-H.M.S. Hood Reference Materials-
1941 CAFO 339 – D.G. Equipment – Adjustment of "M" Coil with Change of Latitude
Updated 25-Jan-2008

This document is a modern transcription of a portion of 1941Confidential Admiralty Fleet Order 339. It concerns procedures for adjusting degaussing equipment aboard Royal Navy warships. Although this document doesn't specifically mention Hood, the ship did indeed carry equipment of this sort (fitted in 1940). The original document is held at the Royal Navy Library at the Royal Navy Base, Portsmouth. This Crown Copyrighted material is reproduced here by kind permission of the Royal Navy Library.

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C.A.F.O. 339. – D.G. Equipment – Adjustment of "M" Coil with Change of Latitude

(S.D.G. 83/41.-13.2.1941.)

The following revised instructions are issued for guidance in the use and adjustment of "M” coils of D.G. equipments.

           2. "M” Coils without Adjustment for Latitude.- If no means are provided for adjusting the strength of the "M” coil, the following rules should be followed:-
                (a)  Ships Built in the Northern Hemisphere.
                      (i)  Use full current when north of the equator; the current is to circulate round the "M” coil in an anti-clockwise direction when viewed from above, i.e., to give a "Red” pole up.
                      (ii)  Switch off the current in the "M” coil when between the equator and 25° south latitude.
                      (iii)  Reverse the current in the "M” coil in higher southern latitudes.

                (b)  Ships Built in the Southern Hemisphere.
                      (i)  Use full current when south of the equator; the current in the "M” coil should circulate in a clockwise direction when viewed from above, i.e., to give a "Red” pole down.
                      (ii)  Switch off the current in the "M” coil when between the equator and 25°
                            north latitude.
                      (iii)  Reverse the current in the "M” coil in higher northern latitudes.

           3. Adjustment of "M” Coil with Change of Latitude.- Adjustment of "M” coil for change of latitude is based on the fact that the vertical magnetism consists of two parts:-
                (a)  The "Induced” Vertical Magnetism, which is directly proportional to the earth’s field. For ships sailing in latitudes where they were built this constitutes approximately 75 per cent of the total vertical magnetism, although the figure is by no means certain.

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                (b)  The "Permanent” Vertical Magnetism, which changes very gradually if the ship spends a long time in a vertical field of opposite direction to that in which she was built. Thus a ship built in England will have "permanent” north pole downwards, which she will retain on sailing into the southern hemisphere; but if she stays there for a long time this will be shaken out and the ship will acq2uire permanent south pole down, just as if she had been built in the southern hemisphere. The time required for a complete reversal is not yet known, but it is probably greater than 8 months.

           4. It is therefore clear that the vertical permanent magnetism of ships which have sailed or are sailing between the north and south hemispheres cannot be specified unless the ship has been recently ranged. The following rules for the adjustment of the "M” coil should therefore be used:-
                (i)  Range as often as possible when changing latitude.
                (ii)  When in the approximate latitude of a range over which the ship has been tested, use the value of the "M” coil recommended at the latest testing on this range, unless, since this ranging, the ship has spent upwards of 3 months in a latitude where the earth’s vertical field is of the opposite sign to that of the range in question (see (v) below). In this case the results of the latest ranging (on any range) should be used, adjusted accordingly to (iii).
                (iii)  A ship ranged in a latitude where the vertical component of the earth’s field is Vo and recommended to use Xo ampere turns in the "M” coil, on proceeding to a latitude where the earth’s vertical field is V should use Xo – m (Vo – V)b ampere turns, where b = the beam if the ship in feet and m a constant tabulated below for ships of different classes. (Note a change from - .20 gauss to - .30 gauss is considered as a decrease and the corresponding decrease in the "M” coil is 0.1 mh ampere turns.

Values of "m”


Class of Ship.


Deck Coil

Coil or


Internal Coil.

Monitors           . .         . .         . .         . .

Capital ships, paddle minesweepers   . .

Cruisers, leaders and destroyers, sloops, minelayers ("Adventure” and "Latona” classes), netlayers ("Guardian” and "Protector”), minesweepers ("Aberdare” and "Halcyon” classes).

Trawlers,* corvettes, minesweepers ("Bangor” and "Bathurst” classes), coastal minelayers ("Redstart,” "Ringdove,” "Linnet” and "Plover”), tugs, boom defence vessels ("Bar” and "Net” classes), surveying vessels.

Aircraft carriers, "Unicorn” †     . .

A.M.C.s, O.B.V.s, depôt and repair ships.









33 (flight deck).












13 (waterline).

19 (waterline).

23 (waterline).



29 (waterline).
34 (Close up under upper deck).


24 (lower hangar deck).
28 (waterline).
37 (close up under upper deck).

           * Trawlers’ "Q” Coils.- In trawlers, the vale of the "Q” coil should be adjusted similarly to that of the "M” coil. The change made in the "Q” coil should be one-third of the change made in the "M” coil and always in the same direction as the change in the "M” coil.
           † Will probably require individual recommendations.
           Notes.-(a) The beam is the total beam, including the bulges in the case of capital ships. In the case of paddle minesweepers, the beam does not include the paddle boxes.

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           (b)  The ships above are classified according to the ratio of moulded depth to beam, which ratio determines the value of "m”. Ships not appearing in the list should adopt a value for "m” on this basis, or apply to the Admiralty, if necessary.

                (iv)  Thus, for example, a cruiser of beam 51 ft., with a deck coil, is ranged at Alexandria (Vo = .28) and is recommended to use 350 ampere turns in her "M” coil. On proceeding to Home Waters (V = .43), she should use 350 – 27 X 51 (.28 - .43) = 350 + 1,375 X .15 = 556 ampere turns, until she has had an opportunity to range in Home Waters. Similarly, on proceeding to Aden (V = .05) she should use 350 – 27 X 51 (.28 - .05) = 350 – 1,375 X .23 = 34 ampere turns.

                (v)  The exception mentioned in (ii) above is best dealt with by the following illustration.

                      A cruiser, ranged in England is recommended to use 900 ampere turns in the "M” coil. She then proceeds to Australia and at the end of 10 months’ stay in the southern hemisphere is recommended to use – 1,000 ampere turns in her "M” coil. It is clear that she has acquired a permanent south pole downwards. On returning to Home Waters, the Australian range recommendation should be used, adjusted in accordance with the rule given in (iii), until such time as she is re-ranged in Home Waters. This is necessary because the 10 months-old recommendation of the Home Range is now clearly invalid, due to the change in the permanent vertical magnetism of the ship.

           5. Charts showing the earth’s horizontal and vertical fields respectively are shown in C.A.F.O. Diagrams 348A/40 (1 and 2).

           6. Except where the low voltage motor generator system is fitted, it will seldom be possible to obtain the exact strength required in the "M" coil. The nearest setting available to the calculated value should be used.

(C.A.F.O.s 845/40, 1651/40, 1652/40, 2064/40, and 2241/40 are cancelled.)

End of transcription