The Gathering Storm: The Second World War (104 page)

Soon it transpired that both acoustic and magnetic firing devices were being used by the enemy in the same mine, which would therefore respond to either impulse. In addition, many anti-sweeping devices appeared, designed to keep the firing mechanism inactive during the first or any predetermined number of impulses, or for a given period of time after the mine was laid. Thus, a channel which had been thoroughly swept by our minesweepers, perhaps several times, might still contain mines which only “ripened” into dangerous activity later. Despite all these fruits of German ingenuity and a severe set-back in January, 1941, when the experimental station on the Solent was bombed and many valuable records destroyed, the ceaseless battle of wits continued to develop slowly in our favour. The eventual victory was a tribute to the tireless efforts of all concerned.

Passive Defence – Degaussing

It is common knowledge that all ships built of steel contain permanent and induced magnetism. The resulting magnetic field may be strong enough to actuate the firing mechanism of a specially designed mine laid on the sea bed, but protection might be afforded by reducing the strength of this field. Although complete protection in shallow water could never be achieved, it was evident that a considerable degree of immunity was attainable. Before the end of November, 1939, preliminary trials at Portsmouth had shown that a ship’s magnetism could be reduced by winding coils of cable horizontally round the hull, and passing current through them from the ship’s own electrical supply. The Admiralty at once accepted this principle; any ship with electric power could thus be given some measure of protection, and whilst pressing on with further investigation to determine the more precise requirements, no time was lost in making large-scale preparations for equipping the Fleet with this form of defence. The aim was to secure immunity for any ship in depths of water over ten fathoms, whilst mine-sweeping craft and other small vessels should be safe in much shallower depths. More extensive trials carried out in December showed that this “coiling” process would enable a ship to move with comparative safety in half the depth of water which would be needed without such protection. Moreover, no important interference with the ship’s structure and no elaborate mechanism were involved, although many ships would require additional electric power plant. As an emergency measure temporary coils could be fitted externally on a ship’s hull in a few days, but more permanent equipment, fitted internally, would have to be installed at the first favourable moment. Thus, in the first instance there need be little delay in the normal turn-round of shipping. The process was given the name of “degaussing,” and an organisation was set up under Vice-Admiral Lane-Poole to supervise the fitting of all ships with this equipment.

The supply and administrative problems involved were immense. Investigation showed that whereas the needs of degaussing would absorb fifteen hundred miles of suitable cable every week, the industrial capacity of the country could only supply about one-third of that amount in the first instance. Although our output could be stepped up, this could only be done at the expensce of other important demands, and the full requirements could only be met by large imports of material from abroad. Furthermore, trained staffs must be provided at all our ports to control the work of fitting, determine the detailed requirements for each individual ship, and give technical advice to the many local authorities concerned with shipping movements. All this refers to the protection of the great mass of ships comprising the British and Allied merchant fleets.

By the first weeks of 1940, this organisation was gathering momentum. At this stage the chief preoccupation was to keep ships moving to and from our ports, particularly the east coast ports where the principal danger lay. All efforts were therefore concentrated on providing temporary coils, and the whole national output of suitable electric cable was requisitioned. Cable-makers worked night and day to meet the demand. Many a ship left port at this time with her hull encased in festoons of cable which could not be expected to survive the battering of the open sea, but at least she could traverse the dangerous coastal waters in safety and could be refitted before again entering the mined area.

Wiping

Besides the method described above, another and simpler method of degaussing was developed which came to be known as “wiping.” This process could be completed in a few hours by placing a large cable alongside the ship’s hull and passing through it a powerful electric current from a shore supply. No permanent cables need be fitted to the ship, but the process had to be repeated at intervals of a few months. This method was not effective for large ships, but its application to the great multitude of small coasters which constantly worked in the danger zone gave much-needed relief to the organisation dealing with “coiling” and yielded immense savings in time, material, and labour. It was of particular value during the evacuation of Dunkirk, when so many small craft of many kinds not normally employed in the open sea were working in the shallow waters round the Channel coasts.

Degaussing of Merchant Ships

My colleagues will be aware that one of our most helpful devices for countering the magnetic mine is the demagnetisation or degaussing of ships. This affords immunity in waters of over ten fathoms.

The number of British ships trading to ports in the United Kingdom which require to be degaussed is about 4,300.

The work of degaussing began in the middle of January, and by March 9, 321 warships and 312 merchant vessels were completed. Two hundred and nineteen warships and approximately 290 merchant vessels were in hand on the same date.

The supply of cable, which has up to the present governed the rate of equipment, is rapidly improving; and it is now the supply of labour in the shipyards which is likely to control the future rate of progress.

It would be a substantial advantage if part of the work of degaussing of British ships could be placed in foreign yards. The number of neutral ships engaged in trade with this country is about seven hundred. Neutral crews and in particular the crews of Norwegian ships are beginning to be uneasy about the dangers from enemy mines on the trade-ways to our ports. The importance to us of the safety of these neutral ships and of the confidence of their crews is a strong argument for disclosing to neutral countries the technical information which they require to demagnetise their ships which trade with this country.

Against the substantial advantages of arranging for some British ships to be demagnetised in foreign yards and of extending demagnetisation to neutral ships must be set any disadvantages of a loss of secrecy. If the enemy is informed of the measures which we are taking, he may (
a
) increase the sensitivity of his mines, or he may
(b)
mix mines of opposite polarity in the same field. If secrecy could be preserved, its advantage would be to delay these reactions of the enemy. But technical details of our degaussing equipment have had to be given to all ship-repairing firms in this country. Information which has been so widely distributed almost certainly becomes quickly known to the enemy.

Moreover (
a
)
and (
b
)
have the disadvantages to the enemy that –

(
a
) would make the mines easier to sweep and reduce the damage to non-degaussed ships by placing the explosion further forward or even ahead of the ships; and

(
b
) reversal of polarity would only be effective against certain ships which are difficult to demagnetise thoroughly, and would also require a sensitive setting of the mine.

The above position has altered since the arrival of the
Queen Elizabeth
at New York and the subsequent publicity given to the subject in the press. The enemy now knows the nature of the protective measures we are taking and, knowing the mechanism of his own mine, it will not be difficult for him to deduce the manner in which degaussing operates. He can therefore now adopt any counter-measures within his power. The press notices have had the further effect of increasing demands for information from neutrals, and to continue to refuse such information conflicts with our general policy of encouraging neutral ships to trade in this country.

It is considered, therefore, by my advisers that we shall not be losing an advantage of any great importance by ceasing to treat the information as secret.

The Admiralty recommend, therefore –

(i) that shipyards in neutral countries be used, if necessary, to supplement resources in this country for the degaussing of British merchant ships;

(ii) that technical information of our methods of demagnetisation be supplied as and when necessary to neutral countries for the degaussing of neutral ships trading with this country.

W. S. C.

Appendix I, Book II

EXTRACT FROM WAR DIARY OF
U.47

On 29.11.39 the following entry was made in the war diary of Admiral von Doenitz: “Following the report that
U.47
had torpedoed a cruiser, Propaganda claimed a sinking. From the service-man’s point of view, such inaccuracies and exaggerations are undesirable.”

Appendix J, Book II

CULTIVATOR NUMBER 6

Note by the Author

During these months of suspense and paralysis I gave much thought and compelled much effort to the development of an idea which I thought might be helpful to the great battle when it began. For secrecy’s sake this was called “White Rabbit Number 6,” later changed to “Cultivator Number 6.” It was a method of imparting to our armies a means of advance up to and through the hostile lines without undue or prohibitive casualties. I believed that a machine could be made which would cut a groove in the earth sufficiently deep and broad through which assaulting infantry and presently assaulting tanks could advance in comparative safety across No-Man’s-Land and wire entanglements, and come to grips with the enemy in his defences on equal terms and in superior strength. It was necessary that the machine cutting this trench should advance at sufficient speed to cross the distance between the two front lines during the hours of darkness. I hoped for a speed of three or four m.p.h.; but even half-a-mile would be enough. If this method could be applied upon a front of perhaps twenty or twenty-five miles, for which two or three hundred trench-cutters might suffice, dawn would find an overwhelming force of determined infantry established on and in the German defences, with hundreds of lines-of-communication trenches stretching back behind them, along which reinforcements and supplies could flow. Thus we should establish ourselves in the enemy’s front line by surprise and with little loss. This process could be repeated indefinitely.