How can we make faster fins?

Speed, specifically how fins can make us go faster, got some scientific attention recently. A study by Alhoush Elshahomi and colleagues out of Australia found that putting grooves along the leading edge of fins led to a 13% reduction in drag in their model. Their paper is open access - worth at least skimming through if you want to learn the language of computational fluid dynamics modeling.

The two nearly identical setups, with the side profiles in (A) and (C), and the view from the front in (B) and (D). I wish they designed these to be Futures. This is Fig. 1 of the paper.

The researchers used Computer-Aided Design (CAD) to design two nearly identical sets of fins, with the only difference being grooves on the inside of the leading edge of one set. They attached these to a board with zero rocker and simulated fluid flow at about 12.5 mph, a speed they cited from established surfing research. By adjusting the angle of attack - think the angle at which the fin box is glassed into the board - they analyzed pressure distribution around the fins.

At a 30-degree angle of attack, quite steep for a real board, they found the pressure in front of the leading edge was about 13% lower for the grooved fin versus the regular twin fin. The reason for this drag reduction mirrors the physics of golf ball dimples. Those small surface irregularities create a turbulent boundary layer around the ball. A dimpled surface creates a smaller wake behind it compared to a smooth one, reducing drag by up to 50% in golf balls. Similar physics apply to these fins, though the reduction is less dramatic due to speed differences, water versus air as a medium, and total coverage of the surface.

The researchers noted that while drag decreases with grooved fins, the lift generated over the airfoil shape also drops. However, the roughly 4% reduction in lift is more than balanced by the drag reduction, leading to an 11% improvement in lift-to-drag ratio. This tradeoff between speed and maneuverability sits at the heart of fin design.

Drag is intuitive - less drag equals more speed. Lift requires more explanation. Fins are shaped with curved outside faces and flat inside faces for a reason (ignore the swooped shape of the fin for now, we’ll get to that another day). When flow separates around the fin, water travels further around the outside than the inside in the same amount of time. This speed difference creates a pressure difference, with faster flow leading to lower pressure, generating force on the fin. In our setups, this force points outward from the board's center toward the rails. This lift force, combined with rail engagement, enables turning.

This shape, which is what makes planes fly and sailboats sail, is called an air foil. Surfboard fins are designed around this shape as well. By Olivier Cleynen - Own work.

The study also investigated how these grooved fins performed at different angles of attack, finding that the drag reduction versus traditional fins remained relatively consistent across various turning angles. This suggests that the speed benefit might persist through different surfing maneuvers, not just during straight-line surfing.

These grooved fins slightly reduce turning ability, but the tradeoff brings increased speed. Since these results come from an idealized model, real-world performance may vary if these enter production. Maybe they will saw through kelp? Hopefully we'll see some hit the market - because who doesn't love more speed?

Further Reading:

The Paper

Air Foils

Golf Ball Dimples