Fins Part One: Why does fin shape matter?

Speed, acceleration, stability, turning - all these qualities we seek in surfing are dramatically shaped by those plastic buggers sticking out from the bottom of our boards. The fin might be one of the most overlooked components of surfboard design, but its impact on performance is profound.

Two recent research papers provide rather neat insights into fin hydrodynamics. Falk and colleagues studied computational fluid dynamics around thruster setups, while MacNeill and Barkdoll investigated optimal shapes for balancing stability with speed. Their findings explain what many surfers feel intuitively but struggle to articulate.

The fin base - that horizontal distance where the fin connects to your board - creates the foundation for acceleration and drive. When you're pumping down the line or pushing through a bottom turn, the base length determines how efficiently your energy translates into forward movement.

Figure 1 of the MacNeill and Barkdoll paper lays out the parameters we’ll discuss.

The physics suggests that a longer foundation distributes pressure across more surface area, providing greater leverage against the water. In practical terms, this aligns with what many surfers experience: boards with longer fin bases tend to maintain speed better through turns and feel more “drivey” when pumping.

The trade-off, though, is predictable. As MacNeill notes, "The greater the base length, the faster acceleration the fin can provide to the surfboard." But that same quality makes the board resist directional changes. Those quick pivots in the pocket become more challenging as base length increases. Still, keel fins are fun.

Fin depth - the measurement from base to tip - controls what surfers typically call "hold." Deeper fins penetrate further into undisturbed water, providing more lateral stability.

MacNeill's research may confirm what noseriders already know: "A longer fin depth provides more hold, which allows the surfer to have more lateral stability. As fin heights are reduced, it could be easier for undesirable lateral board sliding to occur."

When pushing through a steep section or making a hard bottom turn, deeper fins resist sliding out. But this comes at the cost of freedom and speed - sometimes you want that controlled slide, especially in smaller, weaker waves where breaking the tail free to spray your buddy becomes all-important.

Aspect ratio - the relationship between depth and base - is therefore a meaningful thing to discuss. Higher aspect ratios (taller, narrower fins) typically generate less drag for a given amount of lift, similar to how sailboat keels or airplane wings work. That's why many high-performance fins feature this design element - less drag means more speed.

Figure 10 of the same MacNeill and Barkdoll paper, although measuring angle of attack, shows a visualization of the turbulent eddies that increase drag. We’ll talk about angle of attack more soon, I promise.

A bit harder to put a number to, that graceful curve from the fin base to its tip - technically called sweep or rake - definitely changes how your board turns. Falk's computational models show how flow patterns change around fins with different sweep angles, with more upright fins creating different flow characteristics that may contribute to tighter turning. These fins pivot more quickly around their base, making for responsive transitions from rail to rail. This is why you'll often see shorter, more vertical fins on boards designed for small, punchy waves where quick directional changes are essential.

Fins with more sweep (angled back toward the tail) resist pivoting and instead favor drawn-out carving turns. Next time you watch someone laying down beautiful arcs on a wave face (a favorite being Mikey February and his keel fins), take note - they're likely riding fins with moderate to high sweep angles that maintain speed through extended turns rather than pivoting sharply.

The interaction between these three dimensions creates countless performance possibilities. Swap out your fins, and you've got a different board. Parts Two and Three will cover these papers in a bit more depth, and Parts Four and beyond will be about other setups. I hope you like fins, we’re going to be here a while.

Further Reading:

MacNeill and Barkdoll Paper

Falk Paper