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Compass Errors

Compass Errors as seen in Power & Motoryacht Magazine

Now more than ever, a compass adjuster can keep you moving in the right direction.

By Capt. Chris Kelly

I don't need a compass, I've got a GPS! This is the cry of the new boater and one that Jens Jacobs hears almost daily. As a professional compass adjuster and president of North Sea Navigator, Jacobs makes a point of educating newcomers and old salts alike about the virtues and necessity of the magnetic compass.

Jacobs says, "When you buy a piece of electronics, 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 explains that neither a GPS nor a loran receiver has a "compass," and they never will. Boaters are sometimes confused because these electronic navigators depend on position changes to find course over ground (COG) made good. But as Jacobs points out, "That information is only historical it tells you what you were doing, not what you are doing. Secondly, once you stop or slow the boat, the GPS has no idea what your heading is. That's the job of the compass." A compass, Jacob explains, shows not only a change, but indicates this change as it occurs- "it shows what you are doing instead of what you did."

The magnetic compass is a vital piece of navigation equipment-so much so that U.S. Coast Guard regulations require that every compass aboard its vessels must be checked for accuracy each year. And it's reliable: It doesn't run on batteries, is not affected by the weather, and can't be scrambled by the government.

Yet for all its benefits, a compass must still be periodically checked by a compass adjuster. The first check happens when you take delivery of your new boat. The builder has installed the compass but not checked its accuracy, location, or alignment, so the adjuster must determine that the compass' north/south "rhumb line" is parallel with the boat's keel line. Otherwise, readings will be erroneous on all headings. Moreover, the builder may have mounted your compass too close to interference inducing equipment, wreaking havoc on compass accuracy (an error called “deviation"). The compass must also be rechecked whenever you add or remove a piece of electronics near it.

I tagged along with Jens and his daughter Connie as they made a house call on a sick patient-in this case, the compass aboard a 34-foot express cruiser. The owner 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 inaccurate (the one onboard the 34 footer was off by more than 40 degrees), the first step in compass adjusting is to look for sources of interference around the compass. Jacobs told me the worst offenders are the big magnets in stereo speakers. He warns, "The bigger the magnet, the farther away it has to be from the compass."

In this case Jens immediately spotted a suspect: a waterproof speaker mounted along the inwale. He pulled it out, rotated it 360 degrees, and watched the compass. Since the compass remained steady, he determined the speaker was not the problem.

Next, he checked the instrument panel, where blowers, handheld microphones, and windshield wiper motors can all be sources of compass error. Turning each of these items on and off also produced little interference. Finally, Jens found the main source of interference in the cabin: a 1,000-watt speaker mounted in the aft bulkhead just beneath the compass. By rotating it he created compass errors of up to 60 degrees, so he promptly pulled it out and set it aside.

Next he checked the compass for "swing," a test you can do yourself. While sitting at the dock, note the compass heading, then place a screwdriver next to the compass; this will pull the compass card to one side. Note the new reading, remove the screwdriver, and then make sure the compass returns to its original heading. The compass on our boat did, so it passed that test. If yours doesn't, it needs professional repair. (Never try to repair your compass or add oil to it.)

The last step is to take the boat out and swing her through 360 degrees, comparing compass headings to known magnetic headings. To do this a compass adjuster employs a pelorous to determine headings based on the known azimuth (direction) of the sun. There was too much cloud cover on our test day so Connie used a distant landmark as a reference point instead.

With Connie at 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 "mark" at each heading, Jens noted the compass reading and then carefully turned the compass' internal magnets to make the readings agree. But since the internal magnets will only deflect the compass+/-15 degrees and this compass was off by more than that, Jens had to install an external correcting magnet. Afterwards, a final swing and check through 360 degrees showed no deviation or error.

His last step was to put the boat's electronic compass through its automatic compensation mode, and when that was complete, the magnetic and electronic compasses finally agreed to within two degrees on all headings. When the Jacobs’ complete a job like this, they always leave a deviation card, indicating any small error they were unable to remove. The entire procedure takes about an hour, and they charge $150 for this service and $65 for each additional service. (If there are multiple compasses aboard, all must be adjusted at the same time.) This is cheap insurance and money well spent.

So the next time someone tells you he's got a compass built into his GPS, congratulate him. After all, he's got the only one in the world.

For more information, contact
North Sea Navigator,
7 Logan Hill Rd.,Northport,NY11768.
(631) 757-7169

If you have an interesting experience or article to share with the us we would love to hear from you!


Another informative article on the Compass and GPS can be found on E.S.Ritchie’s website  

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