The set up in photo B would be ideal for adding balance to the trimtab, but the tab would still be running in the wake of the rudder. A balanced pendulum, which is only a trimtab which turns in the same direction as the rudder and adds power rather than subtracting from the rudder power spends most of its time running in clear water. That plus the longer moment arm and yaw damping makes it a more powerful tool. 

The Jan Alkema RHM has the pendulum advantages for just a little more construction complication, and can be arranged to lift clear of the water for close in manoeuvring and when in harbour. As your ideas are developing, would it not be worth having a re-read to the Catalyst Jan Alkema article and check its advantages of power and yaw damping, for the same vane input?

Cheers, Slieve.

My initial thought was "correct". But then I thought "what if you put a 1:2 fool's purchase in the lines?" That would make the drum appear to be 15cm diameter.

It has to be picture B. Picture A makes the tab tiller way too long. With B, the tab tiller can be 20cm long, and the negative feedback will be correct at that. Then, if using a flip-flop vane, the drum would need to be about 14cm diameter; or if using a vertical axis vane, the lever arm of the latch gear would need to be about 20cm.

It's OK to have the tab spaced away from the rudder somewhat. There have been successful steering gears like this, on Folkboat-style rudders with vertical tabs mounted purely on the rudder head.

Bonjour

The trim tab (a tab in fact) acts by creating an artificial camber to the rudder plate that builds a relative depression near the trailing edge of the rudder plate on the side opposite to the tab orientation (the water fillets have a longer course and so run faster). The depression creates a force, that creates a torque that turns the rudder.

So the tab should be as near as possible and parallel to the trailing edge of the rudder plate. It’s orientation to the vertical is not relevant. A narrow and long tab is much more efficient than a wide and short one because the impacted area of the rudder plate is much bigger and the effort (size of the wind vane) to turn it is much less.

Eric

Does the trim tab have to be absolutely vertical?

Because of the transom angle, my new rudder is angled aft some 30 degrees, and even more above the waterline. For a vertical trim tab I would need very long ”arms”.