When installing my sailboat’s autopilot a number of years ago, I mounted the fluxgate compass in several locations before it worked properly.
Trying to position the compass close to the boat’s center, I temporarily mounted it under the galley sink using double-faced tape. When I checked it against my accurate cockpit compass, the readings differed significantly.
I tried another location under the galley sink, and this time, the readings differed by only 1 degree. Apparently the first location was too close to my 4,000-pound steel keel. Ever since, the compass has performed extremely well when checked against my GPS.
Recently while under power on a long straight run between buoys, I turned on the autopilot so I could enjoy a quick sandwich. I could see the distant target buoy, but the boat abruptly turned 45 degrees. The autopilot heading and cockpit compass reflected the difference. I rebooted the autopilot with the same results.
While eating, I kept the boat on course by frequently pressing the autopilot’s plus and minus buttons. Afterward, I turned off the erratic autopilot and resumed steering with the wheel.
Later that day, I went below to inspect the area beneath the galley sink where I also keep a few pots and a large can of hand cleaner.
The previous day, a large wake thrown by a passing powerboat must have tossed the pots and hand cleaner near the fluxgate compass. When I removed the metal objects and turned on the autopilot, its reading agreed with the cockpit compass.
Lesson learned: Always keep metal objects away from the autopilot compass.