Made this one with 1 point-something lb density EPS foam. We ordered a 2 lb block, but based on the bead size, I think we got something with a lighter density. Regardless, I made this twinfin with 4 oz uni-directional carbon fiber perimeter stringers and glassed it with “Resin-X.” Single concave running to a semi double concave between the fins and flat at the tail. I say semi double concave because it’s really a single with a little roll in the middle of it, sort of a raised V. The fins inspired by Greg Webber are made from a skateboard deck using the concave to get the inside curve. Finished weight, right at 5 lbs. 6-2 x 20 (dead center) x 13.5 nose, 15.5 tail. I have since added twinzer finlets to it in an effort to increase the board’s drive. It’s light, loose, and strong! Definitely got some spring to it.
BTW- some of the pics are of the board pre-sanding.
edit: BammBamm808 was a huge help giving me insight on the nuances of using Resin-X, lots of great advice.
Deck:
Bottom:
Bottom Presand:
Presand showing the fins’ inside curve:
I guess this one could go in Shwuz’s plywood fin thread (note the hole where one of the truck bolts would go):