COMPASSES
INTRODUCTION
Changes
in Compass Technologies
This chapter discusses
the major types of compasses available to the navigator, their operating principles,
their capabilities, and limitations
of their use. As with other aspects of navigation,
technology is rapidly revolutionizing the field of
compasses. Amazingly, after at
least a millennia of constant use, it is now possible (however advisable it may
or may not be aboard any given vessel) to dispense with the traditional
magnetic compass.
For much of maritime history the only heading reference for navigators has been the magnetic compass.
A great deal of effort and expense has gone into understanding the
magnetic compass scientifically and
making it as accurate as possible through elaborate compensation techniques.
The introduction of the
electro-mechanical gyrocompass relegated the magnetic compass to backup status
for many large vessels. Later came the development of inertial navigation
systems based on gyroscopic principles. The interruption
of electrical power to the
gyrocompass or inertial
navigator, mechanical failure,
or its physical destruction
would instantly elevate the magnetic compass to primary status for most
vessels.
New technologies are
both refining and replacing the magnetic compass as a heading reference and
navigational tool. Although a magnetic compass for backup is certainly
advisable, today’s navigator can safely avoid nearly all of the effort and expense associated with the binnacle- mounted magnetic compass, its compensation, adjustment,
and maintenance.
Similarly, electro-mechanical gyrocompasses are being supplanted by far lighter, cheaper, and more dependable ring laser gyrocompasses. These devices do not
operate on the principle of the gyroscope (which is based
on Newton’s laws of motion), but instead rely on the principles
of electromagnetic energy and wave theory.
Magnetic
flux gate
compasses, while relying on the
earth’s magnetic field
for reference, have no moving parts
and can
compensate themselves, adjusting for both
deviation and variation to
provide true heading, thus completely eliminating the process of
compass correction.
To the
extent that one depends on the
magnetic compass for navigation, it should be checked regularly and adjusted when observed errors exceed certain minimal
limits, usually a few degrees for most vessels. Compensation of a magnetic compass aboard vessels
expected to rely on it offshore during long voyages is best left to professionals. However, this
chapter will present enough material for the competent navigator to do a
passable job.
Whatever type of compass is used, it is advisable to check
it periodically against an
error free reference to determine
its error. This may be done when steering along any range during
harbor and approach
navigation, or by aligning
any two
charted objects and finding the difference between their
observed and charted bearings.
When navigating offshore, the use
of azimuths
and amplitudes
of celestial
bodies will also suffice.
MAGNETIC COMPASSES
The Magnetic Compass and Magnetism
The principle of the
present day magnetic compass is no different from that of the compasses used by
ancient mariners. The magnetic compass consists of a magnetized needle, or an
array of needles, allowed to rotate in the horizontal plane. The superiority of
present day magnetic compasses over ancient ones results from a better
knowledge of the laws of magnetism which govern the behavior of the compass and from greater precision in design and
construction.
Any magnetized piece of metal will have regions
of concentrated magnetism called poles. Any such
magnet will have at least two poles of opposite polarity. Magnetic force (flux)
lines connect one pole of such a magnet with the other pole. The number of such
lines per unit area represents the intensity of the magnetic field in that
area.
If two magnets are placed close to each other,
the like poles will repel each other and the unlike poles will attract each
other.
Magnetism can be either
permanent or induced. A bar having
permanent magnetism will retain its magnetism when it is removed from a magnetizing field. A bar
having induced magnetism will lose its magnetism when removed
from the magnetizing field. Whether
or not a bar will retain
its magnetism on removal from the magnetizing field will depend on the strength of that
field, the degree of
hardness of the iron (retentivity), and upon the amount of physical
stress applied to the bar while in the magnetizing field. The harder the
iron, the more permanent will be the magnetism acquired.
Terrestrial
Magnetism
Consider the Earth as a
huge magnet surrounded by lines of magnetic flux connecting its two magnetic
poles. These magnetic poles are near, but not coincidental with, the Earth’s
geographic poles. Since
the north seeking
end of a compass needle is
conventionally called the north pole, or positive pole, it must
therefore be attracted to a south pole, or negative pole.
Figure 602a illustrates
the Earth and its surrounding magnetic field. The flux lines enter the surface of the Earth at different angles to the horizontal at different
magnetic latitudes. This angle is called the angle
of magnetic dip,
, and increases
from 0 at the magnetic equator
to 90 at the magnetic poles.
The total magnetic field is generally considered as having two components: H,
the horizontal component; and Z, the vertical component. These components
change as the angle changes,
such that H is at its maximum
at the magnetic equator and decreases in the
direction of either pole, while Z is zero at the magnetic
equator and increases in the direction of either pole.
Since
the magnetic
poles of the Earth do
not coincide
with the geographic
poles, a compass
needle in line with the Earth’s
magnetic field will not indicate
true north, but magnetic north. The
angular difference between the true
meridian (great circle connecting the geographic
poles) and the magnetic
meridian (direction of the lines of
magnetic flux) is called
variation. This variation
has different
values at different locations on the Earth. These values of magnetic
variation may be found on pilot charts and
on
the compass rose of navigational charts.
The poles are not geographically static. They are known
to migrate slowly,
so that variation for most areas undergoes
a small annual
change, the amount of which is also noted on
charts. Figure 602b and Figure 602c show magnetic
dip and variation for the world. Up-to-date information on geomag-
netics is available
at http://geomag.usgs.gov/dod.html.
Ship’s Magnetism
A ship under
construction or repair will acquire permanent magnetism due to hammering and
vibration while sitting stationary in the Earth’s magnetic field. After
launching, the ship will lose some of this original magnetism as a result of vibration
and pounding in varying
magnetic fields, and will eventually reach a more or less stable magnetic
condition. The magnetism which
remains is the permanent magnetism of the ship.
In addition
to its permanent magnetism, a ship acquires induced magnetism when
placed in the Earth’s magnetic field. The magnetism induced in any given piece
of soft iron is a function of the field intensity, the alignment of the
soft iron in that field, and the physical properties and dimensions of the
iron. This induced magnetism may add to, or subtract from, the permanent
magnetism already present in the ship, depending on how the ship is aligned in the magnetic field. The softer the
iron, the more readily it will be magnetized by the Earth’s magnetic field, and
the more readily it will give up its magnetism when removed from that field.
The magnetism in the various structures of a ship, which tends to change as a result of cruising,
vibration, or aging, but
which does not alter immediately so as to be properly termed
induced magnetism, is called
subpermanent magnetism. This magnetism, at any instant, is part of the ship’s permanent
magnetism, and consequently
must be corrected
by permanent magnet correctors.
It is
the principal cause of deviation changes on a magnetic compass.
Subsequent reference to permanent
magnetism will refer to the
apparent permanent magnetism which includes the existing permanent
and subpermanent magnetism.
A ship, then,
has a combination of permanent, subpermanent, and induced magnetism. Therefore, the ship’s
Magnetic
dip for the world
Magnetic
variation for the world
apparent
permanent magnetic condition is subject
to change
from deperming, shocks, welding, and vibration. The ship’s
induced magnetism will vary
with the Earth’s magnetic field
strength and with the alignment
of the ship in that field.
604. Magnetic Adjustment
A narrow
rod
of soft iron, placed parallel to
the
Earth’s horizontal magnetic field, H, will have a north pole induced in the end
toward the north geographic pole and a south pole
induced in the end toward the south geographic
pole. This same rod
in a horizontal plane, but at
right angles to the horizontal
Earth’s field, would have no magnetism
induced in it, because
its alignment in the
magnetic field precludes linear magnetization, if the
rod is
of negligible
cross section. Should the rod be
aligned in some horizontal
direction between those headings which create
maximum and zero induction, it would be induced by
an amount
which is a function of the
angle of alignment. However, if a similar
rod is
placed in a vertical position in northern latitudes so as to be aligned with the vertical
Earth’s field Z, it will have a south pole induced at the upper end and a north pole
induced at the
lower end. These
polarities of vertical induced magnetization
will be reversed in southern
latitudes.
The amount of
horizontal or vertical induction in such rods, or in ships whose construction
is equivalent to combinations of such rods, will vary with the intensity
of H and Z, heading, and heel
of the ship.
The magnetic compass
must be corrected for the vessel’s permanent and induced magnetism so that its
operation approximates that of a completely nonmagnetic vessel. Ship’s magnetic
conditions create magnetic compass deviations and sectors of sluggishness and
unsteadiness. Deviation is
defined as deflection right or left of the magnetic meridian caused by
magnetic properties of the vessel. Adjusting the compass consists of arranging
magnetic and soft iron correctors near the compass
so that their effects are
equal and opposite to the effects of the magnetic material in the ship.
The total
permanent magnetic field effect at the compass may be broken into three components, mutually 90 to each other, as shown in Figure 604a.
The vertical permanent
component tilts the compass card, and, when the ship rolls or pitches, causes
oscillating deflections of the card. Oscillation effects which accompa- ny roll are maximum
on north and south compass
headings, and those which accompany pitch are maximum
on east and west compass headings.
The horizontal B and C components of permanent mag-
netism cause varying deviations
of the
compass as the ship swings in heading on an
even keel. Plotting these deviations
against compass heading yields
the sine and cosine curves
shown in Figure 604b.
These deviation curves are called
semicircular curves because
they reverse direction by 180 .
A vector analysis
is helpful in determining deviations or the strength of deviating
fields. For example, a ship as shown in Figure 604c on an east magnetic heading
will subject its compass to a
combination of magnetic effects;
namely, the Earth’s horizontal field H, and the deviating field B, at right angles to the field H. The compass needle will align itself in the resultant
field which is represented by the vector sum of H and B, as shown. A similar
analysis will reveal that the resulting directive force on the compass would be maximum on a north
heading and minimum on a south heading because the deviations for both
conditions are zero.
The magnitude of the deviation caused by the
permanent B magnetic
field will vary
with different values
of H; hence, deviations
resulting from permanent magnetic fields
will vary with the magnetic
latitude of the ship.
Components of permanent magnetic field
Permanent
magnetic deviation effects.
General
force diagram.
Effects of Induced Magnetism
Induced
magnetism varies with the strength
of the
surrounding field, the mass
of metal,
and the
alignment of the metal in the
field. Since the intensity of
the Earth’s
magnetic field varies over the Earth’s surface, the induced magnetism in a ship will vary with latitude, heading, and heeling angle.
With the ship on an even keel, the resultant vertical induced
magnetism, if not directed
through the compass itself, will create
deviations which plot as a semicircular deviation curve. This is true because
the vertical
induction changes magnitude and polarity only with magnetic
latitude and heel, and not with
heading of the ship. Therefore,
as long as the ship is in the same
magnetic latitude, its vertical
induced pole swinging about the
compass will produce the
same effect on the compass
as a permanent pole swinging
about the compass.
The Earth’s field induction in certain other unsymmetrical
arrangements of horizontal
soft iron create a constant
A devia- tion curve. In addition
to this
magnetic A error, there are constant A deviations resulting from: (1)
physical misalign- ments of the compass,
pelorus, or gyro; (2) errors in calculating
the Sun’s azimuth,
observing time, or taking bearings.
The nature, magnitude,
and polarity of these induced effects are dependent upon the disposition of
metal, the symmetry or asymmetry
of the ship, the location
of the bin- nacle, the strength of the Earth’s magnetic field, and the
angle of dip.
Certain heeling errors,
in addition to those resulting from permanent magnetism, are created by the
presence of both horizontal and vertical soft iron which experience changing induction as the ship rolls in the Earth’s
magnetic field. This part of the heeling error will change in magni-
tude proportional to changes of magnetic latitude
of the ship. Oscillation
effects associated with rolling are maxi- mum on north and south headings, just
as with the permanent magnetic heeling errors
Adjustments
and Correctors
Since some magnetic effects are
functions of the ves- sel’s magnetic latitude and others are not, each
individual effect should be corrected
independently. Furthermore, to make the corrections, we use (1) permanent magnet
correc- tors to compensate for permanent magnetic fields at the compass,
and (2) soft iron correctors to compensate for in- duced magnetism. The compass
binnacle provides support for both the compass and its correctors. Typical
large ship binnacles hold the following correctors:
1.
Vertical permanent
heeling magnet in the central vertical tube
2. Fore-and-aft B permanent magnets in
their trays
3. Athwartship C permanent magnets in
their trays
4. Vertical soft iron Flinders bar in
its external tube
5. Soft iron quadrantal spheres
The heeling magnet is the only corrector
which cor- rects for both permanent
and induced effects. Therefore, it
may need to be adjusted for changes in latitude if a vessel permanently changes its normal operating area.
However, any movement of the heeling magnet will require readjust- ment of
other correctors.
Fairly sophisticated magnetic compasses
used on smaller commercial craft,
larger yachts, and fishing vessels, may not have soft iron
correctors or B and C permanent magnets. These compasses are adjusted by
rotating mag- nets located inside the base of the unit, adjustable by
small screws on the outside. A non-magnetic screwdriver is nec- essary to
adjust these compasses. Occasionally one may find a permanent magnet corrector mounted near the com- pass, placed during the initial installation so as to remove a large,
constant deviation before
final adjustments are made.
Normally, this remains in place for the life of the vessel.
Figure
606 summarizes all the various
magnetic condi- tions in a ship,
the types of deviation curves
they create, the correctors for each effect,
and headings on which each cor-
rector is adjusted. When adjusting the compass, always apply the correctors symmetrically and as far away from the
compass as possible. This preserves the uniformity of mag-
netic fields about the compass needle.
Occasionally,
the permanent
magnetic effects at the lo-
cation of the compass are so large
that
they overcome the Earth’s directive force, H. This condition will not only create
sluggish and unsteady
sectors, but may even freeze the com-
pass to one reading
or to
one quadrant,
regardless of the heading of the
ship. Should the compass become
so frozen,
the polarity of the
magnetism which must be attracting
the compass needles is indicated;
hence, correction may be
ef- fected simply by
the application
of permanent
magnet
correctors
to neutralize
this magnetism. Whenever such ad-
justments are made, the
ship should be steered
on a heading such that the unfreezing
of the
compass needles will be im-
mediately evident. For example,
a ship whose compass is frozen to
a north reading would require fore-and-aft
B cor- rector magnets with the positive
ends forward in order to
neutralize the existing
negative pole which attracted the com-
pass. If made on
an east
heading, such an adjustment would be evident when the
compass card was
freed to indicate
an east heading.
correctors
to neutralize
this magnetism. Whenever such ad-
justments are made, the
ship should be steered
on a heading such that the unfreezing
of the
compass needles will be im-
mediately evident. For example,
a ship whose compass is frozen to
a north reading would require fore-and-aft
B cor- rector magnets with the positive
ends forward in order to
neutralize the existing
negative pole which attracted the com-
pass. If made on
an east
heading, such an adjustment would be evident when the
compass card was
freed to indicate
an east heading.
Another
source of transient deviation is
the retentive error. This error results from
the
tendency of
a ship’s
structure to retain induced magnetic
effects for short periods
of time. For example,
a ship traveling
north for several days,
especially if pounding
in heavy seas, will tend to retain some
fore-and-aft magnetism induced under
these conditions. Although this
effect is transient, it may
cause slightly incorrect observations
or adjustments.
This same type of error occurs
when ships are docked on
one heading
for long
periods of time. A short shakedown, with
the
ship on other headings, will tend to remove
such errors. A similar sort of
residual magnetism is left
in many
ships if the degaussing circuits are not secured by the correct reversal sequence.
A source of transient
deviation somewhat shorter in duration than retentive error is
known as Gaussin error. This error is caused by eddy currents set up by a changing number of magnetic lines of
force through soft iron as
the ship changes heading. Due to
these eddy currents, the induced magnetism on a given heading
does not
arrive at its normal value until
about 2 minutes after changing course.
Deperming and other magnetic treatment
will change the magnetic condition of the vessel and therefore require compass
readjustment. The decaying effects of deperming can vary. Therefore, it is best to delay
readjustment for sev- eral days after such treatment.
Since the magnetic fields used for such treatments are sometimes rather large
at the compass locations, the Flinders bar, compass, and related equipment
should be removed from the ship during these operations.
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Sources of Transient Error
The ship must be in
seagoing trim and condition to properly compensate a magnetic compass. Any
movement of large metal objects or the energizing
of any electrical equipment in the vicinity of the compass can cause errors. If
in doubt about the effect of any such changes, temporarily move the gear or cycle power to the equipment while observing the compass card
while on a steady heading. Preferably
this should be done on two different
headings 90 apart, since
the compass might
be affected on one heading and not on another.
Some magnetic
items which cause
deviations if placed too close to the compass are as
follows:
1. Movable guns or weapon
loads
2. Magnetic cargo
3. Hoisting booms
4. Cable reels
5. Metal doors in wheelhouse
6. Chart table drawers
7. Movable gyro repeater
8. Windows and ports
9. Signal pistols racked near
compass
10. Sound powered telephones
11. Magnetic wheel or rudder mechanism
12. Knives or tools near binnacle
13. Watches, wrist bands, spectacle frames
14. Hat grommets, belt buckles, metal pencils
15. Heating of smoke stack or exhaust pipes
16. Landing craft
Some electrical items which cause variable
deviations if placed too close to the compass are:
1. Electric motors
2. Magnetic controllers
3. Gyro repeaters
4. Nonmarried conductors
5. Loudspeakers
6. Electric indicators
7. Electric welding
8. Large power circuits
9. Searchlights or flashlights
10. Electrical
control panels or switches
11. Telephone
headsets
12. Windshield
wipers
13. Rudder
position indicators, solenoid type
14. Minesweeping
power circuits
15. Engine
order telegraphs
16. Radar
equipment
17. Magnetically
controlled switches
18. Radio
transmitters
19. Radio
receivers
20. Voltage regulators
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