You can employ a compass adjuster to swing your compass or you can do it yourself quite easily. There are several ways to do it, but the quickest and easiest is to have the helmsman steer due north (000°C) on the steering compass while another person uses a hand bearing compass to sight along the centerline of the boat while standing at the stern, well clear of any magnetic interference.
Make sure there is no deviation at your chosen spot, by taking a bearing of a chartered transit and checking it on the chart. Note the course steered and the reading from the hand bearing compass, then turn to 030°C and repeat the procedure.
Continue recording bearings every 30° until you have completed a full 360°. Plot the results on graph paper to produce a curve giving the amount and direction of deviation for each course steered.
If deviation exceeds about 6° on any course, employ a compass adjuster to correct the compass.
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