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Know-How - compass


8 tips for using a magnetic compass

8 tips for using a magnetic compass

Aside from the bills for financing, documenting or registering, as well as licensing, maintaining, repairing, equipping, docking and storing, what is present with all boats? The magnetic compass. When Murphy and Mother Nature conspire to do-in your electrical system, your magnetic compass can be relied upon not to fail.

Since GPS gained popular acceptance, a trend has developed toward ignoring the magnetic compass — a practice that’s simply not safe, in my opinion. I doubt if more than a handful of boaters have truly effective compasses on their boats. I can’t provide instructions on how to make your compass useful and reliable in the space here, but here’s a summary.

1. Zeroing-in: Compasses must be zeroed-in before installation to remove errors in aligning the compensating magnets found on almost all worthwhile compasses. These internal magnets are adjusted by turning small brass screws on the outside of the magnet labeled N-S and E-W. Turning the screws will move the compass card. This has to be done away from magnetic influences, like your electrically powered wristwatch, steel eyeglass frames, knives and tools. Carefully rotate the compass (sited on a board resting on a book, parallel edges in contact) until it reads north. When the board holding the compass is reversed and its parallel edge is aligned with the edge of the book, which wasn’t moved, it should read exactly 180 degrees. If not, remove half of the error by turning the N-S compensating screw. Reverse the compass again and, by correcting by half a turn, zero-in the compass for N-S. Do the same for E-W. Now you can install the compass.

2. Compasses respond to the earth’s magnetic lines of force. Magnetic influences near the compass distort those lines of force, and the amount of distortion varies with the orientation of those influences. In other words, a magnetic (compass) error is introduced that varies depending on which way the boat is headed: north, east, south, west and points in between. Reduce those errors to a minimum and create a deviation table for reference to correct any residual errors.

3. Compensating: You can compensate a compass by steering accurate courses on a range (two objects visibly aligned with one another) or between two objects (preferably fixed) at least a half-mile to a mile apart. Note the heading. Run an exactly reciprocal course, not by compass but by sight, and at the end of the run note the compass heading. It should read 180 degrees from the original heading. If not, you’re back to half error corrections after each run. When compensating, start with east-west runs, then advance to north-south. This will keep you off the streets and out of the bars, but a compensated compass and deviation table may keep you off the rocks.

4. Deviation tables: A way to determine deviation is to locate your boat very close to an aid to navigation and sight on a charted object seven to 10 miles away. Draw a line from that object to the aid and record the magnetic bearing. If the distant object is about seven or eight miles away and your boat stays within 200 feet of the aid, your error should be about half a degree. While on different headings, take bearings on the distant object, recording the difference in bearing from your plotted magnetic bearing. Those are your deviations. Plot them, and they should form a sine wave. Smooth the plot and make a deviation table. Differences less than the magnetic bearing are easterly deviations; those that are greater are westerly deviations.

5. Corrections: When applying corrections from true to compass, east is least (subtract) and west is best (add) for both variation and deviation. From compass to true, reverse this rule.

6. Variation and deviation: Variation is the angular difference between true and magnetic north at your locale. Deviation is the angular difference between magnetic direction and the direction shown on your compass for a given heading.

7. When Murphy’s Law and Mother Nature put you in deep fertilizer, steering by compensated compass and applying corrections from your deviation table should allow you to steer an accurate course even in reduced visibility.

8. For more information contact a Power Squadron ( ) or Coast Guard Auxiliary Flotilla (, or read Chapman’s or Bowditch.