Magnetic compass should be ideally placed in a place far away from magnetic material or material that shows magnetic properties. This is to make sure that only the earth’s magnetic field affects the compass. This has been a major concern in modern ship manufacturing companies as ships these days are mainly built using steel. Steel has permanent, sub permanent & induced magnetism which can disturb the earth’s magnetic lines of forces, thus causing an error in the compass. These errors are not fixed, thus if not adjusted/corrected, the compass headings and bearings cannot be relied upon. Ships are made of steel. Steel is a ferromagnetic material. They are magnetized during the construction of a ship. Magnetic properties induced on steel can be permanent or temporary, and mostly they are permanent due to energy imparted during cutting and welding. Magnetism can also be induced by earth’s magnetic lines of forces and also when placed near a permanent magnet.
Effects of permanent magnets onboard the ship can be combined into one single force called permanent magnetic force. Permanent magnetism exist through the life of a ship. The force doesn’t change with heading or at poles. Its force does change very slowly over time. This change is also known as sub-permanent magnetism and corrections to this will be carried out together with permanent magnetism. Sub-permanent magnetism is the process of changing the force as a result of cruising, vibration, or ageing.This magnetism, at any instant, is part of the ship’s permanent magnetism, and consequently must be corrected by permanent magnet correctors. Induced magnetism on ship changes depending on ships heading, the strength of earth lines of forces, magnetic latitude etc. This magnetism isn’t constant as it depends on ferromagnetic properties.