COMPASS ERRORS

The compass lines itself up with the magnetic field of the earth. However, the magnetic field does not operate in straight lines. Local influences can also modify it. That means that the heading your compass shows may not be the heading your boat is actually traveling on. These effects are collectively known as errors. This does not mean the compass is defective. Rather, compass errors – there are several kinds – are anomalies that change the instrument's performance, and they must be accounted for when you navigate.

The compass does not usually point to the North Pole, which is called true north. A GPS will indicate your course over the ground in terms of true north. A compass, by contrast, points instead to the north magnetic pole (called magnetic north), which is located in northern Canada. The difference between true north and magnetic north is known as variation. The exact amount of variation will differ at different locations and, if needed, you can compensate as you navigate. Your chart will have a number of compass roses printed on it. The outside ring of each rose will show true directions. The inner ring will show magnetic directions. In the center will be the amount of variation for that location. For example, if the variation is listed as "11 W," that means magnetic north is 11 degrees west of true north. It will also mention the rate and direction of change. Magnetic north moves around, though its movement is so slow that you can generally ignore it.

Variation is a function of location, and it affects all compasses the same. A second error you must be aware of is deviation, which refers to the effect that metals and other magnetic influences have on your compass . Deviation obviously differs from boat to boat, and at different headings. It can even change as ferrous material is added, subtracted, or moved about the boat from time to time. I once sailed as navigator on a boat going to Bermuda. In preparation for the voyage, the skipper had rearranged the stowage on the boat, and his changes had an effect on the deviation. We didn't discover this until we were putting to sea and I realized the compass was showing us the wrong course. It made for some very challenging navigation.

There are other sources of compass error but they are generally minor and of no great concern. There may also be some local anomalies, but these are usually noted on the chart.

COMPENSATING THE COMPASS

The presence of iron and steel on your boat can affect your compass. If there is not too much of it, many compasses can be adjusted – compensated – to neutralize the error. If the boat itself is made of steel, you can still compensate the compass using steel balls on either side, but this is a very technical process and requires the services of a professional.Gene BjerkeCompass, chart and electronics are the navigator's primary tools.

For other boats, the owner can often do the job. On some, usually older, boats, the compass has been compensated by placing small magnets around it. Most modern compasses come with compensating magnets installed, which allow you to cancel out your boat's deviation if it is not too large. They are usually set by turning slotted pins, one for N-S compensation and one for E-W compensation. The magnets should come zeroed out on a new compass.

There are a number of ways to compensate your compass, sometimes called "swinging ship." You can hire someone to do it for you, but I have had good luck with the following simple method. Make a shadow pin by inserting a dowel into the center of a straight line drawn on a flat board. The pin needs to be perfectly square with the board. You will also need a coin or a non-ferrous screwdriver – some compasses come with a plastic tool – to turn the pins. What you are going to do is use the sun as a reference to be able to run the boat in precisely opposite directions. It is true, the sun moves, but it takes four minutes to move one degree and you can make your adjustments in less time.

This is best done under power on calm water, with at least two people aboard. Have one person steer the boat due north by compass. Set up the shadow pin on a level surface in the sun and arrange it so that the shadow falls along the line on the board. Then the helmsman should turn the boat around and run in the opposite direction, steering to keep the shadow on the line. The compass should read due south. If it doesn't, slowly turn the N-S compensating magnet to remove half the difference. For example, if the compass reads 186 degrees instead of 180 degrees, adjust it to read 183 degrees.

That done, the helmsman should now run due south by the compass. Re-adjust the shadow pin so the shadow falls on the line, and repeat the process heading north. A couple of such runs should either remove any N-S deviation or demonstrate that all the deviation can't be dealt with.

Repeat the same process on east-west courses, and then make one more quick check on north-south to see if it has been affected by the east-west compensation. If the process successfully neutralizes any deviation, you can use the magnetic rose on your chart for courses and bearings. This simplifies life a great deal.