Compass and Magnetism

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

Mounting 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 on the difference in variation. He then makes correction to the compass or on the ships 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

• 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 11

List two things that make good watchkeeping practice in keeping a check on compass performance

Answer 11

• Comparing compasses (Gyros & magnetic)
• Checking for compass error with bearings or transit and comparing them on a chart.

Question 12

What is Variation

Answer 12

Magnetic variation, also known as magnetic declination, is the angle between magnetic north and true north at a particular location on the Earth’s surface. This angle can change over time due to polar wandering.

Question 13

Describe with the aid of a sketch the cause and effect of magnetic deviation.

Answer 13

The diagram showing a ship with a permanent magnetism port and starboard of the vessel. It can be observed that the effect of the permanent magnetism in the vessel will cause a deflection of the compass needle towards the opposite pole. When the ship is heading due North, the effect is in line with the Earth’s field and the effect will decrease.

As the vessel turns off magnetic north the needle is attracked or repelled by the ships permenant magnetic. The amount of deviation is recorded and on the deviation card to assist in calculating heading so that the vessel can be plotted on navigational chart. Of course Variation will have to be added or subtracted to calculate precisely.

Question 14

List two sources that the mariner may obtain the magnetic variation

Answer 14

Compass rose on the chart
and an Almanac of the area.

Question 15

When must a compass error (check) be carried out by the OOW

Answer 15

At least once a watch or every major change of course.

Question 16

What is magntism ?

Answer 16

Magnetism is a physical phenomenon that occurs when objects exert forces on each other due to their electric charges or magnetic moments. Magnetism can cause objects to attract or repel each other, or to change the direction of moving charges. Magnetism is one of the two aspects of electromagnetism, the other being electricity.

Question 17

List the steps in finding the error of the compass

Answer 17

• 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 18

Other than using a transit to check a gyro error, what might we use?

Answer 18

Rising and setting bearings of the sun

Question 19

When do we do a compass check?

Answer 19

1. Pre departure
2. Once a watch
3. After any significant alteration of course

Question 20

How do we do a compass check?

Answer 20

1. Transits
2. Celestial objects, sun rise and sun set
3. Dock

Question 21

Permenant Magnetism and induced magnetism

Answer 21

Permanent deviation on yachts refers to the consistent magnetic deviation caused by the presence of permanent magnetic materials onboard.

Unlike permanent deviation, induced deviation can vary depending on factors such as the vessel’s heading, speed, and the presence of nearby magnetic sources.