A matter of course: the magnetic compass

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I was surveying a large motoryacht not long ago and found myself on the bridge, where there was a prodigious array of electronics but no compass. GPS is a great navigation tool, but it’s no substitute for a properly adjusted magnetic compass. A magnetic compass works independently, without electricity or another power source, so it can be relied upon at all times to determine the direction the boat is headed.

For a compass to be effective, it has to be properly mounted and adjusted, tasks that to some extent are dependent upon each other. The compass should be mounted on a flat, level surface that’s as far as is practical from ferrous metal fittings (iron, steel), magnets (speakers, microphones), and electronics and other current-carrying devices, all of which can interfere with accuracy.

The compass typically is mounted on the steering pedestal in the cockpit aboard sailboats, and on the dash or console aboard powerboats. It should be in the helmsman’s direct line of sight so it’s easy to read when the navigator gives a course to steer.

To calculate the error, or deviation, in the compass, it must be “swung,” whereby the boat is put on a known heading that’s checked against the compass reading. Typically this is done by lining up a set of transit marks and comparing the boat’s course with the indicated reading. Any error is corrected by adjusting the built-in magnets, which are attached to compensator rods.

A compass should be routinely checked, and bearing lines are perfect for this. John Rousmaniere devotes a lengthy chapter to compasses and compass adjustment in The Annapolis Book Of Seamanship. Of course, you can call in a professional compass adjuster to help with all of this.

A correctly adjusted compass points toward magnetic north, not true north. The difference between true and magnetic north varies depending on where you are on the planet; this is known as variation, which must be compensated for when plotting a position. Variation is shown in the compass rose on navigational charts and tells the navigator how many degrees to add or subtract (east or west of true north) to the bearing.

Once the compass has been swung and adjusted, there are likely to be errors that cannot be compensated for, so a deviation card is produced. A professional compass adjuster will provide this card, but you can also work up your own. Deviation should be no more than a few degrees on each heading, so both deviation and variation must be accounted for when working out the plot on the chart or when communicating a compass course to steer to the helmsman.

A compass has a lubber line — two pins or some type of marking — that must be parallel to the centerline of the boat when the compass is mounted. As the boat turns, the compass continues to point at magnetic north, and the course is shown (relative to magnetic north) in reference to the lubber line.

Practice steering a compass course rather than following the chart plotter. You’ll have more situational awareness and less strain on your eyes. On a sailboat, steering by compass can keep you in tune with the wind. As the wind shifts, you may be able to harden up or crack off a few degrees, rather than trying to follow that line on the plotter.

A properly installed and adjusted compass is a valuable navigation tool. Buy the best one your budget will allow and take good care of it — you’ll be rewarded with years of service.

This article originally appeared in the October 2016 issue.