Get yourself a copy of "Self-Steering for Sailing Craft" by John S Letcher jnr. Now out of print but a search through Amazon will probably find you a copy. That's how I got mine.
It's a few years old but the principles haven't changed any.
The first question is where is your rudder and can it be fitted with a trim tab!?
If so you can consider a half way solution and buy an air vane and wires from Scanmar.
Otherwise consider making your own auxilliary rudder with trim tab.
Are you contemplating long offshore passages?
Also check "Yacht Wind Vane Steering" by B Belcher, David and Charles London, 1982. It gives a good range of different gear for the DIYer from trim tabs through to servo pendulums. The Amateur Yacht Research Society also has some good stuff. I thought long and hard about building one and then bought a Monitor instead. /forums/images/graemlins/smile.gif
You could look at the Dutch DIY stainless steel windvane Bouvaan. You can order the parts and make it yourself (will take some time!) I installed one in the winter and it works quite good (on a 32 footer). See http://www.bouvaan.nl/index.htm The site is in Dutch but you can mail the builder in English.
Chris
Thanks to all for your very constructive replies. I have searched some & getting plenty of material so that should keep me quiet for a while. Passage is hopefully round UK next year, either before or after the JAC, if I can persuade SWMBO to let me go of course!
Boat is a long keel Bowman 26, similar to a slightly beefed up Folkboat, so raked transom will make trim tab a "challenge" as they say on management courses. Was looking at servo pendulums & reckon there is enough data about to make one, although with increasing years & decreasing eyesight, maybe not! We will have a go.
Thanks again guys, much appreciated.
Mike
It is / was. Designer/seller (Sven Heesterman) is a very nice and very helpful chap. I can highly recommend him and his system if this is the way you want to go (says he who hasnt tested his kit !). I am sure it will work very well. The jury is still out however regarding "atlantic" durability.