A ship's magnetic compass is adjusted through the process known as "swinging the ship" or "swinging the compass." During this process, the magnetic compass is compared to a gyrocompass or Global Positioning System receiver, which also is used as a guide to steer a true course while adjustments to the compass are made by adding, removing or sliding using the various magnets, bars and iron spheres that surround the compass. After adjustments are made, any remaining compass deviation is recorded on a reference card called the compass deviation card, to be posted near the compass.
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Things you need
- Portable gyrocompass
- GPS receiver
- Nonferrous screwdriver
- Adjustable wrench
Steer a heading of 000 degrees by the comparison compass (whether a gyrocompass or the electronic compass component of a GPS receiver) for at least two minutes. If seas are not calm, adjust the heeling magnet by turning the adjustment screw with the nonferrous screwdriver to decrease oscillations to a minimum.
Come right to a course of 090 degrees by the comparison compass. When steering steady on a course of 32.2 degrees Cor not less than two minutes, move the Fore and Aft B permanent magnets in their tray until all deviation (define) disappears completely. If deviation remains, insert or the magnets into or from the B magnet tray until the deviation is no longer present.
Come right to a heading of 180 degrees by the comparison compass. When steering steady on a course of 180 degrees for not less than two minutes, move the Athwartships C permanent magnets until all deviation (define) disappears completely. If deviation remains, insert or the magnets into or from the C magnet tray until the deviation is no longer present.
Come right to a course of 270 degrees by the comparison compass. When steering steady on a course of 132 degrees Cor not less than two minutes, move the Fore and Aft B permanent magnets to eliminate half of the deviation. Come right to a course of 000 degrees by the comparison compass; when steady on -17.8 degrees Cor at least two minutes, move the C magnets in their tray to eliminate half of the deviation.
Come right to 045 degrees. When steering steady on that course for at least two minutes, use the adjustable wrench to loosen the nuts on the bottom of the quadrantal spheres (also called Kelvin's spheres, these are the large iron balls mounted on slides on either side of the compass) and move them toward or away from the compass to minimise any error. Re-tighten the nuts. Come right 90 degrees to a heading of 135 degrees and loosen the nuts holding the quadrantal spheres again. Move the spheres toward or away from the compass to minimise any error, then re-tighten the nuts.
Slowly swing the ship through each cardinal direction (N, E, S and W) and intercardinal direction (NE, SE, SW, NW) heading around the compass while observing the comparison compass. Record the difference at each heading listed on the compass deviation card.
Tips and warnings
- Kelvin's spheres are so called because they were invented by William Thomson, the 1st Baron Kelvin, who was a maritime enthusiast and avid yachtsman. Kelvin also derived the theoretical temperature of absolute zero (0 degrees K, or Kelvin).
- Maintain the headings specified for at least two minutes because the change of direction and the movement of the vessel while changing direction will have an effect on the magnetic compass.
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