Seamanship Techniques 2011 E Part 9 docx

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 betwee

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 onrotator

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.

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 simpleprocess

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 Thereare 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 pastthe impeller (turbine principle) As previously stated, designs vary, thetwo 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.

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.)18 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

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 leatherThe different materials indicating the various marks are distinctive toallow the leadsman to feel rather than see the difference during thehours 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 takethe cast and call up the sounding to the officer of the watch The leadline is rarely used in this manner today, but the soundings are stilloccasionally called in a traditional manner of stating the actual number

of fathoms last For example,

At 7 fathoms ‘by the mark seven’

At 714 fathoms ‘and a quarter seven’

At 712 fathoms ‘and a half seven’

At 734 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 measureddistances marked off on deck, and tuck the fabrics of the marks throughthe lay of the line

Benefit of the Lead

This is the term used to describe the length from the base of the lead tothe 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 theship

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 comparisonwith 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 bereflected 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

control

Depth indicator or recorder

Illumination control Recording

paper

Amplifier (maybe built as an integral part of the recorder) Reflected sound energy

Range

selector

Receiving oscillator Transmitting

Laminated nickel plate pack

Reflector plate

Weld Thin rust-proof

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

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 therecorder unit, and if it is not set at ‘zero’, then a false time andrecording will be obtained

3 Pythagorean error This error is encountered with separated

trans-ducers rather than with the combined transmit/receive unit Theerror is caused by the measuring of the ‘slant distance’ as opposed tothe 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’sspeed, and abrupt movement of the rudder either way, to sweep awayformed bubbles

False Echoes False bottom echo

This may occur if the echo-sounder is incorrectly set in such a mannerthat in deep water a returning echo is received after the stylus hascompleted one revolution

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

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 andlongitude may be determined by a combination of sextant, chronometerand nautical almanac readings

This precision instrument is based on the principle, enunciated by theFirst Law of Light, that when a ray of light is reflected from a planemirror, then ‘The angle of incidence of the ray equals the angle ofreflection’ 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 isreflected in this way by two plane mirrors, then the angle between thedirection 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 andPlate 7a)

Ray of light from observed object

Index mirror Telescope (in collar)

Observer’s eye Index arm

Scale

Micrometer Arc

Shades Frame

Figure 3.1 Sextant.

Shades Horizon mirror

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

α H

Figure 3.2 Principle of the sextant.

7a Marine sextant.

(iii) In triangle HIL

2β= ∅ + 2Xtherefore from equation (ii) and (iii)2α + 2X = ∅ + 2X

and 2α= ∅i.e twice the angle between the mirrors is equal to the angle betweenthe initial and final directions of a ray of light which has undergonetwo 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 themariner A fourth error, for ‘collimation’, can also be corrected, with careand attention, but only to an older sextant where telescope collars arefitted 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 thearc, remove the telescope, and look obliquely into the index mirror,observing the true and reflected arcs of the sextant Hold the sextanthorizontal, arc away from the body If the true and reflected arcs are not

in line with each other, then an error of perpendicularity must beconsidered to exist (Figure 3.3)

To correct the error, adjust the screw at the rear of the index mirroruntil 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 ofchecking if this error is present The first is by observing a star Hold thesextant in the vertical position with the index arm set at zero, andobserve a second magnitude star through the telescope If the true andreflected 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 sideerror that the true and reflected stars are coincident If this is the case, asmall amount of side error may exist, but a minor adjustment of themicrometer should cause the true star to appear below the reflectedimage Should, however, the reflected image move to one side ratherthan move in a vertical motion, side error may be considered to exist.The second way is by observing the horizon Set the index arm atzero and hold the sextant just off the horizontal position Look throughthe 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 theplane of the instrument at the back of the horizon mirror, to bring eitherthe star and its image into coincidence or the true and reflected horizonsinto line

The third error, index error, is caused by the index mirror and the

(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.

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 – 272 = 4.5′ off the arc

This error must be subtracted from the future sextant readings

The accuracy of the observations may be checked by adding thenumerical values of both readings together and dividing the number byfour The resulting value should equal the semi-diameter of the sun forthe period at which the observation was taken.

Sometimes an instrument suffers from side error and index errorcombined Should this undesirable condition be apparent, the marinercan resolve the problem by removing each error a little at a time, asshown in Figure 3.7 The correction is made by turning the second andthen the third adjustment screws alternately, by a small amount eachtime, until concidence of image is achieved

Collimation error

This is an error caused by the axis of the telescope not being parallel tothe plane of the instrument To check whether the error is present,insert the inverting telescope, setting the eyepiece so that one pair of thecross wires are parallel to the plane of the sextant

To check by observation of two stars (selected about 90° apart), movethe index arm to bring the two stars into exact contact with each otherresting on the wire nearest to the plane of the sextant Now tilt thesextant upwards so as to bring them on to the wire which is furthestfrom the plane of the instrument Should the images diverge or convergefrom 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 tothe plane of the instrument

This error can be corrected by adjustment of the two screws in thecollar or telescope mounting The screws are moved together, one beingtightened, the other slackened, to align the stars on the top intersectionwhich 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 isnot 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 inconjunction 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 isknown to be correct, then the first and last graduations on the drumshould always be aligned with graduation marks on the arc.The manufacturer tables all the non-adjustable errors and issues thesextant with a certificate usually secured inside the lid of the case.The combination of the above four errors is known as ‘InstrumentError’

Figure 3.7 Dealing with combination of side and index

error.

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 halfturns 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.

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 clockwisedirection

Under no circumstances must the hands be turned anti-clockwise, as thiswill place excessive strain on the mechanism and may cause seriousdamage

Starting the chronometer should be done in conjunction with a radiotime signal, once the mechansim has been fully wound It will be necessaryfor any person restarting a chronometer after it has stopped to give thetimepiece a gentle circular twist in the horizontal plane This effectivelyactivates the balance and sets the mechanism in motion

After starting, the chronometer should be rated on a daily basis againstreliable time signals Any error, either fast or slow, should be recorded inthe chronometer error book, small errors being taken account of innavigation calculations

THE GYRO COMPASS

The Sperry, Anschutz and Brown are three well-known makes of gyrocompass and one of them will be found in most deep sea ships Thecompass 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 morespecialist literature

Description and Application (The Three Degrees of Freedom)

The free gyroscope consists of a fast spinning rotor, mounted to providethree degrees of freedom: freedom to spin; freedom to turn about a

Horizontal axis

support

Vertical axis

Freedom to turn (rotate)

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.

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

1 2 3 4

32

31

30 29 28

20

5 6 7 8

19

Figure 3.9 Anschutz Standard 4 gyro compass.

1 Dimming resistance for illumination.

2 Clip-on engaging arm.

13 Narrow conducting band.

14 Window of liquid container.

26 Inner gimbal ring.

27 Outer gimbal ring.

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

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.

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

LIQUID MAGNETIC COMPASS

This compass is illustrated in Figure 3.14

Compass Bowl

Manufactured in high quality non-magnetic brass, this has a clear glassface and a frosted glass base to diffuse the underside lighting The olderdesigns were fitted with chambers to allow for the expansion andcontraction of the fluid, but the modern compass is fitted with a corrugateddiaphragm (elastic membrane) at the base of the bowl for the same purpose.Most compass manufacturers include a graduated verge ring roundthe clear glass face plate Both the face plate and the frosted glass base aresecured 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.