Seamanship Techniques 2011 E Part 9 docx

23Speed and Depth
bronze to resist the corrosive action of salt water. The rotator is a hollow
tube having curved vanes attached to the sides and seized to a hollow
frog (often referred to as a bottle) by a short length of sennet laid line
(stray length). The opposite end of the frog receives the log line and is
secured in the manner shown in Figure 2.2.
To ‘Hand’ the Log (heaving the log back aboard)
1. Disconnect the bridge connection to the bridge repeater.
2. Stop the governor from rotating and bring in a little of the log line
by hand.
3. Unclip the Englefield clip from the governor.
4. Continue to heave the log line inboard, taking the bare end to the
opposite quarter of the vessel. Pay out the bare Englefield clip end
as the rotator is heaved in.
5. Allow time for any kinks in the rope caused by the rotator to be
‘turned out’. Heave in the line, coiling down left-handed.
6. A light grease should be applied to the log clock after removal of
any salt crust on the casing. All equipment should then be returned
to a safe stowage place, except for the line, which should be left to
dry naturally.
When heaving the log back aboard, mariners should be aware that the
rotator when breaking the surface has the tendency to fall back into and
under the stern. This could cause damage to the vanes of the rotator and
render it useless for future operation.
Length of Log Line
The length required for reasonable accuracy will vary; it is found by
experience when comparing logged distance against observed distance.
However, as an approximate guide for vessels with the following speeds,
the recommended length is approximately:
(a) 75 m to 95 m for speeds of about 12 knots.
(b) 100 m to 125 m for speeds of about 15 knots.

(c) 130 m to 160 m for speeds of about 20 knots.
The length of the log line will effectively change as a vessel changes her
draught especially in high freeboard vessels when in ballast. To this end,
small adjustments to the real length of line can be made if it is secured
to the governor as indicated in Figure 2.1, one of the half hitches being
removed to add length to the line or the half hitches spaced out to
shorten the real length. In practice, it is normal to check the log against
Frog Sennet laid line
(stray length)
Shaped fin on
rotator
Rotator
Figure 2.5 Frog and rotator.
Rotator logs are now limited in use with the
advent of various impeller and/or Doppler logs
becoming the norm.
24 Seamanship Techniques
observed positions and allow for the log reading fast or slow, in preference
to continual adjustment of the length of line, although that is a simple
process.
IMPELLER LOG
The impeller log may be considered an electric log, since its operation
is all electrical, except for the mechanical rotation of the impeller. There
are several designs in general use, but probably the most common is the
‘Chernikeeff’.
The principle of operation is based on turning an impeller by a flow
of water, the speed of rotation being proportional to the rate of flow past
the impeller (turbine principle). As previously stated, designs vary, the
two most popular being one with a ring magnet attached to the spindle
In the retracted

stowed position
Check tube
Leads to amplifier and electromagnetic counter
Log
shaft
Coil
Spindle
Water-lubricated
bearing sleeve
Magnet
Impeller
Impeller unit
Guard ring
Sluice
valve
Log housing
Valve
wheel
Ship’s hull plate
Log shaft
In the operational position
Figure 2.6 Impeller log.
25Speed and Depth
and one with the magnet incorporated in the blades of the impeller. In
either case a pick-up coil transmits the generated pulses via an amplifier
to an electromagnetic counter. This signal is then displayed by a speed
indicator and distance recorder.
Additional sensors will provide the opportunity for various repeaters
to include a direct link to allow speed input into True Motion Radar.
Operating power is normally 230/240 volts.

It is worth noting that the load on the impeller is negligible; consequently
the slip, if any, on the impeller is minimal and can be ignored. The
extended log, when in operation, projects approximately 14 in. (35 cm)
below the ship’s hull, usually from the engine room position. The log
shaft should be housed in the stowed position for shallow water, drydocking
etc. The sea valve sluice need only be closed if the log is to be removed
for maintenance. However, it must be considered good seamanship practice
to close the sluice each time the log is housed.
Performance of the log is in general considered to be very good, but
obvious problems arise in dirty water areas with a muddy bottom and
heavily polluted canals (see Figure 2.6).
HAND LEAD
The normal length of the hand lead line is about 25 fathoms, and the
line used is 9 mm
(1 in.)
1
8
untarred cable-laid hemp (left-hand lay). A
rawhide becket attached to an eye splice in the end of the line secures
the lead, the weight of which is 7–9 lb (3.2–4 kg) when operating from
vessels moving at less than 6 knots.
From the eye splice, i.e. ‘lead out’, which has the extra safety factor of
the length of the lead, or ‘lead in’, measured from the base of the lead,
the markings are as follows:
At 2 fathoms a piece of leather with two tails.
At 3 fathoms a piece of leather with three tails.
At 5 fathoms a piece of white linen.
At 7 fathoms a piece of red bunting.
At 10 fathoms a piece of leather with a hole in it (leather washer).
At 13 fathoms a piece of blue serge.

At 15 fathoms a piece of white linen.
At 17 fathoms a piece of red bunting.
At 20 fathoms a piece of cord with two knots.
Markings of metric hand lead line are as follows:
1 and 11 m – 1 strip of leather
2 and 12 m – 2 strips of leather
3 and 13 m – blue bunting
4 and 14 m – green and white bunting
5 and 15 m – white bunting
6 and 16 m – green bunting
7 and 17 m – red bunting
8 and 18 m – yellow bunting
26 Seamanship Techniques
9 and 19 m – red and white bunting
10 m – leather with a hole in it
20 m – leather with a hole and 2 strips of leather
The different materials indicating the various marks are distinctive to
allow the leadsman to feel rather than see the difference during the
hours of darkness. The intermediate whole fathom values, i.e. 1, 4, 6, 8,
9, 11, 12, 14, 16, 18 and 19 fathoms, are known as deeps.
The leadsman used to stand in the ‘chains’, from where he would take
the cast and call up the sounding to the officer of the watch. The lead
line is rarely used in this manner today, but the soundings are still
occasionally called in a traditional manner of stating the actual number
of fathoms last. For example,
At 7 fathoms . . . ‘by the mark seven’.
At
7
1
4

fathoms . . . ‘and a quarter seven’.
At
7
1
2
fathoms . . . ‘and a half seven’.
At
7
3
4
fathoms . . . ‘a quarter less eight’.
At 8 fathoms . . . ‘by the deep eight’.
Should the bottom not be reached, then ‘No Bottom’ is reported.
Constructing a New Line
Splice the eye into one end of the line, then soak and stretch the line,
possibly by towing astern. Mark the line off when wet from measured
distances marked off on deck, and tuck the fabrics of the marks through
the lay of the line.
Benefit of the Lead
This is the term used to describe the length from the base of the lead to
the eye spice. The actual distance is about 12 inches (30 cm) and is always
‘beneficial’ to the soundings, giving more water for the benefit of the
ship.
Arming the Lead
This describes the action of placing tallow into the ‘arming recess’,
found at the base of the lead. The purpose of the soft tallow is to act as
a glue to obtain the nature of the sea bottom. If tallow is not available,
a soft soap will be equally good. The information is passed to the Officer
of the Watch with the depth of sounding. It allows an additional comparison
with the charted information.

ECHO-SOUNDING
Principle of the Echo-sounder
The echo-sounding depth recorder emits a pulse of sound energy from
a transmitter, and the time this pulse takes to reach the sea bed and be
reflected back to the vessel is directly related to the distance. Speed of
Figure 2.7 Principle of the echo-sounder.
Draught
Rx
Tx
Distance to
sea bed
27Speed and Depth
Sensitivity
control
Depth indicator or
recorder
Illumination
control
Recording
paper
Amplifier
(maybe built as an
integral part of the
recorder)
Reflected
sound energy
Oscillation
generator
Transmitted
sound vibration

Reflected
sound energy
Range
selector
Receiving
oscillator
Transmitting
oscillator
Figure 2.8 Echo-sounder.
Power junction box
Watertight gland
Rubber seal
Tank side
Laminated nickel
plate pack
Reflector plate
Weld
Sound energy
Thin rust-proof
steel plate
Parabolic
reflector
Air
Fresh
water
Figure 2.9 Echo-sounder’s transmitting oscillator
(magnetostriction type).
28 Seamanship Techniques
sound through water being the known value of 1500 metres per second
(see Figures 2.7 to 2.9).

However, that value will vary with water temperature and salt content
(salinity).
Let us work out an example:
Let the velocity of sound in water = v metres per second.
Let the time between transmission and reception of the pulse = t seconds.
Let the distance to the sea bed and back = 2s metres.
But the distance = speed × time
∴ 2s = v × t
∴

s =
vt
2
metres
Therefore, s represents the depth of water under the vessel.
Possible Errors of Echo-sounding Equipment
1. Differences of the velocity of propagation. Owing to the differences of
salinity and temperature encountered in various parts of the world,
adjustment tables are available, published by the Admiralty.
2. Transmission line error. This is caused by the misalignment of the
reference ‘zero’ on the scale. Reference ‘zero’ sets the timer of the
recorder unit, and if it is not set at ‘zero’, then a false time and
recording will be obtained.
3. Pythagorean error. This error is encountered with separated trans-
ducers rather than with the combined transmit/receive unit. The
error is caused by the measuring of the ‘slant distance’ as opposed to
the vertical distance under the keel.
4. Aeration. The presence of air in the water will affect the speed at
which sound travels through it, since the velocity of sound through air
is much less than that in water (330 m/s compared with 1500 m/s).

The main causes of aeration are:
(a) Turbulence caused by having the rudder hard over.
(b) Having a light ship which is pitching heavily.
(c) Having sternway on the vessel.
(d) Having broken water over shoals.
(e) Entering an area where prevalent bad weather has left pockets
of air bubbles over comparatively long periods.
Possible cures for the above include stopping or reducing the vessel’s
speed, and abrupt movement of the rudder either way, to sweep away
formed bubbles.
False Echoes
False bottom echo
This may occur if the echo-sounder is incorrectly set in such a manner
that in deep water a returning echo is received after the stylus has
completed one revolution.
29Speed and Depth
Multiple echoes
These are caused by the transmitted pulse being reflected several times
between the sea bed and the water surface before its energy is dispersed.
Such multiple reflection may cause multiple echoes to be recorded on
the trace of the sounding machine. They can, however, be reduced in
strength by decreasing the sensitivity control on the equipment.
Double echo
This type of echo is a double reflection of the transmitted pulse. It
occurs when the energy is reflected from the sea bed and then reflected
back from the surface of the water before being received by the transducer.
A double echo is always weaker than the true echo, and can be expected
to fade quickly with a reduction in the sensitivity of the equipment.
Other causes
Side echo may come from objects not directly under the keel of the

vessel reflecting the sound energy, e.g. shoals of fish or concentrations of
weed or kelp. There may be electrical faults or man-made noise in and
around the hull. In addition, turbulence may be caused by the vessel
herself, with or without interaction between the shore or other shipping.
Deep scattering layer
This is a level of several layers believed to consist of fish and plankton
which will scatter and reflect sound energy. The layer has a tendency to
move from as much as 450 m below the surface during the daylight
hours to very near the surface at night. It becomes more noticeable
during the day when the cloud cover is sparse than when sky is overcast.
3
MARINE INSTRUMENTS
SEXTANT
The sextants purpose is to measure angles, either vertical or horizontal
to obtain the necessary data to check the vessels position. Latitude and
longitude may be determined by a combination of sextant, chronometer
and nautical almanac readings.
This precision instrument is based on the principle, enunciated by the
First Law of Light, that when a ray of light is reflected from a plane
mirror, then ‘The angle of incidence of the ray equals the angle of
reflection’. In the sextant a ray of light is reflected twice by two mirrors,
the index and horizon mirrors, in the same plane. When a ray of light is
reflected in this way by two plane mirrors, then the angle between the
direction of the original ray and the direction of the final reflected ray
is twice the angle between the mirrors (see Figures 3.1 and 3.2 and
Plate 7a).
Ray of light from observed object
Index mirror
Telescope (in collar)
Observer’s

eye
Index arm
Scale
Clamp
Micrometer
Arc
Shades
Frame
Figure 3.1 Sextant.
Shades
Horizon mirror
31Marine Instruments
Principle of the Sextant
The principle of the sextant is based on the fact that twice the angle
between the mirrors HAI must equal the angle between the initial and
final directions of a ray of light which has undergone two reflections.
Proof
Let α represent the angle between the mirrors.
Let ∅ represent the angle between the initial and final directions of a ray
of light.
The required proof is:
2α = ∅
Construction
Extend the ray of light from the object to intersect the reflected ray from
the Horizon Mirror H at point L.
Proof of theory
(i) The angle between the mirrors
α
is equal to the angle between the
normals to the mirrors.

(ii) In triangle HIK
β = α + X
and 2β = 2α + 2X
Ray of light from
observed object
Norm
A
α
φ
K
L
t
t
l
α
H
Figure 3.2 Principle of the sextant.
7a. Marine sextant.
32 Seamanship Techniques
(iii) In triangle HIL
2β = ∅ + 2X
therefore from equation (ii) and (iii)
2α + 2X = ∅ + 2X
and 2α = ∅
i.e. twice the angle between the mirrors is equal to the angle between
the initial and final directions of a ray of light which has undergone
two reflections in the same plane, by two plane mirrors.
Errors of the Marine Sextant
There are three main errors, which can quite easily be corrected by the
mariner. A fourth error, for ‘collimation’, can also be corrected, with care

and attention, but only to an older sextant where telescope collars are
fitted with adjusting screws.
The first error, of Perpendicularity, is caused by the index mirror not
being perpendicular to the plane of the instrument. To check if this error
is present, clamp the index arm between a third and half way along the
arc, remove the telescope, and look obliquely into the index mirror,
observing the true and reflected arcs of the sextant. Hold the sextant
horizontal, arc away from the body. If the true and reflected arcs are not
in line with each other, then an error of perpendicularity must be
considered to exist (Figure 3.3).
To correct the error, adjust the screw at the rear of the index mirror
until the true and reflected arcs are brought together in line.
The second error, side error, is caused by the horizon mirror not being
perpendicular to the plane of the instrument. There are two ways of
checking if this error is present. The first is by observing a star. Hold the
sextant in the vertical position with the index arm set at zero, and
observe a second magnitude star through the telescope. If the true and
reflected stars are side by side, then side error must be considered to exist
(Figure 3.5). It is often the case when checking the instrument for side
error that the true and reflected stars are coincident. If this is the case, a
small amount of side error may exist, but a minor adjustment of the
micrometer should cause the true star to appear below the reflected
image. Should, however, the reflected image move to one side rather
than move in a vertical motion, side error may be considered to exist.
The second way is by observing the horizon. Set the index arm at
zero and hold the sextant just off the horizontal position. Look through
the telescope at the true and reflected horizons. If they are misaligned,
as indicated in Figure 3.6, then side error must be considered to exist.
To correct for side error, adjust the centre screw furthest from the
plane of the instrument at the back of the horizon mirror, to bring either

the star and its image into coincidence or the true and reflected horizons
into line.
The third error, index error, is caused by the index mirror and the
First
adjustment
screw
Back of
mirror
Index
glass
Index
arm
Figure 3.3 Adjustment screw on index mirror.
Second
adjustment
screw
Clear
glass
Third adjustment
screw
(for index error)
Frame of
sextant
Figure 3.4 Adjustment screws on horizon mirror, seen
from behind.
Figure 3.6 Indication of side error.
Side error present
True
Reflected
horizon

Horizon
No indication of side error
Figure 3.5 Images of true and reflected stars, showing
side error.
33Marine Instruments
horizon mirror not being out of parallel to each other when the index
arm is set at zero. To check whether index error is present by observing
a star, look through the telescope when the sextant is set at zero, and if
the reflected image of the star is above or below the true image, then
index error must be considered to exist. Should the true and reflected
images be coincident, then no error will exist. To check by observing the
horizon, set the index arm at zero, hold the sextant in the vertical
position, and observe the line of the true and reflected horizons; if they
are seen as one continuous line, then no error exists, but if the line
between the true and reflected horizons is broken, an adjustment needs
to be made to remove the error. This adjustment is made by turning the
screw nearest to the plane of the instrument. Index error may also be
checked by observing the sun. Fit the shaded eye piece to the telescope.
Clamp the index arm at about 32′ off the arc and observe the true and
reflected images to the position of limb upon limb. Repeat the observation
with index arm set at about 32′ on the arc, and note the two readings of
both observations. The numerical value of the index error is the difference
between the two readings divided by two, and would be called ‘on the
arc’ if the ‘on the arc’ reading were the greater of the two, and ‘off the arc’
if the ‘off the arc’ reading were the greater.
Let us consider an example:
Adjust the micrometer to bring the true sun into contact with the
reflected sun
Note the reading, for example
RS

TS
0° 36′ off the arc.
Repeat the observation, but with images the other way about.
Note the reading, for example
TS
RS
0° 27′ on the arc.
Take the difference of the two readings and divide by 2.
Index error is
36 – 27
2
= 4.5
′
off the arc
This error must be subtracted from the future sextant readings.
34 Seamanship Techniques
The accuracy of the observations may be checked by adding the
numerical values of both readings together and dividing the number by
four. The resulting value should equal the semi-diameter of the sun for
the period at which the observation was taken.
Sometimes an instrument suffers from side error and index error
combined. Should this undesirable condition be apparent, the mariner
can resolve the problem by removing each error a little at a time, as
shown in Figure 3.7. The correction is made by turning the second and
then the third adjustment screws alternately, by a small amount each
time, until concidence of image is achieved.
Collimation error
This is an error caused by the axis of the telescope not being parallel to
the plane of the instrument. To check whether the error is present,
insert the inverting telescope, setting the eyepiece so that one pair of the

cross wires are parallel to the plane of the sextant.
To check by observation of two stars (selected about 90° apart), move
the index arm to bring the two stars into exact contact with each other
resting on the wire nearest to the plane of the sextant. Now tilt the
sextant upwards so as to bring them on to the wire which is furthest
from the plane of the instrument. Should the images diverge or converge
from the top intersections of the wires, it must be assumed that an error
of collimation exists, and that the axis of the telescope is not parallel to
the plane of the instrument.
This error can be corrected by adjustment of the two screws in the
collar or telescope mounting. The screws are moved together, one being
tightened, the other slackened, to align the stars on the top intersection
which will bring the telescope back to parallel with the sextant frame.
(Not all sextants, however, have adjustable collar screws.)
Non-adjustable errors
1. Centering error. This error could be caused by wearing of the pivot
on which the index arm moves, perhaps because the index arm is
not pivoted at the exact point of the centre of curvature of the arc.
2. Prismatic error. This error is caused by the two faces of the mirror not
being parallel to each other.
3. Shade error. This is an error caused by the faces of shades not being
parallel to each other. If it is known to exist, the telescope is used in
conjunction with the dark eyepiece.
4. Graduation error. This error may be encountered on the arc itself or
on the vernier or micrometer scales. If the micrometer drum is
known to be correct, then the first and last graduations on the drum
should always be aligned with graduation marks on the arc.
The manufacturer tables all the non-adjustable errors and issues the
sextant with a certificate usually secured inside the lid of the case.
The combination of the above four errors is known as ‘Instrument

Error’.
Figure 3.7 Dealing with combination of side and index
error.
35Marine Instruments
MARINE CHRONOMETER
The chronometer represents a fine example of precision engineering.
The instrument is manufactured and tested under stringent quality-
control methods to comply with marine authorities’ regulations. The
mechanical movement of the timepiece is manufactured as near to
perfection as is humanly possible.
It is used for the purpose of navigation and is generally the only
instrument aboard which records GMT (Greenwich Mean Time), all
other clocks tending to indicate local mean time or zone time. It is
normal practice for two chronometers to be carried by modern vessels,
as a safeguard against mechanical failure or accident.
The chronometer is stowed if possible in a place free of vibration and
maintained at a regular and even temperature. It must be accessible to the
navigation officer but not so exposed as to allow irresponsible handling.
By experience it has been found that the chartroom or wheelhouse area
are ideal positions for this most important of ship’s instruments.
The timepiece itself is slung in a gimbal arrangement, which can be
locked in position, should the instrument have to be transported, the
whole being encased in a strong wooden box fitted with a lock and
binding strap. Most vessels are fitted with a glass-covered well which
holds the ship’s chronometers. These wells are often padded to reduce
vibration effects, while the glass acts as a dust cover and permits observation
of the clock.
Usually a brass bowl is made to encase the mechanism. The bowl is
maintained in the horizontal position by the gimbal arrangement set on
stainless steel pivot bearings. A sliding, spring-loaded dust cover set in the

base of the bowl allows access for winding.
Regularity is achieved via a torque-equalising chain to a fusee drum.
The main spring is non-magnetic (of platinum, gold or palladium alloy),
and is fully tested before the instrument is released.
The chronometer is fitted into an inner guard box fitted with a
hinged, glazed lid. The outer wooden protective box is normally removed
once the instrument has been transported to the vessel and secured in
place.
Two-day chronometers should be wound daily at the same time. The
winding key, known as the ‘Tipsy key’, is inserted into the base of the
instrument after inverting the bowl in the gimbals and sliding the dust
cover over the key orfice. Chronometers are manufactured so that they
cannot be overwound, the majority being fully wound after 7

1
2
half
turns of the key anticlockwise. At this stage the person winding will
encounter a butt stop which prevents further winding. A small indicating
dial, on the clock face also provides indication that the instrument is
fully wound.
Should the chronometer have stopped through oversight or other
reason, it may become necessary to reset the hands on the face before
restarting the mechanism. If time permits, it is best to wait until the time
indicated is arrived at twelve hours later, then just restart the instrument.
7b. Marine chronometer.
36 Seamanship Techniques
However, this is not always practical, and if the hands need to be reset,
they can be by means of the following method:
1. Unscrew the glass face plate of the chronometer.

2. Fit the ‘Tipsy key’ over the centre spindle, holding the hands.
3. Carefully turn the key to move the hands in the normal clockwise
direction.
Under no circumstances must the hands be turned anti-clockwise, as this
will place excessive strain on the mechanism and may cause serious
damage.
Starting the chronometer should be done in conjunction with a radio
time signal, once the mechansim has been fully wound. It will be necessary
for any person restarting a chronometer after it has stopped to give the
timepiece a gentle circular twist in the horizontal plane. This effectively
activates the balance and sets the mechanism in motion.
After starting, the chronometer should be rated on a daily basis against
reliable time signals. Any error, either fast or slow, should be recorded in
the chronometer error book, small errors being taken account of in
navigation calculations.
THE GYRO COMPASS
The Sperry, Anschutz and Brown are three well-known makes of gyro
compass and one of them will be found in most deep sea ships. The
compass provides a directional reference to true north and is unaffected
by the earth’s magnetism and that of the ship.
A brief description follows but readers requiring more information
on the theory and construction of the compass should consult more
specialist literature.
Description and Application (The Three Degrees of Freedom)
The free gyroscope consists of a fast spinning rotor, mounted to provide
three degrees of freedom: freedom to spin; freedom to turn about a
Horizontal
axis
Spin axis
Rotor

Gimbal
support
Vertical
axis
Freedom to turn (rotate)
about the ‘vertical axis’
Freedom to tilt about the
horizontal axis, in
azimuth
Freedom to spin about
the ‘spin axis’
Figure 3.8 Degrees of freedom of rotor of free gyroscope.
37Marine Instruments
vertical axis; and freedom to tilt about a horizontal axis. As the rotor is
so constructed, to have a high mass, in relation to its dimensions, such a
gyroscope displays two important properties:
(a) gyroscopic inertia (rigidity in space) whereby it will point in space
to a fixed direction and thus follow the apparent motion of a fixed
star;
(b) gyroscopic precession – the angular velocity acquired by the spin
axis when torque is applied to the gyro in a plane perpendicular to
the plane of the instrument.
These properties are made use of in the gyro compass, where a rotor
spins at very high speed in nearly frictionless bearings, mounted with
freedom to turn and tilt. The axle of the gyro is constrained by a system
of weights producing a torque which causes the axle to precess (under
the influence of gravity) in such a manner that it remains horizontal and
in the meridian. The rate of precession of the gyro is equal to the rate at
which the axle of the free gyroscope would appear to tilt and drift as the
result of the earth’s motion.

The Properties of the Free Gyroscope
It is important that the mariner understands the properties of the free
gyroscope in order to understand the gyro compass.
Gyroscopic inertia
This term is often referred to as ‘Rigidity in Space’ which better describes
this property. It is the ability of the gyroscope to remain with its spin axis
pointing in the same fixed direction in space regardless of how the
gimbal support system may turn. The term may be illustrated by considering
the direction of a star in space. If the free gyroscope is set spinning with
the spin axis pointing to that star, then it will be seen that, as the earth
turns, the spin axis will follow the apparent motion of that star.
Precession
If a torque is applied to the spin axis of the free gyroscope then it will
be observed that the axis will turn in a direction at right angles to that
applied torque. This movement, by the rotor, due to the applied force, is
known as precession.
Torque
Torque is defined as the moment of a couple or system of couples
producing pure rotation. For a rotating body, torque is equal to the
product of the moment of inertia and the angular acceleration.
38 Seamanship Techniques
12
3
4
32
31
30
29
28
27

26
25
24
23
22
21
20
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Figure 3.9 Anschutz Standard 4 gyro compass.
1. Dimming resistance for illumination.
2. Clip-on engaging arm.
3. Coupling block.
4. Centring ball of gear plate.
5. Follow-up motor.
6. Slip rings.
7. Inspection window for temperature

readings.
8. Thermometer.
9. Spider leg.
10. Supporting ring with suspension springs.
11. Outer sphere.
12. Gyrosphere.
13. Narrow conducting band.
14. Window of liquid container.
15. Compensating weight.
16. Follow-up amplifier.
17. Symmetrical transformer.
18. Motor with fan.
19. Shock mounts.
20. Bolt connecting binnacle to pedestal.
21. Air duct.
22. Rubber skirt.
23. Broad conducting band.
24. Binnacle.
25. Liquid container.
26. Inner gimbal ring.
27. Outer gimbal ring.
28. Thermostat.
29. Top plate/supporting plate.
30. Micro-switch.
31. Cable connections.
32. Dimmer knob.
340 320 300 280 260 240 220 200
330 310 290 270 250 230 210
39Marine Instruments
1

2
3
4
5
6
7
12
13
14
15
16
17
N
8
9
10
11
Figure 3.10 Anschutz gyrosphere for Standard 4 gyro
compass.
1. Damping vessel.
2. Sealing ring of gyrosphere.
3. Gyro stator.
4. Narrow conducting band.
5. Narrow conducting band.
6. Gyro.
7. Repulsion coil.
8. Gyro casing 1.
9. Narrow conducting band.
10. Broad conducting band
11. Gyro casing 2.

12. Lower calotte (conducting dome).
13. Oil sump.
14. Capacitor.
15. Broad conducting band.
16. Terminal strip.
17. Upper calotte (conducting dome).
020 0
30 10
40 Seamanship Techniques
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Figure 3.11 Anschutz Standard 12 gyro compass
equipment.
1. Hood covering.
2. Dimmer switch for card illumination.
3. Lubber line.
4. On/off switch for follow up system.
5. Supporting plate for: 2, 13 and 14.

6. Transmitter.
7. Outer casing.
8. Outer sphere.
9. Servomotor.
10. Gear assembly.
11. Support plate carrying the outer sphere.
12. Compass card.
13. Amplifier.
14. Symmetrical transformer.
41Marine Instruments
8. Binnacle in hardwood finish.
MAGNETIC COMPASS
This is without doubt the most important of all instruments aboard even
the most modern vessel, and it is probably the most reliable. Its origins
go back as long ago as 2300 BC, but the Chinese development of the
compass card dates to the fourteenth century, and the sophisticated
instrument we know today became established with the advent of steel
ships in the nineteenth century.
The compass bowl is supported in a binnacle usually constructed of
wood, but, increasingly, many binnacles are being made in fibreglass
(Plates 8 and 9). The natural resilience of fibreglass absorbs vibration
from machinery and requires little maintenance.
The main function of the binnacle is to provide support and protection
for the compass bowl. However, the structure also provides the ideal
support for the standard correction elements, namely the quadrantal
correctors, the flinders bar, and the fore and aft and athwartships permanent
magnets. Heeling error magnets are placed in a ‘bucket’ arrangement on
the centre-line of the binnacle directly under the central position of the
compass bowl (see Figure 3.12). The effect of heeling error magnets can
be increased or decreased by respective adjustment of the chain raising

or lowering the bucket.
42 Seamanship Techniques
LIQUID MAGNETIC COMPASS
This compass is illustrated in Figure 3.14.
Compass Bowl
Manufactured in high quality non-magnetic brass, this has a clear glass
face and a frosted glass base to diffuse the underside lighting. The older
designs were fitted with chambers to allow for the expansion and
contraction of the fluid, but the modern compass is fitted with a corrugated
diaphragm (elastic membrane) at the base of the bowl for the same purpose.
Most compass manufacturers include a graduated verge ring round
the clear glass face plate. Both the face plate and the frosted glass base are
secured via rubber gaskets to prevent leaks from the bowl. Special paints,
used both internally and externally, are ‘stove baked’ on to the compass
9. Modern binnacle manufactured in glass-reinforced
plastic.
43Marine Instruments
body so that they will not cause discoloration of the fluid, and they last
for many years. Many paints are magnetic, especially blacks, and their use
on and around the binnacle should be limited. Brackets are fixed to the
outside to connect to the gimbal system and a support is secured on the
inside of the bowl to accommodate the pivot. The ‘lubber line’ is marked
on the inside of the bowl, in alignment with the ship’s fore and aft line.
Compass Card
Usually made of glass melamine or mica, it must be the correct size for
the bowl to which it is fitted. Should another sized card be used, then
fluid disturbances could make the compass unsteady. The diameter of
cards varies, but a 10 in. (254 mm) compass bowl could expect a 7

1

2
in.
(191 mm) diameter card, unless a specially reduced card is provided.
In the course of manufacture the card is corrected for its magnetic
moment, to limit its speed of movement, then checked for friction.
Cards are normally screen-printed to indicate three-figure notation in
degrees, and have the cardinal and half cardinal points identified.
Intermediate and by-points are indicated but not individually lettered.
A single circular magnet is secured beneath the card to produce the
directive force required of the compass. The magnet system may consist
of two parallel circular magnets disposed on either side of the central
line.
Flinders bar
case
Removable cap to allow
fitting of a pelorus
Quadrantal correctors
(Kelvin’s balls)
Compass bowl
Bucket guide for
heeling error
(vertical magnets)
Access panels for
athwartships and
fore and aft magnets
Slewing bolts
Telescopic reflector
unit
Inclinometer
Bracket

Figure 3.12 Magnetic compass. The chain adjustment
for the heeling error bucket can be reached
via the panel under the compass bowl into
the light chamber.
Perspex (clear) helmet
44 Seamanship Techniques
Pivot Point
This is made of polished iridium, a member of the platinum family, and
care should be taken in manufacture that the iridium element is neither
too hard, or it may shatter, nor too soft, or it may collapse. The bearing
point is an industrial jewel, usually a sapphire, fitted into the base of a
float set into the centre of the compass card.
The pivot point effectively lowers the centre of gravity of the card
below the point of suspension. This arrangement is achieved by the
inclusion of the dome-shaped float in the centre of the card. The magnetic
system may be in the form shown, with a needle arrangement slung
beneath the card, or a single circular magnet may be encased below the
float. The casing is generally made of brass, to prevent rusting and loss of
magnetic effect.
One of the main advantages of the liquid compass over the dry card
compass is that it is not as sensitive. Consequently, it makes an excellent
steering compass. Oscillations of the card are greatly reduced by the
dense liquid within the bowl and any induced movement is practically
eliminated.
The term ‘dead beat’ applied to the liquid compass means slow moving,
with a steady card. Undesirable oscillations of the card are kept to a
minimum by the liquid.
Liquid
The older style of liquid magnetic compass contained a mixture of two
parts distilled water to one part ethyl alcohol, providing a fluid with low

viscosity and a small coefficient of expansion. The idea behind the mix
was that the alcohol would reduce the freezing point of the mixture in
10. Liquid compass bowl in gimbal arrangement.
45Marine Instruments
cold climates and the water would reduce evaporation in the warm
tropical climates. The modern liquid compass employs an oily liquid
derived from ‘Bayol’, which not only provides additional flotation for
the card but also lubricates the pivot and reduces motion on the card.
In the manufacture of modern compasses the bowl, once assembled,
is passed through a vacuum before being filled. The actual filling is
carried out at an ambient temperature, and any final air bubbles are
removed by manual joggling of the instrument (see Plate 10).
Gimballing
This means slinging the compass in such a manner that it remains
horizontal at all times, even in a heavy sea. Keeping the compass card
horizontal at all times may be achieved in two ways:
1. Raising the point of support of the compass card above the centre
of gravity of the card.
2. Maintaining the compass bowl in the horizontal position by two
axis/gimbal rings, one in the fore aft line and the other in the
athwartships line.
It is usual to have the fore and aft axis secured to the outer gimbal ring
FORWARD
Flinders bar
Compass
card
Sphere
Sphere
Bearing
carrier

Quadrantal correctors
Quadrantal correctors
Gimbal
Sphere
collar
Bracket
Graduations on brackets
face aft
Bracket
Gimbal arrangement
Lens in base of
compass faces forward
Figure 3.13 Siting of magnetic compass’s correctors.
Float
Gasket
Compass
needle
case
Iridium pivot
Frosted glass
Rubber seal
gasket
Liquid
Clear glass
face plate
Verge ring
(engraved 0°–360°)
Sapphire bearing
Expansion
diaphragm

Compass
card
Compass
bowl
Figure 3.14 Liquid magnetic compass.
46 Seamanship Techniques
rather than the inner ring, as this reduces the possibility of the lubber’s
line mark travelling to port or starboard when the vessel is rolling heavily.
Order of Placing Correctors
Mariners are advised that to attempt to cover compass adjustment
within the bounds of this text would be impractical. The reduction of
deviation effecting the magnetic compass is complex and should be
studied in depth. Use of correctors to compensate for permanent and
induced magnetic effects must be carried out in a correct and orderly
procedure.
A method of adjustment within the mercantile marine employs
coefficients A, B, C, D, E and J for heeling error. These coefficients are
types of deviation which vary in accordance with some ratio of the
compass course.
For example: coefficient B is a deviation which varies as the sine of
the compass course.
A thorough knowledge of the use of the coefficients together with
a sound background of general magnetism must be considered essential
to any mariner attempting the adjustment of the marine magnetic
compass.
Marine students seeking further information should refer to: The
Ship’s Compass by G.A.A. Grant and J. Klinkert (Routledge and Kegan
Paul).
1. Flinders bar.
This bar usually comes in lengths of 12 inches (30.48 cm), 6 inches

(15.24 cm), 3 inches (7.62 cm), and 1

1
2
inches (3.8 cm), all of 3
inches (7.62 cm) diameter, with similar size wood blocks to raise the
level of the bar and bring the pole of the bar, level with the magnets
of the card. The pole is assumed to be one-twelfth the length from
the end. This is explained by the fact that the pole of a bar magnet
is never at the very end of the bar (Figure 3.15).
2. Spheres.
Employed in various sizes from 2 inches to 10 inches (5.08 to
25.4 cm) diameter, they may be of a solid or hollow construction.
They are placed with their centres on a level with the magnets of
the card but not closer than one and a quarter times the length of
the longest needle in the card.
3. Heeling error magnets.
Hard iron magnets 9 inches in length (22.86 cm) by

3
8
inch
(0.93 cm) diameter, they compensate for heeling error due to field
‘R’ and vertical soft iron. They also induce magnetism into the
flinders bar and spheres, which helps the heeling error correction.
4. Horizontal magnets.
These are 8 inches in length (19.32 cm) and either

3
8

or

3
16
inch
(0.93 or 0.46 cms) in diameter. They compensate for the effects of
the fore and aft and athwartships components of semi permanent
magnetism.
Figure 3.15 Lines of magnetic force.
SN Bar magnet
Lines of magnetic force
47Marine Instruments
11. Reflector unit.
Telescopic Reflector Unit
The majority of compass manufacturers will now supply ships’ binnacles
with or without telescopic reflector units (Plate 11), depending on the
requirements of the ship owner. The reflector unit was an acceptable
advance within the industry, since it obviated the need and the cost of
providing a steering compass.
The idea of achieving a through-deck repeater from the standard
compass on the ‘Monkey island’ is based on the development of the
submarine periscope. The unit is fitted under the forepart of the compass
in order to reflect the lubber’s line and the foremost section of the
compass card within the standard compass. The reflector unit is not
centrally positioned, as the operation of the bucket containing the heeling
error magnets would obstruct its use.
A typical reflector unit would be manufactured in PVC, with brass
fittings. It usually incorporates moisture-free, sealed-in mirrors, which
are adjustable at eye level inside the wheelhouse, together with a detachable
anti-glare shield.