On a long passage the autopilot is the hardest-working member of the crew. Its failures usually start not in the electronics but at the mechanical link to the rudder, in its power supply, or in a poor calibration.
Parts of the system
- Drive unit
- Linear, hydraulic or wheel type. It must be sized to the boat and rudder load.
- Mechanical link
- Attachment to rudder stock or quadrant; reinforcement and play at the mounting point.
- Rudder angle sensor
- Reports the rudder's actual position; a loose sensor gives an unstable course.
- Compass and heading sensor
- Positioned away from magnetic interference, and calibrated.
- Course computer and network
- Data connection to the navigation instruments; up-to-date software.
- Power
- Cable size and fuse; the drive draws high current under load.
Testing on sea trial
- Holding course. In calm water and in waves; oscillation and correction rate.
- Turns. Course changes on command.
- Wind and track modes. On a sailing yacht, steering to the wind.
- Current draw. The drive unit's draw, with the clamp meter.
- Disengaging. Handing back to manual steering quickly and safely.
How each part is tested
An autopilot has several parts: the control head, the compass or heading sensor giving direction, the drive unit that applies force to the steering, and usually a rudder angle sensor. A problem in any one affects the reliability of the whole. On survey each part is tried separately and the system as a whole.
The drive unit
The drive unit takes heavy loads where it attaches to the steering. The strength of the attachment, its reinforcement and the drive's sound are examined. On hydraulic drives, leaks and fluid level are checked; on mechanical drives, play and wear.
Heading sensor
The heading sensor must sit away from metal and cables that affect the magnetic field. A speaker, battery or cable added nearby later can make the pilot wander. Calibration follows the maker's steps.
On the sea trial
The pilot is engaged at different speeds and sea states; how well it holds course, its behaviour in waves and its disengagement are watched. Being able to return quickly to hand steering matters for safety.
Power
On long passages the autopilot can be one of the heaviest power users aboard; its consumption goes into the energy plan.
Backup
On boats making long passages, spares for the pilot's critical parts or a second system are considered; a pilot failure means a tiring passage for a small crew.
Records
Pilot model, installation date and calibration notes go into the boat's file; they save time when a fault needs tracing.
For the owner
Even with the pilot engaged, someone on watch is good seamanship; the pilot holds the course but does not look out. Its attachment bolts and reinforcement are checked at each haul-out along with the rudder and bearings, since a failure there takes both pilot and steering out at once.
Pilot settings are reviewed as loading and sea state change. When new equipment is added, or metal gear fitted near the heading sensor, calibration is checked again; many cases of a wandering pilot trace back to exactly such changes, made months earlier and forgotten.
Logging changes with dates in the boat's file means the cause of a wander is found quickly, and needless part replacements are avoided.
A tidy file is the first place to look when anything misbehaves at sea.
Calibration is quick.