Ships are magnetised through the mechanical stress of being used and being constantly exposed to the Earth's magnetic field, ships and submarines slowly acquire their own magnetism.  Through this magneto-mechanical effect, a magnetisation is  “hammered in” to the steel of the hull and superstructure. In fact, because of the distribution of the major steel components in different vessels, each has its own characteristic magnetic signature. Marine mines can detect this “magnetic signature” and just one mine can sink a ship or submarine.

To do this the crew wraps the ship in thick copper cables which are heavy, slimy, muddy and generally just nasty. You can see the cables hanging vertically on the hull in photo above.  Once the ship is wrapped like a solenoid, then pulses of current - sometimes thousands of amps, are passed through the cable. First one way and then the other, shaking up the magnetic orientation of the steel.  Sort of like making a magnet out of a nail and some wire like we did in grade school science class only on a much larger scale. The deperm process lasted about a year. New, quicker processes are now in use.

A popular misconception is that all marine mines are magnetically attracted to the hull of a vessel. It is true that in the past some mines were detonated through mechanical contact  but most modern mines are more likely to be lying in wait on the seabed. These are triggered by a sensitive device that looks for variations in the ambient magnetic field. Variations such as a distortion of the Earth’s field caused by many thousands of tons of ship steel passing overhead. The situation would be even more dangerous for those onboard if that steel itself was magnetised.

Ships are required to be checked every six months. The ship testing requirements are:

                      (1)           After new construction.
                      (2)           Before and after a major dry-docking availability.
                      (3)           After a major shock to the hull from a nearby explosion.
                      (4)           After grounding or collision.
                      (5)           As feasible, before entering mined waters.
                      (6)           Before issuing a Casualty Corrected of any Casualty Report degaussing equipment.
                      (7)           At every opportunity, when entering or leaving a port with checking facilities.

NEW Mine Countermeasures Ships