an irregular permanent vertical magnetization, the ship
and a south pole at the bow. This is a reversal of the
is scheduled to report for deperming.
conditions on northerly headings, when a north pole is
induced at the bow and a south pole at the stern. The
longitudinal component of induced magnetic field also
FREQUENCY OF RANGING
changes, to some extent, as the ship pitches.
The athwartship induced magnetic field changes
All minesweepers and landing craft utilities (LCUs)
when either the magnetic latitude or the heading changes
are required to be checked quarterly by a degaussing
and when the ship rolls or pitches. When a ship is on an
range. All other ships that have a degaussing installation
east heading, a north pole is induced on the port side and
must be checked semiannually. Submarines must be
a south pole on the starboard side (fig. 10-2, view B),
checked annually. Any ship that exceeds check range
which is the athwartship component of induced
limits must undergo calibration ranging or magnetic
magnetization. The magnitude of the athwartship
treatment as soon as possible.
magnetic field depends on the magnitude of the
horizontal component of the earth's magnetic field at
SHIPBOARD DEGAUSSING
that latitude. The horizontal component is maximum at
INSTALLATION
the magnetic equator for a ship on an east-west heading,
and zero at the magnetic poles or for a ship on a
north-south heading.
A shipboard degaussing installation consists of the
following items:
MAGNETIC RANGING
One or more coils of electric cable in specific
locations inside the ship's hull.
A ship is said to be ranged when its magnetic field
A means of controlling the magnitude and
is measured at a magnetic range, commonly called a
polarity of current to these coils and therefore the
degaussing range or degaussing station. A degaussing
magnetic field produced by them.
range or station measures the magnetic field of ships that
pass over measuring equipment located at or near the
The ships degaussing folder.
bottom of the channel in which the ships travel.
A dc power source to energize these coils.
RANGING PROCEDURES
Compass-compensating equipment, consisting
of compensating coils and control boxes, to
Ships are ranged before they are depermed to
compensate for the deviation effect of the
determine the direction and magnitude of their fields.
degaussing coils on the ship's magnetic
Magnetometer garden measurements also are required
during and after the deperming process to evaluate the
Used properly, these items will greatly reduce the
quality and effectiveness of the treatment. The most
magnetic signature of the ship and help to prevent
common ranging procedure, called check ranging, uses
detection by magnetic sensitive instruments.
the coil range. Check ranging usually occurs during a
ship's normal entry into port. After passing over the
range, the ship receives a report of its magnetic
DEGAUSSING COILS
characteristics. If the strength of its magnetic field
exceeds a safe operational level, the ship is scheduled to
The distortion of the earth's field caused by the
report for calibration ranging. Here the ship makes a
ship's permanent magnetic field (vertical, longitudinal,
number of passes over the range while its shipboard
and athwartship components) and the ship's induced
degaussing coils are adjusted and calibrated from
magnetic field (vertical, longitudinal, and athwartship
information supplied from the range hut. When the new
components) is neutralized by degaussing coils. The
settings for the degaussing coils have been determined,
degaussing coils are made with either single-conductor
new degaussing control settings are placed in the
or multiconductor cables. The coils must be energized
degaussing folder.
by direct current. This current is supplied from 120-volt
or 240-volt dc ship's service generators or from
If the ship is unable to compensate adequately for
degaussing power supply equipment installed for the
its magnetic field because of excessive permanent
specific purpose of energizing the degaussing coils.
longitudinal or permanent athwartship magnetization or
10-5
