I am looking for a design that I can execute myself, out of stock components, in a week or two. Since I am redoing the rudder blade, it makes sense to make the trim tab part of the effort. Having a trim tab provides the same sort of amplification as a servo-pendulum, but without any fancy stainless steel gears.
I expect that the trim tab will get a lot of use: hand-steering, especially in heavy following seas; hooked up to the tiller pilot, for precise
compass courses when sailing free, and hooked up to wind vane, for beating for days on end (headwinds seem to be attracted to me as fleas to dogs). So, I think I am OK with having the trim tab integral to the design, especially as it can be secured in
line with the rudder to provide extra turning force when maneuvering.
I think I have all the pieces more or less thought up: a 1'x4' horizontally pivoted wind vane paddle with a pushrod linkage to the trim tab, which is 20% of rudder blade. It's the coupling between the two that's giving me headaches.