Magnetism

Question 1

What ships are required to carry a magnetic compass?

Answer 1

All seagoing ships 150GT and over

Question 2

What ships require a spare magnetic compass?

Answer 2

All ships 150GT and over

All passenger ships

Question 3

When should a magnetic compass be adjusted?

Answer 3

1. When first installed
2. When it becomes unreliable
3. After any structural alterations or repairs which may affect the permanent or induced magnetism
4. When any electrical or magnetic equipment is added, altered or removed close to the magnetic compass
5. More than 2 years since the last adjustment
6. If no record of deviations has been kept
7. Deviations are excessive
8. The compass shows signs of defect or damage

Question 4

What is permanent magnetism?

Answer 4

Magnetism that is remained in the material when the magnetic field is removed

Question 5

What is induced magnetism?

Answer 5

Magnetism that is lost when the material is removed from the magnetic field

Question 6

What is hard iron and soft iron?

Answer 6

A metal is defined by how much carbon it has in it and is either (soft) with little carbon content such as iron or (hard) with large carbon content such as steel.

Hard iron maintains its magnetism whereas soft iron loses its magnetism easily

Question 7

What is magnetic dip?

Answer 7

The angle that the flux lines between the magnetic poles enter the surface of the earth
Dip = 0 degrees at the equator and 90 degrees at the poles

Question 8

What is deviation?

Answer 8

The deflection of the compass needle to the right or left of the magnetic meridian

Question 9

What are the components of ships permanent magnetism?

Answer 9

Fore and Aft component (B)
Athwartship component (C)
Vertical Heeling component (Forms part of B)

Question 10

What are the formulas for a sine curve and a cosine curve?

Answer 10

Sine - Y = Sin(x)
Cosine Y=Cos(x)

Question 11

What variables affect induced magnetism?

Answer 11

1. Strength of the surrounding magnetic field
2. Mass of the metal
3. alignment of the metal in the magnetic field

Question 12

What ship movements will change the induced magnetism?

Answer 12

1. Change of latitude
2. Heading
3. Heel of the ship

Question 13

What are the 5 components of ships magnetism?

Answer 13

A - Constant error from misalignment of compass, calculation error from position fix and small asymmetrical horizontal induced magnetism

B - Fore and Aft Permanent and vertical induced

C - Athwartship Permanent

D - Symmetrical Horizontal induced

E - Asymmetrical horizontal induced

Question 14

What is heeling error?

Answer 14

when the ship rolls within the earths magnetic field, there is changing induction of the soft iron as the ships alignment changes and also the alignment of the ships permanent magnetism and earths magnetic field changes.

Question 15

What correcting magnets are within the compass binnacle?

Answer 15

1 - Vertical heeling magnet hanging inside the binnacle corrects permanent and induced heeling error
2. Fore and Aft permanent magnets
3. Athwartship permanent magnets
4. Vertical soft iron flinders bar in an external tube
5. Soft iron spheres either side of the compass

Question 16

What causes a sticky/sluggish needle and how is it fixed?

Answer 16

When the magnetism of the ship is so great that it overcomes the directive force of the earths magnetic field so the needle stays attracted to the same point on the ship even when the heading is changed.

Fore and aft magnets moved closer to the compass with opposite polarity to the ship until the force is neutralised (Red pole forward if bow is blue)

Question 17

What precautions should be taken prior to and during compass correction?

Answer 17

1. make sure there are no bubbles in the compass card fluid
2. Secure all magnetic equipment in normal seagoing position
3. Bring the the ship upright on even keel.
4. B and C magnets should be symmetrical either side of the compass
5. The “magnetic circle” is a sphere around the compass which should always be kept clear of any magnetic or electrical equipment

Question 18

What are the steps for compass correction of each permanent coefficient?

Answer 18

1. Correct B (Fore and Aft) first on a 90 degree heading with fore and aft magnets. 270 corrected in opposite direction
2. Intercardinal deviation can be calculated (adjacent intercardinals 0.7 x cardinal correction value in the same direction)
3. Correct C (Athwartships) on 000 heading with athwartship magnets) 180 corrected in opposite direction
4. Intercardinal deviations calculated again

Question 19

How is induced magnetism corrected?

Answer 19

Vertical asymmetrical magnetism using the flinders bar (also corrects vertical permanent magnetism)

Horizontal symmetrical magnetism using the spheres.

Question 20

How can you estimate the coefficients of magnitude?

Answer 20

Deviation = A + BSin(heading) + CCos(heading) + DSin2(heading) + ECos2(heading)

A = Mean of all deviations on card
B= Mean of deviations on 000 and 180 (180 polarity reversed)
C = Mean of deviations on 090 and 270 (270 polarity reversed)
D = Mean of inter cardinal deviations (135 and 315 polarity reversed)
E = Mean of cardinal deviations (090 and 270 polarity reversed)

Question 21

What is Kelvins rule?

Answer 21

1. Correct deviations on East or West courses by using fore and aft B magnets when ship has arrived at places of weaker magnetic field
2. Use the flinders bar when she has arrived at places of stronger vertical magnetic field

Fore and Aft (B) magnets are then used for remaining B error and sphere adjustments are refined.

Question 22

What methods can be used to find deviation?

Answer 22

1. The variation and gyro error can be applied to the gyro heading to give a magnetic heading and compared to the compass heading.
2. A pelorus can be used clamped to the compass card in the direction of the suns magnetic azimuth. The sun is then lined up with the mirror. The magnetic heading will be shown on the lubbers line. The mirror is moved to keep the sun aligned as the ship changes heading.
3. Compare the compass heading of the sun using a compass circle and the calculated suns azimuth
4. Use of terrestrial transits and charted bearing

Question 23

What additional errors in the ships magnetism occur?

Answer 23

1. Transient error - effects of magnetic material or electronics too close to the compass

• Magnetic cargo
• Crane booms
• Cable reels
• Metal doors
• Heating of exhaust metal
• Metal clothing/watches/phones
• portable radios

2. Retentive error - Induced magnetism doesn’t immediately dissappear after being on a steady heading for a long time. Altering courses should solve this
3. Gaussin error - eddies in the ships magnetic field as it changes heading. Induced magnetism will not settle until 2 minutes after changing heading

Question 24

How is heeling error corrected?

Answer 24

• The vertical permanent heeling magnet corrects both permanent and induced magnetism.
• Corrected after radical latitude changes and polarity changed after hemisphere changes.
• It is adjusted by moving the heeling magnet up or down while the vessel is on N or S heading until compass oscillations are reduced.

Any resultant deviation changes are corrected with fore and aft magnets

Question 25

When should compass errors be calculated?

Answer 25

1. After every large alteration of course
2. At least once every watch

Question 26

In the UK who can correct the compass?

Answer 26

All adjustments should be made by a qualified compass adjuster holding a UK certificate of competency as a compass adjuster

When necessary, a Master can adjust the compass but it must be done again by a compass adjuster asap.

Question 27

Where should spare compass be stored?

Answer 27

Away from the bridge in case the bridge is damaged and not accessible

Question 28

How often should the deviation card be produced?

Answer 28

Annually