Compass Work

Question 1

(a) Define magnetic variation.

Answer 1

a) The angular distance between Magnetic North and True north measured east
or west of true north (3)

Question 2

(c) Define magnetic deviation

Answer 2

c) The angular distance between magnetic north and compass north, measured
east or west of magnetic north (3)

Question 3

(d) Describe how the OOW would use a transit bearing to check the ships deviation
table or curve.

Answer 3

Step:

1. Note ships head
2. Determine True brg from chart
3. Compare with compass brg from ship
4. Determine comp error and apply var’n to obtain dev’n
5. Compare the dev’n calculated with that in the dev’n table for the ships head.
   (11)

Question 4

1. State why the error of the ships’ magnetic compass changes with alterations in the
   ships head.

Answer 4

Ships magnetism induced by earths field changes as ships angle to the field changes.
Alignment of ships permanent field and earth’s field varies with the ships head.

Question 5

2. What arrangements are incorporated within the compass binnacle to reduce
   deviation?

Answer 5

Permanent magnetism
• Permanent magnets athwart ships, fore and aft and below the compass for Heeling Error
Induced Magnetism
• Soft iron: Kelvin’s balls, Flinders Bar for induced magnetism.

Question 6

1. Compass Best Error West
2. Compass Least error East

Answer 6

Question 7

Interpolation

Answer 7

Question 8

CoSt Gyro to W/T

Answer 8

Question 9

Variation calculation

Answer 9

Question 10

Compass Error

Answer 10

Question 11

Where is the local value of Variation of the standard compass to be found?
(5)

Answer 11

Variation charts
Compass rose

Question 12

Give three main factors that cause Deviation of the Magnetic compass.
(10)

Answer 12

a) Ships permanent magnetism
b) Ships induced magnetism (temporary changes with heading)
c) Equipment sited close to compass

Question 13

Describe three methods of minimising this deviation.
(5)

Answer 13

a) Place compass on centre line
b) Place compass clear of equipment
c) Adjust compass using magnets and soft iron

Question 14

Explain the causes of both permanent and induced magnetism. (10)

Answer 14

Permanent
Caused by heating hammering etc. Takes on direction of the local
magnetic field. Ships magnetic field determined by orientation during
building, complicated by pre fabrication. A Permanent field only
changed by further heating, hammering etc.
Induced.
A temporary magnetism caused by magnetic field and
orientation of ship within said field, changes when ships head changes.
(10 marks) :

Question 15

List the five circumstances that the magnetic compass should be checked
and adjusted by a compass adjuster, as required by the SOLAS 2002
regulations
(5)

Answer 15

1. First installed
2. Become un reliable
3. Structural change
4. Electrical equipment near compass
5. After 2 years when no record of compass deviations has been kept.
   (5marks)

Question 16

How frequently should the OOW check the compass error?
(2)

Answer 16

large alteration of course and once a watch (2 marks

Question 17

State 3 considerations when sighting a magnetic compass on a ship.
(3)

Answer 17

1. View all round horizon.
2. Viewable from steering position.
3. On centre line.
4. Away from magnetic influences.
5. In the centre of ships’ magnetic field.
   (3 marks)

Question 18

Draw a Petal Diagram

Answer 18

Question 19

On a steel yacht state the type of correctors used to correct the compass for the
effects of induced magnetism. (4 marks)
Kelvins spheres and flinders bar; (soft iron for soft deviation) 4 marks

**State where these correctors may be placed if the compass is situated in the wheelhouse in the fore part of the vessel. (6 marks) **

Answer 19

Balls either side, for atwartships component 3
Flinders bar either for’d or aft depending on the vertical component of the induced deviation.
3