DEVIATION AND COMPENSATION
The earth magnetic field can be segmented in two components : horizontal and vertical. · The vertical component affects the horizontality of the compass card and pulls it to dip towards North or South. This natural force varies according to the geographical location : a compass balanced in Lorient is not horizontal in Sydney. · The horizontal component exercises an influence on the card directivity. The compass environment on board and the various sources of interference create a specific, local magnetic field, which is different from the earth magnetic field. The compass does not point towards magnetic North.

DEVIATION
The course discrepancy (in degrees) between the compass north and the magnetic north is called the deviation. It can be negative or positive. In order to minimise this error, your compass should be installed as far as possible from objects generating local magnetic fields : compass, fire extinguisher, loudspeaker, electric wires and equipment, metallic parts of steering system, camera, tools, analogic instruments... Once the deviation errors are known quantities and allowed for, the compass is a perfectly reliable navigation instrument. Deviation is then recorded graphically on a deviation curve, always handy for future reference. Deviation must be checked and updated once a year.

· Step 3 : Establish the deviation curve. To assure accuracy on all headings, check for deviation every 30° (North, 30°, 60°, East, 120°, etc.) and record any deviation (positive or negative) on the deviation card.

How to draw a deviation curve
Check that the compensation screws are in neutral position (according to the compass model, the screw slot will either be horizontal, or aligned with the dot -). Although very simple, the procedure to calculate the deviation must be carefully carried out. Deviation can be checked very effectively by comparing different headings read on your steering compass, with those obtained from a handbearing compass held well clear of any interference. Standing at the stern of the boat is usually the easiest, provided it is a "non magnetic" area. Example : if the handbearing compass reads 30° and the steering compass reads 34°, deviation on a course of 34° is - 4°. · Step 1 : Find a position that is well away from any source of interference. On a nice day with a smooth sea, run the boat under power. Select a distant object or landmark (at least 3 miles away from the boat), whose bearing in known. Sight the landmark with the handbearing compass and steer the boat slowly round in circles. If the bearing remains constant, it means that you are in an area well clear of any interference. If not, repeat the operation, re-siting yourself in another part of the deck. · Step 2 : Compare headings from handbearing and steering compasses. Reversing directions, compare the 2 compasses on each heading. At this stage, the difference you may notice is only due to the deviation on the steering compass.

How to read a deviation table
If deviation is no more than ± 7°, simply draw a deviation table assessing the error and keep it for future reference when calculating the true course. If the deviation curve shows values between ± 7° and ± 20°, the compass must be corrected with the compensation box. A new deviation curve must then be drawn. If deviation is superior to ± 20°, your compass should be re-located to another place on board, to keep well away from local magnetic fields.

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