The gyrocompass may sometimes wander. This could be caused by faults in the system or by fluctuation/interruption in the electric supply. The ship would then follow the erratic course of the gyrocompass, undetected by the autopilot or the course recorder, both of which are solely dependent on the gyrocompass. To call attention to gyro wander, a 'Magnetic Compass Off Course Alarm' is included, on many ships, as part of the autopilot. This unit usually has two input control knobs:
Course input control: This is a rotary dial, marked in degrees, to feed in the course as per the magnetic compass
Tolerance control: This is a rotary dial to feed in the limiting number of degrees of yaw within which the alarms would NOT be activated. Setting this to zero would shut off this entire unit.
Once the ship has settled on a gyro course, the observed reading of the magnetic compass and reasonable value of tolerance is fed in. Thereafter, if the compass course differs from the pre-set course by more than the value of tolerance fed in, audible and visible alarms would be activated.
At least once in a watch, this alarm should be tested.
This is done as follows:
Suppose the course by magnetic compass is 045° and the tolerance is set at 8°. Turning the Course Input Control to less than 037° or more than 053° should cause visible and audible alarms to be activated. Resetting this input to any value between 037º and 053° would de-activate the alarm.