These are not the type of fins we’re talking about. Photo: Michael Olsen//Unsplash
Surfboard design is often more of an art than a science. Even when surfers and shapers do prefer a more analytical approach, it can be all but impossible to surf repeatable, predictable waves to properly test equipment out in a scientific manner (although this may change with the advent of wave pools). As a result, much of the reasoning behind surfboard design comes from institutional knowledge. Granted, it’s institutional knowledge with years and years of experience behind it, but it’s not necessarily backed up by hard data.
This also leaves plenty of room for manufacturers to market alternative designs that claim to use real science to improve your surfing. These mavericks often operate in a sort of sweet spot between magical thinking and pseudoscience where technical terms will get thrown around a lot, but not necessarily in a way that holds up to close scrutiny.
Nowhere is this more apparent than in surfboard fins. Since as long as there have been fins on boards, there have been alternative fin shapes that claim to increase performance despite the fact that they generally look like some sort of freakish mutation ripped off a radioactive dolphin. Being the science-loving writer that I am, I decided it was time to to see if some of the various crazy looking fin designs I’d seen over the years had any sort of basis in physics.
Enter Riccardo Rossi and his company RED Fluid Dynamics. They use Computational Fluid Dynamics to apply a science-based approach to surfing, and have consulted for companies like Futures, Firewire, Tomo and Dakine. Riccardo himself has worked at Stanford and the University of Bologna, and has over 15 years of experience in the field of Computational Fluid Dynamics, so rest assured that when it comes to the physics of water, he knows what he’s talking about.
With that in mind, I asked Rossi to take a look at six different alternative fin designs and tell me what he thought.
Note: Rossi was careful to qualify his statements, saying, “I haven’t done the simulations myself or made any tests, so I can’t make a final statement on the designs. I’m just guessing based on my knowledge.”
Photo: Cooper Gegan
Angular/Trapezoid/757 Fin
They go by a myriad of names, but each of these templates has one thing in common: corners. In Essential Surfing, George Orbelian writes, “Curves hold onto water flow, whereas straight edges and corners cause water to break away cleanly. Straight edges free the fin to pivot and turn with less drag than a fin designed with curves.”
Rossi told me that straight lines and hard edges wouldn’t necessarily have the radical effect Orbelian was talking about. Because the fin is so thin, the difference between a rounded or an angular template profile “wouldn’t affect the flow much,” said Rossi. It’s a rather insignificant variable, compared to other factors such as foil and sweep. This is not to say that corners would have a negative effect, but rather it’s theoretically not much different than a similarly sized fin with a rounded profile.
Are you ready for some football? Photo: Cooper Gegan
Football Fin
This design has been around since at least the ’80s, used in single-fin setups for longboards. Orbelian wrote about these too, saying, “The narrow base makes it easier to lean the board rail to rail, the tip area is maintained or increased so there’s plenty of fin in the water when the board is leaned on edge. These fins will resist sliding and spinning out in critical situations.”
Rossi agreed that some of the claims regarding the ability to tilt the fin could be accurate. He wrote, “I guess there should be in fact less resistance when tilting the board during turns, due to the area of the fin removed. However, since the missing area is located near the base of the fins, the effect is not going to be as significant as reducing the tip area instead.”
In addition, Rossi said that not only would the reduced surface area of the template reduce lift, but the s-shape of the leading edge could potentially generate “vortical structures” (read: spiraling water) that may lead to increased drag.
George Washington’s preferred fin. Photo: Cooper Gegan
Hatchet Fin
Essentially the same concept as the football fin, but inverted, with a larger surface area towards the back of the fin tip. “The majority of the fin sits deep in the water, creating lift and acceleration,” a description for a popular modern iteration of the design states, “the cut-away base allows for quick direction changes and maneuverability.”
This one works, in theory. “It is true that a larger surface area leads, in general, to more lift,” says Rossi. However, he added the caveat that, “The location of the added surface makes quite a difference. Our models show in this case the trailing-edge region of the fin generating much less lift than the leading-edge region (15 percent vs 85 percent of the total lift). Therefore, despite sitting deeper in the water being located in the tip region, the added area will most likely have less impact on performance than tweaking the design in the leading-edge region.”
Rossi also noted that the hatchet design most likely works better than the football shape, because it avoids the potential drawback of vortices created by the football’s curved leading edge.
This one warps space, time, and generally held beliefs about the behavior of vortices. Photo: Cooper Gegan
Curved Fin
“The mid-face curve creates what we call a spiral propulsion,” says the creator of a curved fin design, “Water moves in vortices, which is a circular motion, and it catches that inside mid-face curve and creates a spiral uplifting to the bottom of the board.”
“To be honest most of the claims made by the manufacturers go against my personal knowledge about fluid dynamics,” wrote Rossi. According to him, the idea of the fin creating a sort of propulsive, propellor-like vortex behind it doesn’t hold up. “If that’s the case, this would simply induce a lot of drag, since vortical motion is generated at the expense of (i.e. draining energy from) the main water flow,” he wrote. Rossi also doubted that vortices could significantly affect water pressure acting on the board, and that even if they did, “a strong vortical motion would create a lower pressure on the board and thus work against lift.”
A technical report from wind tunnel testing did indicate that curved fins were more efficient than standard fins. However, Rossi wrote that he “was unable to find evidence online of the science claimed by the manufacturer for this design and the explanation for the benefits shown by the wind tunnel tests might be pretty far from what they suggest on their website and socials.”
For when your fin needs fins. Photo: Cooper Gegan
Star Fin / Winged Fin
Originally designed by Cheyne Horan and Ben Lexcen, the star fin and its variants have two small winglets at the tip. On his website, Horan claims, “With the starfin you still have a fin vertical in the water allowing cutbacks to gain acceleration.” The design supposedly has the same benefits as a winged keel in sailing.
“This template was indeed inspired by the breakthrough design introduced by Ben Lexcen for Australia II, that went on to take the first victory of the America’s Cup in 132 years by a non- American boat,” said Rossi. ”There are multiple benefits associated with it, but they are mostly and specifically associated with adopting the wings on a keel, which has a very different function in a boat compared to a fin (i.e. keep the boat balanced while sailing).”
However, I was surprised to learn that Rossi believed there might still be some use to this striking design, “When it comes to surfboards, I think it’s fair to believe wing tip vortices might be mitigated by this design, thereby reducing drag and provide more speed on a straight line (not acceleration).”
FORE. Photo: Cooper Gegan
Dimpled Fin
Dimpled fins claim to leverage the same aerodynamic principles used in the design of golf balls. A manufacturer of one model of dimpled fins claims the “design utilizes the turbulent flow created by the dimples on the fins, this turbulent flow draws the fluid back over the fin’s surface, reducing cavitation and drag during extreme changes of direction (high angles of attack) and at high speeds.”
“Cavitation is the most abused term in the surfing world.” says Rossi. “Cavitation means that the water is boiling. The pressure in the water goes so low that it falls below the vapor tension.” It’s a phenomena that happens with, say, a torpedo, but a surfboard almost never goes fast enough to create this effect.
But back to the dimples. “A golf ball is what you call a bluff body, the opposite of a streamlined body like a fin,” says Rossi. He explained that because a golf ball is, well, a ball, it creates a large wake behind it, which increases drag. Adding dimples activates turbulence on the surface and reduces the size of that wake. The tradeoff is that you have more friction, but in the case of a golf ball, this is far offset by the benefit of a smaller wake.
However, the opposite is true when applied to fins. “For something like a fin, that is thin, the friction is very important, and the wake usually is not really big. If you put the dimples on the fins, you’re increasing friction and you’re not really helping the fin to reduce the wake,” says Rossi. “That’s why there’s no wings with dimples.”
Between specious claims and surprising plausibility, a recurring theme started to emerge: manufacturers drawing from successful designs in other fields – the angular shape of an aircraft wing, the winged keel of a sailboat, the dimples on a golf ball. However, over and over we see that when you take a certain technology and apply it to a surfboard, it doesn’t necessarily work in the same way. “It’s totally fine to be inspired by other technologies or other fields,” says Rossi. “It’s always a good thing to look around you and find inspiration, but then there’s a process where you should develop the knowledge and prove that what you took works in your application. With surfing that’s always the piece that is missing.”
The innovation of these sort of designs is admirable, especially in a sport where the equipment has been essentially the same for decades. However, without peer reviewed studies and scientific testing to back them up, radical performance claims are just speculation. In the end, it’s hard to beat the years and years of institutional knowledge that have gone into the tried and true designs we see every day. It turns out that even science backs the advice inevitably trotted out when someone starts asking if a board or fin design is right for them: ask your local shaper.
