FWIW, Insatiable I is an IOR Mk2 designed by Franz Maas and built by Palmer Johnson. Typical old IOR shape -- big max beam, pinched ends and tumblehome galore. We did a bit of racing
in her before going cruising and she was a handful downwind with a kite up.
So, when cruise
time came around, I built her a windvane
loosely based on the Autohelm
(vane, not pilot) principal.This is a nearly balanced auxillary rudder with a trim tab. The tab is driven by a sloped axis air blade which runs push-pull cables
. After some teething problems, it worked a treat! The big thing was that the additional rudder area way aft seemed to make her way less squirrely downwind, even when hand steering
with the vane locked off. It drove the boat all the time we were at sea for the 17 years we cruised in her -- approximately 75K miles all told.
As to the mounting problem -- the sloped transom will be an issue no matter what vane you select. The one disadvantage of the aux rudder vane for you is that the mounting must be stronger than that for a servo pendulum vane, since its rudder is actually steering
in Sausalito still manufactures the Autohelm vane, and by now has very likely already designed a suitable mounting for your boat. They are pretty friendly chaps, and would be worth contacting. Incidentally, this type of vane is easily adapted to remote
adjustment (as are many others for that matter).
One other option is a German outfit called WindPilot. They make both aux rudder and servo type vanes, and their mounting is ideally suited to sloped bums. You might have a look at their website (don't have a specific link handy).
Hope that this helps, mate, and if there are other questions, fire away!