I think you misunderstand the typical hydraulic auto-pilot for
mid-size boats where the manual steering is not hydraulic. They
typically have a dedicated reversible hydraulic pump driving (as Glenn
points out) a balanced cylinder. The AP electronics turns the electric
motor driving the pump on and off in the required direction.

I was kinda thinking the same thing. Infact, at first, I thought this unit
was a electro-hydraulic unit.

Since I'm not going to drive the hydraulics off the engine I really need to
find a compact efficient 12vdc power unit.

I don't think I need to get into a continious running pump since the boat
has a full keel and maintains it's course very well. 90% of the time, the AP
linear drive is just holding the tiller in position (static load). Something
that a hydrualic cylinder it very good at. The main reason the AH3000 linear
isn't satisfactory, is because in heavy weather, the rudder tiller exerts
enough force against the linear drive to force the motor to turn in reverse.
I'm afraid of what this is doing to the planetary gear reduction.

Thanks to your comments and that of Glenn, I will look farther for a 12vdc
(reversiable) PowerPack.. If anyone knows of something like this, please let
me know.. The pumps that Glenn referenced were just the pumps.. gotta be a
complete unit someplace, like truck power lifts, etc.

Steve
s/v Good Intentions