I have three fin ideas that I don’t have time to implement and test so I am throwing them out there. They probably aren’t all novel, but they are novel to me. Any DIYer can use them and any small volume on-site manufacturer for free (if they actually amount to something) but if Rainbow starts doing one I’ll take action.
I have a Ph.D. in Mechanical Engineering but I am more of a machine design guy than a fluids guy so these are all highly speculative.
Twinline fins. Larger flex fin in front and smaller trailer. That is, two tandem longboard fin boxen (or a longboard box in front and a fixed mount like FCS in back). Might be loose for gentle turns but grip tighter for harder turns. Flex fin in front might give some of the thruster feel.
Reflex fin (see attachment). If there are no flutter or other problems, should pull the tail down in a turn but allow flatter trim going straight.
Swiveling single fin with spring preload. Only swivels/breaks away when the preload is met and then responds in a roughly linear fashion to torque on fin. Obviously, a rake fin will be required for this idea. The reasoning behind this is that gentle longboard turns look very smooth and arclike but harder turns look forced. It would be good to have a point at which the fin can break a bit to permit a tight, albeit somewhat slipped or slidden turn. Of course, flex fins permit this but the thing I don’t like about that idea is that the flex is always there, even going down the line and thus always permits some side slip. The preload would of course be adjustable by a knob or screw.
Note that I mean swiveling about an axis perpendicular to the board (the yaw axis) not about the roll asix.