Hi! I just happened across this forum and throught some of you might be interested in my amature experiences with building a trim-tab self-steering system.
We built the tab back in 2004 and we've used it for about 5,000 miles on it in some nasty stuff. For boats that can accomidate a trim tab design it's amazingly simple and can be a very powerful steering system, especially downwind in big waves.
Anyway, there are two documents on our website. The first is about building and designing the trim tab and the second is dedicated to wind-vanes and autopilots using the trim tab (stability, range, gain, etc.). The second document is much more technical and contains a lot of information.
Check it out and see if it's helpful to you. http://www.sailsarana.com/selfsteering.htm
You can leave feedback on our website if you have questions or comments.