CHAPTER 9
NAVIGATIONAL DUTIES
Navigation is the art or science of determining the
position of a ship or aircraft and directing that ship or
aircraft from one position to another. It can be
regarded as an art because its application involves the
exercise of special skills and fine techniques, which
can be perfected only by experience and careful
practice. On the other hand, navigation can be
regarded as a science inasmuch as it is knowledge
dealing with a body of facts and truths systematically
arranged and showing the operation of general laws.
Navigation has been practiced for thousands of years;
however, modern methods date from the 18th century
invention of the chronometer, a precision timepiece.
As a Signalman, you may be required to assist the
navigator by taking bearings, using the bearing circle
or alidade. You may assist the officer of the deck
(OOD) by sounding whistle signals and by being alert
to aids to navigation.
To better prepare you for your navigational duties,
this chapter contains information on navigational
equipment, aids to navigation, and Rules of the Road.
NAVIGATIONAL EQUIPMENT
LEARNING OBJECTIVES: List and explain
the use of navigational equipment that you will
come in contact with as a Signalman.
The equipment described in this section is the
equipment most likely to be used by you in performing
navigational duties.
COMPASSES
There are two types of compasses in general
shipboard use: the magnetic compass, which depends
on Earth's magnetic field for its directive force, and
the gyrocompass, which operates on the gyroscopic
principle of the spinning wheel.
When you studied as a seaman, you learned that
the magnetic compass points to the magnetic rather
than the true North Pole, and that the magnetic pole is
located some distance away from the true pole. You
also discovered how the Navy standard compass is
made, and how its needle is deflected by magnetic
materials either in a ship itself or by magnetic
materials brought near the compass.
The gyrocompass, on the other hand, points to true
north by operation of the gyroscopic principle. It may,
however, have a slight mechanical error of a degree or
two, which is known and for which due allowance is
made.
Magnetic Compass
The ship’s magnetic compasses are named or
classed according to their use.
The standard compass is the magnetic compass
used by the navigator as a standard for checking other
compasses on the ship. It is so located that it is least
affected by the internal magnetism of the ship.
Courses or bearings given from it are designated per
standard compass (PSC).
The steering compass is located near the
helmsman. Along with the gyro repeater, it is the
compass by which the ship is steered. Courses or
bearings given from it are designated per steering
compass (PSTCO).
Gyrocompass
The gyrocompass is not affected by variation and
deviation. Headings or bearings from it are designated
per gyrocompass (PGC).
When in proper running order, the gyrocompass
points constantly to true instead of magnetic north. It
may have a slight mechanical error, called gyro error,
which is computed easily and remains constant for any
heading.
Despite the excellence of the gyro mechanism, it
is the magnetic compass—not the gyro—that is
standard aboard ship. The reason is the magnetic
compass operates through the attraction exerted by
Earth. Consequently, the magnetic compass will never
go out of commission because of power failure.
The gyrocompass, on the other hand, is powered
by electricity. If the supply is cut off, the gyro is
useless. Being an extremely complicated and delicate
instrument, it is also subject to mechanical failure.
9-1
