Many boaters think that, in these days of modern electronic-charting aids, compasses are no longer needed.

Compasses, used on boats for centuries, work because a permanently magnetized needle always points to north. Irrespective of the position of the boat. A compass is needed more now than ever. A magnetic compass requires no electricity to operate. It accurately could be the one piece of equipment that will still operate on your boat when the you-know-what hits the fan.

For a compass to work well, it has to be correctly installed and properly adjusted. Unfortunately, on a large number of boats, the compass has been installed incorrectly. And with the ever-increasing strain on dashboard real estate, the compass is often pushed out, literally. Electrical interference from chartplotters, radios, and other electronic aids affect compasses if they are too close to each other..

The skipper needs to be able to easily see the main steering compass. This means that it must be placed directly in front of the helm position with what is known as the lubber line — two pins or some type of marking — parallel to the centerline of the boat. The skipper merely glances down to see the course being steered.

When North Is Not North

In a perfect world, a compass would always point to true north, but there are factors that make this not so. Two errors have to be accounted for: variation and deviation. Magnetic north is not the same as true north. This difference is written on the compass rose on the chart of the area you’re cruising. The difference, in degrees, between true and magnetic, is variation, which must be compensated for when plotting a position.

Compass Rose

A nautical chart has two compass roses, one inside the other. The outer one always points to true north, and the inner shows, in degrees and minutes, the variation in the area, either east or west of true north. Variation, is caused by differences in Earth’s structure. It differs from area to area, and changes by a very small amount each year. This shows up on the chart inside the inner compass rose , variation changes from about 16 degrees west in Maine to 6 degrees in Florida and 0 degrees in Louisiana.

Another compass error that shows up is deviation. Deviation refers to errors in the compass itself that cannot be adjusted. Things that affect deviation include nearby boat electronics, electrical wiring, metal fittings, and radio equipment. Other things, such as the boat’s engine, may also affect deviation. Anything magnetic (such as speakers) placed close by will surely increase deviation. To identify the error in the compass, it must be “swung. That’s where the boat is put on known headings that are checked against the compass reading. An error is then corrected by adjusting the built-in magnets on the compass.. Compass fluid damps the needle’s responses to sudden movements of the boat.