I have a
Autohelm self
steering unit (from Scanmar marine) on my 37 foot cat.
One of the nice things about the
Autohelm is that the vane portion is separate from the
rudder. They are attached only with
cables that can be routed via blocks. That way you can put the vane as far as you want from the rudder. They do not even have to line up. I would think that the vane would be better on the boat's center line to avoid
wind shadows on either tack. I would not think that, on a
catamaran, the rudder position would be hurt if off center. It is, of course, better to minimize the number of turns in the
cables to reduce friction.
Another thing I like about the Autohelm is that it is very simple. There is nothing on it that you can't
repair or replace in a pinch with
hardware store stuff.