Wouter,

Thank you for sharing your ideas!
I have played with a similar idea, but do not have the time or workshop to test it out.
My proposal is similar to Wouter's in the sence that the horizontal foil/winglet may pivot when the rudder kicks up, and it is fixed in horizontal position during sailing by a vertical rod inside the rudder.

However, my proposal is to fix the foil/winglet by having the Boyer-type kick-up push down the vertical rod. Hence, when you lock the rudder to down-position by fixing the kick-up rod, you simultanously fix the vertical rod in down-position and lock the foil/winglet. When the rudder hits the bottom, the kick-up rod releases and the horizontal foil/winglet will be unlocked and turn while the rudder kicks up.

This setup will not allow adjustment of foil/winglet angle during sailing. However, if the vertical rod is made of two pieces permitting screw-adjusted length, it will be possible to adjust angles with some device that rotates the upper part of the rod.

Catamarans ususally come with two rudders and designing a device that reliably works both rudders is not trivial.

I ask all you innovative sailors to report results if you try out any of these systems.

Stein