamics and aerodynamics teach that lower sweepback angle increases lift while decreasing drag for a given surface area.  But foil selection is critical because lower sweepback-angle fins are more prone to stalling than higher sweepback-angle fins.  But greater sweepback angle on a fin that is being turned places the entire planform obliquely to the turning direction and functions more as a brake than a higher-aspect ratio fin, especially if the fin does not have the appropriate high-lift, low drag foil section. 

            Hydrodynamics and aerodynamics teach that a high-aspect ratio planform generates more lift with less drag than lower aspect ratios.  Aspect ratios of 2:1 or more are preferred over lower aspect ratios.

            Hydrodynamics teaches that underwater foils should have appropriate thickness, and should not be too thin, or cavitation will occur, a condition that is aggravated by turning.  Underwater foils should be between a 9 percent and a 15 percent thickness, comparing fin width to the local chord length.

            Hydrodynamics teaches that fins used as rudders should not be too thin, and that certain types of foil sections maintain laminar flow necessary to produce lift with a minimum drag over a wide variety of angles of attack as contrasted with other types of foil sections.  NACA 0010 and 0012 foil sections have a demonstrated history of effectiveness.  Maximum foil width should be no greater than 35% aft of the leading edge, and point of maximum width 30 half of the leading edge is demonstrated as being particularly desirable for rudders as in NACA 0010 and 0012 series foils.

            Hydrodynamics teaches that a rounded nose section, as exists with NACA 0010 and 0012 foil sections, is better for rudder design because such rounded nose sections facilitate lift production over a wide range of yaw angles.  Existing fin design typically are sharp or angular at the nose.  In addition to decreasing the effective useful range of fin before stalling, the design is dangerous when it strikes surfers, because of the sharp surfaces, especially the tip.

            Hydrodynamics teaches that the end of fins should have the same shape as the cross-section of the foil shape within the fin itself, thus it should be a foil-shaped foil tip.  Thus a fin should not be rounded or chopped off, because it loses its effectiveness as a lifting surface, and aggravates tip-vortex drag.

            Hydrodynamics teaches that foils should have a comparatively small taper ratio: the chord length at the fin tip should be between 40 to 60 percent of the chord length at the fin base.  With some exceptions, existing surfboard fins generally have a much longer chord length at their bases, at the fin root, than they have at their tips, and they thus have thus high taper ratios, and typically such fins have a short tip span, and a low aspect ratio.  Although this design combination assists with strengthening the fin, it aggravates drag.  Hydrodynamics principles teach that underwater appendages such as keels and rudders, or analogously, surfboard fins, should have high aspect ratios and comparatively short root lengths and taper rations between 0.4 and 0.6 in order to maximize lift while minimizing drag. 

            Aerodynamics and hydrodynamics teach that endplates, fences, wings, or winglets placed at the end of wings, keels, or other hydrofoils can be effective at reducing the loss of lift that occurs at the end of such surfaces due to downwash and tip-vortex drag.  But if improperly designed, used, or placed, such devices will increase surface area to such an extent that overall drag is increased, and there is no net benefit demonstrated by the use of such endplates, fences, wings, or winglets. Aerodynamics and hydrodynamics principles teach that an endplate, wing or winglet, a surface oriented generally perpendicular to the fin and parallel to the path of water travel past the fin, decreases or prevents drag-inducing and lift-decreasing downwash.  Downwash is the tendency of a fluid on the high-pressure side of a wing, keel, or fin to move to the low pressure side, in a circular motion.  Plates or wings are effective at preventing that movement from one side of the wing, keel, or fin, but at a penaltysurfingthe plate or wing adds surface area to the wing, keel or fin.  Hydrodynamics studies and experiments, however, teach that wingletssurfingsmall wings with chords significantly shorter than the fin chord itself and with significantly smaller areas that the wing, keel or fin to which attachedsurfingproduce the same or similar downwash-canceling effects as wings, but with a much smaller surface area, and thus with a much smaller drag penalty.  Thus the incorporation of winglets, as opposed to wings, increases lift while decreasing drag.  Moreover, winglets assist in maintaining lateral lift that otherwise would be lost when a surfer rolls the board to one side in a turning maneuver thus placing the surfboard fin at an angle to the vertical, shortening the vertical length, and creating a tendency of the fin to pop out of the water, losing all turning control of the fin.  To be effective and to avoid increasing drag, the winglets themselves must be effective lift-producing surfaces, must be correctly sized and placed or they risk increasing drag by virtue of their added surface area.  Increasing lift with low drag increases wing, keel, and fin efficiency and speed.

            Aerodynamics and hydrodynamics teach that winglets, as opposed to endplates, fences, or wings, have proven effective at reducing induced drag while increasing lift in greater proportion than the increased area and associated additional form drag of the winglet, and thus greater lift with less drag than an equivalent increase in planform area or span length.  Winglets have a shorter chord length than the wing tips to which the winglets are attached, as distinguished from endplates, fences, or wings.  Winglets should themselves be effective lifting surfaces using the principles discussed. 

            Aerodynamics and hydrodynamics teach that elliptical wings or fins yields tip vortices that are less concentrated at the tips, the downwash is spread more evenly across the wingspan.  Here, the term “elliptical” does not necessarily refer to the shape of the planform, which planforms generally do exhibit elliptical lift, but to the distribution of lift across the planform.  Elliptical lift distribution is desirable, and rectangular fin planforms and wings yield a close approximation to elliptical lift distribution.

The Wavegrinder Surfboard Fin, Model R925:

            Existing surfboard fins typically do not incorporate the aerodynamic and hydrodynamic principles discussed above.  For example, surfboard fins typically are heavily raked or swept back from the vertical, often to the point where the leading edge of the surfboard fin is approximately 35 or 40 degrees to the perpendicular to the fin root chord.  This condition encourages downwash, the situation in which water flowing horizontally past the fin moves from one side of the fin to the other, which at the fin tip creates a large vortex behind the fin as it travels though the water.  Moreover, the high-sweepback angle contributes to the loss of the laminar flow of water past the fin, especially when turning past a certain point, such that the water on the back half of the fin is turbulent as opposed to smoothly flowing, and thus such fins stall earlier and lose lift and turning ability at a shallower angle of attack than a fin of low sweepback angle, and with a means of preventing or of reducing the tip vortex.  Turbulent conditions as encountered with typical surfboard fins should be avoided in order to minimize drag while maximizing lift.  In airplanes, this loss of lift results in the plane dropping from the sky.  In surfing, stalling typically results in the loss of the wave.  Ever catch a good wave, turn hard right or left, right at the lip, then come almost to a dead stop, end up in foam and watch that great wave pass you by?  Yup, you have just experienced fin stall, i.e. fin braking by turning.

F-22 Raptor	F-35 Joint Strike Fighter	F-4 Wildcat, circa 1942
Compare the modern rectangular tail planforms on the F-22 and on the F-35, similar to the Wavegrinder shape, with the rounded tail planform of the vintage F-4, circa 1944, common to most surfboard fins today.  The rounded shape persists among surfboard fins today.  But why?

            Existing surfboard fins typically have no recognizable hydrodynamic section or foil shape; they appear to not be designed or engineered other than to look good, and today they generally all look like one another.  Indeed, many surfboard fins are nearly flat in section, particularly when used as side fins.  When a surfboard with such flat-sectioned fins turn or yaw such that the angle of attack between fin and moving water no longer is straight ahead, or a zero angle of attack, many thin surfboard fins quickly stall.  Stalling is the critical loss of foil lift, the angle of attack at which fins cease functioning as fins, and begin working only as brakes, creating drag but no lift.  In airplanes, the airplane dropping from the sky illustrates wing stalling, whereas in surfing, fin stalling generally results in the board slowing or stopping, and in losing the wave, which continues uninterrupted.  Consequently, a shortcoming of existing surfboard-fin design is that they are typically too flat in section, and are not engineered to incorporate low-drag foil sections that produce lift with minimum drag over wide range of yaw angles.  Aerodynamics and hydrodynamics teach that certain foil sections, though perhaps thicker than intuition would have us design, work very effectively, thus allowing boats to sail upwind at speeds greater than the wind, as they make their own apparent wind my movement, increasing the lifting force, and thus speed.

Interested In More Surfboard-Fin Science?

            If you are interested in hearing more about how these principles come together in the Wavegrinder fin, you might be interested in this article about Surf Science. The team from SurfScience are the experts at seeing how new surfing technology can be applied to help you surf better. We sat down together and came up with this summary: Wavegrinder Rethinks Fin Design.

Some Helpful Surfboard-Fin Science Terms and Definitions: