Deviation of the magnetic compass caused by the magnetic properties of a steel ship. Components which cause deviation is best left to a compass adjuster,although I have done about 70 small vessels ranging from 25' to 100'. I learned how to do this from a great old guy named Paul Parks from Portland, Or. I will just go over the basic concepts and terminology. The various magnetic components or parameters of the total magnetic field of a vessel are called coefficients, and different correcting magnets are used to compensate for their effects on the compass. To identify their polarity, the ends of compensating bar magnets used in binnacles are color-coded, the north end being painted red, and the south, blue.
The total local magnetic effects which cause deviation of a vessel's compass can be broken down into a series of components called coefficients.
Coefficient A is constant on all headings and may be a combination of other parameters or may be mechanical, as from an incorrect! placed lubber's line.
Coefficient B is maximum on compass headings east or west and zero on compass headings north or south.
Coefficient C is maximum on compass headings north or south and zero on east or west. Coefficients Band C are caused by permanent magnetism.
On small craft constructed mainly of wood and/or fiberglass, adjustment is normally made only for these coefficients. Most boat compass mountings will have built-in small correcting magnets for this purpose.
Coefficient D is quadrantal deviation. It is maximum on intercardinal headings: 045°-135°-225°-315°, and zero on cardinal compass headings: north-south-east-west.
Coefficient E is quadrantal deviation which is maximum on the cardinal compass headings and zero on the intercardinal headings.
Coefficients D and E are caused by induced magnetism in horizontal soft iron and are compensated for by the use of the soft iron quadrantal spheres normally mounted on brackets athwartship on the binacle. These spheres should be used on all vessels constructed of steel.
J Coefficient Heeling error is defined as the change of deviation for a heel of 1 ° while the vessel is on compass heading 000°. It is, in effect, the error caused because the compass, with its gimballing arrangement, remains in a horizontal plane while the ship, with its magnetic field, rolls and pitches. This change in deviation caused by the motion of the ship can cause the compass card to oscillate. Coefficient J is compensated for by a heeling magnet placed in a vertical tube directly below the center of the compass.
To compensate for or neutralize any induced magnetism in vertical soft iron, a Flinders bar is used. This consists of sections of soft iron having no permanent magnetism; as many sections as required are installed vertically in a tube on the side of the compass opposite to the effective pole of the ship's field.
The theory of compass adjustment can be found in the Handbook of Magnetic Compass Adjustment and Compensation, DMAHC Pub. No. 226.
There are 5 methods for coming to a magnetic heading and finding the deviation.
1. By taking azimuths of a celestial body.
2. By comparison with a gyrocompass.
3. By comparison with a magnetic compass knowing the deviation.
4. By taking bearings of a distant object.
5. By using ranges.
