'Trim tab' self steering

[petery]

\'Trim tab\' self steering

I'm looking for a self steering system for a transom-hung rudder on a 27' boat and am considering a system that uses a vane to move a 'trim tab' that is simply a small pivotted extension to the rudder's trailing edge - ie it is not a paddle that moves the tiller via lines.

While I can understand how the forward motion of the boat provides significant forces on the tiller when a servo pendulum system is used, how does a trim tab work? Is it any different to moving the complete rudder? Have you experience of using this system?

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[qsiv]

Re: \'Trim tab\' self steering

The trim tab works by converting the rudder into what is effectively a cambered aerofoil, thus generating lift (in this case it's acting horizontally at the stern, and will turn the boat). Just like flaps on an aeroplane wing generate extrat lift, except that a rudder is (normally) a symmetrical foil with a 0 degree angle of attack - thus it generates no lift until it's turned.

Yes I have used them - my father always used to make his own, and they work well, but tend to need bigger windvanes than servos. Dont forget the trick of connecting a small tiller pilot to the TRIM TAB 'tiller'. This will vastly reduce the current drain of the autopilot - and still allow you to 'self steer' when motoring. Dont forget to mount it on the opposite side to that which you would conventionally mount it when attaching it to the tiller.

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