Remembering where the compass points, by Dick Everitt

A. True North is a fixed position at the top of the earth where all the meridians of longitude join up – so all the vertical chart grid lines point to True North. All bearings shown on the chart, such as tidal streams and light directions etc, are given in degrees True as they don’t change.

B. Magnetic North, however, gradually moves each year and can be slightly east or west of True North, depending on where you are in the world.

C. Charts show the difference between True and Magnetic North and how much it is changing. Here it was 40 west in 2000 and moving 9’ east each year. This difference is called variation – you can remember this term by remembering that the angle looks like a ‘V’ on the chart. Magnetic compasses point to Magnetic North and gyrocompasses (which are rare on small craft) point to True North. GPS compasses point to True North as a default, but can also be set up to point to Magnetic North if you wish.

Magnetic compasses can also be affected by things on the boat. Check whether running the engine, navigation instruments, radar, radio, lights, windscreen wipers etc alters the compass reading. I’ve been on some boats where just turning the metal wheel on the helm turned the compass!

One way to check your compass is to compare it to an accurate hand bearing one. Take a bearing of a landmark several miles away, or the moon or a star, and gradually turn the boat in a large circle. If the bearing remains constant there is no deviation in

E. When heeled a metal keel moving sideways can upset some compasses. Steel boats used to have their fluxgate compass sensor (F) up the mast to get it away from the metal hull, but heeling can also affect this. Nowadays steel boats often have the sensor in the hull at the centre of rotation (G).

H. Steel boats can also become magnetised by sailing for days in the same direction, which can affect the compass reading when turning on to a new heading. that spot on the boat. Now sight along the centreline of the boat and compare the readings of the steering compass against the hand bearing one, on several different headings. If your compass has a shadow pin (K) then watch the shadow as you turn

I. If compass errors cannot be removed with correcting magnets, a deviation curve is drawn up. Remember the term deviation by remembering the curve is like a ‘D’. Here, if we want to steer a course of 0900 the compass should read 0950. through 3600 – if it doesn’t move there’s no deviation. If checking against GPS make sure there is no crosswind or tide, as the ‘track’ is where you have been, and if using a ‘bearing to waypoint’ make sure you can see it dead ahead.