I just thought I’d give some feedback about The FCS K-Flex fins (soft polymer coating over rigid core). I’ve been using them on a 6’3" thruster now for about 3 months. I originally got them for the safety factor, having gashed my foot prior to making the board. The gash wasn’t bad but my toe and whole inside of my foot had that tingly, dead skin sensation for about a month.
My take on the fins is that they feel a little “sticky.” What I think happens with regular rigid fins is that as you finish your turn and the water slows down over the fin and the turn radius tightens, they cavitate and slip a little. That helps set up your next turn. The K-Flex have the last 1/2" of the trailing edge that is soft and what I think happens is that they flex and delay the cavitation/release, thus the sticky feel at the end of the turn, like they don’t want let go.
I won’t say yet that that’s bad. Maybe if I get used to it it could be positive (a little squirt at the end of the turn?). But for the moment they just feel a sticky.