Surfboard fin science

Doug

Doug

In designing our fins, I used concepts from my background in sailing, naval architecture, fluid dynamics, and physics, and put them to use in surfboard fin science–first designing the original Wavegrinder fin for longboards and SUP. We’ve sold a couple thousand of those to surfers all over the world and have had great feedback–I get emails from surfers all the time–see some of them on our testimonials page.

More recently, I’ve designed two sizes of shortboard fins, in two different flexes. The shortboard fins can be used in thruster setup, in a quad setup, as sidebites or, with an adapter, as a center fin in a longboard box.

Fins Aren’t Magic–the Magic Is Science!

Probably I shouldn’t say this. But fins aren’t magic. Not ours, not anybody else’s.  Ever read stuff about why a certain brand of fin works better than others, and come away wondering what they’re talking about? Seems like most of the explanations are memorized folklore more than scientific fact.  And everything has “excellent hold” and “more drive.”

Fins do two things, and only two things. In terms of science, fins create two forces, and two forces only. Surfboard fins create lift, and fins create drag. That’s it. In general, you want more lift per unit of drag–just like with airplane wings, boat keels, propellers, helicopter blades, and car spoilers. The laws of physics aren’t suspended for surfboard fins.

Lift, drag

Scientifically speaking, lift and drag are the only two forces

When we’re talking about lift, we’re mostly talking about sideways (horizontal) force–it either helps you turn or resist turning by giving you something to push off of.  (Of course I say mostly because fin splay (tips set wider than the fin bases) or in the case of our fins, winglets, give some component of lift vertically).

In general, a fin moving faster generates more lift (side force) than a slower-moving fin. The design takeaway is that IF your fin is capable of producing good lift, THEN you can make it smaller, and THEN it will go faster, and THEN will produce even more lift–and THEN go even faster still.  In surfing, we feel “more drive” as efficient fins generate lift (sucking force) with more speed.  This lift or sucking force is why efficient sailboats like America’s Cup catamarans can sail toward the wind at several times the speed of the wind. Wings–and surfboards, are effectively sucked forward by the force of lift, and increasingly so with speed. In the case of airplanes, the sucking force of lift puts gets them airborne.

Drag is what holds you back, what slows you down–like brakes. It comes in several flavors: skin-friction drag, interference drag, form drag, and dynamic drag.

3 Foils CL CD Polars

3 Foils CL CD Polars

Skin-friction drag is drag that slows you down because of surface roughness, and because of the surface area of whatever fin you have. The rougher the fin and the bigger the fin in area, not just height, the more drag it’ll have. We know intuitively that a small Prius has less skin friction than a large SUV–the Prius has less skin surface area causing skin friction. The same is true with surfboard fins. Smoother and smaller fins have less drag and will go faster (and that means they will generate more lift!).

Interference drag is drag caused by the connection of the fin to the board. It can be decreased by small cutaways, by bulbous forward sections, and by shorter chord lengths. So our fins do all three of these things. We use cutaways. We use bulbous forward projections. And we have short chords–i.e., they have short bases, short in the fore-and-aft direction. Look at the wings and tails of airplanes, and generally you’ll see a subtle forward-projecting section of the wing or tail where it meets the fuselage. And ships use bulbous bows to decrease drag at the water surface/bow intersection. These features also work for surfboard fins. It’s just science.

Form drag is drag by virtue of the shape of the fin. We might imagine that a blunt forward edge on a fin might be slow–that’s an example of form drag. But more subtle is the planform–the up-and-down fin shape, including the aspect ratio (high aspect ratio is tall and thin versus low, which is short and fat), the taper ratio (tip chord length compared to base chord length), and the sweepback angle or rake. In general, higher aspect ratios, lower taper ratios, and lower sweepback angles have less drag than their opposites–especially when turning. Thinner fins will generally be faster in a straight line, but they stall easily when turning. Another form-drag issue is the foil section, some foil shapes are better than others, especially when it comes to turning. Look around and notice how long airplane wings are, how tall and narrow state-of-the-art sailboats’ keels and sailplans are. We use these features in our fins, as well as proven foil sectional shapes, known for low drag and high lift. Again, there’s no magic here, it’s just science.

Dynamic drag is drag that occurs by virtue of a fin’s movement through the water. Drag increases with speed, at whatever ratio compared to lift, depending on the design. But other things such as vortices occur, both robbing fins of lift and causing drag. Vortices occur around the end of the fin as water tries to move from the high-pressure side to the low-pressure side of the fin. This is especially true while turning. Other vortices occur at the trailing edge of fins. We use winglets to minimize the tip vortices, and a specially designed trailing edge to decrease the vortices and cavitation at the trailing edge. Trailing-edge vortices often cause humming or singing, which actually is a back-and-forth vibration of the fin to such an extent that the fin begins to resonate at an audible frequency. It’s just science.

Surfers often talk about drive and hold, and most fins claim to have the best of both. Maybe that makes sense as part of surfing folklore handed down for decades. But in terms of science, the only forces at work are lift and drag. Our fins don’t look like fins that might have been designed based on tradition, folklore, fashion, or “what looks good” to a leading old-timer in the sport. Our fins are unique. And they’re patented.

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Some Surfboard Fin History

Below are some photos that show surfboard-fin evolution from about 50 years ago. Some very well-known surfers designed some of these fins. These fins are often called dolphin or dorsal fins, because they were patterned after dolphins’ dorsal fins or, in some cases, tail flukes. Why folks took the dorsal fin from the center of a dolphin’s back and figured it would be good on the tail of a surfboard, I don’t know. But everybody started doing it.

Historic Fins That Look Like Dolphin Dorsal Fins

(Images courtesy of Geoff Cater, http://www.surfresearch.com.au/f.html)

Historic Fins That Look Like Dolphin Tail Flukes

(Images courtesy of Geoff Cater, http://www.surfresearch.com.au/f.html)

Some other fins from the 1960s look like they were patterned after dolphin tail flukes. Lots of today’s fins, especially longboard fins, look pretty much like dolphin tail flukes. Surfboard fin design has changed little over the past 50 years.

Our Fins Are Based on Modern Science, Not Mother Nature

Mimicking nature is, well, natural. Nature once was the primary basis for ship design. For over 500 years, from before Columbus sailed the Nina, Pinta, and Santa Maria, and into the 1900s, ship shapes emulated fish shapes. They used codfish bows and mackerel tails. Why not? Science had not progressed to justify a better design.

Codfish bow

Ship design mimicked nature for over 500 years with codfish bows and mackerel sterns. Science had not yet demonstrated a better design.

In airplane design, Otto Lilienthal made his gliders look like birds, as did other glider and airplane designers. The Wright brothers used wing- bending to turn, mimicking birds. Science had not yet developed a better solution.

Le Bris 1868 Albatross Glider

Le Bris’ 1868 Albatross Glider looked like a bird

 

Lilienthal 1894 bird glider

Lilienthal’s 1894 glider looked like a bird

 

Our fins compared to typical fins

Our fins look like modern airplane wings and modern boat keels

 

Surfboard Speed, Acceleration, and Maneuverability

Surfing (and SUP) is all about catching lots of waves and having long surf sessions before tiring out. We designed our fins to help people catch more waves, maneuver better, and have longer surf sessions. We did this by increasing lift and decreasing drag. By lift we mean SIDE FORCE, not up-and-down force. Our winglets sometimes give folks the impression that our fins lift them and their boards out of the water like a hydrofoil. But the winglets don’t. The winglets are just way too tiny–the size of a postage stamp. The purpose of the winglets is to keep flow over both sides of the fin as much as possible without stalling. The idea is to maximize the efficiency of the fin area, so that there is no wasted effort lost to unnecessary or inefficient surface area. Winglets make fins work as though they were 25 to 30 percent larger than they are.

Winglets Make Fins Work Better by Decreasing Drag

The first thing people notice about our fins is the winglets. Winglets make fins behave as though they are larger, but without the penalty of drag. Here is a short video about this design concept:

Southwest Airlines says that winglets save fuel costs, allowing for cheaper fares. In surfing, winglets give you more hold with a smaller fin. This is good, because the bigger the fin and the larger the surface area, the more drag you have.

Drag is resistance–resistance to acceleration and speed. Drag is what makes paddling harder and surf sessions shorter. More about drag in 60 seconds in the old movie clip below.

Winglets Work by Decreasing Downwash and Tip Vortices

When a fin exerts force on the water, as when turning, water tends to move circularly from the high-pressure side around the fin tip to the low-pressure side. And as the fin travels forward, the circular motion of water elongates and becomes a vortex. This robs the end of the fin of effectiveness. Because it robs fins of effectiveness, more area is necessary to make up for the inefficiency. By contrast, fins with winglets decrease the tendency of water to migrate around the fin tip, decreasing the circular motion of water, decreasing fin-tip vortices, and increasing the fin’s efficiency–so the fin can be smaller while producing the same lift. This is the key concept–winglets make fins behave like larger fins, but with the drag of smaller fins.

Aspect Ratio–Tall and Thin Is Better Than Short and Fat

Another thing people notice right away about our fins is that they are pretty upright–they don’t have much rake or sweepback–and they are pretty thin and pretty rectangular. Together, these features create a high-aspect shape. A high-aspect shape is better at producing lift–again, side force or “hold” as surfers like to say–than low-aspect-ratio shapes. Tall and thin is better than short and fat. That’s why we see high-aspect shapes all around us. Below are examples of high-aspect-ratio shapes and low-aspect-ratio shapes.

High-aspect-ratio shapes

Low-aspect-ratio shapes

Interference Drag, Cutaways, and Bulbous Forward Sections

Interference drag occurs where a surfboard fin meets the surfboard. In boats, naval architects have minimized interference drag by reducing the length of the keel-to-hull intersection via a cutaway at the trailing edge of the keel. Although helpful, the cutaway trailing edge tends to be less effective than a bulbous forward section. Even though today’s cutaway designs have huge cutaways, a cutaway need not be large to be effective. The region of disturbed water flow is relatively small and close to the board’s bottom. The size of the proper cutaway can be approximated to the fin’s width. Thus the Wavegrinder fins’ cutaways are small. Bulbous forward sections function like bulbs on the bows of freighters and navy ships. Bulbous bows decrease interference drag that occurs at the intersection of a ship’s bow and the water.

Surfboard-Fin Foil Section

Upright fins require a foil cross-section that counteracts stalling. Upright fins are more prone to stalling than more raked fins. But fins with more rake have more fin area presented obliquely to the turning direction during a turn. This acts as a brake. We use NACA double-zero foil sections because they have an anti-stall property. NACA double zero foils inhibit stalling over a wide range of angles of attack better than most other foil sections. In other words, even in an upright fin, the NACA double zero foil section keeps going without stalling, even when turning, and does so better than most other foil sections.

NACA double-zero foil section

NACA double-zero foil section

Underwater foils should have appropriate thickness. If foils are too thin, cavitation and vibrations will more likely occur, conditions aggravated by turning. Foils should be between a 9 percent and a 15 percent thickness. Maximum width should be no greater than 35% aft of the leading edge. Thirty percent aft of the leading edge has been demonstrated as being particularly desirable for rudders, and is the shape incorporated into NACA double zero series foils. Hydrodynamics teaches that a rounded nose section, as exists with NACA double zero foil sections, as well as on missiles, is better for rudder design than sharp-nosed foil sections. Rounded-nose sections maintain lift over a wide range of yaw angles, and are low-drag shapes, which is why rounded sections are used on airplanes, rockets, and missiles.

Surfboard-Fin-Tip Design and Taper Ratio

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

For greatest efficiency, 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. In other words, the fin should be more rectangular in shape, and less triangular. This puts a lot of fin area where it is most effective at producing lift without drag–at the tip, away from the interference drag of the board at the fin’s 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. Thus they have high taper ratios–they are pretty triangular, and typically such fins have a short tip span, and an overall low aspect ratio. Although this design combination assists with strengthening the fin, it aggravates drag. Moreover, more triangular fins (high taper ratio) put the least amount of fin area where it would be most effective–at the tip, away from the interference drag caused by the board at the base of the fin. Underwater appendages such as keels and rudders, or analogously, surfboard fins, should have high aspect ratios and comparatively short root lengths and taper ratios between 0.4 and 0.6 in order to maximize lift while minimizing drag.

Elliptical wings, or fin shapes that produce elliptical lift, yield tip vortices that are less concentrated at the tips. The downwash from rectangular wings is spread more evenly across the wingspan. Here, the term “elliptical” does not necessarily refer to the shape of the fin or wing. Instead, “elliptical” refers to the overall pattern of lift from the surfboard fin combined. Elliptical lift distribution is desirable. Rectangular fin shapes and wings yield a close approximation to elliptical lift distribution. Thus airplanes have evolved from rounded shapes to rectangular shapes. 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. But why?

Fin Sciences’ Surfboard Fins

Areas Compared - WG to Typical Dolphin Fin

Because we make our fin very efficient, it can be smaller in area, making it faster.

Few surfboard fins these days appear to use any of the aerodynamic and hydrodynamic principles discussed above. Because Fin Sciences’ fins have high lift and low drag, they can be a bit smaller than other fins, yet behave as larger fins.  The savings in size means more speed, acceleration, and maneuverability because the fins have less drag.  Or we can make fins the same size as other fins, and give them more hold per square inch of drag.  Our Wavegrinder original longboard fin is a bit smaller than typical 9-inch fins (0urs is 30.54 sq. inches in area), whereas our shortboard fin (the large size) is about 14.5 square inches, just about typical for other medium fins.

Surfboard fins typically are heavily raked or swept back from the vertical. This encourages downwash, the situation in which water flows from one side of the fin to the other. Heavily raked fins tend to stall during hard turns, because the fin tip downwash creates a large vortex behind the fin as it travels through the water. In airplanes, stalling 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. Existing surfboard fins typically have no recognizable hydrodynamic section or foil shape. They appear to be shaped by hand by-gum-and-by-gosh based on tradition, and not on science. Few companies explain why their fins look as they do.

This NASA diagram depicts stalling, drag that occurs when laminar flow is lost, as occurs when some foils are turned too sharply (or when planes take off too abruptly). Planes that stall drop like rocks; surfboard fins that stall cause you to lose the wave, or otherwise inhibit your surfing, as in tail sink during sharp cutbacks.

Stalling, aka inclination drag.

Stalling, aka inclination drag.