Marine Compass

The Magnetic Compass The Magnetic compass was first used when people discovered that a piece of magnetised iron, if suspended or balanced, would point towards the north. In fact it does not point towards the North Pole, which would be True North, but does point towards the Magnetic Pole, in the Arctic regions of Canada, giving us Magnetic North. The difference between Magnetic North and True North is called the Magnetic Variation and changes by a small amount every year, as the location of the Magnetic Pole moves slightly all the time. Details of the local magnetic variation are printed on every nautical chart and Ordnance Survey map for the year they were published.

The essential part of a compass is a magnetic needle. Because of this you must not allow any other magnets or objects containing iron or steel near the compass as this will affect the accuracy of the compass. Interference caused by such metal objects or magnetic fields near the compass is called Compass Deviation. Most Sea Scouts will be familiar with the “Silva” Compass, used mainly on land for hiking or orienteering. This type of compass can also be used on small boats and canoes if taped down in front of the helmsman and placed right on the centre line of the boat with the arrow facing right ahead.

To use a ‘Silva’ type compass which has been fitted to your boat, turn the bezel line up the course you want to steer with the index mark on the dial. Alter your course so that the red half of the magnetic needle lines up with the north pointing arrow. When they are lined up, the boat’s head is in the direction you want.

Marine Compass

Modern compasses use 360° notation and mark North, East, South and West, the cardinal points. Before 360° notation was used, people referred to the compass points for navigation.

Nowadays the only use for cardinal and intermediate points is for wind direction, when the direction given is the direction the wind is coming from, and tidal streams, when the direction given is the direction the tidal stream is going.

Compass courses and bearings are always given in degrees, using three figures. For example, ‘North East’ is written as 045°, and would be spoken as ‘zero four five degrees’ while south is 180° and spoken as ‘one eight zero degrees’. A Course is the directionyou in which are going to travel.

A Bearing is the direction of one position from another. It is possible to get a rough bearing with a Silva type compass.

Other types of compass designed for marine use are available and are fitted to larger vessels. These are very expensive and not suitable for use in small boats.

Also available is a hand-bearing compass, which will have some form of sighting device which can be used to take more accurate bearings.