Submarine Onboard Degaussing: HMS Gotland
Recent commentary on another blog revealed some submariners were apparently unaware of onboard degaussing technology. I had only been reminded of it while writing this, so there was a chance others might benefit from an update, too:
Background from Naval Historical Center
Degaussing is a process in which a system of electrical cables are installed around the circumference of ship's hull, running from bow to stern on both sides. A measured electrical current is passed through these cables to cancel out the ship's magnetic field. Degaussing equipment was installed in the hull of Navy ships and could be turned on whenever the ship was in waters that might contain magnetic mines, usually shallow waters in combat areas. It could be said that degaussing, correctly done, makes a ship "invisible" to the sensors of magnetic mines.
History from Wikipedia:
The original method of degaussing was wiping, which simply dragged a large electrical cable along the side of the ship with about 2000 amps flowing through it. This induced the proper field into the ship in the form of a slight bias. It was originally thought that the pounding of the sea and the ship's engines would slowly randomize this field, but in testing this was found not to be a real problem. A more serious problem was later realized: as a ship travels through the Earth's magnetic field it will slowly pick up that field, counteracting the effects of the degaussing. From then on captains were instructed to change direction as often as possible to avoid this problem. Nevertheless the bias did wear off eventually, and ships had to be degaussed on a schedule.
A more permanent solution was found in the form of electromagnets built into the ships in the form of a coil, known simply as coiling. In addition to being able continually to bias the ship, coiling also allowed the bias field to be reversed in the southern hemisphere, where the mines were set to detect "S-pole down" fields. British ships, notably cruisers and battleships, were well protected by about 1943. Smaller ships continued to use wiping through the war. (emphasis added)
There are several different types of degaussing systems in the fleet today. The basic differences between them can be characterized as to what type of power supply they use, and what type of control circuitry they use.
Yet, most of us Cold War submariners went through secretive degaussing ranges at some point(s).
Open source documents now reveal that an AIP submarine, HMS Gotland, utilizes onboard degaussing technology that is 95% effective. Moreover, the technology is being used routinely not only for submarines but for surface combatants at least since the Gotland's installation, perhaps earlier.