A magnetic compass should align itself steadily to the earth’s magnetic field, pointing to magnetic north. However, when the compass is installed on a boat, metals and electrical equipment may cause magnetic fields that distort the earth’s field at the compass location. The effects of these onboard magnetic fields change with the vessel’s heading. The difference between compass north and magnetic north on the different headings is called the deviation. Compass adjustment is the compensation of these deviating forces with magnetic correctors. For various reasons such as design, location and practical expedience, all the deviation may not be removed. The residual deviation is recorded as a table or a curve of deviation against the compass headings.
- After periods of lay up.
- When a new compass is installed.
- When deviation exceeds 5 degrees.
- On a new vessel or in a new area of operation.
- After trauma, such as lightning strike, grounding, fire, etc.
- When compass performance is unsatisfactory or unreliable.
- When a record of compass deviation has not been maintained.
- After alterations and additions to the vessel’s structure and equipment.
- After repairs involving welding, cutting, grinding, etc. which may affect the compass.
- When electrical or magnetic equipment close to the compass is added, removed or altered.
- When compass deviation does not appear to correspond with that shown on deviation card.
The magnetic compass is an essential navigational instrument. On the water, knowledge of direction is of vital importance, especially when visibility is poor or visual clues of direction are not available. A correctly functioning magnetic compass shows the vessel’s heading. The magnetic compass is independent of power supply, making it a most reliable navigational instrument. The compass is also a key tool for determining the risk of collision (Colregs rule 7).
GPS has a built in electronic compass. However it only works when you are on the move. A GPS requires satellite signals and electricity to function. It is an excellent tool but power failure or electronic break down renders it useless.
Deviation is the difference between a Compass reading and a True reading after allowing for magnetic variation. Good seamanship recommends that all vessels should have a deviation table for the magnetic compass.
Deviation is caused by the vessels magnetic pull. Steel or electrical sources can affect the pull on the magnetic compass. Common sources include electronics, audio speakers, batteries and wire cables.
The time to accurately assess and compensate a compass varies. However, the time should not exceed two hours. The vessel must leave the quay to manoeuvre around the compass headings (swinging the compass). During this swing, distant bearings or other appropriate methods determine the vessel’s magnetic heading.
Yes, you can check for error in your compass by aligning the vessel on known headings and noting the compass reading.
During its life, your compass card will pivot on its jewel thousands of times. Sooner or later, the pivot will become worn, and the compass card may fail to respond properly due to increased friction. You can check for a worn or defective pivot by bringing a magnetic object such as a small magnet or a screwdriver close enough to the compass to deflect it about five or ten degrees. When the magnetic object is removed the compass should return quickly to the original bearing. Repeat this procedure several times, deflecting the card in opposite directions by changing the location of the magnetic object relative to the compass. If the compass doesn’t return to exactly the same bearing, the pivot is either worn or defective, and the compass is due for overhaul or replacement. An overhaul can return a compass to new condition, so don’t discard an expensive piece of equipment without first getting a professional evaluation. Just because you rarely use your boat doesn’t mean that the pivot can’t wear. A boat kept on a mooring is constantly changing direction 24 hours a day, wearing away at the pivot jewel.