I have a plastic fin for my board and i've been told its awful. Until such a time arises that i can buy a new fin, could i increase the performance of the fin by trying to improve the shape of the foil. Will i see any noticeable difference or am i just making a bad fin worse?
The main problem with plastic fins is their extreme flex,so improving the foil will only have a marginal affect. Making it shorter and using with a smaller sail will reduce the flex, so may be as one of those supper short free style fins, that's a remote possibility.
But I think you're better of throwing it in the round filling cabinet.
Was given a Andrew Regan Caveman 40cm double angle (why?) weedy some years ago.
Big, cumbersome and actually grabbed weed.
Grant (KA43) also had one & cut it down, so thought I'd have a go using his outline.
Tried as best I could to get an Eppler foil shape (thanks Decrepit for info & diagram in another topic much earlier).
Fin now performs way beyond expectation (may have been a bit low!) 
Have had it out with 5.0, 5.8 & 6.5 sails with conditions varying from flattish weed to Botany washing machine chop.
Good lift & doesn't let go.
Now a 26cm weedy!
Fin before cutting
Post cut & before profiling
Finished & used 
Multi function tool with half round blade - cut through like butter.
Then used rasp for rough trim, orbital sander & fine flat file.
Followed by sanding pad on MFT and finally wet & dry.
Used a profiler (& lots of pencil marks) to try and keep to a reasonable foil shape.
Be very patient....slowly slowly.
Peter yours is definitely fibreglass, probably G10, I'm not sure what Oblix's is but I doubt it's the supper crappy flexi molded plastic Mmyer is talking about. So he's more likely to follow in Oblix's footsteps.
Peter glad you had success, that looks like a decent weedy now.
Select to expand quotedecrepit said..
About 42% from memory Mark
Is that a speed thing? Wave and general use kinda fins seem to be around 30-35% (?)
There's also the naca 63..about 45%, which of course may vary according to the amount of ginger beer consumed!
Hi Maui Meyer
I have an old fibreglass american box fin you can borrow for a while.
Not as nice as the newer style fins, but a massive improvement on the plastic one you've currently got.
Otherwise try on ebay/gumtree. You see them occasionally for pretty cheap (less than $20).
Clarence
Mine was a soft plastic one. The tip was worn out and way too soft.
The fin still works, but doesn't go well up wind, so had to upgrade.
Wineman - that. one precise cut. Nice work.
Select to expand quoteMark _australia said..decrepit said..
About 42% from memory Mark
Is that a speed thing? Wave and general use kinda fins seem to be around 30-35% (?)
Yep, makes for a finer entry, goes downwind faster, but not as forgiving at lower speeds upwind.
