The autopilot disengages or zigzags

An autopilot is three things that have to work together: a sensor that knows where the boat is heading (the electronic compass), a computer that decides on the correction, and a mechanism that pushes the helm (ram, arm or motor on the steering system). When the boat zigzags or the pilot gives up with an alarm, the fault can come from any of the three, but also quite simply from a boat that is badly balanced under sail. This guide helps you look in the right order. Your autopilot manufacturer’s manual always has the last word, especially for calibration and settings.

1. What you notice

• The boat zigzags either side of the course, in regular S-bends.
• The pilot disengages with an alarm (“off course”, “no sensor”, “low voltage”).
• It holds well under engine in a calm sea but lets go downwind or in gusts.
• The displayed heading drifts while the steering compass stays steady, or differs a lot from it.
• A knock, a jerk or a new noise with each helm correction.

2. Safety first

An autopilot is not a lookout. As soon as it shows signs of weakness, take the helm back by hand and keep someone on watch: a pilot that disengages near an obstacle, another boat or a coast does not always warn you early enough. Know how to disengage the pilot in a single movement, without thinking, and train the crew to do it. Never put your hands near the ram, the steering quadrant or the cables while the pilot is powered: the mechanism can move suddenly and with great force. In heavy weather, at night or close to land, do not look for the fault under way: steer by hand, and diagnose in port.

3. Likely causes, from the most common to the rarest

• A disturbed compass — the electronic compass is sensitive to anything metallic or magnetic nearby: tools, tins, a phone, a loudspeaker, an electric cable running close to it. An object stowed there during the week is enough to throw the heading off.
• A badly balanced boat — too much sail, mainsail sheeted in too hard, a boat with weather helm that wants to round up: the pilot spends its time fighting, uses a lot of power and ends up disengaging. This is not a fault, it is a matter of sail trim.
• Unsuitable gain settings — the “response” or “gain” sets how vigorously it corrects: too high, the boat zigzags; too low, it rounds up before the pilot reacts. The right settings change with the sea state.
• Missing or out-of-date calibration — after installation, a change of equipment near the compass or a software update, the pilot must be recalibrated (compass swing, rudder direction, end stops) according to its manual.
• A voltage drop — low batteries, cable too thin, corroded connection: the ram lacks strength just when it needs it most, and the computer resets or shows an alarm. See batteries that run down.
• The ram or the arm — a mounting that moves, a worn ball joint, a slipping clutch, a tired motor; on a tiller pilot, a worn pin or socket causes play.
• Play in the steering system — slack cables, a quadrant or chain that slips, a rudder stock worn in its bearings: the pilot corrects, but the helm does not follow straight away.
• The rudder angle sensor — a loose or misaligned linkage makes the pilot believe the rudder is somewhere else: excessive corrections, a boat that turns to one side.

4. What you can check yourself, safely

• Around the compass — find where it is installed (often under a floorboard or in a low locker, in the middle of the boat) and remove any metal object, phone, speaker or tool stowed within about a metre.
• Comparing headings — under engine, in a calm sea, in a straight line: compare the pilot’s heading with the steering compass on several different headings. A difference that changes with the heading suggests a disturbed or badly calibrated compass.
• Balance under sail — reduce sail, ease the mainsail a little, and see whether the pilot calms down. If hand steering needs constant effort, the pilot feels it too.
• The settings — note the current gain, response and sea-state settings, then try, following the manual, the ones intended for the conditions of the moment. If you lose track, returning to the factory settings is done according to the manual.
• The voltage — with a multimeter, where the pilot’s power cable arrives if it can be reached without dismantling anything, with the pilot working: a voltage that collapses during corrections points to a cable, a connection or batteries that are too weak.
• The ram and its mounting — pilot switched off: by hand, look for play in the fixings, the ball joint, the bracket; see whether screws have moved or rub marks have appeared.
• Play in the steering — pilot disengaged and switched off: move the helm, or have it moved while someone watches the rudder or the quadrant. Any clear dead time between the helm and the rudder should be reported.
• The rudder angle sensor — by eye, pilot switched off: linkage firmly fixed at both ends, ball joints without play, no visible damage.

5. When to stop and call a professional

Call a marine electronics technician if the displayed heading stays wrong despite a clear compass, if calibration fails or has to be redone often, if the computer resets or shows error codes, if the ram gets hot, knocks or slips, or if you find play in the steering system or the rudder stock — this last point is also a job for a boatyard, because it is the steering itself that is at stake. Before a long passage, a full check of the pilot and its mechanism is a reasonable precaution: see preparing a transatlantic crossing.

6. Stopping it from happening again

Keep a space free of any metal around the compass, and tell the crew: no phone or tools stowed there. Every season, check the ram mounting, the tension of the steering cables and the rudder angle sensor; grease according to the manual. Redo the calibration after any change to the installation near the compass. Trim the sails so that the boat is almost neutral on the helm: the pilot uses less power and holds better. And check the battery bank regularly, since it is what keeps the pilot alive.

Record in the Hullteam log every calibration, every changed setting and every job on the pilot or the steering, with the date and the invoice: it is this history that will make the boat’s value at resale — see which proofs count.