PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION

By S. H. AYLIFFE, M.Sc, Ph.D.*

(The paper was first received 29th December, 1945, and in revised form 13th February, 1946. // was read before THE INSTITUTION 5th April, 1946.)

SUMMARYAs an alternative measure to coiling certain vessels for their protec-

tion against mines, processes known as "wiping" and "flashing"have been used. The process known as "deperming" has beendeveloped to remove or compensate the permanent longitudinal andathwartship magnetisms, which frequently lead to dangerous fieldseven in the case of coiled vessels. These three processes are separatelydescribed, and the various techniques which have been employed arebriefly discussed. Particular attention is paid to the methods developedfor ensuring the maximum stability for the treatment.

(1) INTRODUCTIONIt has been made clear in another paper that the magnetism

of a ship can be regarded for practical purposes as consistingof three components in mutually perpendicular directions, andthai each of these components comprises a part which is inducedand a part which is more or less permanent. It has also beenshown that an approximate compensation of these componentsmay be effected by fitting the ship with coils. As a supple-mentary measure to coiling, or in some cases as a completealternative, methods have been developed for partially demag-netizing the ship and for introducing magnetism into the ship ina reverse direction to its natural magnetism to provide theapproximate compensation required.

There are several reasons why the development of such pro-cesses has been necessary. First, smaller vessels may not havesufficient spare dynamo power to energize D.G. coils. Secondly,the coiling of a vessel takes considerable time, which cannot beeasily sacrificed during war time, whilst the wiping, flashing anddeperming processes can be carried out comparatively rapidly.Thirdly, the removal of some part of the permanent magnetismwill save power for a coiled vessel by leaving less magnetism forthe coils to cope with, and in certain circumstances will leave abetter signature than would be obtained with coils alone.Finally, as is made clear in another paper, it is only in certaincases that a ship is fitted with a complete set of coils to deal withall three components. For the many vessels which are equippedwith an M-coil only, the removal of the Permanent LongitudinalMagnetism (P.L.M.) is generally a necessity if reasonable safetyis to be provided.

On the other hand, the limits of these demagnetizing processesalone should be stated at the outset. It will be clear that nosuch process can compensate for variable effects such as changesin the induced magnetism occurring when the ship changes itslocality or heading. The processes are, therefore, limited to thecompensation of the permanent components or, alternatively, ofthe permanent plus induced components on one heading or inone locality. There is also the question of stability to be con-sidered. Unless the permanent magnetism is largely removed, orunless the compensating magnetism introduced is so "hard" thatit will not be readily removed by the constant vibration of thevessel in the earth's field, which is always present, retreatmentwill be necessary at frequent intervals. No solution has beenfound to the problem of avoiding retreatment, and this remains

. the major drawback of any demagnetizing process, but it will be

shown that, by the development of appropriate technique,stability has been gradually and consistently improved. Theprocesses of wiping, flashing and deperming will be described inturn, with particular attention to this problem of stability.

(2) WIPING(2.1) Normal Wiping

The process known as "wiping" has been developed mainlyfor the purpose of providing compensation for the verticalmagnetism of those ships which have insufficient power toenergize an M-coil. It depends upon the principle of magnetizingferrous materials by stroking (or "wiping") with a magnet orconductor carrying a current. In practice a cable is wrappedtemporarily around the vessel in a horizontal plane and is movedup or down while a heavy current is passed through it (Fig. 1).

Wiping Cable Hemp Lanyard

Higher PositionoP Cabl€

• S.D.G. Department, Admiralty.

To Power Supply Yower PositionoF Cable

Fig. 1.—Ship rigged for wiping.

This leaves remanent magnetism in the portion of the ship's hullclose to the path of the cable, in a direction depending upon thedirection of the current in the cable. In the Northern hemi-sphere, it is clear that wiping must induce a South poledownwards to compensate for both the permanent and inducedcomponents of the ship's normal vertical magnetism. Each sideof the ship may be wiped separately. The remanent magnetismprovides approximate compensation of the ship's magnetism ina reasonable depth of water, and the protection afforded isroughly equivalent, while the remanent magnetism remains, tothat obtained by an M-coil carrying a fixed current.

The currents required for this simple wipe are of the order of1 000-2 000 amp and are normally supplied from storage cellshoused either on shore or carried in a barge or drifter to theship being treated. The wiping cable is hauled up or let down asrequired by ropes handled by a working party on the vessel.The process is controlled by taking vertical field readings directlybeneath the keel, either by mooring the vessel during treatmentover a fixed range or by suspending a magnetometer from thevessel itself. Readings are taken before and after the wipe,and currents are determined by experience and trial and error.If insufficient effect is obtained after the first wipe, the process isrepeated with a larger current. A ship which is overwiped maybe wiped back with a reverse current to give the required sig-nature. Local peaks may be removed by sectional wiping, i.e.wiping only that part of the ship where the local peak exists,

[508 ]

AYUFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION 509

though this process is avoided wherever possible since it resultsin differential ageing of the various parts of the ship.

The simple wiping process described above was applied tomany vessels of all types in the period following the outbreak of•war, since it afforded a rapid means for immediate protection.Very soon it was restricted in the main to ships such as smallcoasters, inshore craft and submarines, which vessels have reliedupon wiping for their protection throughout the war. Sub-marines are especially difficult to coil, because little power canbe spared and the coils and gear are difficult to install in thealready overcrowded spaces. The first mines laid by the enemycould be actuated only by an increase of field, and accordinglythe early wipes were made in such a way as to ensure that thefield due to the ship was predominantly negative (i.e. directedupwards) beneath the ship. Since it was anticipated that theenemy would develop mines actuated by either increase ordecrease of field, the policy was soon altered to provide areasonably compensated signature below the keel. In practice,the ships were still left somewhat overcompensated after wiping,to allow for the ageing effect which takes place; the aim was toleave the vessels in a state of compensation over as long a periodas practicable.

Immediate questions which arose in the early days wereanswered in most cases by carrying out trials on actual vessels,and occasionally by small-scale experiments on models andplates. It was found, for example, that the direction of wiping(upwards or downwards) was immaterial, except that in practicethe cable tended to stick to the sides of the vessel when heavycurrents were used and therefore had to be pulled upwardsrather than let down. The effect of a wipe was largely inde-pendent of the speed at which the cable was moved. Repeatedwipes with the same current gave an increased effect, the greatesteffect occurring on the first wipe and successively smaller effectswith each succeeding wipe. It was further found both onexperimental and theoretical grounds that saturation of the steel-was not to be expected until considerably larger currents werein use, and that therefore increased wiping effect could alwaysbe obtained, up to the limit of the power available, by increasingthe current.

Very large ships could be treated when necessary by wipingthem in sections. The method had little objection in theory butproved difficult to control in practice and was not used on alarge scale.

(2.2) Overwiping and StabilityThe most interesting problem in connection with wiping is

that of providing stability for the treatment. A simple wipe,such as that described above, has on the average an effectivelife of about 1 to 3 months. There is, of course, a considerablevariation in the ageing rate from ship to ship, and the field limitswhich define when a ship can be considered adequately protectedby wiping have been changed from time to time. There is noneed to consider these limits in detail; the standard has beendetermined in accordance with the mine risk prevailing and thenumber and disposition of the stations available for treatment.It was early realized that if a ship was heavily overwiped andthen wiped or flashed back to within the required limits, improvedstability would result. (During flashing back, the cable is heldat a fixed level and a momentary current passed through it.)The theoretical background for such a belief is obvious. Theinitial heavy overwiping causes the more stable domains to bereorientated in the direction of.the applied field, whilst thesmaller fields used during the wiping back will affect only theless stable domains. The result should be that the "harder"magnetism introduced will have an appreciably longer life inthe vibration encountered by the ship during voyaging. The

question of practical application was more difficult to answer,because it depended upon considerations such as the following:

(a) Could the degree of overwiping be made sufficient withthe available power to provide a worth-while increase instability?

(b) Could the process be controlled sufficiently well to enablea satisfactory signature to be obtained without the need forundue sectional wiping? Too much sectional wiping wouldintroduce differential ageing and might offset the resultantgeneral gain in stability.

(c) Would the extra time involved delay the ships at thewiping station to such an extent that the increased stabilitywould be offset?

The answer to these questions came after many trials andcareful examination of the records of ships which had beentreated. An early investigation failed to show any markedincrease in the stability, probably because the degree of over-wiping applied was insufficient. Experiments on steel platesand model submarines, however, showed that a worth-whilegain in stability was to be expected, and trials on full-scalesubmarines later confirmed the model results. It was still notknown whether the process would lead to difficulties in controlin full-scale routine work, and accordingly 100 ships wereselected at chosen stations, overwiped and their ageing comparedwith normally wiped ships. The result was conclusive; nooutstanding difficulties in technique were experienced, and itwas shown that the period between treatments was at leastdoubled.

As an illustration of the type of work involved and theimprovement in stability obtained, a few results will be con-sidered in more detail. Fig. 2 shows typical signatures obtainedfor a ship during the overwiping treatment. The vertical fieldsbeneath the keel are given (A) before treatment, (B) after theinitial heavy overwipe, and (C) after wiping or flashing back to

Initial untreatedSignature

+80

+40

Signature after250? Overwipe

-20a

Fig. 2.—Typical signatures during overwiping.

510 AYLIFFE: PROCESSES APPIJED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION

within the required limits. A curve (D) is also included whichrepresents the signature after some period of time has elapsedand the treatment has aged. For simplicity, in Fig. 2 threecurves only are given to represent the treatment. In practice theoverwiping and flashing back may consist of several more stages,since the required degree of overwiping and bringing the signatureback within limits may include several wipes, and generally adeperming treatment (Section 4) is also involved towards theend of the process. The degree of overwiping may be mostconveniently represented by amidships values, and OA representsthe initial unprotected field; OB, the field after overwipe; OC,the final field after wiping or flashing back; and OD, the fieldat some future date. The percentage overwipe given is measuredby (OB/OA) x 100, and the percentage decay of the wipe afterany period of time is given by (CD/AC) x 100.

Fig. 3 shows the decay curves for the full-scale submarines

PercentageOverwipe

0 10 20 36 40 50Time in Days after Degaussing Treatment

Fig. 3.—Decay time of overwiped submarines.

tested. It can be clearly seen that those vessels which receivedthe heavier overwiping treatment aged more slowly than thosefor which lesser overwipes were given. This fact is brought outmore clearly by Fig. 4, where the percentage decay of the wipesgiven to those vessels is plotted against the percentage overwipe.Although this curve requires extrapolation to arrive at theoptimum figure for stability after 30 days, it is clear that anoverwipe of about 250% would lead to maximum stability overthis period. This figure was therefore adopted for the trial onthe 100 selected ships, and the results are shown in Fig. 5. Inthis Figure, the curves show the average amidships fields at thebeam depth plotted against time for the selected overwipedships and for 100 similar ships which were normally wiped.There is a practical difficulty in obtaining the behaviour of thesecurves after a long time has elapsed since treatment, becausesome ships then exceed the normal limits for rewiping and there-fore disappear from the statistics. It would obviously be unsafe

50

50 100 150Percentage Overwipe

V200 250

Fig. 4.—Comparison of decay after 30 days with percentage overwipe.

Normally Wiped Ships

+30?

to*5+107.o>u

-10

10 20 30 40Time in Days after Treatment

50

Fig. 5.—Comparison of ageing of ships overwiped with ships normallywiped.

in war time to leave a ship in a dangerous state merely for thepurpose of statistical analysis. But the curves indicate the benefitgained by overwiping. """

A more recent and fuller analysis has shown that if a period*of 3 months is taken as the average interval between normalwipes to keep the ship in a reasonable condition of safety, thisperiod can be extended to 8 months by overwiping.

Overwiping has now been adopted as a standard process, andnormal wiping is mainly confined to vessels where the treatmentcannot be employed because of lack of power or because delayat the wiping station cannot be tolerated. The process takesabout twice as long as an ordinary wipe, but there are inevitablyother delays, such as swinging for adjustment of compass, waitingfor tides, etc., and, since the treatment lasts over twice as longbefore reapplication is required, there is generally a considerablegain. The process introduces one or two minor problems.For example, in vessels such as fishing trawlers it introducesinstability in the magnetic compass unless special measures aretaken. Again, if carried out on an East-West heading, thecomponent of the earth's field athwart the ship introducespermanent athwartship magnetism, and a troublesome athwart-ship deperm may be required. For this reason, the treatmentis generally given on a heading close to North-South, and anypermanent longitudinal magnetism which results is removed bylongitudinal deperming at the end of the treatment. Suchdeperming naturally shakes out some of the wipe, and this effect

AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION 511

has to be allowed for when wiping back. The current neededfor a 250 % overwipe may be up to about 6 000 amp. With suchcurrents there is a tendency for the cable to adhere so stronglyto the ship's sides that damage may be done to it when attemptingto pull it up. If adhesion is too great, the difficulty can beovercome by giving the vessel a succession of flashes instead ofa continuous wipe; the cable is maintained in a fixed positionduring each flash and raised or lowered by a few feet betweenflashes.

Before leaving the subject of wiping, two outstanding achieve-ments should be noted. Some 400 ships, urgently required forthe evacuation from Dunkirk, were treated by degaussing partiesworking day and night. Also, the components used in the con-struction of the Mulberry harbour carried no equipment capableof energizing coils. They were treated, in many instances ofnecessity without control measurements, to bring them safelyaround the coasts and into position across the Channel.

It is perhaps also worthy of mention that the Germans, whotook their degaussing very seriously and built elaborate stationsto apply methods allied to wiping, do not seem to have developedmethods corresponding to overwiping. Thus the correspondingGerman treatment is less stable than the British.

(3) FLASHINGThe term "flashing" has already been used in this paper in

the general sense to describe the application of any field of shortduration to a ship undergoing treatment. This Section is con-cerned with the process which gave rise to the term and is knowngenerally as "flashing," although it would be less ambiguouslydescribed as "vertical flashing."

The process consists essentially in neutralizing the verticalmagnetism in a ship by surrounding the vessel with a horizontalcable near the water-line, and passing a heavy current momen-tarily through the cable. It thus forms an alternative method towiping, and much of the previous discussion concerning wipingapplies equally to the flashing process.

The treatment was initially developed by the French, who wereflashing their ships for immediate protection in the early criticaldays when wiping was being developed in this country. TheFrench set up several flashing stations, and the techniquediffered slightly at each. At certain of the stations the flashingcables were fitted, around the sides of the dock, except at theopen end where they were laid on the sea-bed to allow theentrance of the ship over them. At stations of a different typethe flashing cables were laid on a series of rafts brought in closeto the sides of the vessel so that the cables were generally about ayard away from the hull. This disposition of cable enabledcontrol of the flash to be obtained by moving rafts away fromthe hull at those places where a smaller field was desired. Inthis way, a ship with an asymmetrical signature could be treated,and, in fact, it was found advantageous in most cases to keepthe cables away from the hull at bows and stern, otherwisethe signature became too negative near the ends. At the timeof application of these methods, however, the primary aim wasto remove as much positive field as possible, even if large negativefields remained, and the technique of control was comparativelyunimportant. The currents used were high, and m.m.f.'s of theorder of 20 000-30 000 ampere-turns were distributed betweenabout 6 or 7 cables. The stations were designed to treat shipsup to 10 000 tons, and even larger ones in some instances. Thevalue of overflashing and flashing back in order to improvestability was clearly realized by the French.

A close liaison had been maintained with the French, and thework which they were doing was well known in this country.Nevertheless, British practice concentrated mainly upon wiping,since the latter was by" this time proving to be a very satisfactory

VOL. 93, PART I.

process. The decision to develop wiping rather than flashingwas governed, not by technical grounds, but chiefly by considera-tions concerning the wisdom of constructing the large numberof hign-power stations required, compared with the somewhateasier task of fitting out mobile wiping units or fixed wipingstations with a somewhat lower-power output. Technically, itwas thought that flashing could be made equally as effective aswiping; in fact, that a slightly better compensation of the ship'sfield could probably be produced, particularly at points notdirectly underneath the keel. The fact that the heavily over-flashed French ships usually retained their flashed effect longerthan the normally wiped British ships provided an additionalincentive to the development of overwiping.

A single flashing station was, however, set up at Rosneath,Scotland, by the Americans and later turned over to Britishuse. Its use was by no means confined to vertical flashing,though a few ships have been treated there by methods similarto those described above. At this station the flashing cableconsisted of a loop of 7 banks of 7 turns of heavy cable hanging4 ft below water-level from an inflated rubber hose. One sectionof this hose could be deflated, and the cable sunk to the sea-bedto permit the entry of the ship. The hose could then be againinflated, and the loop drawn into shape around the ship bymeans of ropes to the ship and to the surrounding catwalk.Flashing currents to give up to 40 000 ampere-turns could beprovided, and the resulting treatment proved quite successful.

In general, however, vertical flashing has been confined in thiscountry to wooden-hulled vessels, where wiping is inapplicablebut where treatment is necessary because of the appreciableferrous content such as engines and armament. The methodthen adopted is to wrap the cable around the vessel near the water-line in a manner similar to wiping; sometimes the cable is passedround only those parts of the ship which contain the bulk ofthe magnetic material. Momentary currents are passed throughthe cable, and the vessel is overflashed to provide stability andthen flashed back to within the required limits. The m.m.f.'sinvolved in the treatment of a small wooden vessel are of theorder 6 000 ampere-turns.

Local vertical flashing has been applied to coiled vessels withsuccess in special instances. As an example, certain mine-sweepers carry acoustic hammer-boxes on booms to provide asource of sound capable of detonating acoustic mines. The boomconsists of a long steel frame which is lowered into an almostvertical position in the water when sweeping. Such a frame in thesweeping position was found to give fields which would endangerthe sweeper from magnetic mines. The frames were initiallycompensated by coiling, but it was later determined that localvertical flashing of the frames could give adequate protectionand that reasonable stability could be achieved by the over-flashing technique. This method was therefore adopted asquicker and quite satisfactory.

There is a further local application of vertical flashing treat-ment which is important. This may best be described by theterm "stabilization." As an illustration, it has already beenmentioned that, when fishing trawlers were overwiped, theprocess resulted in an unstable magnetic compass with largeinitial deviations. This was traced to the magnetization of thewheelhouse, funnel and ventilators resulting from the overwipingtreatment. It was found that the instability of the compasscould be considerably reduced by winding a temporary verticalcoil of a few turns around these structures and cycling withvertical flashes of alternate sign, commencing with a high currentand progressively reducing to zero in equal steps. This resultedin stabilization of the structures in the field of the ship andconsiderably reduced the deviations and instability of the mag-netic compass.

31

512 AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION

(4) DEPERMINGThe term "deperming" is used to cover any form of treatment

to which the vessel is temporarily subjected, designed to removeor compensate permanent longitudinal or athwartships mag-netism. The term "longitudinal deperming" applies to thecompensation or removal of P.L.M., and "athwartship deperm-ing" applies to P.A.M. Since comparatively few vessels aresubjected to the latter treatment, the term "longitudinal deperm-ing" is frequently shortened to "deperming" in cases where noconfusion is likely to arise.

(4.1) General ConsiderationsThere is an essential difference between the problem of com-

pensating the horizontal magnetism by deperming and that ofcompensating the vertical magnetism by wiping or flashing.Reconsidering the case of vertical magnetism, it will be realizedthat the ship moves continually in the vertical component of theearth's field (except in the limiting case at the magnetic equatorwhere this is zero), and that this component at one locality isconstant in magnitude and sign with respect to the vessel. Thismeans, in the first place, that the induced vertical magnetism(which, as is indicated in another paper, constitutes about two-thirds of the total vertical magnetism) is always present, and forcompensation an equal and opposite remanent magnetism mustbe introduced by the wipe or flash. In the second place, theearth's vertical field introduces a continual magnetic force on theship, irrespective of heading, and this field tends to reintroduce anypermanent vertical magnetism removed and also to remove anypermanent magnetism introduced by the wipe or flash. In suchcircumstances there is an obvious limit to the duration of thewiped or flashed effect. In the case of horizontal magnetism,the situation is different. The ship experiences a horizontalmagnetizing force due to the earth's field on any particularheading, but when the ship is on the opposite heading the fieldis reversed in sign with respect to the vessel, and over a longperiod of voyaging the time average of the horizontal field inany direction with respect to the ship is approximately zero.The problem thus becomes essentially one of demagnetization.If the ship could be completely demagnetized in a horizontalplane, the necessity for subsequent retreatments would arise onlyinfrequently.

For example, it is known that the natural P.L.M. of anuntreated vessel is acquired mainly during building and fittingout, and that there is a close correlation between head-on build-ing and the P.L.M. acquired. Several ships, whose directionof building has been such that the resulting P.L.M. was originallysmall, have been measured on a large number of occasionsthroughout the war years, and have never shown such largeP.L.M. fields that they have exceeded the normal limits atwhich deperming has been considered necessary. Naturally, itdoes not follow that this is so in all cases. Ships can acquireor lose P.L.M. by vibration during long voyages on one heading,by violent shock or collision, by refitting, and in other ways.An interesting case is that of a destroyer near Alexandria, whichwas successfully but drastically depermed by a near-miss froman enemy bomb while on an East-West heading. Nevertheless,a deperming process which involves complete demagnetizationis clearly the ideal which could be achieved so far as stabilityis concerned.

The problem of demagnetization is unfortunately not so easyas might appear at first sight. The standard method for demag-netizing small bodies is to maintain them in a zero field and toapply a strong alternating field which is gradually reduced tozero. The difficulties encountered in attempting to apply thisto a massive complex body such as a ship are mainly due to thechange of scale. Field-free space cannot easily be produced

over such a large volume, and there is considerable difficulty inobtaining a sufficiently powerful alternating field; the latterdifficulty is greatly enhanced by the fact that the reactance ofany circuit comprising coils surrounding the vessel to alternatingcurrents of the normal frequency of 50 c/s is prohibitively large.Alternating currents with a 50-c/s frequency have been appliedon the model scale to study problems concerning the demag-netization of ships, but it can be shown on theoretical groundsthat the time dimension must be increased by a factor K2 whenapplying model results to full scale, 1/K being the linear scale ofthe model. Thus for a typical 1 : 48 scale model, 50 c/s corre-sponds to about 0-02 c/s on the full scale. It will be realized,therefore, that if a similar process is to be applied on the fullscale, alternate flashes of direct current will have to be substitutedunless special arrangements are made for the generation of low-frequency alternating current.

(4.2) Early DepermsConsideration will first be restricted to the development of

longitudinal deperming, since athwartship deperming is of farless practical importance. It is best to approach the problemhistorically. The first longitudinal deperms were based uponthe principles given above. The longitudinal field through theship was maintained zero by the simple expedient of mooringthe ship on an East-West heading, and the vessel was riggedwith a temporary coil known generally as a "deperming solenoid."This is illustrated in Fig. 6; it consists of a number of turns of

To Power Supply

Elevation Showing Turns

To Power P l a n o F D e c k L e a d s

Supply

Fig. 6.—Ship rigged for longitudinal deperming.

heavy cable passed around the ship and connected in series.Each turn lies in a vertical plane, while the supply cables arearranged to lie on the deck and close alongside the interconnectingcables between the series turns. Since the currents through thesupply and interconnecting cables are in opposite directions,their fields are largely neutralized. Thus the resultant field isthat due solely to the vertical turns and is reasonably uniformover most of the volume of the ship, provided the turns are notspaced too far apart. When such a solenoid has been rigged,a series of longitudinal flashes can be given, commencing withas high a value of the current as possible. The flashes arearranged so that each is of opposite sign to the preceding oneand the current is diminished by a fixed amount.

Although model experiments had previously been made at theAdmiralty Research Laboratory, the first full-scale vessel towhich this type of treatment was applied was the destroyerH.M.S. Wild Swan, in early 1940. In this case, 20 turns wererigged on the vessel and a maximum current of 640 amp wasused, corresponding to about 40 ampere-turns per foot of

AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION 513

solenoid. The current was obtained from a motor-generatorwith controlled field current. After successive reversals withprogressively reduced current, it was found that the P.L.M. wasconsiderably diminished but not completely removed, and theexperiment was thus only partially successful. Trials withseveral other vessels followed, and supplementary tests on modelswere carried out at the Admiralty Research Laboratory. Theyshowed that for an approach to complete demagnetization,much higher currents were needed. These currents, which wereestimated to correspond to 100-200 ampere-turns per foot of.solenoid, were realizable only in small vessels. Accordingly, anearly variation of the method was to bias the flashes in such away that some longitudinal magnetism was introduced during theprocess to compensate for any P.L.M. which had not been re-moved. These early trials, when carefully carried out, ensuredthat the ship's effective P.L.M. was small after deperming, butthe process suffered from the disadvantages that high currents,heavy cable, careful control of currents and specialized operatorswere necessary, whilst what was urgently wantad was a simplemethod which could be rapidly applied on a large scale.

(4.3) Longitudinal WipingMeanwhile, wiping with a horizontal cable was proving an

effective method for compensating vertical magnetism, and itwas decided to test the application of the wiping method to thecompensation of longitudinal magnetism by using a verticalcable. The basis of the method is illustrated in Fig. 7. A

Hauling Line\ To Power Supply

j\ . oop

Fig. 7.—Longitudinal wiping.

single vertical turn of cable is rigged around the vessel, kept asclose to the hull as possible, energized with direct current, anddragged along the ship in a longitudinal direction. By varyingthe current as the cable is moved, the remanent magnetism alongthe ship can be varied and any irregularities in the distributionof the longitudinal magnetism can be compensated. In practice,it was found best to start the wipe amidships and work separatelytowards bow and stern. Currents of the order of 1 000 ampsufficed for most ships. The process was used considerably for ashort period and became known as "deperm A." Among otherobvious disadvantages, it was cumbersome in operation, thecable was frequently fouled by obstructions, and, once again, alarge measure of skill and experience was required on the partof the operator. A modification of the method, known as"deperm B," consisted in using buoyant cable and thus ensuringmore complete contact with the flat bottoms of merchant ships.

(4.4) Deperm C and its DevelopmentsThe solution to the problem of providing an easily controlled

and rapid operation came in the autumn of 1940 with thedevelopment of "deperm C." This process in its simplest formconsists in rigging the ship with a deperming solenoid, asillustrated in Fig. 6, and passing a single short pulse of currentthrough this solenoid in such a way that the effective longitudinalmagnetism is reduced to a small value, i.e. the remanent mag-netism left by the flash balances out that part of the naturalP.L.M. which has not been removed by the flash. In practice,it is not generally possible to achieve this by a single flash, even

if such a procedure were desirable, and usually a few flashes willbe required. But the number involved is considerably less thanused in the earlier deperms described in Section 4.2. Measure-ments are taken before and after each flash to determine itseffect. It is generally necessary to control the process by tworeadings only, these being taken by magnetometers placed atabout beam depth below the keel of the vessel and at a distanceinwards from bow and stern equal to the beam; these are thepositions where the maximum fields due to P.L.M. usually occur.After treatment, a final set of readings is taken along the lengthof the ship to confirm that the vessel has been left in a satisfactorystate.

This method was found to be successful in so far that it wasrelatively simple to apply and easily controlled, and that a goodfinal signature could generally be obtained. The power require-ments were easily within the reach of the stations and seldomexceeded 60 a.t./ft. The problems which arose were such thatthe process could be applied successfully on a large scale untiltime and results were available for their solution. Nearly allthe later deperming processes used in this country have beenderived from the above basic deperm C.

It is possible in this paper to discuss only very briefly a fewof the more important problems of development which havearisen. The main questions which have been the subject ofconsiderable research are (a) the best disposition for the deperm-ing solenoid, (b) the duration of the flashes, (c) the relationbetween the flashes and their effect, and (d) the sequence of theflashes and the effect on stability. These will be consideredseparately.

(4.4.1) Disposition of the Deperming Solenoid.In considering the spacing of turns required on the solenoid,

it must be borne in mind that by far the greater part of the timespent on a deperming operation is taken up in rigging the vessel,and that the fewer the turns necessary, the more rapid will bethe operation. On the other hand, if the turns are spaced toofar apart it will be found that the signature shows a characteristiceffect for each turn. A typical example of rigging the turns toofar apart is shown in Fig. 8, where the separation of each turn

+ 80

Fig. 8.—Effect of turns being spaced too far apart.

from the next is 1 • 2 times the beam of the vessel. A smoothsignature at beam depth is obtained when the turns are riggedwith a uniform spacing not exceeding two-thirds of the beam ofthe vessel. Considerable work has been carried out to determineat what distance the end-turns of the solenoid should be placedfrom the bows and stern of the vessel, and it is generallyestablished that a distance of one-third of the beam from eachend leads to the best results on the average. With such asolenoid, the effect of the flash is roughly the same shape as thenatural P.L.M. for most vessels, and a good signature results.

514 AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION

For certain classes of vessel, non-uniform spacing has beenadvocated from tests on models, but the increased effectivenesshas not been thought worth while and the method is rarelyadopted. It may be pointed out that in large vessels the resis-tance of a solenoid with single turns may limit the current toomuch, and it may be necessary to duplicate some or all of theturns, connecting them in parallel with the original turns.

(4.4.2) Duration of the Flashes.The maximum effect of a flash is not realized if it is applied

for too short a time. This is due to the inductance of the hull,in which eddy currents oppose the build-up of the main current.As a simple illustration, in the battleship H.M.S. Revenge itwas found that, on switching off the flashing currents, eddycurrents persisted for about 30 sec after the break; their effectcould be detected on a galvanometer connected in circuit witha single loop of light wire around the vessel. This problem hasbeen investigated theoretically and on models and full scale.The approximate working answer which governs the routineprocesses allows 20 sec for the flash on ships up to 60-ft beam,and 30 sec for larger vessels.

(4.4.3) The Effect of a Flash.The relation between the strength of a flash and the effect

produced has been investigated in some detail by analysis ofoperational deperms and by model work. Results show that,for an initial flash, the following approximate relation holds

where Pl represents the effective change in P.L.M. produced bythe flash, measured in milligauss at beam depth;/,, the strengthof the flash in a.t./ft; and k is a constant depending on the classof vessel. There is a large scatter from the mean for the valuesof k shown by individual vessels, but the values assumed inpractical deperming are 0 • 02 for warships and 0 • 03 for merchantships. Newly constructed vessels respond more readily to aninitial flash, and for these k may be as high as 0 06. A secondflash in the same direction as the initial one produces no furthereffect unless it is the greater, in which case the total effect isgoverned by the second flash only.

On the other hand, a succeeding smaller flash (/2 a.t./ft) inthe reverse direction produces a greater proportionate effectthan the initial one, since it removes much of the softer mag-netism introduced by the latter. Its effect (P2) depends, there-fore, on the magnitude of the initial flash, and for small flashescan be expressed in the form

Relations such as the above simplify the deperming process byproviding a means of estimating the strength of the flashes whichwill be required. They found their main application in applyingthe deperm C process under conditions where measurementswere not possible (blind deperming). This latter process proveda useful, although somewhat risky, expedient, but was largelyabandoned soon after the horizontal-control measurementsdescribed in Section 4.8 were introduced.

(4.4.4) Sequence of Flashes and Stability*The sequence of flashes is of major importance since the

stability of the deperm is governed by it. In the earliest deperms,the first flash was made sufficient to reverse the sign of the P.L.M.,i.e. the ship was overflashed and then flashed back to the requiredsignature. This overflashing was originally adopted, not forstability considerations, but because it was thought that thelarge initial flash would provide a means of smoothing irregu-larities in the signature. The amount of overflashing was firstfixed at 100%, which means that the initial flash produced a

P.L.M. equal in magnitude and opposite in sign to the originalP.L.M. The figure was later reduced to 50%, since thereappeared to be little gain in achieving smoothness by the largeroverflash. Very little systematic work had at that time beencarried out to examine the effect on the stability, but one im-portant fact was realized early. It was clear that when a vesselchanged heading in the earth's field it was subject to the equivalentof a flash. It can easily be shown by calculation of the field of asolenoid that the earth's field in U.K. waters {H = 0-175 gauss).corresponds to a m.m.f. of about 5 a.t./ft in the solenoid, andthe corresponding field in equatorial waters to about 12 a.t./ft.Thus the ship is liable to receive equivalent flashes up to thelatter amount during voyaging; therefore the reverse flash shouldprovide at least this value to produce what may be described asan artificial ageing of the first flash and thus increase stability.This figure is an underestimate, as will appear from the followingparagraphs.

Meanwhile, interest was rearoused in stability considerationwhen it was shown from examination of early deperms thatthose ships which had been overflashed by large amounts of theorder of 250% were proving the most stable. Model work wasalso confirming this result. Accordingly, in 1942 the deperm Cprocess was standardized to consist of (a) an initial flash orflashes to produce some 250% overcompensation of P.L.M.,(b) a reverse flash to produce approximate compensation, and(c) three or four flashes of alternate sign and magnitude of12 a.tVft to produce artificial ageing equivalent to the maximumfield encountered during voyaging. The process remained inthat state for about two years, and generally resulted in verysatisfactory deperms. It became clear, however, that sufficienttime had then elapsed to allow enough data to become availablefor a thorough re-examination of the stability problem.

Two main approaches were possible, and both were fullyexploited. The first consisted in complete investigation on themodel scale, where controlled conditions could readily beachieved, and the second consisted in a statistical examinationon the full scale of the stability of the various treatments.Luckily for the latter investigation, a sufficient number of thesehad diverted from the procedure outlined above to enable com-parisons to be made. Both space and security considerations donot allow full details to be given concerning the interestingmethods and results obtained, but some of them can be outlinedbriefly.

The general method of investigation on the model scaleproceeded on the following lines. P.L.M. was first introducedinto a selected model by stabilizing it on a heading other thanEast-West, i.e. by cycling from a high value of alternating currentdown to zero in the presence of an ambient longitudinal field.Any desired value of P.L.M. up to a limiting maximum couldtherefore be introduced, by varying the heading of stabilization.The model was then depermed by the application of the processwhose stability was under test. The stability of the process wastested by subjecting the model to a series of a.c. cycles in alongitudinal solenoid, each cycle being commenced at some fixedamount and gradually reduced to zero. This treatment, which isknown as D.A.C. (diminishing alternating cycles), is commencedwith the maximum cycling current small and the latter is thengradually increased for the succeeding treatments, the P.L.M.being measured after each cycle down from the maximum hasbeen completed. A few typical results are shown in Fig. 9,which is representative of the many decay curves which havebeen obtained on several models for various values of the P.L.M.and for different deperming processes. Such curves differquantitatively, but their general qualitative behaviour is similar.

In Fig. 9, the full curve shows the distribution of hardness inthe P.L.M. of the undepermed model. At about 140 a.t./ft

AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION 515

2100

a:

ISo

3

-25

40 60 80 100 120 140D.A.C. Treatment, Ampere-turns per foot

Fig. 9.—Model test of stability of deperm treatment.Undepermed ship.

x — X — x No overflash.0 — 0 — 0 100% overflash.. — • — • 200% overflash.

300% overflash.100% overflash with 12 a.t./ft cycle.300% overflash with 12 a.t./ft cycle.

D.A.C. treatment, some 20% of the original P.L.M. remains.It can be seen by inspection of the curves that a single-flashdeperm is very unstable, some 80%of the original P.L.M. returningin this particular case after D.A.C. treatment commencingat 30 a.t./ft. The decay curve for an overflashed ship is suchthat the D.A.C. first removes more of the effect of the flashbackthan that of the original flash. The P.L.M. therefore reversessign. As the D.A.C. treatment increases, the P.L.M. changesback to its original sign, rises to a maximum at the approximatepoint where the overflash effect has been removed, and thereafterdecays slowly. It will be noted that such overflashing thereforein general leads to an increase in stability. It can also be seenfrom the curves that the effect of the 12-a.t./ft cycles arebeneficial in removing the initial large ageing which takes placeas the softer P.L.M. introduced by the flashback is shaken out.An interesting point is that the position of the cross-over movesinto the higher D.A.C. values with increasing overflash. Itsposition also depends upon the value of the initial P.L.M.

There is a fundamental difficulty in applying considerationssuch as the above direct to full-scale work. This is due to thefact that P.L.M. has been introduced and removed from themodel by electromagnetic treatment, and the latter process isdifferent from that which occurs in the full scale. As has beenpointed out, P.L.M. is mainly introduced into full-scale ships•when they are subjected to stresses in the longitudinal field ofthe earth during construction, refit or vibration on one pre-dominant heading. The removal of this natural P.L.M., or ofany P.L.M. artificially introduced during deperming, is due to therepeated stresses which the vessel experiences during voyagingon randomly orientated headings. In order, therefore, to linkup the model- and full-scale work, some estimate has to bemade of the equivalence between the D.A.C. treatment and thenormal working stresses to which ships are subjected. That

the normal working stresses aregenerally insufficient to remove thenatural P.L.M. completely is shownby the fact that correlation existsbetween head-on building and P.L.M.of undepermed ships, even for vesselswhich were constructed several yearsbefore measurement. On the otherhand, the early deperms show thatthe working stress is sufficient toremove much of the effects of a single-flash deperm within a few months.The problem of deriving a good de-perming process clearly becomes oneof choosing a method which gives aflat ageing curve up to the limits ofthe normal working stress. Com-plete demagnetization is obviouslythe answer, but, as has been seen,the process involves considerablepractical disadvantages except forthe smaller vessels.

The best estimate of the equivalencebetween normal working stress andD.A.C. came by linking the modelwork with the second method ofexamining the problem of stability,i.e. the statistical examination of theresults of full-scale deperming. Thisinvestigation, carried out during1944, took into consideration the re-cords of some 2 000 deperms. Amongthe results, the optimum overflash for

stability for vessels with P.L.M.'s of a definite average valuewas carefully determined. The assumption was then made thatthe optimum overflash for a given P.L.M. had been used ifthe P.L.M. has returned to zero after negative decay (in otherwords, if the cross-over point of Fig. 9 has been reached) onthe application of the average working stress. From a know-ledge of the optimum overflash for a given P.L.M., theposition of the cross-over which corresponds to the averageworking stress could therefore be obtained in terms of D.A.C.It was thus found that the average working stress correspondedto about 30 a.t./ft D.A.C. treatment on the model. The resultwas, of course, approximate and gave the order of magnitudeonly; individual variations were to be expected.

It was also shown during the investigation that initial flashesof the order of 70 a.t./ft actually removed (as opposed to com-pensated) some 60% to 80% of the original P.L.M. Other in-teresting results concerned the problems of providing the moststable methods for deperming new-construction ships. Thesehad always presented a separate problem, because the initialP.L.M. is softer than that of an old ship, whose soft componentshave already been removed by the working stress.

Based upon the results of these extensive investigations, the.deperm C process was modified to its present form, and a newimproved method of deperming known as deperm L.R.I(Limited Reversals, Type 1) was introduced. Details of thesepresent processes remain confidential and cannot be given here.It can be stated, however, that deperm L.R.I is designed toremove as much of the natural P.L.M. as possible and to com-pensate for the remainder by hard P.L.M. introduced by biasingat a high level. An essential feature of the method consists intesting out the stability by cycling at a level estimated to beequivalent to the normal working stresses.

No figures can as yet be quoted for the success of the later

516 AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION

methods, as sufficient time has not elapsed since their introductionto enable the stability to be tested, but it is known that the averagelife of a good deperm C is in excess of a year and it is believedthat the life of a deperm L.R.I will show considerable improve-ment. There clearly exists an upper limit, because any abnormalworking stresses experienced on headings other than East-Westmay introduce P.L.M. in a completely demagnetized vessel.

(4.5) True DepermingMention should be made of the process which may be called

"true deperming." This is the ideal demagnetizing process towhich reference has already been made and consists essentiallyin applying non-biased cycles in azero ambient field, the cycles com-mencing at the highest possible leveland gradually being reduced to zero.As has been pointed out, owing tothe practical difficulties of provisionof high power and control of currentthe method has not been generallyadopted in this country, but it hasbeen found particularly useful inspecial cases of small craft wheresuch difficulties do not arise. As anexample, it has been used in midgetsubmarines, where alternating cur-rents of 1 c/s could be employedand the difficulties of current con-trol eliminated.

fields which occur in deep water, it is generally necessary tostipulate that such measurements must be taken at a depth whichlies between 0-75 and 1*5 times the beam below the centre-lineof the ship. There are standard expressions for making appro-priate allowances for heading effects (I.L.M.) and for convertingvalues from one depth to another within this range.

It frequently happens, however, that the necessary depth ofwater cannot be obtained. An alternative method of controlcan then be employed by arranging the magnetometers to measurehorizontal fields at right angles to the longitudinal centre-line ofthe vessel. The two control points are chosen to be opposite topoints in the centre-line at beam distance in from bow and stern,

Centre Line of Ship

MagnetometerSpar

Magnetometer

Centre Line of Ship

Magnetometer

(4.6) Change of Vertical Field byProcesses Allied to Deperming

It should also be mentioned thatby longitudinal cycling (or for that matter, by cycling in any otherdirection) the vertical field of the vessel can be changed undercertain circumstances, since the vessel becomes more or lessstabilized in whatever ambient vertical field exists at the time.By producing an artificial vertical field (usually by means of a coilin a horizontal plane near the water-line of the vessel) and subject-ing the vessel to a series of progressively reduced cycles, the verticalmagnetism can be brought to within certain desired limits. Theprocess has been mainly developed by the Americans as a counter-part to British wiping and vertical flashing, and details cannot bereleased at the moment. In this country, the method of cyclingin an ambient field, other than that of the earth, has not beenextensively employed in full-scale work. It was, however, usedwith complete success during the degaussing of the midget sub-marines mentioned above.

Stabilization by cycling in the earth's vertical field is, however,the standard process for eradicating past history. On the fullscale, this process is known as "dewiping" since it finds a largeapplication in removing the effects of a past wipe from a shipwhich in future is to be protected by coiling. It has also beenemployed to remove irregularities attributed to abnormal localmagnetization exhibited occasionally in coiled vessels, and thusto enable the coils to provide a better fit to the signature.

(4.7) Control MeasurementsBefore concluding the subject of longitudinal deperming, the

methods of measurement for controlling the operation should befurther discussed. As has been stated, the normal method isto take vertical-field readings at two control points directlybeneath the keel, either by mooring the vessel over a fixed rangeor by suspending magnetometers from the vessel itself. In orderto avoid abnormal results due to irregularities which occur inshallow water, and inaccurate results due to the smallness of the

Fig. 10.—Method of measurement for deperm CH.

and at a horizontal distance from the centre-line equal to thebeam (Fig. 10). In these circumstances, the fields due to verticaland athwartship magnetism are approximately equal at the twomagnetometers and their difference in readings enables anapproximate estimation of the P.L.M. present to be made. Themagnetometers may be mounted on the dockside or at the endsof spars lashed temporarily to the ship, as illustrated. Thedeperm C process when controlled by horizontal measurementsin this way is known as "deperm CH." This development hasenabled many ships to receive controlled treatment under con-ditions where the depth was quite unsuitable for vertical measure-ments. Much time has frequently been saved by the use of theprocess to make possible controlled deperming of ships evenwhen loading or unloading in docks.

Throughout this paper, P.L.M. has been referred to as thoughit were a quantity capable of precise measurement. Actually,on the full scale, the exact determination of the true P.L.M. isdifficult owing to fundamental uncertainties in distinguishingbetween fields due to longitudinal and those due to vertical mag-netism, and approximate methods have to be used. It is com-paratively easy, however, to determine the amount of asym-metry in the East-West keel signature with the M-coil energizedat the compensating value. This latter quantity is usuallytermed "apparent P.L.M." and may differ from true P.L.M. andcontain a component due to misfit of the M-coil with the verticalmagnetism of the ship. It has been the policy in most instancesto remove this "apparent P.L.M." when deperming and thus toobtain the safest possible signature. In most vessels, the"apparent P.L.M." gives a fairly close representation of the trueP.L.M. and the better the coiling, the more closely do the valuesof true and apparent P.L.M. agree. It must be made clear thatthe compensation of M-coil misfit by longitudinal deperming hasbeen adopted on the grounds of expediency, and efforts are con-

AYLIFFE: PROCESSES APPLIED TO A SHIP TO ALTER ITS STATE OF MAGNETIZATION 517

tinually being made to reduce the necessity for it to a minimumby designing well-fitting M-coils.

(4.9) Athwartship DepermingIt has already been noted that the permanent athwartship

magnetism (P.A.M.) cannot be of large magnitude. In fact, avalue of the order of the I.A.M. forms a natural upper limit, and

Fig. 11.—Ship rigged for athwartship deperm.

this is comparatively rare. Nevertheless, for those ships inwhich such magnitudes are exhibited, and where the P.A.M.forms the limiting factor of the ship's safety, the process ofathwartship deperming has been developed successfully as aroutine measure. The ship is rigged with two turns in a verticalplane in the manner shown in Fig. 11, and a few flashes, includingan overflash and flashback after the manner of deperm C, areapplied. The process is controlled by vertical measurementstaken amidships under each beam end. Lack of stability isseldom troublesome; indeed, there is some evidence that the largedemagnetizing factor in the athwartship direction will in timereduce the natural P.A.M. to a small value, since P.A.M. is rarelyexhibited in old-construction vessels.

(5) ACKNOWLEDGMENTSIn conclusion, the author wishes to acknowledge considerable

assistance received from Mr. A. B. Malone during the prepara-tion of this paper.

It must be emphasized that the advances described havebeen due to the combined efforts of numerous workers atHeadquarters and at Degaussing Stations in this country andoverseas. Among the many individuals responsible, the authordesires to mention the following: Messrs. E. C. Bullard, C. F.Goodeve, A. B. Malone, W. C. Potts, J. K. Roberts, andJ. C. Weston.

[The discussion on the above paper will be found on page 522.]