Compensating for the inaccuracy of the compass
If you wish to steer accurate courses by compass, you must know the compass’s deviation, or the amount by which it deviates from the magnetic course. Deviation is complicated—it changes as you swing the boat around and varies for every course, so it’s convenient to make up a graph or a table that can be displayed on a deviation card for quick reference.The process of calculating deviation is known as “swinging the ship.” All it involves is measuring the difference between a known magnetic bearing and what your steering compass says the bearing is. These differences are measured every 15 to 45 degrees while the boat is swung around the clock in a tight circle.The results are displayed on a deviation card, and tell you at a glance what compass course to steer for any given magnetic course, which you can either read straight off a magnetic compass rose on the chart or calculate for yourself from a true course.It’s not difficult to make your own deviation card, and it’s a good skill to learn because deviation usually changes with time. All you need is an inexpensive pelorus from a marine hardware store and a modicum of patience while you swing the ship. There is no great mathematical skill involved, just simple addition and subtraction.There are several other ways to check for deviation and many good books that describe the methods, but Chapman Piloting: Seamanship and Small Boat Handling is the old standby, providing as much detail as you’re ever likely to need.Some small-boat compasses will introduce a surprising degree of error into courses steered if they are not checked for deviation—as much as 20 degrees is quite common. They must, therefore, be compensated, or adjusted, so that deviation is reduced as much as possible—that is, within about 5 or 6 degrees.There are professional compass adjusters who can do this for you, but you can also do it yourself by means of the adjustable external magnets found on most marine compasses. It’s not black magic, and you can hardly do any permanent damage to the compass during your experimentation. If you’re planning to cross an ocean, however, it might be wise to call in an expert, just for peace of mind. Once your compass is corrected, keep it free of nearby magnetic influences such as anchors, binoculars, and steel beer cans.
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