Fin condition vs performance

thermalben's picture
thermalben started the topic in Thursday, 27 Feb 2020 at 1:11pm

First up: I'm not really up to speed with technical fin design. So, excuse any generalisations.

Righto. On Monday, down in Vicco surfing solid, well overhead waves, I powered through a clean bowly section only to find the board slowing down rapidly... as if someone had applied an aquatic handbrake. It was perhaps similar to running into a thick bed of seaweed, except there wasn't any around. Running over bull kelp was a possibility, though I would have experienced a bump, not a steady speed change.

Unsurprisingly, the section mowed me down, and I took a thorough beating.

Paddling back out, I flipped over the board, to find that the top inside layer of glass had peeled back from the base of the fin, about 3-4cm. The glass was holding at about 90 degrees, and was the obvious source of the speed loss.

Not wanting to paddle in, I managed to tear off the fibreglass - though it left a very rough edge.

Anyway, I kept surfing, and whilst the speed issue had been rectified, I didn't really notice any performance issue either. 

So my question is: how important are the condition of the fins, relative to performance? I was expecting the board's performance to be rather compromised but it ended up being perfectly normal. I've known surfers who are meticulous about sanding back their fins regularly, to ensure they're in perfect condition.. but is this really necessary?

As a side note, I'm not sure how it happened - looks like an unusual delam, as there are no dents on the fin's edge (perhaps I hit some rocks on the paddle out - but there's no visible damage).

thermalben's picture
thermalben's picture
thermalben commented Thursday, 27 Feb 2020 at 1:12pm

Another view:

helmet-not-hose's picture
helmet-not-hose's picture
helmet-not-hose commented Thursday, 27 Feb 2020 at 1:31pm

It's on the base, which is the stiffest part of the fin, so you'd expect minimal impact from bad flex. Also looks like it's on the outside edge of the fin, which is where the most turbulence is (compared to smooth inside edge), so the rough section may have been lost in the turbulence.

thermalben's picture
thermalben's picture
thermalben commented Thursday, 27 Feb 2020 at 3:01pm

Lost in the turbulence of my ripping? 

Very unlikely.

BTW damage is on inside of fin.

Pops's picture
Pops's picture
Pops commented Thursday, 27 Feb 2020 at 3:39pm

The roughness would promote a turbulent boundary layer and could lead to cavitation and/or earlier boundary separation. Both would increase drag (depending on speed, bank/trim angles, etc.) on the fin notably, but not massively compared to overall drag. Might expect a penalty on maneovrability rather than straight line speed... I'd expect it would be worse if the damage was closer to the tip rather than the base.

He who hesitates is lost

surfiebum's picture
surfiebum's picture
surfiebum commented Thursday, 27 Feb 2020 at 3:40pm

Outside or inside fin relative to the wave? I'd imagine that would make a difference.

udo's picture
udo's picture
udo commented Thursday, 27 Feb 2020 at 4:23pm

pops - 3D fins ...have smooth leading edge then into rough dimples...whats going on there ?

Pops's picture
Pops's picture
Pops commented Thursday, 27 Feb 2020 at 4:40pm

Could be a similar concept to dimples on a golf ball. At the right range of Reynolds number the separation point actually gets shifted back (reducing drag among other things). Cavitation would be the main pitfall. I haven't closely studied fins (my thesis looked at other areas of board design), but read a fair bit as part of my background research. So take what I say with a grain of salt.

From memory Stu interviewed a professor from Woollongong a while back on similar concepts?

He who hesitates is lost

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 27 Feb 2020 at 4:43pm

A tad off topic, but it triggered a memory.

I remember one early morning at Crescent head pulling into the carpark after sleeping somewhere else in the kombie, sun just coming up and a nice clean shoulder high east swell peeling down the point and no one was out, raced up the point jumped off the rocks, caught a wave and then it was just clunk clunk, stop start, leg rope dragging and then i realised there was hundred or thousands of those big round blue UFO looking jelly fish all down the point.

Really fun waves and no one around but these jelly fish made it almost unsurfable and paddling through those things is also not fun.

Pops's picture
Pops's picture
Pops commented Thursday, 27 Feb 2020 at 4:53pm

The horseshoe shaped ones?
Read somewhere they're actually clusters of eggs.

He who hesitates is lost

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 27 Feb 2020 at 5:05pm

Little clear horseshoe ones you see down south only about 100mm long, yeah they are eggs for a sea snail https://www.environment.sa.gov.au/goodliving/posts/2017/12/mistaken-iden...

But these are those big UFO looking blue ones, dont know how common they are these days, but use to appear in huge numbers sometimes like when paddling the seaway.

https://www.qm.qld.gov.au/Find+out+about/Animals+of+Queensland/Sea+Life/...

simba's picture
simba's picture
simba commented Thursday, 27 Feb 2020 at 5:11pm

indo had a few encounters with these guys at lennox over the years and its usually in clean waves....clunk clunk etc didnt think they stung but they can...seems as if they can turn on the sting if they want.

simba

wally's picture
wally's picture
wally commented Thursday, 27 Feb 2020 at 5:41pm

In 2015, there was a spectacular swarm of blue blubbers off the Gold Coast.

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 27 Feb 2020 at 6:34pm

Wow thats thick.

Yeah they can kind of give a stingy little itch, people often say its sea lice that's on them though?..dont know.

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 27 Feb 2020 at 6:35pm

BTW. speaking of fins, now with a decent pool in Australia would be great place to do some blind test with fins, materials etc.

GuySmiley's picture
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GuySmiley commented Thursday, 27 Feb 2020 at 6:43pm

Three of us pulled up at Lennox once, perfect 4 foot plus no one out apart from 1,000s of dinner plate sized jelly fish. Interesting surf that one which ended with a jelly fish attaching itself to my mate’s bare chest. Lots of pain, antics and of course laughter. Same trip same mate ate a caldron of porridge made up from 1 kg of dry oats at Byron, lots more pain after the fermentation started in his stomach and laughter at his expense that continues to this day.

freeride76's picture
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freeride76 commented Thursday, 27 Feb 2020 at 8:07pm

catostylus

blackers's picture
blackers's picture
blackers commented Thursday, 27 Feb 2020 at 9:55pm

Plenty of blue blubbers in Port Phillip Bay year round Indo. They will sting if you are silly enough to put part of your body into their tentacles. Funnily enough, the only time I have been stung was a Crescent.

morg's picture
morg's picture
morg commented Wednesday, 4 Mar 2020 at 10:53pm

I have habit of damaging the tip section of the same fin on my boards and generally tidy it up with a file. This is more about tidying the look of it up than anything else because I can’t actually feel a difference. I replace them when about 5mm or so has gone.