I've got the WindPilot Pacific Plus on my Pearson
35. It's a pendulum servo vane driving an auxillary rudder which is what I think you are talking about. WINDPILOT - Products: Pacific Plus
It worked fine for me on a recent dead downwind sail to Hawaii
. The trades were a little light and rairly saw 8k relative wind
and often 5k or slightly less. Ran wing and wing DDW for 12 days with the vane steering
all the way. Days runs were from 140-155nm.
I made a lightweight 8"x4' corrugated plastic vane to make the vane more sensitive. Used it most of the way. Changed to the larger plastic vane when the stock plywood
vane was allowing the boat to yaw too much.