Compass Errors

“I don't need a compass...I've got a GPS!" This is the claim of the new boater, and one that Jens Jacobs hears almost daily. As president of North Sea Navigator and himself a professional compass adjuster, Jacobs makes it his business to educate newcomers and old salts alike about the virtues of the magnetic compass. And while today's marine electronics are now a staple of navigation, they aren't making his job any easier.

"When you buy a piece of electronics," says Jacobs, "you've got to understand what it can do, and what it can't do. Whenever someone tells me their GPS already has a compass, I tell them it's worth a lot of money since it's the only one in the world that has one."

Rather than argue with customers, Jacobs takes the time to explain that neither a GPS or loran receiver has a "compass," and they never will. Yet boaters are sometimes confused because these electronic navigators can show Course-Over-Ground (COG) and heading information in degrees magnetic. But as Jacobs points out, "That information is only historical--it tells you what you were doing, not what you are doing. And second, once you stop the boat the GPS has no idea what your heading is. That is the job of the compass."

As all mariners know, the magnetic compass is a mandatory piece of navigation equipment. Its job is to show your ship's heading underway and at rest, and it aligns itself with the earth's magnetic meridian in order to do so. The compass doesn't run on batteries, it's not affected by the weather, and the government can’t scramble it. In short, it tells you your heading no matter what. The compass is so vital, in fact, that the U.S. Coast Guard's own regulations require that every compass aboard its vessels must be checked each year for accuracy.

There are several times in a compass' life when it must be checked by a compass adjuster. The first is when taking delivery of a new boat. More often than not, the builder installed the compass in a preset hole in the dash, and it is absolutely critical that the compass be parallel with the boat's keel line. Otherwise, the compass will be off on all headings. Moreover, the builder may have mounted it too close to sources of magnetic interference, and these can wreak havoc on compass accuracy. The compass must also be rechecked whenever you add or remove a piece of electronics on the dash since they, too, can affect accuracy.

I tagged along with Jacobs and his daughter Connie as they made a house call on a sick patient--in this case, a 34-foot express cruiser. They were called to the scene by an owner who said his magnetic compass didn't agree with his electronic one, and neither of those matched the GPS heading information.

Assuming the magnetic compass is, in fact, inaccurate (ours was off by more than 40 degrees when we arrived!), the first step in compass adjusting is to look for sources of interference around the compass. "The worst thing," says Jacobs, "is stereo speakers. They have big magnets that affect the compass, and the bigger the magnet, the further away it has to be from the compass."

Right off the bat, Jacobs spotted a possible suspect--a waterproof speaker mounted along the inwale. He pulled it out, rotated it through 360 degrees, and watched the Ritchie compass for movement. There was no effect, so it was not affecting the compass. What next? Check the dash.

Here, sources of interference may include blowers, handheld microphones, steering systems, and windshield wiper motors. If mounted too close to the compass, any of these can cause compass error, and that error will vary depending on the vessel's heading (a phenomenon called "deviation"). A check of these items (turn each one on and off) also showed little interference. So now what? Look in the cabin.

And that's where he found it--a massive, 1,000-watt Sony speaker mounted in the aft bulkhead just beneath the compass. By rotating this speaker he created compass errors of up to 60 degrees, so he promptly pulled it out and set it aside.

Next he checked the compass itself for proper swing, and you can do this test yourself. While sitting at the dock, note the compass heading and then place a screwdriver next to the compass. This will pull the compass card to one side. Note the deflection, remove the screwdriver, and then make sure the compass returns to its original heading. Ours did, so it passed that test. If yours doesn't it will need professional repair. (Note: Never attempt to repair your compass or add oil to it.)

The last step is to take the boat out and rotate her through 360 degrees, comparing the compass heading to a known magnetic heading. But how do you know the "real" magnetic heading? You don't, but the compass adjuster does.

Using a pelorous, Connie can precisely determine heading based on the known azimuth (direction) of the sun. On our test day we had too much cloud cover, so she used a landmark as a reference point instead.

With Connie on the pelorous and Jens at the helm, we rotated the boat to eight different headings, stopping to check compass accuracy at each one. As Connie called out "MARK" at each heading, Jens noted the compass reading and then carefully turned the compass' internal magnets to correct the heading. But since the internal magnets will only deflect the compass +/- 15 degrees and this compass was off by more than that, Jacobs had to install an external correcting magnet in just the right spot. A final swing and check through 360 degrees showed no deviation or error after he was done.

His final step was to put the Ritchie FN201 electronic compass through its auto-compensation procedure, and when that was complete, the magnetic and electronic compasses agreed to within two degrees on all headings. When the job is done, Jacobs leaves a deviation card which indicates any small error that could not be removed, and the entire procedure takes about an hour. Jacobs' fee for the service is $150 per compass aboard, and all must be adjusted at the same time. As you can see, this is cheap insurance and money well spent.

So the next time someone tells you he's got a compass built into his GPS, don't argue, just congratulate him. After all, he's got the only one in the world.

For more information contact: North Sea Navigator, Dept. BT.com, 9 Logan Hill Road, Northport, NY 11768. Phone and fax: (516) 757-7169. Ritchie Navigation, Dept.BT.com, 243 Oak St., PO Box 548, Pembroke, MA 02359. (617) 826-5131. Fax: (617) 826-7336.