Why does my boat autopilot keep losing its course?
I have a Raymarine autopilot on my 35-foot sailing yacht, and recently it keeps wandering off course. It holds fine for a few minutes, then suddenly veers 20 degrees or drops out completely. What should I be looking at to fix this?
Answer
An autopilot losing its course stems from a voltage drop at the course computer, a failing rudder reference transducer or magnetic interference near the compass. Start by testing the battery voltage under load and inspecting the mechanical linkages at the rudder quadrant before buying new electronics.
Power and Voltage Drops
Autopilot drive units consume heavy current. A Raymarine Type 2 linear drive or a Simrad hydraulic pump draws 15 to 20 amps when working hard in a following sea. If the voltage at the course computer drops below 11.5V, the processor reboots, drops to standby or loses its heading.
Measure the DC voltage directly at the power input terminals of the course computer (such as the Raymarine ACU-200 or B&G NAC-3) while the drive moves the rudder under load. A voltage drop exceeding 3% compared to the battery posts indicates a wiring fault. Check for corroded spade terminals, loose ground busbar connections or an undersized supply cable. A 10-metre round trip for a 15-amp drive requires at least 10mm² tinned marine cable to prevent voltage sag.
Rudder Reference Unit Faults
The rudder reference unit tells the autopilot computer exactly where the rudder points. This small black stick with a plastic linkage arm connects to the rudder quadrant. Inside sits a simple potentiometer. After five to ten years of constant movement, the internal tracks wear out.
The unit then sends erratic resistance values to the computer. The boat will suddenly veer 20 degrees off course or snake continuously. Test the unit with these steps:
- Disconnect the rudder reference cables from the course computer.
- Set a multimeter to measure resistance in ohms.
- Connect the meter probes to the signal wires (the red and green wires on a Raymarine M81105).
- Turn the steering wheel slowly from lock to lock.
The resistance must change smoothly. This ranges between 2,000 and 5,000 ohms depending on the brand. If the reading jumps wildly or drops to zero, the unit requires replacement at a cost of around £250. Check the small plastic ball joints on the linkage rod too. Just 2mm of mechanical play causes the autopilot to hunt and wander.