Plywood Fin Thickness

Cut some keel fins last night (9mm ext. Ply)

for my 1st shaping attempt a 5’10" twin fin fish

I hope to foil them this w/end, they are 7 1/2 x 4 1/2

glass on’s, should they be a certain thickness,

if so where on the fin do I take the measurements??

Or is it just what looks & feels right??

Intend is put them 6 1/2 from tail, 5 1/2 from stringer

1/8 toe in with 4deg cant, any help/advise would be great.


Hey Ants

I reckon that if they look good go for it, check out some other fins but they are generally about the thickness of 3 ply plus whatever glass you put on them…

Thanks Hicksy,

I forgot to add, do I foil the inside slightly

or leave it flat as per thruster set up’s.


concave on the inside, some one who knows please correct me if i am wrong, will increase maneuverabilty and create less parasitic drag. i need to get some 1/4 inch good quailty birch ply.


Fin positioning sounds perfect… right where they should go :slight_smile:

In terms of thickness and foil, Hicksy is right… It is looks good, go for it. I was measuring some fin thicknesses last night as I’m planning on doing one of Chip’s fin templates for his experiment. It really depends on the fin but I have some as thin as 3/16" and going up to 7/16". The wide point really depends on the fin and how it is foiled. Fish fins are usually foiled with the thickest part towards the front of the fin and thinning out towards the back. You’ll get the idea when you look at a foiled ply fin as you can clearly see the rings of the grain in the ply as to how the fin has been foiled… that template you emailed me a few days ago shows it fairly well.

In terms of foiling the inside of the fin, the fish fins I use have the inside foiled. It is a more subtle foil than the outside foil. That said, you don’t have to foil the inside if you don’t want… Ashley Surfboards in Torquay make a really nice fish that they do custom ply fins on and they don’t foil the inside of the fin. If you get a chance, drop in there on the way to / from a surf and check it out. Very nice glass tints on the fins too.


PS: Good luck on the weekend with the shaping. I hope for your sake there’s no waves to tempt you away from shaping, I hope for my sake there will be great waves… One of us has to win :slight_smile:


If you are foiling both sides, start with 12mm ply, with as many plys as possible, the contours make foiling soo much easier. Concave may be good on the inside face, but a real bitch to foil unless you are pretty handy with a grinder. Good luck