Are My Electronics Impacting the Accuracy of My Compass?

Check to see if your electronics are inducing deviation errors. That test is easy. It can be a little more difficult to get the compass swung right.

30th October 2013.
By Ed Sherman

Question: I’ve been told by a bunch of my buddies to be careful about mounting electronic equipment too close to the compass on my boat because it might induce compass error when the electronics are running. I’m a little intimidated by this because I live on the Maine coast and we get a lot of fog during the summer months. I actually rely on my compass fairly often.

It's very common for magnetic compasses to be surrounded by electronics these days.

It’s very common for magnetic compasses to be surrounded by electronics these days.

Is this really something I should worry about? Here’s a photo of the helm area on my boat; you can see that there is a lot of electronic equipment surrounding the compass. How can I test this?

Answer: I can’t know whether your compass is accurate without knowing whether you’ve had it checked by a professional compass adjuster — but I can tell you how to test to see if your electronics are impacting your compass’s accuracy. That part is easy: With your boat stationary at a dock, watch your compass card carefully and turn one electronic instrument at a time on and off. If your card swings even a couple of degrees in either direction when the instrumeent is on, it’s interfering. It may be that more than one instrument is having an impact, or that there’s no error even with all the instruments on together.

In any case, I do recommend that you get your compass “swung” by an expert compass adjuster. Most areas that have a concentration of boats have at least a few experts who have the necessary equipment to accomplish this task. If a calibration is necessary, you’ll need to consult with the adjuster to decide whether you want the calibration to occur with the electronics turned on or off. Generally you want the compass set with the gear you typically use in place and running at the time of the calibration. Once you‘ve established that the compass is calibrated properly, simply test the impact of your electronics periodically by comparing the indicated compass heading with your gear turned on and off.

With modern electronic equipment such as you show in your picture, interference is generally not a problem. That said, I’ve seen VHF mics changed out and in close proximity to a compass, where the speaker magnet impacts the compass bearing significantly. Simply put, test your compass with the gear on and off and carefully look for any changes. If you see changes, you may need to get the compass re-calibrated with said equipment in place and operating.


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About the author:

Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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http://www.EdsBoatTips.com

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