Many hydraulic steering
systems suffer from "creep"... that is the wheel
position that results in the rudder
being straight ahead slowly changes. This confounds the typical wind vane
. I have been told that well designed hydraulic systems do not suffer from this problem, but I have no personal experience of this.
Any auxiliary rudder vane system will circumvent that problem, and there are others besides Hydrovane, including AutoHelm
(from Scanmar) and Wind
Pilot. Likely are others as well. Further, the very simple Sayes Rig (also sold by Scanmar IIRC) which is a servo that drives the rudder directly would work, but they only are applicable to rudders that are very close to or hung from the transom.
Cheers and good luck with your choice... a good vane is priceless!