The Earth’s magnetic field is the cause of the ship’s permanent magnetic field. The process of constructing a ship in the Earth’s magnetic field generates an appreciable amount of permanent-magnetism in the floating metal structure.
Whenever there is a movement of the floating metal structure, the magnetic lines of force also move with it. This movement of magnetic lines results in the addition or subtraction from magnetic-field of the Earth. And thus a floating vessel under momentum creates its own unique magnetic-signature. This magnetic-signature may be used for triggering a magnetic susceptible equipments. The same principle is used in counteracting the enemy vessels in warfare by a magnetic mine which are being especially manufactured for detecting the magnetic-signatures. To know about the history of warfare in which it had been used extensively, visit http://electricalfundablog.com/blog/degaussing-coils/720/
Bigger ships contains Direct current (D/C) coils system at many places in the ship’s structure to reduce the effect of ship’s permanent magnetic field. It generates a magnetic field that is same and contradictory with the permanent magnetism of the ship. This process is commonly termed as Degaussing method. Though this degaussing process neutralises the permanent magnetism of the ship to a great level, Yet there is still some magnetism left in the metal structure. It is therefore, if in case further de-magnetisation is required, then the floating structure is required to carry out a special form of degaussing, known as Deperming. This Deperming is carried out in a specially designed area termed as Deperming-bay.
In this method, huge coils are placed at some particular locations of the floating vessel. Then a high value Direct current is passed to create an equal and opposite magnetic field so that the residual permanent magnetisation of the floating vessel can be nullified. The same procedure is continued repeatedly so that the floating vessel’s permanent magnetisation can be gradually decreased to a negligible amount. This procedure provides magnetic invisibility (approx) to the floating vessel. After that, the floating vessel can almost cross a mine or any magnetism sensing instrument without being sensed.
The amount of the permanent magnetization on a floating vessel relates on the following:
Magnetic lines of forces due to Earth around the ship building area.
Type of metal used for construction of the floating structure.
Type of method used for construction of the floating structure.
The layout and direction of the floating structure w.r.t. the magnetic lines of forces of the Earth during its construction.
The permanent magnetism of a ship can further be divided in three parts. They are :
- Vertical permanent field component.
- Athwartship permanent field component.
- Longitudinal permanent field component.
These vertical, athwartship, and longitudinal magnetic field components (permanent in nature) are almost consistent. They change very slowly w.r.t. the passing time.