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Compass Deviation

The angular error of a magnetic compass caused by magnetic influences aboard the vessel.

Deviation is the difference between magnetic heading and compass heading and is named east or west. It normally changes with heading because permanent and induced magnetism in the hull, machinery, wiring and equipment affects the compass differently as the vessel turns.

A compass swing is used to determine and, where possible, correct deviation, with residual values recorded on a deviation card. Deviation is distinct from magnetic variation, which is the angle between true and magnetic north at a place and time.

What causes deviation?

Influenced by direction & heal

Permanent influences

  • Any ferrous metals (ferromagnetic), such as iron or steel, for example, the engine or the keel, will impact the boat’s permanent magnetic field.
  • Anything with an electric current will create a magnetic field (electromagnetic), so all the onboard electronics, including the chartplotter, will impact the boat’s permanent magnetic field.
  • Other fixtures, for example, loudspeakers – especially in the cockpit near the steering compass – may impact the boat’s permanent magnetic field.
  • The boat’s batteries have a magnetic field that will impact the compass, especially if physically located nearby.