Surfboard Fin Science

Doug, FinSciences' fin designer

Doug, FinSciences’ fin designer

Not a whole lot of fin designers pay much attention to surfboard fin science.But we’re all about the science of surfing and SUPing.

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 I designed 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.

I’ve since designed two sizes of smaller fins, in two different flexes. These smaller fins can be used in thruster setup, in a quad setup, as sidebites in a 2 + 1 setup or, with an adapter, as a center fin in a longboard box.

Fins Aren’t Magic–If There’s Magic, It’s the Science!

Probably I shouldn’t say this. But fins aren’t magic. Not ours, not anybody else’s.

Ever read stuff about fins, and come away wondering what they’re talking about? I used to read about cutaways, sickle fins, tunnel fins, double foils, and wonder what the heck they were saying, and how they made sense. When I dug deeper and spoke with some renowned surfers, it seemed more like the explanations were a version of memorized folklore, tradition, subjectivity—and not scientific fact. That bugged me. Seems like everybody says “excellent hold” and “more drive.”

But where is the truth? I researched, studied, designed, engineered, developed and ultimately produced fins based on what I found.

Fins do two things, and only two things, in terms of science. A fin creates two forces–and two forces only.

Surfboard fins create lift and fins create drag. That’s it. That’s science. That’s physics and fluid dynamics. The laws of science aren’t suspended for surfboard and SUP fins.

In general, you want more lift per unit of drag—just like with airplane wings, boat keels, propellers, helicopter blades, and car spoilers. More lift per unit of drag makes the fin efficient. Why do we care? Efficient surfboard and SUP fins means easier paddling, quicker acceleration and, combined with other design features, a more maneuverable fin.

Surfboard Fin Science: Lift and Drag Are the Only Two Forces Acting on Fins, Scientifically Speaking

When we’re talking about lift, we’re mostly talking about sideways (sideways to the board, horizontal if your board is flat on the water) force—it either helps you turn or helps tracking by giving you something to push off of.

In surfing, you want enough lift—enough sideways force—so that when you plant a foot for a turn you don’t slip sideways or spin out; instead, the board bites and takes off into the turn. In SUP, you want enough lift—enough sideways force—so that when you paddle, your board tracks well and you don’t turn like crazy with each stroke. In both surfing and SUP, any more fin than just enough to do your turning or to track as you’d like is going to mean more drag that slows you down, or makes it harder to accelerate (making it harder to catch waves, tiring your arms more, or slowing your paddling speed).

Most of the lift force is sideways to the board. But of course if you have a fair amount of splay (tips set wider than the fin bases) in your fins, there will be some upward component to lift.

In general, a faster-moving fin generates more lift (more sideways force) than a slower-moving fin. We know this from riding airplanes. As we roll faster and faster down a runway, we increase lift enough that we can take off. What’s happening is that the lift force increases as the plane goes faster, until the force is strong enough that it actually sucks the airplane up into the air, overcoming gravity. This continues until the plane slows down, loses lift, lands, and stalls.

The fin-design takeaway is that IF your fin is capable of producing good lift, THEN you can make it smaller, and THEN you will go faster, and THEN your fins will produce even more lift—and THEN go even faster still. In surfing, we feel “more drive” as efficient fins generate more lift—more power—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 increasing speed. In the case of airplanes, the sucking force of lift puts gets them airborne. In the case of fins, it makes surfing and SUPing fun.

Another illustration: I had a sailing skiff, a super-light dinghy sailboat with a lot of sail area for lots of go-fast power. The boat’s daggerboard, in combination with the boat’s rudder, provided the lift (sideways force) to get the boat sailing fast in a straight line. But it was real squirrely at super low speeds. The daggerboard and rudder didn’t bite enough at low speeds, just like a taxiing airplane doesn’t take off—the daggerboard was just not generating enough side force (a/k/a lift) at those slow speeds. So I had to lay off the wind a bit, gather speed, and only then would the daggerboard and rudder bite enough to generate enough lift to go straight—just like an airplane needs enough speed to take off. Had the daggerboard-and-rudder system been larger, it would have had more lift at lower speeds, but would have sacrificed top-end speed because more daggerboard-and-rudder area means more drag.

Contents hide
1 Surfboard Fin Science

Drag is a Drag—Surfboard Fin Drag, Standup Paddleboard (SUP) Fin Drag

Drag In General

Drag holds you back. Drag slows you down. Drag is like brakes.

Drag is resistance–resistance to acceleration and speed. Drag makes your arms tire. Drag keeps you from accelerating more quickly. Drag makes you miss waves you should have caught. Drag keeps occasional surfers from winning the lineup race to claim the wave. Drag makes aerials smaller.

Want big air? Decrease drag! Want more waves? Decrease drag! Want longer sessions? Decrease drag!

Drag is what makes paddleboarders’ arms tire, and keeps them from going faster, or longer.

Drag is what makes paddling harder and surf sessions shorter. Drag is a drag.

Drag comes in several flavors: skin-friction drag, interference drag, form drag, and dynamic drag.

 

Drag Video—Drag Explained in 60 Seconds

Skin Friction Drag

Skin-friction drag is drag that slows you down because of surface roughness and because of the amount of surface area of whatever fin you have. The rougher the fin surface 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, so it has less skin-friction drag, making it slip through the air more easily. Similarly, smoother and smaller fins have less drag and will go faster (and that means they will generate more lift!) than rougher and larger-area fins.

Most folks only think in terms of fin length/height, and not of fin area. Fins are often categorized and sold by length/depth, like a 9-inch fin, a 4.25-inch fin, etc., but are rarely categorized or sold by area. The area is often not disclosed, or is an afterthought, even though surface area is very important to fin performance. Why?  Because fin designers don’t even think about it. When comparing fins, you should look into the fin area. Bigger surfers/paddlers, softer rails, flatter boards with less rocker will need more oompf to turn—more power—and this means a larger fin. But with most fins, beware that more power means more drag—it’ll be harder to paddle and catch waves and will accelerate more slowly.

Lift-drag comparison for three surfboard fins

Lift-drag comparison for three surfboard fins

Interference Drag

Interference drag is drag caused by the connection of the fin to the board. The longer the connection, the more drag there is. So a shorter base or chord length means less drag. Typical fins shaped like a dolphin fin put most of their area at the base, and have long fore-and-aft base lengths. While animals might need that for strength, blood flow, and other purposes, science and drag reduction suggest a different shape—a shape with a shorter fore-and-aft base length.

This base length and interference drag can be creased by a cutaway, as appears on some dolphin fins. But the cutaway doesn’t need to be large, as the boundary layer of interference drag is small, and close to the board. Our cutaway is 3/8″ tall, approximately, and also shortens the base length by a similar amount, give or take, depending on the fin model.

Decreasing base length is not the only way to decrease interference drag; that drag also can be decreased by a bulbous forward section. This is what you see on ship bows and often on the leading edge of airplane tails. The bulbous section decreases interference drag.

So our fins use several features to decrease interference drag. We use cutaways, bulbous forward projections, and  short bases or chord lengths.

Form Drag

Form drag is drag by virtue of the shape of the fin. We might imagine that a blunt or square forward edge on a fin might be slow, whereas a curved or sharp leading edge might be fast—that’s an example of form drag. But more subtle is the planform—the up-and-down fin shape that surfers often call the template. This planform or template has several design issues, including the aspect ratio (high aspect ratio is tall and thin versus low aspect ratio, which is short and fat), the taper ratio (tip chord length compared to base chord length), and the sweepback angle or rake. In general, fins with water flowing smoothly across the chord of the fin—in the fore-and-aft direction—are faster, with less drag, than fins that have water flowing not just across the fin, but up the fin, toward the fin tip.

In general, higher aspect ratios, lower taper ratios, and lower sweepback angles have less form drag than their opposites—especially when turning. So more triangular or tapered fins in general will have more drag than more rectangular fins. More raked fins will have more drag than more straight-up-and-down fins. Our fins tend to be upright, with a pretty rectangular shape, for these reasons. Our fins resemble airplane wings or airplane tails because aerodynamics and fluid dynamics share the same principles (although there are differences in scale, speed, and fluid viscosity).

Fin thickness is also a form-drag fin-design issue. Thinner fins will generally be faster in a straight line, but thinner fins tend to stall easily when turning. Another form-drag fin-design issue is the foil section (the cross-sectional shape of the foil). 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. Our fins use these features, as well as proven foil sectional shapes, known for low drag and high lift. Above is a lift-drag chart comparing how three foils compare in terms of lift and drag over varying angles of attack, as when turning from a straight line to a cutback across the wave. Some shapes tend to stall here—lose lift—and become anchors. Our fins, including the foil section we selected, are designed to resist stalling.

Sometimes in surfing when you make that initial cutback as you’re catching the wave you lose the wave—your fins stalled, your tail dropped into the wave, and that perfect wave you wanted goes to the beach without you. Our fins are designed to enhance the likelihood that you’ll catch that wave, because the fins don’t stall so easily.

Many surfers who try FinSciences fins have reported a sense that they are riding higher in the water than with their old fins, especially noticeable when dropping in.  Although our fins are not designed to lift surfers out of the water, the fact that the fins have less drag means that the board tail will seem higher in the water. If you have a lot of drag, as with a too big a fin, you might sense that you’re dragging tail. Probably you are.

Many surfers report that they can just keep going, cutback after cutback, where normally they’d have lost the wave. Many find themselves surfing longer on waves, closer to the beach, because their boards keep going. These comments also make sense. With less stalling, less speed is forfeited with a cutback. And with less drag, you can keep going even as the waves peter out. Check out the testimonials page for more about these kinds of observations.

Some Water Flow Studies (Computational Fluid Dynamics (CFD) Studies) Videos

We try to reduce the fin-tip vortex that robs fins of power. Modern airplane wings use winglets to reduce the wing tip vortex. We use finlets to reduce the fin tip vortex. Compare the CFD fluid flow studies below.

Water Flowing Past A FinSciences Fin Without Tip Vortex
Water Flowing Past A Typical Dolphin Fin—Pronounced Tip Vortex

Dynamic Drag

Dynamic drag is drag that occurs by virtue of a fin’s movement through the water. Drag increases with speed, as does lift. These forces increase in a certain ratio, depending on the fin design. But other things, such as vortices, occur as a fin moves though the water. Vortices rob fins of lift and cause 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. What happens is that when you plant your foot (or stroke with a SUP paddle), you pressurize one side of a fin, and the water wants to swirl around from the pressurized side to the side with lower pressure. Basically, water leaks around the tip of the fin from the pressurized side to the other side—especially at the fin tip. As the fin goes through the water, this creates a vortex—a mini tornado of sorts. While smooth/straight flowing water (laminar flow) is efficient and fast, vortices are slow, draggy, and rob fins of lift. The fin’s trailing edge also creates vortices, unless the trailing edge is razor-sharp. Airplanes use winglets to minimize wing-tip vortices, and a specially designed trailing edge to decrease trailing-edge vortices. On FinSciences fins tips, we use what we call finlets, and, like airplane winglets, they control vortices by restricting the leakage from the high- to the low-pressure side of the fin. We use a special trailing edge (to avoid having a razor-sharp trailing edge) while also decreasing trailing-edge vortices. The water flow past our fins is smooth, as shown in the CFD video above.

Surfers often talk about drive and hold, and most fin companies claim to have the best of both. But they don’t explain why their fins have those characteristics. Our fins are different. By design. Our fins don’t look like fins that designed based on tradition, folklore, fashion, or what looks good to a leading old-timer in the sport who could surf well on a barn door. Our fins are unique. And they’re patented.

 

Some Surfboard Fin History and Surfboard Fin Evolution In Pictures—Surfboard Fins Began Emulating Dolphin Dorsal Fins and DolphinTail Fins Beginning about 60 Years Ago

Here are some photos that show surfboard-fin evolution from about 50 years ago. Some very well-known surfers designed some of these fins. They’re often called dolphin fins or dorsal fins, because they mimicked dolphins’ dorsal fins. 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. Check out a modern-day catalog of typical fin companies, and the fins look pretty much the same today as they did a half-century ago.  (Surfboard-fin images courtesy of Geoff Cater, http://www.surfresearch.com.au/f.html.)

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 even into the 1900s, ship shapes mimicked fish shapes. Ship designers used codfish bows and mackerel tails. Why not? Science had not progressed to justify 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.

But if you board a cruise ship or other boat today, or an airplane, you’ll be on a ship, plane or boat that doesn’t mimic nature. We have modern tools and modern science to make better designs.  

 

Our Fins Look Like Modern Airplane Wings and Boat Keels

Surfing (and SUP) is all about speed and control, catching lots of waves, and having long surf or paddling sessions before tiring out. We designed our fins to help people catch more waves, maneuver better, and have longer surf sessions. Check out our testimonials—we have made lots of surfers and SUPers happy! We did this by increasing lift while decreasing drag. Again, by “lift” we mean SIDEWAYS FORCE, not up-and-down force. Our finlets sometimes give folks the impression that our fins lift them and their boards out of the water like a hydrofoil.

But the finlets don’t lift boards out of the water—they are way too small. The finlets are the size of a postage stamp, or smaller. The purpose of the finlets is to keep flow over both sides of the fin as much as possible to inhibit vortex-production and stalling. The idea is to maximize the efficiency of the fin area, so that there is no wasted effort or speed lost to unnecessary or inefficient surface area. Finlets make fins work as though they were 25 to 30 percent larger than they are. This efficiency is the same reason that modern airplane wings use winglets.

FinSciences Fins are about 20 to 30 Percent Smaller Than Fins With the Same “Hold” and “Drive,” So Our FIns Have Less Drag, and Are Faster.

This is the magic behind our fins—if there is any magic. Because traditional fins are inefficient, they have to add extra area to create enough power (that surfers call drive and hold and SUPers call tracking ability). But because FinSciences fins maximize lift per unit of drag (they are efficient), they can be smaller tan typical fins.

The smaller area means less drag.

In surfing, less drag means it’s easier to paddle, accelerate to catch a wave, and get some air if you feel like it. Also, you can surf summer mush waves—you fin won’t drag you down or hold you back. But still they have all the power your used to.

In standup paddleboarding (SUP), less drag means you use less horsepower per stroke, so your arms don’t tire so easily, or you can paddle faster, but still with all the tracking ability you’re used to. Yes, we’ve measured the horsepower consumption of our fins compared to others.

Surfboard Fin Science 1

Winglets—Finlets—Make Fins Work Better by Decreasing Drag

The first thing people notice about our fins is the winglets, which we call finlets. Finlets make fins behave as though they are larger, but without the penalty of drag.

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.

Read more about winglets on NASA’s website: here and here.

Recap: Finlets 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 finlets 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, which 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.

Interference Drag, Cutaways, and Bulbous Forward Sections

Interference drag occurs where a surfboard fin meets the surfboard. For boats, naval architects minimize 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 doesn’t need to 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. So our 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—FinSciences Fins Are Double FoilsAnd Why

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 foil section

NACA 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% and a 15% 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% 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. More triangular fins increase downwash—the more raked the leading edge, the greater the tendency of water to move toward the fin tip instead of just around the fin, which is more efficient.

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 FinSciences fin 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?

Because Our Fin Is 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 FinSciences’ fins have high lift and low drag, they can be a bit smaller than other fins, yet behave like 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 original Wavegrinder longboard fin is a bit smaller than typical 9-inch fins (ours 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.

Fin Stalling, Cutbacks, Tail Sinkage

Stalling is when the smooth or laminar flow of a fin is lost. When that happens to an airplane wing, it loses lift, and the plane drops.

With fins, stalling occurs when you make sharp turns or cutbacks. The effect is slowing of the board, sometimes loss of the wave, and tail sink. One common review we get from surfers is that they feel like they are riding with their tails higher in the water. Probably that is because a typical stalling fin slows the board, and makes it sink a bit, whereas boards with our fins that stall less are so so prone to tail sinkage.

Stalling Airplane Wing as Described by NASA

Surfboard Fin Science 2