Autopilot Veer

[Rockjumper]

Had the boat on autopilot at idle speed and opened the engine hatch to check the stuffing box drip.
While the hatch was opening the boat veered about 20 degrees off course. Did the same thing when I tried it again later. Any guesses what's going on? Is this normal?

 

[glider4]

Normal. Boat needs reasonable forward speed for autopilot to work properly. I think the technical term is that it “goes nuts” when forward speed below about 2-3 knots. I don’t think the autopilot likes it when the boat is going backwards either.

 

[Rockjumper]

It's not speed related, the system tracks perfectly until I operate the hatch. Seems to be a hydraulic issue.

 

[Full_O_Fish]

The engine hatch lift is typically powered by the trim tab pump, and if you open it too far with tabs down it will not have enough fluid in the reservoir. Sometimes this can cause a tab to retract if one of the solenoids is getting weak. At slow speed the autopilot will have a hard time catching up with the additional drag on one side from changes in the tab positions.

Another alternative is if your autopilot heading sensor is mounted in the vicinity of the hatch, once opened the magnetic field is changed and causes the pilot to think the vessel turned and attempts to compensate. (This can be checked without being underway, just turn the electronics on and note the vessel "heading", then open the hatch and see if the "heading" has changed. If it changes then proximity of your heading sensor is the problem)