ok, after using the FCS II fatboy 10 inch fin for about 6 paddles,
i switched back to the 8 inch dolphin style fin that came with my "imagine surf" inflatable sup.
HUGE difference, with the fatboy fin i felt much greater stability,
back to the dolphin fin today, i had to work much harder. not just to keep it straighter but somehow the board felt "bouncier".
so imo, the money i spent to upgrade the fin was way worth it! ymmv