Compass work

Question 1

Sketch lines of magnetic force showing the earth’s magnetic field and how a compass needle indicates Magnetic North

Answer 1

Question 2

Define Variation

Answer 2

The angular difference between True North and the Earth’s Magnetic
North. Variation is named East or West of True North

Question 3

Define Deviation

Answer 3

The angular difference between Magnetic North and Compass North. Deviation is named East or West of Magnetic North

Question 4

What corrects for the ship’s hard iron component of error?

Answer 4

Fore & Aft and Athwartships permanent magnets &
Vertical (Heeling) magnet

Question 5

List the correctors which correct for the ship’s soft iron component of error

Answer 5

Kelvin spheres (quadrantal correctors) & Flinders Bar &
Vertical (Heeling) magnet

Question 6

In addition to soft iron and hard iron, what else can affect the magnetic compass needle onboard?

Answer 6

Siting of electrical equipment too close to the binnacle

Question 7

What does a compass adjuster do?

Answer 7

He corrects the magnetic compass by swinging the ship and making observations and adjusting correctors. He then produces a Deviation card.

Question 8

What is the residual error after the compass has been ‘swung’ and what is it made up of?

Answer 8

Compass Error. Made up of Variation and deviation.

Question 9

List the steps in finding the error of the compass

Answer 9

Take a Compass bearing of something.

Calculate or find the True bearing.
Find the difference between the two, this is the error of the compass.

Label it East if lower than the True bearing or West if more.

Question 10

List the further steps necessary to check the deviation card is correct for the ship’s head

Answer 10

Take a Compass bearing of something.

Calculate or find the True bearing.
Find the difference between the two, this is the error of the compass.

Label it East if lower than the True bearing or West if more.

Question 11

List the further steps necessary to check the deviation card is correct for the ship’s head

Answer 11

At the same time as making the observation note the ship’s head.
From the chart calculate the Variation.

Remove this from the Compass Error to find the observed Deviation.

Compare this with the Deviation for the ship’s head on the Deviation Card

Question 12

A ship on a heading of 231oC finds that the deviation for each of the following bearings is 2½oW: 103oC,
205oC & 271oC.

Why is the deviation not different for each one?

Answer 12

The deviation is dependent on the ship’s heading, not bearings taken

Question 13

Summarize how a free gyroscope is made into a gyrocompass

Answer 13

It is gravity controlled to make it North seeking,
Then damped to settle on the meridian