How Marine Compasses Work

Card Compass (Marine Compass)

Marine compasses are usually liquid-filled. The liquid has a damping (reducing) effect on disturbances caused by the motion of the vessel in rough seas. The compass card is mounted above the magnetic needle mechanism. It is finely balanced on a needle point, floating and able to swing around to align with magnetic north. The reading is taken from the fixed lubber line on the body of the compass. The lubber line is pointed in the direction of the head of the vessel for steering on a given bearing. An example of a marine compass is shown in cross-section below.

A MAGNETIC COMPASS

(Simplified cross-section)

Gimbals (gymbals) are usually used to mount marine compasses. These keep the compass bowl horizontal, even in heavy rolling and pitching, at sea.

Don't forget that any magnetic compass will point to magnetic north and readings taken will require adjustment for declination (variation).

Deviation is the other problem encountered with magnetic compasses and care is taken to mount the compass away from the influence of unwanted magnetic or electrical fields. Compasses always suffer some deviation caused by electrical equipment nearby, materials used in the construction of the vessel and due to the proximity of other metal objects. A process known as 'swinging the compass' is used to determine the deviation of the compass at each of the eight major compass points. Swinging is best done by professionals skilled in adjusting the compass then creating a deviation card for your vessel. Remember that adding electronics at the helm, a common trend these days, is likely to affect the deviation for your compass as a result of magnetic fields created by electric currents.

Gyrocompasses are used on ocean-going vessels. They are large, heavy and expensive equipment but invaluable on larger ships because of their greater accuracy and reliability. The advantages of a gyrocompass are that it is not magnetic and that it always reads true north. There is no need for correction of compass readings for declination (variation) or deviation. 

The gyrocompass is a sensitive, precision instrument. Its construction is of a freely suspended spinning gyroscope powered by an electric motor. A spinning gyroscope tends to maintain the direction of its axis and in a gyrocompass this constant direction of the axis is towards true north. Thus the compass points to true north and is not affected by magnetic fields, electrical equipment or other metal objects nearby. It is the ideal compass for use with an autopilot.