Sorry no post of circuit. I can't find my old stuff since moving houses. What you can do though is try a pair of rheostats, mount one on the rudder or sail. Mount another on a simple servo. Use a LM339 quad comparator and some logic chips. The comparator reads voltage differences and outputs a logic high or low. Use two comparators for setting the high and low dead zone, reached zone, and the other 2 for the active location. I have done this before and it works great. Use standard H bridge with the setup with any power/ motor setup you want and attach the screw, geared or not. Good luck with the project.