Where candidates fail
The commonest weakness is conflating variation and deviation — using the terms interchangeably or being unable to explain the physical cause of each. A second failure is giving a vague, textbook answer on deviation: candidates say "local magnetic influences" without being able to say what those influences are or what the master's obligations around them are. Examiners also catch candidates who cannot explain the practical consequences of siting errors.
Variation
Variation is the angular difference between True North and Magnetic North at any given geographical position. Its cause is the separation between the geographic and magnetic poles. It is entirely independent of the vessel. Value, named East or West, is taken from the chart for the area; it changes slowly over time (secular change) and is annotated on the compass rose with an annual rate of change. Apply the current year's correction to any charted value.
Deviation
Deviation is the angular difference between Magnetic North and the direction the compass needle actually points aboard that vessel. Its cause is the vessel's own magnetic field — hard iron (permanent magnetism from the hull and fittings), soft iron (induced magnetism from ferrous structure), electrical equipment, and cargo. Deviation varies with the vessel's heading. It is vessel-specific and heading-specific.
The master's obligation is to ensure a valid deviation card or curve is available and used. Deviation must be checked regularly — by comparison with a known bearing (transit, celestial azimuth, GPS COG on a steady heading in calm water) and by swinging the vessel through 360°. If deviation on any heading is found to have increased materially, the compass adjuster must be called. A deviation card that is out of date or derived from an unswung vessel is worthless.
Siting of equipment
The standard magnetic compass must be sited as near the vessel's centreline and as high as practicable, to maximise distance from magnetic disturbances. The following must be kept at a minimum safe distance (verified by the compass adjuster and recorded in the deviation card):
- Electric motors, pumps, generators
- Loudspeakers and switchgear
- Steel structure and fittings
- Electronic equipment (radars, chartplotters, radios)
Moveable items — VHF handsets, tools, steel coffee cups — left near the binnacle are a common practical cause of increased deviation. The master is responsible for ensuring crew understand this.
Fluxgate and digital compasses feed autopilots and electronic chart systems. Their sensors have the same susceptibility to magnetic interference as a magnetic compass bowl; siting rules are equally critical. A fluxgate sensor sited near a bow thruster motor will give corrupted heading data to every system it feeds.
The command-level point
At Master level the examiner expects you to own this: you are responsible for the accuracy of the compass, not just the compass adjuster. If you cannot verify the deviation card is current, you do not have a reliable heading reference.