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julesmoto said..A labor-intensive boat related but at home project might be just the ticket for this Lockdown. Don't suppose you or anyone else has a link which has fairly detailed drawings or video of a hydrovane style unit (independent of existing steering) such as is described in the attached drawing from another forum?

Don't bother with any other self steering gear than a servo assist self steering gear. Even on a light weight trailer sailer. The hydrovane is not servo assisted and uses a rudder to steer the yacht with the boats rudder fixed. If you have a lathe or a pensioner friend with one then go straight to a design with bevel gears, this gives automatic feedback and there is no zig zagging. If you have limited tools and funds then search out one of the Walt Murray designs. These are servo vanes but use a Z crank instead of the bevel gears. The early designs like the 20-20 could be built for $20. That's $20 for the plastic pipes and the rest is supposed to be stuff laying about in your workshop. My first one I built from plastic pipe and stainless that was just laying about the workshop.
The Z crank can be seen here and it slides though that grey plastic block. The oar rotates vertically on gudgeon and pintles. It rotates horizontally on the shaft that is part of the Z crank on plastic blocks. This style steers well but there is a lot of fiddling with the Zcrank and the plastic block to get good feedback. If you can, go straight to a bevel gear design. The Hydrovane uses a Z crank too hidden in that cetre section with a lever for 3 settings.
Walt Murray passed away years ago but you can find some of his drawings on this site.
windvaneselfsteering.com/You will have to hunt about for them though. There are also drawings for Jan Alkema Up side down vane there as well. Mister Vee uses his design. Jan has a good design too that acts directly on a stern hung rudder.
This is another shot of this earlier vane and the Z crank is easier to see.
The first shot is the vane when I took it off my 26 foot quarter tonner. The second one is with an alteration to the leverage ratio and a longer oar blade.