What is a Magnetic Compass, its principle, types and errors?

1. Carrying cargoes which have magnetic properties;
2. Using electromagnetic lifting appliances to load or discharge;
3. A casualty in which the ship has been subject to severe contact or electrical charges; or,
4. The ship has been laid up or has been lying idle – even a short period of idleness can lead to serious deviations, especially for small vessels.

Further to (b), the retentive magnetism can alter a ship’s magnetism, making compasses unreliable. However, a large amount of the magnetism induced by an electromagnet may subsequently decay so immediate readjustment is not advised. Every effort should be made to determine the compass deviation.