Care of the magnetic compass

This page explores the principles of operation and care of the magnetic compass. It then discusses operational checks before looking at the use of deviation and variation to determine the true north from compass north.

Describe the principle of operation of a magnetic compass

The magnetic compass uses a freely rotating magnetised needle to indicate the local magnetic meridian, thereby indicating the magnetic north.

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Outline the checks and steps required to care for the magnetic compass

SOLAS V/19 makes the carriage of a standard magnetic compass, together with a pelorus and a means of correcting the heading for variation and deviation, mandatory for vessels irrespective of size.

This piece of equipment must be regularly checked to ensure its continuing and efficient operation. Checks to be performed include:

The compass bowl must be topped up fully with liquid (glycol and distilled water solution) so that no air gaps or bubbles form. Any bubble greater than 20mm should be rectified.
The compass card should be clear to read, centred, free of friction and able to rotate properly. To check that the card is free of friction, a magnet can be brought up to the bowl so that the card is deflected by 10 degrees. Once the magnet is removed, the card should promptly return to its original heading.
The binnacle bolts should be securely mounted.
The fore/aft and port/starboard correctors are in the correct position, as recorded by the previous compass adjustment.
The azimuth ring should fit correctly and be free to move through a full rotation.
The azimuth mirror should be in the correct place and rotate correctly. Checking the alignment takes the azimuth of a celestial body in both the arrow-up and arrow-down positions. If there is a difference, the mirror is not aligned with its plane.
The light should work correctly and should not cause a deflection in the compass card when the supply current is switched on.
The lubber line should be correctly centred. To check this out, take an azimuth off the forward mast headlight.
It should be easy to view the magnetic compass from the steering position and make a precise reading.
Finally, the Master should ensure that the compass remains reliable by regularly checking the compass error book. This stops massive changes in deviation from going unnoticed.

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What are the causes of a large or sudden change in deviation?

If a sudden large deviation appears with no possible or likely explanation, then it could be for several reasons:

due to the effect of an outsized localised magnetic anomaly
changes in the vessel's equipment or arrangements, such as the ship's cranes
cargo has magnetic properties such as bulk metal, iron ores, or containers, for example
if all the above has been considered and found not to be the case, then it may be suspected that the correcting magnets have been moved or fallen out of position.

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When and by whom should the magnetic compass be adjusted?

A certified compass adjustor should adjust the magnetic compass when:

the vessel has spent time laid up either for commercial expedience or dry-docking
compass deviations become excessive and unreliable
compass deviation record has been maintained, for whatever reason
the physical impact of stresses on the fabric of the vessel caused by a collision, grounding, or other serious impact has altered the magnetic properties of the vessel.
new steel structures and equipment have been added (this alters the magnetic signature of the ship)

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What can the Master do when a local compass adjuster is unavailable?

If it is absolutely necessary to adjust the compass, but no certified adjustor can be found in the port, then a Master Mariner may adjust the compass.

However, the compass must then be adjusted again by a certified adjustor as soon as practicably possible.

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What is magnetic variation, and how is it obtained?

Variation is the difference in angle between the bearings of true and magnetic north poles. It is a measure of the difference in the locations of the magnetic north pole from the geographic north pole.

The geographic north pole is the centre of rotation of the Earth as it spins on its axis and its location is permanent.

The magnetic north pole is the point where the horizontal component of the Earth's magnetic force is zero, and the vertical component is at its maximum. The magnetic north is not fixed and is constantly moving, meaning that it is hard to predict its location exactly.

As the value of variation changes according to the observer’s position on the Earth, the best estimate of variation is found on local navigational charts.

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What is magnetic deviation, and how is it obtained?

The deviation is the difference in angle between magnetic north and compass north as observed on the vessel.

It is the sum of all the magnetic vectors originating from the ship’s structure and fabric. Its various components interact, and the total sum will be designated East or West according to the resultant sum of these components.

Correctly determining the value for deviation is of paramount importance because, taken together with variation, the difference between true and compass north.

The deviation is usually obtained once per watch by comparing the angles of the magnetic north and compass north.

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What factors can affect the deviation?

The deviation is the sum of all the components of the ship’s magnetic characteristics, which deflect the needle from the true north meridian. The various components which combine to cause deviation are the vessel’s structure, keel, machinery, lifeboats, deck cranes and fittings and electrical equipment. Ships can be magnetised in the following ways:

permanent ferromagnetism of the ship’s steel, occurring when it is newly built or berthed on one heading for a long period (e.g. in dry dock)
semi-permanent magnetism caused by significant physical forces such as pounding in heavy weather
temporary magnetism induced by the Earth’s magnetic fields acting on the ship’s soft iron
temporarily induced magnetism caused by electromagnetic equipment such as antennae

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Why is variation and deviation so important to the marine navigator?

If a ship suffers a catastrophic failure of its compasses, then the magnetic compass will allow the navigator to determine where the true north is and plot a safe passage to the nearest place of refuge. The true heading to the nearest port can be found on the chart, but in order to steam along with this heading, the navigator needs to truly go north accurately.

This can be obtained by using the following mnemonic:

True Virgins Make Dull Companions.

Sample readings are as follows:

True: 300⁰ (true course to be steered to the nearest safe port)

Variation: 5⁰W (variation obtained from the chart)

Magnetic: 305⁰ (magnetic heading)

Deviation: 3E (deviation calculated for each watch)

Compass: 302⁰ (compass course to be steered to make 300⁰ (T)

Remember that error west means compass best, and error east means compass least

#RT @Seeker: Researchers believe they've uncovered why Earth's magnetic north pole keeps shifting. pic.twitter.com/IiNz9VARpP
— Dr. Olaf K. Krueger 🇦🇺 (@drokkrueger) January 22, 2023

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Describe the various methods for finding the deviation

There are six methods for determining the deviation of a compass, each of which involves a comparison of the ship's head with a second line of bearing as follows:

True bearing of a known fixed object, with variation applied
The azimuth/amplitude of a celestial body
Reciprocal bearings between the ship and a fixed object ashore
The ship transiting on a line of known and fixed headings
Average readings taken during a compass swing
A comparison with the gyro heading. This is the least attractive option due to gyro errors, but can occasionally be used as an alternative to the above methods.

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