A sea-going metal-hulled ship or submarine, by its very nature, develops a magnetic signature as it travels, due to a magneto-mechanical interaction with Earth's magnetic field. It also picks up the magnetic orientation of the earth's magnetic field where it is built. This signature can be exploited by magnetic mines or facilitate the detection of a submarine by ships or aircraft with magnetic anomaly detection (MAD) equipment. Navies use the deperming procedure, in conjunction with degaussing, as a countermeasure against this.
Specialized deperming facilities, such as the United States Navy's Lambert's Point Deperming Station at Naval Base Kitsap, or Pacific Fleet Submarine Drive-In Magnetic Silencing Facility (MSF) at Joint Base Pearl Harbor-Hickam, are used to perform the procedure. During a closed-wrap magnetic treatment, heavy-gauge copper cables encircle the hull and superstructure of the vessel, and high electrical currents (up to 4000 amperes) are pulsed through the cables. This has the effect of "resetting" the ship's magnetic signature to the ambient level after flashing its hull with electricity. It is also possible to assign a specific signature that is best suited to the particular area of the world in which the ship will operate. In drive-in magnetic silencing facilities, all cables are either hung above, below and on the sides, or concealed within the structural elements of facilities. Deperming is "permanent". It is only done once unless major repairs or structural modifications are done to the ship.
During World War II, the United States Navy commissioned a specialized class of degaussing ships that were capable of performing this function. One of them, USS Deperm (ADG-10), was named after the procedure.
With the introduction of iron ships, the negative effect of the metal hull on steering compasses was noted. It was also observed that lightning strikes had a significant effect on compass deviation, identified in some extreme cases as being caused by the reversal of the ship's magnetic signature. In 1866, Evan Hopkins of London registered a patent for a process "to depolarise iron vessels and leave them thenceforth free from any compass-disturbing influence whatever". The technique was described as follows: "For this purpose he employed a number of Grove's batteries and electromagnets. The latter were to be passed along the plates till the desired end had been obtained... the process must not be overdone for fear of re-polarising in the opposite direction." The invention was, however, reported to be "incapable of being carried to a successful issue", and "quickly died a natural death".
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- Payne, Craig M. (2006). Principles of Naval Weapon Systems. Naval Institute Press. p. 208. ISBN 978-1-59114-658-2. Retrieved January 3, 2011.
- Holmes, John J. (2008). Reduction of a Ship's Magnetic Field Signatures - Volume 23 of Synthesis lectures on computational electromagnetics. Morgan & Claypool Publishers. p. 19. ISBN 978-1-59829-248-0. Retrieved January 3, 2011.
- Lecky, Commander S.T.S., Wrinkles in Practical Navigation, first published 1881, 19th Edition, George Philip & Son Ltd., London, 1917, p. 36