How Does a Magnetic Compass Work on Metal Ships?

by | Last updated Feb 3, 2024 | Equipment, How-to Guide, Navigation | 0 comments

The two words that seem contradictory here are magnetic compass and metal ships.

We all know that magnets attract iron, and a vessel is a huge block of metal floating on the vast oceans.

But every merchant ship of any type employs a magnetic compass that points to the magnetic North.

How does it function? Does the surrounding metal interfere with its operation? Let’s discuss them here in this article.

Key Takeaways

  • A magnetic compass is a critical navigational instrument mounted on a ship that determines the ship’s heading relative to the Earth’s magnetic field.
  • Careful installation of compasses away from magnetic interference is crucial for a more accurate performance.
  • Correctors like permanent magnets and soft iron compensators counteract a ship’s magnetism and minimize deviation.
  • SOLAS regulations, specifically Chapter V Regulation 19.2.1, mandate the presence of a properly adjusted standard magnetic compass on all ships.

What is a Ship’s Magnetic Compass

A ship’s magnetic compass is a critical navigational instrument mounted on a vessel that consists of a magnetized needle allowed to rotate in the horizontal plane.

This design allows the device to determine the ship’s heading relative to the Earth’s magnetic field.

Like any compass used in direction finding, it uses a compass rose, a circular card with directional markings surrounding the needle, indicating cardinal and intercardinal points.

Why is a Magnetic Compass essential on board?

A magnetic compass is very important because it serves as a reliable backup in case of technical failures or power outages. 

Even if gyro compasses are more accurate, they are prone to disruption, especially during power failures.

Magnetic compasses are relatively simple mechanical devices that don’t rely on external power sources. Their simplicity makes them robust and dependable in various conditions.

Two Types of Magnetic Compass

Standard Compass

This is a magnetic compass for navigation purposes. It is on a suitable binnacle containing the required correcting devices.

Additionally, this compass has an azimuth reading device.

Steering Compass

This is a magnetic compass for steering purposes. It is on a suitable binnacle containing the required correcting devices.

Basically, both compasses are the same. They just differ in their primary usage as well as their mounting.

A standard magnetic compass installed on the monkey island with its soft iron spheres colored green and red.

How Does a Magnetic Compass Work on Ships?

The basic operating principle of this device is magnetism – a compass needle aligns itself with Earth’s magnetic poles.

But ship hulls are made of materials like steel that can affect magnetic fields.

So, how does a compass maintain accuracy?

Here are the details:

  1. Quality compasses are specially constructed using strong magnets and minimal friction.

This allows the needle to rotate easily to find the magnetic north pole.

  1. The compass is carefully installed away from sources of magnetism.

Thus, you can find it on the monkey island.

  1. A binnacle housed the compass, and within it are two magnet correctors. These are the permanent magnet correctors and soft iron correctors.

Permanent magnet correctors compensate for permanent magnetic fields at the compass. Soft iron correctors compensate for induced magnetism. 

  1. A Heeling magnet also corrects for both permanent and soft iron correctors.

It may need adjustments for changes in latitude.

However, adjusting the heeling magnet also requires adjustments to the permanent and soft iron correctors.

  1. These correctors significantly minimize deviation, allowing the magnetic needle to align more closely with Earth’s magnetic field lines.  These lines orient from the magnetic south pole to the magnetic north pole.
  1. The compass is also mounted on gimbals, which keep it level despite the ship’s motion, and fluid-filled dampers minimize disturbances. 
  2. Even with reduced metal exposure, minimized electrical components, and corrective measures in place, the compass still has some degree of deviation error.

To counter this, a ship is provided with a deviation card.

What is a Deviation Card/ Table?

A deviation card or table is a record of the errors in the magnetic compass at various headings.

If you notice on annual compass adjustments, the captain maneuvers the vessel 360°. During this time, the technician records the errors at each heading and plots them on the deviation card. 

You normally find the deviation card posted near the chart table for reference during inspection and taking compass errors.

A Sample of Magnetic Compass Deviation Certificate of a merchant vessel.


Deviation is the difference between the compass heading and the magnetic heading.

It is caused by the permanent magnetism of the ship, as well as nearby electrical equipment.

To compensate for deviation, refer to the deviation card and note the readings on where your ship is heading.


Variation is the angle between the magnetic and true meridian at a given location. 

The magnetic anomaly of a particular locality causes variation and the error can be in a westerly or easterly direction.

Variation changes every year. You can find it by checking your ECDIS or looking it up in your GPS receiver.

Parts of a Ship’s Magnetic Compass 

A vessel’s magnetic compass consists of several key parts, including:

  1. Compass Card – A circular card marked with directions and degrees that are free to rotate and indicate the ship’s heading.
  2. Compass Bowl – A liquid-filled bowl that houses the compass card. The liquid (often oil) dampens the oscillations of the compass card.
  3. Binnacle – The binnacle is a housing for the compass, designed to protect it from the elements and to provide support for the correctors used to compensate for the ship’s magnetic influences.
  4. Correctors – Used to compensate for the magnetic influences of the ship. These include permanent magnet correctors and soft iron correctors.
  5. Azimuth Circle – A graduated horizontal circle attached to the compass bowl, used for taking bearings.
  6. Lubber’s Line – The lubber’s line is a fixed line on the compass bowl or binnacle that references the ship’s heading.
Basic Parts of a magnetic compass.

Compass History

Before the introduction of the magnetic compass on ships, the only way navigators determined their position and direction was by using landmarks marked on a chart.

Coupled with their skills in celestial navigation and the knowledge passed down by their peers, that was all they needed to find where they were and where they were going next.

But it was inaccurate.

Everything changed with the invention of the magnetic compass 2,000 years ago in China. 

Navigation improved significantly, and exploration increased as well.

For much of maritime history, the only heading reference for navigators has been the magnetic compass.

However, as ship-building advanced, metal ships entered the seas, greatly impacting the compass’s accuracy.

Carriage requirements

Even though 99% of merchant ships today are made of metal, this equipment is still widely used on board.

The invention of gyro compasses may take away its spotlight, but SOLAS Chapter V Regulation 19.2.1 still requires it, irrespective of vessel size.

All ships, irrespective of size, shall have a “properly adjusted standard magnetic compass” or other means, independent of any power supply, to determine the ship’s heading and display the reading at the main steering position.

Furthermore, IMO Resolution A.382(X) lays out its performance standards and carriage requirements on ships.

May the winds be in your favor.



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