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To shield your marine compass from magnetic interference, you have to understand what affects your compass. First, your boat's permanent magnetism is always there, coming from things such as the bolts that hold your steering wheel in place. The second factor is induced magnetism--changes caused by the earth's magnetic field. Together, these are called “magnetic variation.” The third form is sub-permanent magnetism, the kind that happens when you carelessly leave a tool by the compass. You use this form to shield your compass from the other two by inducing "magnetic variation" with magnets and iron bars.
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Keep things such as knives, small radios, stereo speakers, your flashlight, or any tools away from the compass. These things either contain a magnet or an iron-based product that can influence your compass.
Permanently mount your compass as close to the center line of your boat as possible, so that the influence from the permanent steel parts of your boat is stable. If your compass is adjusted, then moved, the magnetic variation--the boat's effect on the compass--will change.
Spend a few hours on the water with a marine technician certified to adjust compasses. The technician will have you follow certain courses and make certain maneuvers while moving the magnets and iron bars in your compass that compensate for the magnetic variation induced by your boat.
Use the most current chart when planning your voyage and pay attention to the note on changes in the Earth's magnetic field located at the top center of the chart. This note will tell you how to adjust your course to allow for changes in the location of the North Magnetic Pole (the "North" your compass points toward) over time. The note will vary from chart to chart because the Earth's magnetic field isn't the same everywhere.
- If your compass is five degrees off, you'll be a full mile off course after running only 11.5 miles. This could lead to an uncomfortable situation if you're making landfall in poor visibility or if you're navigating across a lake with a known hazard you must pass.
- Your boat's compass should be adjusted by a professional when you purchase the boat and once a year thereafter.
- If you have a GPS receiver and your compass "goes haywire," use your GPS's compass function. There are a handful of places on the ocean where a magnetic compass just won't work correctly. The places aren't anything like the "Bermuda Triangle"; they are harmless, well-known, and well-documented geomagnetic phenomena.
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