If an accurate adjustment of the magnetic compass is desired, efforts should be made by the person(s) responsible, to ensure that all necessary precautions are followed so that the compass is properly adjusted.
Precautions to be observed when Swinging the Vessel
- The vessel should be in a seagoing condition, that is, all derricks, cranes, davits etc. should be stowed and secured. Since anchors are masses of steel, the anchors should be housed and secured.
- The vessel must be upright and have her normal seagoing, or close to normal seagoing trim.
- Before bringing the compass onboard, test it ashore for excessive friction by deflecting the compass to one side by few degrees (using a hand held magnet) and letting it come to rest after the removal of the magnet. Note the heading and repeat the same procedure by swinging the card to the other side and noting the heading again once the card settles. The above test is to be repeated several times and the readings should be consistent. If the readings are different, there is excessive friction at the pivot and such a compass should be sent ashore for repairs/testing by the maker.
- The accuracy of the lubber line should be checked to ensure that it is parallel to the ship's fore and aft line.
- The azimuth mirror must be checked for accuracy by taking the bearing of an object with 'arrow up' and then with 'arrow down'. An accurate azimuth mirror will show the same bearings.
- Remove all metal objects howsoever small, from the vicinity of the compass.
- All persons who are required to be near the compass when the adjustments are being made should ensure that there are no magnetic material, like keys, etc. on their person.
- As a ship is a large steel object, there must be no vessel within 3 to 4 cables.
- Test the Flinders bar - note the compass head (preferably when heading east or west as the Flinders bar causes maximum effect on these headings) and hold the vessel steady by the steering compass or by the gyro compass. Now invert the Flinders bar, heading of the compass must not change. Test by reversing a few times and there must be no change in the compass heading. If the readings change, there is residual magnetism in the bar and it must be de-magnetised (To demagnetise the Flinders Bar, is explained later).
- The soft iron spheres are tested by rotating them after slacking the mounting nuts. If the compass heading changes as the spheres are rotated, the spheres have residual magnetism and must be demagnetised (To demagnetise the Flinders Bar, is explained later).
How to Demagnetise the Flinders Bar?
To demagnetise the Flinders bar or the soft iron spheres, they must be heated to 'Red Hot' and then allowed to cool slowly. Test them in position again and if retained magnetism is still found, the process of heating and slowly cooling is to be repeated again till no deflections of the compass is achieved, when the spheres are rotated.
If only slight magnetism is found to remain in the spheres, it can be go rid of by rolling them on deck.