Thanks for the help. I now see that Raymarine
has both mechanical and hydraulic linear drives, so that's why I was confused.
This linear drive is the last original component of an old Autohelm
autopilot that who knows when it was originally installed. Probably mid-90's??
In turning the wheel while the autopilot was engaged, I was not applying a lot of force - only enough to determine that the drive was no longer capable of holding the rudder
straight. The boat
likes to turn left under power, and just the prop torque was enough that the boat
would not hold a course.
I guess when the autopilot computer figured out something was wrong, it disengaged the drive? It was certainly trying to steer up until the time it disengaged. Based just on that, I'm guessing it's a gear
problem more than an electrical