There are three different types of compasses; Magnetic, Gyro, Fluxgate.
The earth is magnetic, therefore, a freely suspended bar magnet or magnetic needle will align itself parallel to the earth’s magnetic lines of force. This is the basic principle used in a magnetic compass.
A typical compass consists of a number of bar magnets or magnetic needles, a compass card, made of magnetic material, mounted on a wire frame. The magnets are attached to the frame with the N-S markings on the card. The card and magnets are usually suspended to reduce friction.
This mechanism is fitted inside a bowl, with a transparent, sealed cover, filled with liquid to dampen the swing of the compass card. The liquid is mainly distilled water with some alcohol to prevent it from freezing in cold conditions and an expansion bellows is fitted to allow for changes in temperature.
This type of compass is sometimes referred to as a ‘dead beat compass’.
Magnetic compasses are affected by variation of True North and Magnetic North, and by deviation caused by a vessel’s own magnetic field.
A gyrocompass is non-magnetic; it operates on a fast spinning wheel to maintain its direction. By holding its orientation in space, it is able to sense changes in the direction of the vessel.
A gyrocompass is not affected by the same errors that affect a magnetic compass, providing if gets a steady power supply. Fluctuations in the voltage may cause it to run slightly high or low. Many modern gyrocompasses can be run on a 24 volt battery in the event of a power failure.
The fluxgate compass works by reading the earth’s magnetic field. It contains two coils carrying an electric current, one mounted horizontally and the other vertically. Any change in the vessel’s heading in the earth’s magnetic field will result in a change in the flow of current in one coil relative to the other.
Fluxgate compasses are affected by the rolling and pitching of a vessel and should be well gimballed and mounted in a position of little movement in the vessel.
These compasses are subject to magnetic variation and deviation and require a steady power supply, but far less power than a gyrocompass.
Installation of Compasses
The USL Code Section 13 Appendix B and NSCV Part C list the rules for installation of compasses on small vessels. These are summarised below.
Are to be located so that the view of the horizon from the compass position shall be
uninterrupted for a minimum arc of 115O from right ahead on either side of the vessel.
Are to be located forward of the steering position in such a manner that it can be easily
read from the normal steering position.
Are to be fitted with an efficient means of illumination together with a device for
dimming the illumination
Are to be suspended by gimbals so that the bowl remains horizontal when the binnacle
is tilted 40 degrees in any direction.
Are to be located in such a position as to permit proper adjustment.
Must be provided with a compass card according to the following table:
Vessel Length Diameter of Compass card
Less than 10m 75mm
Less than 20m 100mm
20m and over 125mm
Should not be affected by a power failure
Should be supplied with at least 24 hours of emergency power
Should not have electrical equipment placed closer than the ‘safe distance’
recommended by the manufacturer.
Should have all electrical equipment close to it checked in both operating
and non-operating mode to test for any affect on the compass.
Compasses should be mounted on the centreline of the vessel. If this is not practicable, mount the compass so that a line through the centre of the compass card and the lubber line is parallel to the centreline of the vessel. The lubber line is a vertical line on the inside of the compass bowl to indicate the vessel’s heading.
This alignment is important in order that the deviation can be properly calculated and adjusted for.
If a Compass Behaves Erratically
The magnetic compass card is supported only in its centre on a pivot. Erratic behaviour may indicate that the pivot point is worn, there are air bubbles in the liquid, or the compass card is being obstructed through other means.
A New Error in the Compass
If a magnetic compass develops a new and sudden error, it may indicate a large metallic or electrical influence from a newly installed device, or interference from such mundane objects as a metal tea mug, a spanner or a portable radio.
A magnetic compass points to the magnetic north pole. This unfortunately is not the same as the geographic or True north pole. Magnetic north is currently situated in Canada and is moving approximately 3 miles south each year.
The difference between these two north poles is called the magnetic variation and is measured east or west of true north. Magnetic variation changes in different locations around the world. The direction, amount and annual rate of change are shown in the compass roses on nautical charts. Although again the accuracy of this can be affected in some locations by magnetic anomalies in the earth’s surface.
Most vessel’s have their own particular magnetic field. This can be caused by the material used in the construction of the vessel, electrical circuits and the proximity of metal and electrical equipment.
This magnetic field can influence the compass and deflect it east or west of magnetic north. This is called deviation, and the amount and direction of deviation changes with the vessel’s heading.
A compass technician can reduce the deviation and also produce a deviation card tabling the errors, to be used to adjust compass bearings and headings to true bearings and headings.
Magnetic variation and deviation makeup the total compass error.