I did a rough cardboard mock-up of an idea. None of the dimensions are exactly right; it's just to illustrate the principle.
The trim tab is made up of two sections. The lower section is connected to the back of the rudder blade using gudgeons and pintles and to the upper section using a U-bracket and a through-bolt. The upper section's upper end slides through a slot hinged to the back of the transom-hung rudder body. Its middle section is V-shaped and mates with a fork at the end of the trim tab tiller, but only when the rudder blade is down. As the rudder blade comes up, the upper section of the trim tab backs away from the fork. Note that when the blade is down, the axes of the trim tab's top section slider pivot and the bottom pivot are aligned, allowing the trim tab to turn, but as it comes up, the axes are no longer aligned, constraining the trim tab to the same plane as the rudder.
Omitted from the model is the windvane linkage. This would be a bell crank permanently connected to the wind vane tiller about 20% back of the rudder pivot point (adjustable to get the right balance of responsiveness and damping), as shown here:
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Here are photos of the model in operation.
This design achieves all of my objectives: make the trim tab integral to the rudder blade; disengage automatically when the blade kicks up; stay in the plane of the rudder when kicking up; and, be really really simple to build.
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