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Deviation

The difference between a vessel’s actual compass heading and the magnetic heading indicated by the compass.

Deviation is caused by the magnetic fields created by the vessel’s metal components and electronic equipment, which can affect the accuracy of the compass reading. Deviation can be corrected by applying deviation correction values to the compass heading to obtain the correct magnetic heading. These corrections are found on a Deviation Card.

What causes deviation?

Any ferrous metal, such as the engine, electronics, navigation equipment, speakers and mobile phones, has a magnetic field that will disrupt the compass, causing it to deviate from pointing at magnetic north.

Influenced by direction & heal

It’s made more complicated as the impact of deviation changes depending on the vessel’s heading, the location of the compass and, to some extent, heal.

Permanent influences

Many permanent fixtures on a boat have a magnetic field that can impact the compass, including:

  • 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.