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."
Mind your thumb. Webber's 'Extractor' model fins with sharp trailing edge, and a narrow tip to boot
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.
Comments
A couple of years ago i was surfing Geraldtons best man made wave on a two foot day and somehow landed on the fins behind my knee.
This is the result:
Luckily i was wearing a wetsuit otherwise im sure it would have cut through those tendons or hamstrings or whatever they are called that are behind your knees.
Ive always wondered whether the trailing edge needs to be so sharp, it does seem very dangerous. Especially if your leggy pulls your board back towards your face after a wipeout.
I've landed hard, right on the end of my shrivelled snake before on my fin. Coldish water so had some rubber on but it hurt so much, I was sure there was the head of the snake had been neutralised. Paddled to shore and peeling my wetsuit down was one of the most intense cliff-hangers of my life - will it or wont it be there. It was, and is still able to slither on.
Thought we were finding out where your nicked bone name came from for a sec.
Oh nah. That was my super steel snake marvel origin story. Nick bone is simple but not as riveting. A portmanteau of Nick (first name) and Bonesister - Nick Bone.
Now you are no longer lost, doggy.
Haha classic story Nick..reminds me of Johnny Knocksville in one of his stunts where the motorbike lands on his penis.
Portmanteau? A new word for me. I had to look it up.
In this context, either meaning will suffice but I like the first one.
1. a large travelling bag, typically made of stiff leather and opening into two equal parts.
2. a word blending the sounds and combining the meanings of two others, for example motel or brunch.
Could this be where the peculiar name for an old solid school bag comes from in Queensland? Port.
Probably....but I was hinting at another large bag....notwithstanding a near miss from a sharp trailing edge - potentially split into equal parts. I liked the 'stiff leather' reference too..........
I'll go now.
No need I was talking to myself.
Years back I stupidly took off on a slabbing inside middling section of a wave here. Wave doubled up and went dry as I was popping up. Air drop without contact to my board that turned upside down. Fell arse first onto fin which felt penetrative then got took back up and over the falls and slammed on the rock shelf. Got in and felt my finger go thru wetty into a deep hole in my arse cheek. Asked guy in car park to have a look. Yep you better get to hospital. Doctor cleaning wound said I can see your sciatic nerve down there you’re lucky. Nice scar on butt cheek now
A friend had a wipe out at a south coast bommy, board and surfer pushed under, he slowed the bounancy of the board by catching the back of fin in the sack....stiches required.
.Your Switchblades from Mitchell Rae Stu are they Blunt?
Our ones not sharp on trailing edge.
AB used to make the sharpest fins. Could fillet fish with em.
I had this conversation with Richard Harvey recently. He makes fins for the boards I shape. Blunt leading edge, thin trailing edge but not sharp. "Not necessary and makes it dangerous"
Love the cdrives performance and feel but fuk i worry about landing on them ..just dont see the reasen for fins to be so pointy and sharp same goes for pointy noses on boards ..knock an inch of em and round em seen a few incidents over the years
I've been giving my fins a once over with 180 wet sand paper for a wee while now. Not just to dull sharp trailing edges, but to get rid of imperfections and bumps. Sometimes it takes a few iterations before a fin stops humming etc.
Doesn't necessarily prevent fin cuts; have had my own 7" Bonzer fin poke a deep hole in my shin (6 stitches), and just three weeks ago someone ran over my heel and gave it a nice cut. Missed my achilles, thankfully.
Dolphins & whales have flexible fins with rounded edges.
Slow fish have flexible fins with sharp spines to ward off predators.
Greenough rides a mat & advised progressive surfers against pointy noses. Slater listened.
Dont look at these injuries below while eating or with family....
eg. Tom Carroll coped a fin up the ar** in Japan.
https://www.theinertia.com/surf/5-of-the-goriest-wipeouts-in-surfings-hi...
After years of surfing with countless near misses, scars & seeing others bloody injuries, I opted for booties, gaff helmet & tube suit on coral reefs; then dealing with NSW windswell; led to roundnose & roundtail, plastic fins, wetsuit & gaff; always with a longer legrope. Less injuries, means more waves : ). Plastic fins means the boards slide abit under pressure ....the biggest challenge /drawback of the lot.
Started surfing on a Crozier flextail; a real sharp weapon....so maybe my injuries taught me lessons earlier than some....
https://www.surfresearch.com.au/00000322.html
https://lh5.ggpht.com/-NZ-Ov14pl7o/U2CY-F3UJ0I/AAAAAAAATa4/c1MK4YyJv3g/s...
"Martin Potter’s escaped intestines"
"In 1997 the former world champion and now WSL commentator Martin Potter was lucky to escape with his life after the nose of his board pierced his stomach. Six inches of his surfboard snapped off cutting him so deeply that his intestines were coming out of the wound. He needed 40 stitches and spent three months out of the water."
https://www.mensjournal.com/travel/surfing-injuries/
"Recent research suggests that lacerations account for almost half of all surfing injuries.
Body parts most frequently injured
Surfers most often sustain injuries to the leg (46%). Head and facial injuries are also common (26%), followed by injuries to the trunk/back (13%) and the shoulder and arm (13%).
Cause of injury
The main cause of injury is contact with a surfer’s own board or that of another surfer (45%). ‘Wiping out’ accounts for 36% of all injuries and striking the seabed accounts for 18% of injuries."
Reference: https://sma.org.au/resources-advice/surfing/
Reference for above surf injury stats
Taylor D et.al. Acute injury and chronic disability resulting from surfboard riding. Royal Melbourne Hospital. October 2003.
Rory Russell at Ulu comes to mind... Big single fin in the Guts.
that Martin potter injury reminds me of a guy down my way who wiped out and the tip of the board went up his ass and tore it and he was out of action for months, completely messed up
Fins can be made a much safer shape.
The simple nubster style.
With a bunch of them,
(Greg webber style)
Less chance of being speared by fin tip.
Might need 8-16 of them but yeah, safer for wipeouts!
Hey Caml, you mean have a cluster of nubsters replacing the normal fins?
Def not these
Speaking of Mitchell Rae’s Switchblade fins, which perform very well from my experience, has anyone noticed the similarity of the new fang dang FCS super expensive fins that appear to be an exact copy of Mitchell’s ones that have been around for years??
Similiar yes ...exact copy nah.
very interesting topic .
Bob Conneelly told me you can leave the trailing edge flat . in the context of perfection of foiling the trailing edge .
turns out he was right . have seen many fins work with a flat. would like to see some scientific study of that
Same concept as transom stern compared to rounded, lots of studies on displacement hull efficiency
Spot on
Courtney...this would be an interesting Trailing edge for a Surfboard Fin ?
https://www.becker-marine-systems.com/products/product-detail/becker-twi...
Thanks Udo That is really interesting I’ll give it a try for sure thanks
Not really the same guys - a fin is an appendage (underwater) not a hull (on the surface).
A hull creates a surface wave which is way more complex.
More appropriate to look at skeg, rudder or shaft bracket theories if that floats your boat!
There's heaps of info out there on Fine Tuning of Rudder Trailing edges
Square - Assym etc for Noise and Vibration - very interesting stuff.
For sure and generally it’s aligned with noise and vibration (anti singing etc) as you say and not claiming much increase in performance. If anyone is going to get into the theory on performance of surfboard fins I’d suggest they start with the leading edge. A lot more possibilities for efficiency gains on the outside fins on thrusters that are flat on the inside.
I've often wondered what the carry-on with sharp fins was. I nearly sliced my achilles as a grommet and have sanded my fins for safety ever since...singles, sidies, twins, thrusters, c-drives, the lot. I've been warp speed and not experienced hum/growl or noticeable cavitation as a result of sanding the trailing edge. Perhaps if you're a top-shelf performer it becomes an issue but for the average surfer it's just needlessly dangerous.
Copped a wayward Mal fin to the calf one time and needed 14 internal (muscular) stitches and a bunch externally. Bloke told me he didn't need a leash on his Mal. Apparently bloke can also ditch his Mal if unable to duck dive a 2 ft wave... drove home from 13th bleeding and in pain to my (Nurse) fiancée who gave me no sympathy and a band aid - at least she took me to emergency. Still bear the scars today. (Not just physical).
Hell hath no fury...
Craig yes thats right. (its not a typo 8-16 of them ) Either fins like bonzers , nubsters etc. Or even just fins built in to the channells. Like the finless shapers have done.
so if the rider surfs over another surfer there is no fin to hit them. Its more like a skateboard has rails to slide on the ramp edge. (Perfect in seaweedy breaks, you can surf on without drag) for crowded metro beaches.
Plus sharp nosed board shapes is not only danger to self but its dangerous to others. Why cant surfers see that this is unnecessary danger ?
Greenough has always told us,
it was his words mostly that influenced me to have round nose board shape. Cheyne horan aswell way back in the 80s & 90s, nowadays Dave rasta another top surfer with the round nose shape- ripping.
Some HB's also rounded nose for same reason
First thing I do with a new board is attach one of those rubber tips. It ain’t perfect but it’s a damn sight better than nothing.
Would happily buy rounded nose boards. Has zero effect on performance, so why have them
i wonder if someone is clever enough to make a 16 finner for hollow waves, bit of side slipping into the barrel then being able to engage rail when required. I'd be keen to try if someone was game enough to try shaping one. Extra glassing to handle the floggings :)
I remember seeing that post and completely forgot about it!! GW could be the man for the job cheers udo
Yep, gotcha.
And I'm all for rounding off noses, no functional use to them at all.
https://www.mivision.com.au/2010/10/exposing-the-danger/
Awesome links Udo and interesting reading.
I noticed that in a lot of those incidents the leg rope stretching then sling shotting back at the victim seems to be a reoccurring theme as well.
On Wednesday I pindropped through a closeout and came up without being sucked over but my board did get sucked over forcing the leggie to stretch out and after a few seconds the board sling shotted out of the water upside down fins first and hit me full impact straight below the lip. It ended in a nasty deep gash that went through my chin to the inside of my mouth and a face surgery with 20+ stitches around the goatee area and inside of my mouth as well as smashing one of my teeth leaving the nerve exposed and a lot of pain and swelling. Fragments of the JJF future rear fin were all through the wound and also in my teeth.
It has been a pretty miserable week drinking through a straw but you have to pay to play and I am just so lucky the fin didn't hit me in the eyes.
My main take away's from this injury are sanding back the sharp trailing edge on the fins should help and I am thinking off now exclusively using thick heavy duty 7ft+ leg ropes from now on, as it should prevent the sling shot effect of the shorter lightweight leggies
Even if I was using a rounded trailing edged fin, the force of the slingshot from the short leg rope would of done some serious damage regardless so I reckon using a longer chunky thicker leggie from now on is the go.
How much force is stored in a 7 foot heavy leggy stretched full compared to a short light leggy ? Think about the science. Maybe an experimental comparison of destructive force in the back yard after a few beers.
Thanks udo thats a good article supporting the safety nose shape
Yeh im all for the Rounded Noses
How did they come up with the 75mm Rule ..75mm is bigger than an eye socket ..... ?
https://surfingdoctors.com/4284/surfing-related-eye-injuries/
http://www.mltj.online/wp-content/uploads/2020/06/Nathanson.pdf 35mmm mentioned here
Thanks guys for the call I appreciate not being recognized for fin innovation :) by any of the surf industry. But to put things straight a fin should have a squared off edge just like a racing boat so the water will exit the foil with out grabbing to a curved surface. This reduces wake and no flag flapping ( high/ low pressure fighting over pressure zones) clean exit = faster fins. Grab a square block square off the trailing edge and any fin will go better.
bingo
Square it off up to a mm...then lightly /finely knock the Square edges of that....
so it leaves Trailing edge similiar looking to the top half of a Hexagon..
Dilute the Mix Tommy
https://surfeuropemag.com/features/surfing-ripped-me-a-new-arsehole-the-...
The worst fin injury I have ever heard of happened at the eastern end of Portsea at Sphinx, this is second, third hand, or fourth hand so details may be sketchy. Surfing alone but with a couple of guys surfing a different bank, not far away, a bloke on a single fin or a twin landed on his upsidedown board ass first. Split him open from his notya (not your ass not your balls) to his coxic bone. Apparently, he had internals hanging out and lost an extreme amount of blood before they could get him all the way back to Portsea to get him choppered out.
He survived, but he was a Jehovah's Witness and wouldn't accept blood, they managed to get synthetic blood from America and save him. Not too sure on the synthetic blood transfusion part, but pretty sure he was a Jehovah's witness.
https://journal.surfersmedicalassociation.org/case-report-severe-fin-lac...
A good mate of mine speared his leg on a beautiful brand new board from a north coast shaper. He was way up the beach bleeding out big time. Luckily with a mate who saved his life. He was a beginner and the fins were like razors which was totally unnecessary
If anyone can tell the difference in performance between a razor sharp fin and a small flat I reckon they are dreaming. It’s fun to try to apply hydrodynamic theories and perhaps you’d be able to measure a result in a towing tank - but the surfboard in practice
is way too dynamic and individually engineered.
dawnperiscope Agree. Maybe surfing 50+ ft Nazare or Shipsterns etc. a razor sharp trailing edge may make a difference but bouncing around at your crowded local ... forget it.
CP of 3d fins , great to have your explanation, including the race boat rear end reference.
Udo the 45° angle idea is good
Windsurfers are the fastest fuckers
on the Water.....they all say Square Trailing edge
A European backpacker died some years back in West Oz, the trailing edge sliced his inner groin artery at Smiths Beach and he bled out before making the beach. Safety comes first.
Pinched this from Swaylocks :
A pointy nose on a surfboard is like a pointed toe on a cowboy boot. Real cowboys don't wear them except for an unfair advantage in a bar room brawl and killing cockroaches in the corners of a room. Aloha, OL>>> Using nose guards is great, but it's kinda like putting your kid's car > seat in the back seat so he won't be killed by the air bag you bought.>>> Pointy noses are lethal, and they have no functional purpose. For a board > builder pointey noses represent future law suits, and guilt over > needlessly contributing to an injury or death. I won't shape one, and as a > pro I wouldn't sell one. I can just see myself trying to explain to a jury > why a surfboard needs a dagger on its nose.>>> Sure, go ahead and spend money to cover the problem up.
Better yet, don't > build a problem.
I cnc my fins and have tended toward a dolphin fin.
I start with 12mm thick 2x 6mm marine ply with carbon fibre centre,
everything but sleek and sharp, rounded and fat,
when I can get an organic flex into them I'll be happy.
Look at pictures of dolphin fins, they're rounded and low profile,
they move through the water @ <10-20 m/s, surfing is <5-15 m/s.
Surfing should have fins designed for speeds slower than dolphins i.e. squatter.
A round trailing edge is within the turbulent field around the foil and doesn't generate excessive turbulence after the trailing edge and won't create worse drag.
Got some pics of yours Fins Bigtree?
Had a Grubb board about 8"10 with fixed fins ,
Was enjoying riding it for about 6 months , along came a clean 18s swell and every wave the fins hummed badly. Had never happened before and i had ridden some solid waves on it.
I Believe it was the speed from the faster moving swells @ 18s
Some people here commenting say that you cant notice the difference with a bad foil , probably arent surfing bigger peroid swells so they cant notice any problems with the fin foils ...