ship, in effect, becomes another magnet. We do not
intend to give a detailed explanation of how this force
affects a magnetic compass, but where deviation exists,
it must be taken into account. Although it remains a
constant amount for each compass heading, it gradually
increases, decreases, increases, and decreases again as
the ship swings through a complete 360° circle.
Deviation must be considered in correcting
compass error, consequently the deviation for any given
heading of a ship must be known. Before the ship puts
to sea, it is swung through the complete circle from 0°
to 360°, and the amount of the compass deviation is
noted at every 15° swing. Compass deviation is
calculated by various methods, generally by comparison
with the gyrocompass, or by reciprocal bearings on a
compass on the beach, which would be unaffected by
the metal in the ship.
The results are compiled into a table called the
deviation table (table 5-2). Every 15° is considered close
enough, and in using the table, you should use the
deviation for the heading nearest the heading you are
checking. In other words, if you look in this table for the
amount of deviation for a 17° heading, you would select
the deviation for 15°, a deviation of 10°W.
When studying the compass, a coxswain should
have the following definitions memorized.
Deviation The error caused by the magnetic
properties of any metal in the immediate vicinity of the
compass. Deviation for a particular compass should be
shown on the deviation table mounted near the
Variation The angular difference between true
north and the direction of Earth's magnetic field.
Variation is marked on charts because it changes from
year to year and from place to place.
Compass error Deviation and variation together
are known as the compass error.
True course The angle between true north and the
path along the ocean floor over which the boat is
traveling, measured from true north in a clockwise
Magnetic courseThe angle, measured clockwise,
from the magnetic meridian to the track. Correcting the
magnetic course for variation gives the true course.
Compass course The reading of a particular boat's
compass when the boat is following a definite track.
Correcting the compass course for deviation gives the
Correcting The process by which both deviation
and variation corrections are applied in converting a
compass course to a true course.
CORRECTING COMPASS COURSE
The course you take from a chart usually is a true
course. You must convert the true course from a
magnetic compass to a compass course. To do this, you
apply the compass error (variation and deviation) to the
true course. Changing a true course to a compass course
is called UNCORRECTING, and changing a magnetic
course to a true course is CORRECTING. You can
Table 5-2.Typical Deviation Table