There are a few books
that describe in detail how to build a windvane
self steering system.a good one is "Ocean Voyaging" by David M Parker,Bernard Moitessier also explains the fundamentals in his book"The Long Way".I think Hiscock also has some info.I have built them before,they don't look as pretty as a Monitor
,but work just as well,and are easier to repair.All you need is S.S. pipe for shaft with straps welded on, an inner rod for trim tab,a few fittings,some wood for vane,a counterweight. Rudder
and trim tab are antifouled wood, same size as main rudder
and a trim tab 1/4 the area of rudder,on pintle and gudgeon attached to rudder.Vane should be about same area as rudder.To lock vane into set position,a pulley wheel
,fan belt and highfield lever.Maybe $500. in total.