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Compass compensation

Apr 22, 2014


How to remove a gross deviation for a boat with a non-steel hull:

1. Pick a calm day. All the boat’s equipment, magnetic items in particular, should be in their normal location. Electrical equipment that normally would be running should be running. Remove any knife, cell phone or other magnetic influence from your person.

2. Locate visible charted objects (lighthouse, buoy, conspicuous tower, etc.) within about 10º of magnetic north, east, south and west of your (known!) position, and distant at least a mile or so. Suitable ranges may be available. If using GPS, set it to read the bearing to each object. Do not attempt to use the GPS course made good! The GPS should be set to give readings in TRUE, to avoid any errors in the algorithm it uses to convert true to magnetic (see note 4 in notes below). Even a single buoy which may be approached safely from N, S, E and W may be used, if you are prepared to approach it sequentially from different directions as required. Bottom line, we need known northerly, southerly, easterly and westerly courses (magnetic directions, not true).

3. Head for the northerly object. When the course is steady, read the compass while heading towards the object. Adjust the athwartships compensating magnets to make the compass read the correct magnetic bearing. If using GPS, remember to add the correct local variation to the true bearing given by the GPS (subtract if east) to obtain the magnetic bearing to the object.

4. Head for the southerly mark. When the course is steady, read the compass. If the compass does not read the correct bearing, there is slew. In this case, loosen the compass mounting screws so that the compass may be rotated slightly in its mount. Correct half the error by rotating the compass.

5. Again head for the northerly object. Repeat steps 3 and 4 until any error is less than a degree or so, whatever you consider negligible.

6. Repeat steps 3 to 5, but using the known easterly/westerly courses and adjusting with the fore and aft compensating magnets. At step 4, there will be little or no error, as slew will have been pretty well eliminated so that no realignment of the compass should be required.

7. Repeat the whole procedure. One more time will be all that’s worth doing. If the compass mounting was loosened at step 4, carefully tighten the compass mounting screws.

The boat must now be swung to discover any remaining deviation. Note that our procedure has eliminated deviation in the cardinal directions, N, S, E and W, so that no significant deviation will be observed in these directions. Try for measurements every 30°, but at the very least, check deviation close to the inter-cardinal directions, NE, SE, SW and NW. Swing ship at least twice. You should get very near the same results each time. If not, if the results are erratic, you’re looking at errors in reading the compass and/or in steering, and not deviation. If any real (repeatable) deviation is observed, plot the deviation against either magnetic or compass heading. Draw a fair curve, even though it may pass through few, maybe none, of the plotted points. It is very important to construct your deviation table from your plotted curve, not from the raw data.


1. The objective is to cancel (compensate) any stray, boat-related, magnetic field in the vicinity of the compass, so that the compass “sees” only the Earth’s magnetic field. The method is based on instructions by Ritchie compasses at:

2. If slew is observed (step 4), it must be possible to rotate the compass in its mount to remove this slew. A compass mounted on a bulkhead may be rotated using suitable wedges. If the compass cannot be rotated in its mount, and slew is detected at step 4, alter the compass mounting so that the compass may be rotated slightly to correct the observed slew.

3. To obtain local magnetic variation, either use the compass rose on a current chart, or go to for an up-to-date calculation. This is in fact the source for the magnetic variation indicated on your chart.                                            

4. A GPS unit will calculate in true. If you configure it to display magnetic bearings, it converts these true bearings to magnetic. To check if magnetic bearings on your GPS are correct, enter any waypoint. Have the GPS unit display the bearing from your present position to the waypoint in true and in magnetic. If the difference between the values is equal to the magnetic variation calculated at the URL above, for your actual present position, then GPS magnetic readings are correct. If not, there is an inadequacy in the GPS’s algorithm to obtain magnetic bearings and this particular instrument must be set to display in true for compass compensation application, as described.

5. How it works: The principle of the method is that if there is no external, boat related, magnetic influence deviating the Earth’s magnetic field in the vicinity of the compass, then the magnetic field at the location of the compass is in fact the Earth’s magnetic field only and the compass must read correctly. If the compass reads correctly when headed north, it will read correctly when headed 180° or very nearly 180° from this direction, i.e., when headed south. Ditto east-west. The athwartships compensating magnets will correct a boat-related magnetic field affecting the direction of the Earth’s magnetic field at the location of the compass when headed north/south. The fore-and-aft magnets will correct such a field affecting the direction of Earth’s magnetic field at the location of compass when headed east/west. A correctly installed compass which reads correctly when headed north is free of any magnetic influence in the athwartships or transverse direction, i.e., perpendicular to the Earth’s magnetic field. If there is slew (error observed at step 4 above), then a magnetic field must (erroneously) be applied in the athwartships direction to “correct” the reading when headed north, and it is this field which causes the error on turning south. Ditto east/west, only here we are dealing with corrections in the fore-and-aft directions, which are perpendicular to the Earth’s magnetic field on the east-west course.


Emanuel Laufer owns a C&C 32 named Ceol Mor and lives in Halifax, Nova Scotia.

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