With the modern marine electronics found on most boats these days, a compass can seem almost quaint. But not all boats are full outfitted for electronic navigation. Case in point: It had been a perfect summer morning when we set out from the marina in a friend's 19-foot runabout, headed for an island visible just 20 miles away. But halfway there, the horizon started to get hazy and, almost before we knew it, we were deep in a fog that would have made London proud. With the visibility reduced to just a few yards we slowed down to a crawl, posted lookouts, and crept along. It was at that point that my friend asked what direction we should be steering. Having made the crossing many times, I knew that the course was about 185 degrees and gave him that bearing. There was a moment of silence, and he said, "Uh, you know, I don't think this compass is accurate."
Let's just say that this was a very, very long couple of hours until the fog suddenly lifted and we saw that we were far off course. What should have been a simple case of steering slowly through the fog on a known course had turned into a scary ride in unknown waters. The moral is that every — every — boat should have a compass and someone who knows how to use it.
The compass is the most basic of all navigational tools, and it's been used by seamen since long before Columbus set sail. The simple magnetic compass is the backbone of even the simplest navigation since you always need to steer in the proper direction to reach your goal.
Today's electronics are great, with GPS receivers that can not only tell you exactly where you are, but what direction to head for home. But when their battery dies or their innards go "zap", it's going to be the simple, zero-power magnetic compass that gets you home safely.
Most boaters barely pay attention to their compass since, like my friend, it's rarely used. The compass arrived on your new boat and you didn't even have a choice of the type of compass you wanted. In fact, you probably haven't ever considered that it might be inaccurate. But a surprising number of new boats are delivered with compasses that are wildly inaccurate.
A compass is used to point to magnetic north and every compass has a compass card and a lubber line. The compass card usually floats in clear liquid on a tiny bearing so it can turn freely, and it is marked off in degrees. The lubber line is a small pin or line on the compass used to indicate the direction the boat is headed.
The best type of compass for most boaters is a compass with a flat or curved card and a lubber line on the far side, making it easy to see which direction to steer when you're off course. The aviation-type compass with a dome-shaped card and a lubber line on the near side of the compass is more difficult for beginners because it seems to move backwards to the direction of the bow.
For most powerboats, compasses are available in two mounting styles: flush-mount and bracket-mount. Flush-mounted compasses are recessed into a horizontal surface with only the dome showing, while bracket-mounted compasses can be installed on a variety of angled surfaces.
Choose a good quality compass, and you might follow one expert's suggestion to pick compasses that can be repaired, since they are usually better made. A compass that is flimsy and meant to be thrown away is probably not a good bet with your life.
Choose a compass that has a large and easily readable card, which will make long periods of steering by compass easier on your eyes. Stability is also a factor, and some compasses have special dampening to keep the card from jumping around on performance boats. Last, be sure that your compass has illumination for use at night.
The two most common compass problems are misalignment and improper adjustment. Misalignment occurs when the lubber line on the compass isn't lined up precisely with the center axis of your boat. In today's mass production boats, it's fairly easy for a compass to be out of alignment by a few degrees which, on a day cruise, can mean you'll miss your objective by miles.
To check alignment, run a string from the bow to the center of the stern, and measure the distance off center from the string to the lubber line of your compass. At the bow, measure the same distance off center and put a tape marker on the bow rail at that point. When standing behind the compass and sighting along the center pin and the lubber line of your compass, the tape mark should also be directly in line. If not, your compass is misaligned and you should re-position it correctly.
Improper adjustment is more complex and something that many skippers overlook completely. Since a compass points to magnetic north, it stands to reason that any ferrous metals near the compass will affect it. Put a couple of steel washers next to your compass, and you'll see it swing toward them. Put an 800-pound block of steel — your engine — in the vicinity, and you'll get the same effect.
Even worse, magnetic compasses are affected by stray electrical currents and electric motors. Since your compass is probably on top of your dashboard, it's going to be affected by the windshield wipers, the horn and light switches, and the wires running to your electronics. It might even be affected when you use the electronics, such as the microphone on your VHF radio. You can test for these problems by turning on and off the various circuits, and watching to see if your compass moves.
The amount that your compass is affected by the metal and electricity on your boat is called deviation, and you not only need to correct the compass for the largest part of the deviation, but you also need to make a deviation card that shows the small differences on various headings.
The easiest way to correct your compass for deviation, called "swinging the compass", is to hire a professional compass adjuster who will spend a morning on your boat. For most boaters, however, you can swing your own compass with sufficient accuracy for most uses.
On a chart, find two objects that line up directly north magnetic, such as a pair of radio towers. Steer toward the objects and keep them directly in line while checking your compass. If it doesn't read 0 degrees or north, it needs to be adjusted.
Most compasses have a pair of compass adjusting screws marked N-S and E-W. Turn the N-S screw until the compass reads directly north, and then turn the boat south, keeping the same objects in line while you again adjust the screw to remove half the southerly error. Turn north once more and remove half the error again, which should give you a good N-S adjustment.
To correct the compass for east-west error, repeat the procedure with a pair of objects aligned in those directions. To complete the process, use other known objects to check the deviation every 20 degrees to 30 degrees around the entire compass and make a deviation card that shows the error on each course. Part of a deviation card might read:
|30 degrees||31 degrees|
|60 degrees||63 degrees|
|90 degrees||89 degrees|
|120 degrees||88 degrees|
Make copies of the deviation card and laminate one in plastic so you can leave it at the helm to check in all conditions.
With a good compass that is properly installed and aligned, you're ready to head out of sight of land with the confidence that comes from knowing that your most basic navigational tool is ready when you need it.
To learn more about navigating with both modern electronics and traditional tools, be sure to check out How to Navigate a Boat article and videos.
Editor's note: this article was updated in July of 2017.