Compass Courses
Why you must adjust for variation and deviation
If your steering compass is out by only 5 degrees, you will be 1 full mile off course for every 11.5 miles run. This illustrates the importance of having your compass swung, to test its accuracy. You can have it swung by a professional, who can also correct it so that it always shows the correct magnetic course. Or you can swing it yourself and make a simple correction chart that provides the correct allowance to make for deviation. It’s a relatively easy process that’s described in detail in several boating books.There are two effects that prevent a magnetic compass from pointing to true north. The first is the fact that the Earth’s magnetic poles, to which the ends of a compass needle are drawn, are not in the same spot as the geographic poles. The difference, in degrees, is known as magnetic variation. And vary it does, from place to place on the globe. Depending on your location, the magnetic north pole might lie to the west or east of the true north pole by 20 degrees or more. Luckily, magnetic variation is well recorded and shown on all charts. It changes slowly over time, however, so if you’re using an old chart (which you shouldn’t be), you may need to allow for this. The rate of change is usually given in the compass roses printed on the chart.The second effect is known as magnetic deviation. It’s generated by metal and magnetic currents present on your own boat, to which the compass needle may be attracted or repelled. The mass of a steel engine near the compass will affect its accuracy, as will strong magnets in a radio or speaker box or even a knife in the pocket of a crew member.Every boat is individual in this respect and, to complicate matters, the amount of deviation varies with the heading of the boat. It is usually greater on some courses than others. Deviation up to about 5 degrees is common and does not require correction. Simply consult your deviation card and steer a little left or right of the correct course to compensate. But if your deviation is more than 5 degrees, as it probably will be on a steel boat, the compass should be corrected.Some compasses have built-in corrector magnets that you can set with a small screwdriver so that the compass points to magnetic north on all courses. Then all you have to worry about is the variation mentioned previously. If you don’t trust yourself to do this important work, a professional can set the corrector magnets for you or cure the deviation with external magnets and/or iron masses.If your steering compass is behaving suspiciously, you can quickly check it with a hand bearing compass. Stand exactly on the centerline of the boat and sight with your hand bearing compass directly forward over the middle of the bow, making sure you are 5 feet or more away from any magnets or magnetic metal. This small compass should then be free of variation. Note its heading and read the steering compass at the same time. The difference gives you the deviation of the steering compass and will alert you if something is seriously wrong. By the way, if you wear glasses with steel parts, take them off when you use the hand bearing compass; even the tiny steel hinge screws can affect the compass at close range.Wires carrying electrical current create magnetic fields that can affect the compass, so be sure that the wires leading to your red compass light are twisted around each other, canceling out the magnetism. Look out for portable radios, headphone sets, or other electronic devices left near the compass—they have powerful magnets. Make sure there’s nothing within at least 3 feet (preferably 5 feet) of the compass that could affect its accuracy, including tools, cans, and engines. It’s difficult to achieve this on a small boat, of course, but do your best and expect the worst.See also Compass Bearings;
