I have a few chips out of the tip and leading edge of my 46cm fin - approx .5 cm on tip and .25cm on leading edge. Just wondering in real terms if you would notice the difference in effficiency of the fin compared to it being new. I know the answer is it would not be as efficient and it depends how badly beaten up it is, but I don't think it is really performing badly at all.... Just wondering what peoples' experiences are using hacked up fins.
Cheers

You do notice a damaged fin a fare bit, they also tend to spinout, i try to keep all my fins in top condition, any chips get sanded or filled then sanded, sometimes the graphics even make them spinout, well if there printed on top of the glass that is..

I have had some pretty mangled fins that worked ok. No doubt they were not as fast as they should be and maybe they would spin out a bit earlier but I couldn't tell. I'm inclined to clean up my fins though. If I'm particularly looking for speed I'll be particular about my fin. Its that whole thing of subjective judgement. I can't tell within about 10% whether something is faster or slower.

On the speed forum a month or so ago, there was a pic of the guys fin who did 50kt, apparently he hit a rock or something and the fin was a mess. Nobody could believe he'd gone that fast on it.

decrep you seem to know much about finsmy question if i had a nicked fin which i have a lot and wanted to sand it, do you sand it from base to tip or from leading edge to trailing and how do you figure if one side had enough sanding

I've got this device, it's lots and lots of thin steel rods held in a loose clamp, when you press it down on a surface the ends of the rods take on the shape of the surface. This way I can compare sides of the fin against each other and against the required foil shape.

If I'm starting from scratch with a slab of fiberglass I use a sweeping action with a 7" right angle grinder from base to tip. once I get close to the required shape, I use a soft sanding block across the fin.
If it's just repairing a nick that I've refilled, it's the hand block again, but I just bring it down to the old surface which ever direction seems best.

If I'm resanding the whole fin, depends how deep the nicks are, real deep I may use the grinder with even sweeps from base to tip, then hand sand back across the fin, if they aren't very deep it's just a hand sand.

If it's a molded fin be careful of taking the outside skin off, most of them I've seen rely on the outside layers for their strength.

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decrepit said...

I've got this device, it's lots and lots of thin steel rods held in a loose clamp, when you press it down on a surface the ends of the rods take on the shape of the surface.

Not sure of the exact name of the tool, but we call it a 'shape copier' used to match timber skirtings/architraves and mouldings.......sorry, couldn't help myself

Just to add a question.....some clown (namely me), always leaves it too late to dismount, and has worn the outer coating off the bottom leading edge of the fin, back to the glass fibre, or mesh. Will this affect the fin greatly?

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decrepit said...

I've got this device, it's lots and lots of thin steel rods held in a loose clamp, when you press it down on a surface the ends of the rods take on the shape of the surface. This way I can compare sides of the fin against each other and against the required foil shape.

If I'm starting from scratch with a slab of fiberglass I use a sweeping action with a 7" right angle grinder from base to tip. once I get close to the required shape, I use a soft sanding block across the fin.
If it's just repairing a nick that I've refilled, it's the hand block again, but I just bring it down to the old surface which ever direction seems best.

If I'm resanding the whole fin, depends how deep the nicks are, real deep I may use the grinder with even sweeps from base to tip, then hand sand back across the fin, if they aren't very deep it's just a hand sand.

If it's a molded fin be careful of taking the outside skin off, most of them I've seen rely on the outside layers for their strength.

so it's just line of sight for figuring out how much to sand off on both sides then..i've tried doing it once before manual wet sanding with i think 300# attached to a table vice grip hard work with sweats all over and sticky itchy stuff it turned out nice but when on the water i noticed it didn't have the neat water slice sort of cavitating on one side

The fins on my wave boards are all pretty mangled at the tip from the beach (walking upwind etc). To be honest I don't notice the difference at all. On of my fins I backed into the garage door with the board still on the roof, result, 0.5" off the tip and a massive gouge out of the back edge. I just sanded it smooth again and away I went. It remains like that to this day and I never noticed a difference (except the slightly shorter fin).

I'm sure when you doing 40+kts it might make more of a difference as drag increases with the square of velocity (what component of total hydrodynamic drag is fin drag is an area of debate) but I think for most of use cruising around at 15-25kts its no problem :D

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Sailhack said...
[>>>>>
has worn the outer coating off the bottom leading edge of the fin, back to the glass fibre, or mesh. Will this affect the fin greatly?

Probably not, to any great extent.
But if it's thickened the front edge up significantly, or has a rough surface that will increase drag somewhat.

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jp747 said...


so it's just line of sight for figuring out how much to sand off on both sides then..i've tried doing it once before manual wet sanding with i think 300# attached to a table vice grip hard work with sweats all over and sticky itchy stuff it turned out nice but when on the water i noticed it didn't have the neat water slice sort of cavitating on one side

You could have changed the foil slightly I guess, I'd check the trailing edge, make sure it isn't rounded at all. It doesn't have to be razor sharp (that's dangerous, I have stitches to prove it), maybe .5mm thick but with a square edge. The last third of the foil should also be straight or even slightly hollow on a good modern speed foil. the closer to parallel the rear surfaces are the better.