Compass

Question 1

Draw and label a compass binnacle

Answer 1

Compass
Soft iron sphere
Flinders Bar
Heeling magnets:
Athwartship corrector magnets
Fore and aft corrector magnets

Question 2

What is variation

Answer 2

Variation is the angle between the geographic (true) and magnetic meridians at any place. It is measured east or west from true north.

Question 3

Why does deviation depend on ships heading

Answer 3

It’s because the ship is long and skinny, hence if you imagine we have a large anchor and several shackles of cable on the bow which is highly magnetic we could say the ship has a positive or negative force from the bow!

Hence:
• There is no deviation on compass north and south when the pull is in line with the
compass needle.
• There is maximum deviation on compass east and west when the pull is at right angles
to the compass needle.
• On the other directions of the ships head the deviation varies
A ship depending on which way she is pointing will deviate the compass needle from pointing “North” hence the compass adjuster swings the vessel and places the permanent magnets to compensate for this (hopefully) but a small amount will remain and hence we have a deviation card. Showing how much deviation we have on different courses.

Question 4

How to check a compass

Answer 4

This is a very important question and will come up!

• Freedom of movement of the gimbal.
• Card floating freely and level and rotating without any friction.
• Liquid free of bubbles and clear.
• Compass card clear no distortion or discoloration.
• No liquid leaks around seals or filler plugs.
• No tools or magnetic equipment in the area.

• Optical system (if any) correctly adjusted and clean.

Question 5

• When to swing a compass

Answer 5

Magnetic compasses should be adjusted when:

• They are first installed.
• They become unreliable.
• structural repairs or alterations that could affect its permanent and induced magnetism.
• Electrical or magnetic equipment close to the compass is added, removed or altered.
• A period of two years has elapsed since the last adjustment and a record of compass.
• Deviations has not been maintained, or the recorded deviations are excessive.
• When the compass shows physical defects.
• Collision or major change of latitude.