Well, the designer of the wind vane is me - I don't have the cash to shell out for a Monitor or an Aries. Plus, I am replacing the rudder blade (core rot) so why not add a trim tab while I am at it? There are two reasons to add the trim tab: one is that I want a wind vane, for going against the wind for long periods of time without having to man the helm; two is that I have a Simrad tillerpilot, which is cheap enough for me, but too wimpy in heavy seas, so I want to link it to the trim tab instead of the rudder itself.
The pivot of the trim tab will be back of the pivot of the rudder itself. Trim tab blade will be about 20% of the area of the blade. I want a linkage that will disengage when the rudder kicks up and reengage when it goes back down. Since the swing of the trim tab is only +/- 20 degrees or so, I am think of putting a horizontal steering plate right above the pivot point of the rudder blade with a v-slot in it, and having the trim tab have an interesting-shaped protrusion at its top that sits in the v-slot of the wishbone plate when the rudder blade is all the way down. When the blade kicks up, the protrusion should gently slot itself into the bottom of the rudder body, which consists of two vertical boards that hold the rudder blade, holding the trim tab in the same plane as the rudder blade instead of flopping side to side and causing (even more) erratic steering. (Steering gets quite interesting when the blade kicks up; as in, get your heel on the tiller because your arms are too small.)
The designer is working with a later version of the same transom-hung rudder design (a better one: more balanced) so we'll probably end up with slightly different designs.