Handbook of Magnetic Compass Adjustment - Maritime Safety ...

(2) If the above method is impractical, the Flinders bar length will have to be set approximately by:
(a) Using an empirical amount of Flinders bar that has been found correct for other ships of similar structure.
(b) Studying the arrangement of masts, stacks, and other vertical structures and estimating the Flinders bar length
required.
If these methods are not suitable, the Flinders bar should be omitted until data is acquired.
The iron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the top.
Wooden spacers are used at the bottom of the tube to achieve such spacing.
408. Having adjusted the length of Flinders bar, place the spheres on the bracket arms at the best approximate position. If the
compass has been adjusted previously, place the spheres at the best position as indicated by the previous deviation table. In
the event the compass has never been adjusted, place the spheres at midposition on the bracket arms.
409. The next adjustment is the positioning of the heeling magnet by means of a properly balanced dip needle, as discussed
in Chapter XI.
410. These three adjustments at dockside - Flinders bar, spheres, and heeling magnet - will properly establish the conditions
of mutual induction and shielding on the compass, such that a minimum of procedures at sea will complete the adjustment.
411. Expected errors. Figure 318, "Summary of Compass Errors and Adjustment", lists six different coefficients or types of
deviation errors with their causes and corresponding correctors. A discussion of these coefficients follows:
The A error is more generally caused by the miscalculation of azimuths or by physical misalignments, rather than magnetic
effects of asymmetrical arrangements of horizontal soft iron. Thus, if the physical alignments are checked at dockside, and if
care is exercised in making all calculations, the A error will be insignificant. Where an azimuth or bearing circle is used on a
standard compass to determine deviations, any observed A error will be solely magnetic A error. This results from the fact
that such readings are taken on the face of the compass card itself rather than at the lubber's line of the compass. On a
steering compass where deviations are obtained by a comparison of the compass lubber's line reading with the ship's
magnetic heading as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and
mechanical A (misalignment). These facts explain the procedure wherein only mechanical A is corrected on the standard
compass by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compass
by realignment of the binnacle (see article 405). On the standard compass, the mechanical A error may be isolated from the
magnetic A error by making the following observations simultaneously:
(1) Record a curve of deviations by using an azimuth (or bearing) circle. An A error found will be solely magnetic A.
(2) Record a curve of deviations by comparison of the compass lubber's line reading with the ship's magnetic heading as
determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A.
The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A
found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount. It is neither
convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro
may be removed by rotating the binnacle in the proper direction by that amount.
The B error results from two different causes, namely: the fore-and-aft permanent magnetic field across the compass, and a
resultant asymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-andaft
B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Inasmuch as the
Flinders bar setting has been made at dockside, any B error remaining is corrected by the use of fore-and-aft B magnets.
The C error has two causes, namely: the athwartship permanent magnetic field across the compass, and a resultant
asymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C
magnets, and the latter by the use of the Flinders bar to port or starboard of the compass; but, inasmuch as this vertical
induced effect is very rare, the C error is corrected by athwartship C magnets only.
The D error is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by
spheres, generally athwartship of the compass.
The existence of E error of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangements
of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres, as described in Chapter VI.
As has been stated previously, the heeling error is most practically adjusted at dockside with a balanced dip needle. (See
Chapter XI.)
412. A summary of the above discussion reveals that certain errors are rare, and others have been corrected by adjustments at
dockside. Therefore, for most ships, there remain only three errors to be corrected at sea, namely the B, C, and D errors.
These are corrected by the use of fore-and-aft B magnets, athwartship C magnets, and quadrantal spheres respectively.
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