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Degaussing

A steel-hulled ship is like a huge floating magnet with a large magnetic field surrounding it. As the ship moves through the water, this field also moves and adds to or subtracts from the Earth's magnetic field. Because of its distortion effects on the Earth’s magnetic field, the ship can act as a trigger device for magnetic sensitive ordnance or devices which are designed to detect these distortions. The degaussing system is installed aboard ship to reduce the ship's effect on the Earth's magnetic field. In order to accomplish this, the change in the Earth's field about the ship's hull is "canceled" by controlling the electric current flowing through degaussing coils wound in specific locations within the hull. This, in turn, reduces the possibility of detection by these magnetic sensitive ordnance or devices.

The ship's permanent magnetization is the source of the ship's permanent magnetic field. The process of building a ship in the Earth's magnetic field develops a certain amount of permanent magnetism in the ship. The ship's induced magnetization depends on the strength of the Earth's magnetic field and on the heading of the ship with respect to the inducing (Earth's) field.

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. For example, some units use silicon-controlled rectifier solid-state power supplies (as in the SSM system). Other systems use motor generator sets (as in the RM-5 type systems), and still others use a mixture of the two (as in the GM-1A units).

The Main coil (M) compensates the induced and permanent vertical components of the ship's magnetic field (Z zone). It is installed in the horizontal plane at the waterline. As the ship changes hemispheres the coil current polarity must be manually adjusted.

The Forecastle permanent - Quarterdeck permanent coils (FP-QP) compensate for the longitudinal permanent component of the ship's magnetic field. The FP coil encircles approximately the forward 1/3 of the ship in the horizontal plane at the main deck. QP coil encircles approximately the after 1/3 of the ship in the horizontal plane at the quarterdeck. Any coil current changes require manual adjustment. The Forecastle induced - Quarterdeck induced coils (FI-QI) are located in the same area as the FP-QP coils, they compensate for the longitudinal induced component of the ship's magnetic field. The FI-QI current is proportional to the horizontal component of the Earth's magnetic field along the ship's longitudinal axis. The FI-QI coil current is manually changed, by shifting the "H zone" switch on the switchboard, when the ship's location changes H zones. The degaussing system automatically compensates for heading changes by converting a gyro input signal to a magnetic heading.

The Athwartship coil (A) is installed in the vertical plane and extends from the keel to the main deck. It compensates the athwartship induced and athwartship's permanent components of the ship's magnetic field. The A coil current consists of permanent and induced components.

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