Do Iron Ships Pose Particular Problems For Magnetic Compasses?

The magnetic field of the iron body of the ship itself affects the reading on the compass.

When iron and steel ships became common, many scientists studied the problem. One of the earliest was the Astronomer Royal, Sir G.B. Airy, who in 1838 used the iron steamer Rainbow for his experiments. Airy thought of a method of neutralizing a ship's magnetism by placing magnets and pieces of unmagnetized iron near the compass.