Correcting Heeling Error
Contemporary binnacles correct the vertical field by a magnet called the heeling error magnet. The accepted practice of correcting the vertical field is to place the heeling error magnet at a position and orientation to bring the vertical field at the compass to an equal multiplier to the horizontal field.
With a typical assumption of the mean horizontal magnetic field at the compass being 80% of the ambient earth’s horizontal magnetic field, the heeling error magnet is set to cause 80% of the ambient earth’s vertical magnetic field at the compass site.
This is a departure from the compass adjusting goal of correcting like with like; permanent magnets for permanent magnetic fields and soft iron for induced magnetic fields.
If a vessel’s heeling error magnet is set in a location of ambient vertical field strength of -40µTeslas the resulting vertical field at the compass would be 80% of -40 = -32µTeslas. If the vertically induced soft iron causes the mean vertical field at the compass to in fact be 120% of the ambient earth’s vertical magnetic field, the heeling error magnet is correcting for -16µTeslas of induced magnetism.
Should the vessel voyage to a location of ambient vertical field strength of +40µTeslas, the vertical field strength at the compass will be +16µTeslas from the heeling error magnet plus +48µTeslas from the ambient earths field by the multiplier of 120%. This is a total of 64µTeslas, which is double the field strength of the required +32µTeslas to have the vertical field of an equal multiplier of the horizontal field.
This vertical field will cause heeling error when the vessel is not level. As the vessels rolls, the compass card will oscillate, especially on northerly or southerly headings. The contemporary binnacle design allows the user to adjust the heeling error magnet to reduce the compass card oscillations on a northerly or southerly course. This change in the heeling error magnet may effect the fore & aft field at the compass due to the changed induction of the flinders bar, which in turn will cause deviations on easterly and westerly headings. Subject to the positioning of other soft iron correctors, the athwartship field may also be altered. This is clearly not conducive to correcting a compass for a global voyage.
The ideal magnetic correctors would allow for the correction of the compass to be valid for a vessel’s voyage and in all geographic areas of operation. The improved binnacle design achieves this.
Earth’s Vertical Magnetic field in nanoTeslas