Get To The Point: Should Trailing Edges Be Sharp On Fins?
This week came media reports of a Perth surfer who badly cut themselves on their fin. It follows news of Manly surfer Max Hyett who also sliced himself on the thin trailing edge of his own fin. You may have seen Max surfing in the recent Deadmans edits, and the reason for pointing this out is to show injuries - even self-inflicted injuries - can happen to the most skilled surfers.
Surf Life Saving Australia report that 45% of "surfboard injuries" are caused by a surfer hitting their board, though the exact numbers get hazy (read: aren't available) when trying to find which part of the board caused the damage: nose, fin, tail, or rail.
Anecdotally, and writing as someone who's copped their (un)fair share of injuries, fins, and particularly the trailing edge, account for some of the more gruesome kind. That they're often self-inflicted adds a twist: Could we have done something to avoid it?
It's often assumed that a sharp trailing edge on fins is necessary for good performance but, considering the risk of legrope recoil and other tangles, is that really the case?
To find out, Swellnet asked three fin designers.
Greg Trotter has five decades experience in the surf industry, three of them working exclusively with fins. Greg currently runs SOAR fins from a backyard at Bawley Point on the NSW South Coast and supplies fins to shapers all around the world.
"I don't see any reason for the trailing edge of a fin to be sharp," says Greg. "There's no performance gain from it. None at all."
"I'm sure you've run your fingers up the sharp edge of a fin," Greg asks, and I assent. "The thought of that thing coming at you isn't pleasant."
Yet surfers value performance over risk, so I ask at what point does a blunt trailing edge effect performance. "It's hard to put a finite number on it," answers Greg, "but for instance, 2mm is too thick, you're likely to get cavitation and maybe unwanted flex, so around 1mm is ideal."
When working with fins, Greg says, "I'll always take the arris [the sharp edge] off with a quick pass of the sander. A fin can still cut you if it comes at speed, though danger is reduced if the sharp trailing edge is removed."
Phil Way is Australia's all-knowing fin guru. Phil began focussing on the minutiae of fin design back when shapers were simply asking 'one fin or two?' Phil passed on his body of knowledge to his stepson Nathan Bartlett, who now runs Alkali Fins, and it was Nathan who I spoke to about knife-like edges.
"There isn't any reason for the trailing edge to be sharp," says Nathan when I ask. "Despite what surfers think, surfboards just aren't going through water quick enough for there to be any discernible difference off the trailing edge."
"I think they feel precise, that might be why people like sharp edges," speculates Nathan, "plus removing the sharp edge takes a little longer in the production process."
"It might simply come down to that."
Though he's more known as a surfboard shaper, Greg Webber has designed many fins. In fact, at various times both Nathan and Greg Trotter have worked with Greg's designs.
Unsurprisingly, Webber has a differing opinion: "[A sharp edge] is needed so that when low pressure forms on the curved side of the fin, water from the high pressure flat side can't wrap around the back edge."
"If it does," says Webber, "the water replaces the low pressure at the trailing edge on the foiled side. If that does happen, then this is what is called cavitation."
"The low pressure can even form bubbles, which sounds crazy," admits Webber, "but if the pressure is low enough then standard water temperature can be enough for the water to boil. Well it really vaporises and doesn't boil but it's the same effect - water boils at a lower temperature when the pressure is lower."
Slightly less esoterically, Webber explains what happens when a sharp edge is used: "Then both sheets of water meet again at the back edge and the water will shear off super cleanly and cannot wrap around."
That said, there are tolerances than can be...erm, tolerated. "There's no worry with a slightly rounded edge, say 0.5mm or less."
While chatting, both Nathan and Greg Trotter noted that it wasn't just the trailing edge that presents danger.
"A few years ago I had my board surface slowly, then quickly shoot back at me," says Nathan, "the fin hit me right in the eye. I couldn't see for about an hour - thought I'd split my eyeball."
After a time, Nathan regained sight and the incident was chalked up as a close call, however the accident occurred because the shape of the fin tip was less bulbous, more pointed. Greg Trotter also mentions the threat posed by pointed tips and cites some fins, such as Mitchell Rae's Switchblade, which has the tip removed, as designs that would limit such injury.
The other thing Nathan and Greg Trotter agree on is the necessity for a rounded leading edge. In this case, the desire isn't safety but performance. "You can have various foils, but always that leading edge has to be rounded, says Greg Trotter with Nathan also adhering to that theory.
Like Greg Trotter says, any fin can cut you if it's coming fast enough, however self-inflicted fin chops are often caused by the trailing edge and keenness of the edge is often the deciding factor.
As Max Hyett says of his recent injury: "I didn't even feel the leggy tug hard and just as I landed I felt a light whack in my bicep."
And the result? "The fin chopped through 90% of the bicep muscle."
Max is now out for six weeks just as the autumn swell season kicks into gear. That's reason enough to reconsider how sharp those trailing edges have to be.