A Scientific Explanation of Surfboard Fins with Dr. Cliff Kapono and Matt Rott

2025年8月26日

I see tons of surfboards and fins here at Hawaiian South Shore, and have always been fascinated by the hydrodynamic principles that make them work. We have done a few different tutorials and blogs over the years about the different aspects of fin design and how they affect the way a surfboard rides (as well as the different advantages and disadvantages of the various fin configurations).

However, virtually all of the information that people have shared with me over the years about fins have been non-scientific explanations largely gleaned from personal experience or general descriptions from shapers and other surfers. Interestingly, surfer/scientist Dr. Cliff Kapono recently discovered a peer-reviewed scientific journal article looking at how fins actually work, and shared the study in an Instagram post. Today, Hawaiian South Shore’s resident surf nerd Matt Rott goes over the information in Dr. Kapono’s video and breaks it down for us in layman’s terms.

Why Surfboard Fins Matter

Fins have always been the unheralded, underappreciated driver of our surfboards. They provide thrust, stability, and lift, and even slight tweaks in design or size can have a huge impact on how a board rides—far more so than similar tweaks in the design of actual surfboards. The article covered by Cliff is the first one I’ve heard of that breaks down the hydrodynamics of fin performance into peer-reviewed scientific data, which was pretty excited.

About the Scientific Study

First, a few caveats. The study by Stefan Kneisburges and his colleagues, titled “Measurements of the hydrodynamic pressure on a surfboard fin during surfing” and published in Scientific Reports, focused primarily on lift and only tested fins on an artificial, standing wave in a river in Germany (as opposed to real ocean waves, which have different energy bands and which are surfed differently). That being said, the special 3D-printed fins that they used (based on the FCS G5 fin) was equipped with sensitive instruments that measured pressure, which is a relative constant when it comes to fins moving through water.

Instagram Post by Dr. Cliff Kapono

How Fins Work Like Airplane Wings

The important thing to understand is that side fins operate on the same concept as airplane wings, which is, of course, lift. These fins typically have a curved outer side and a flat (or less curved) inner side, just like an airplane wing. What causes airplane wings to work is the fact that the air moving over the curved top surface of the wing has to travel a longer distance than the air moving along the flat under-surface of the wing. In order to do so, the air on the curved side moves faster, creating a low-pressure environment. Meanwhile, the air on the underside is moving slower, which creates higher pressure. This high pressure below and low pressure above causes the wing to lift—and is why airplanes can fly!

Diagram comparing airplane wing airflow lift with surfboard fin water flow lift

What the Study Found

The same thing happens in water when fins—which are foiled, similar to an airplane (i.e., rounded on one side and flat on the other)—travel along the face of a wave. However, the fins are arranged vertically (facing up and down), whereas airplane wings are arranged horizontally. Thus, the lift that is generated by surfboard fins does not push up—it pushes out (from the stringer toward the rail). This is exactly what the scientific study measured. The sensors in the fins found that, regardless of the direction the surfboard was traveling (left or right), there was always pressure pushing from the stringer toward the rail—the greater the speed, the greater the pressure. In fact, they measured as much as 300 Newtons of pressure, which is equivalent to the force of gravity on a 67-pound child. (Pick up your keiki next time they walk by, and imagine the amount of force it takes for you to lift them—except that force is being exerted by the water on your fin as you surf along a wave).

Why Lift Matters for Surfers

So, what good does this lift do for us surfers, since we aren’t flying on our surfboards (unlike foilers, who use horizontal wings to fly, just like airplanes do)? The water is pushing against the inside of the fins (from the stringer toward the rail), which creates pressure. If there were only one side fin, the water would push against the fin and cause the board to spin out. But because there are two symmetrical fins working together—both experiencing equal but opposite force pushing them away from each other—it stabilizes the board in the face of the wave, making it easier to control. At the same time, the lift generated by the fins results in an equal and opposite force that generates thrust and pushes the board more quickly through the water.

Balancing Lift and Speed

The interesting thing about lift is that the design features that create lift also slow objects down. Another way to put this is to say that the more curved and fat a wing or fin is on the top, the more lift it will create (because the difference between the curved side and the flat side is greater), but it will also move more slowly through the air or water, due to the fact that it is less aerodynamic or hydrodynamic. Over the years, surfboard and fin designers have dialed in the perfect combination of foil on fins to ensure that they provide the optimum amount of both lift and drive. They may not have used scientific instruments to do so, but trial and error over a hundred years and millions of surfboards is pretty effective, as well!

Frequently Asked Questions

1. What does hydrodynamic lift from surfboard fins do for surfers?

Lift generated by the fins pushes outward—from the stringer toward the rail—creating stability when both fins oppose each other. It also produces forward thrust, helping the board move faster through the water.

2. Why don’t surfboards spin out with two fins?

A single fin would allow water pressure to unbalance the board, causing a spin. However, two symmetrical side fins experience equal and opposite outward forces, which balance each other and stabilize the board.

3. How much force do surfboard fins experience?

The study measured pressure on 3D-printed fins equivalent to about 300 Newtons—the weight of a 67-lb child. That represents the substantial sideways force fins handle while surfing.

4. How do fin shapes impact surfing performance?

Thicker, more curved fins generate more lift—but also more drag, which slows the board. Thinner, streamlined fins offer less lift but allow faster speeds. Designers balance foil to maximize both lift and drive over years of trial and error.

5. Do fins work like airplane wings?

Yes. Like wings, fins have a curved surface on one side and a flatter surface on the other. This shape creates low pressure above and high pressure below, generating lift—except the lift pushes sideways instead of upward.

6. Can these study results apply to real ocean waves?

The study was conducted on an artificial standing wave in Germany, not on dynamic ocean waves. That introduces limitations. However, pressure measurements still offer valid insight into how fins perform in water.

7. What fin configurations benefit from this science?

Any setup with symmetrical side fins—like twin or thruster (tri-fin)—benefits from balancing lift and thrust. Single-fins lack this opposing force and are more prone to instability under pressure.

8. How does fin foil balance lift and speed?

The foil design influences hydrodynamic performance: greater curvature increases lift, while thinner profiles reduce drag, boosting speed. Surf designers refine foil to optimize both aspects based on experience.