The M.L. first detecting the noise hoisted a signal (flag by day and coloured electric light by night), giving the direction from which the sound came (see Fig. 22). The next ship in the line to receive the sound on its instruments then hoisted a signal, also giving the bearing--_i.e._ N.N.W., E.S.E., etc. If the two coincided in regard to direction, the attack commenced. If, however, they did not agree in this important respect, the line of patrol ships advanced another mile and listened again.

The flag-ship of the unit on receiving the direction from one or more ships marked the lines of sound on a chart (as in Fig. 22), and when this was substantiated by another ship the point where the two lines crossed was known to be the position of the hostile submarine, and the attack was ordered.

As to the exact method of an anti-submarine attack little need be said here beyond the fact that the ships advanced at full speed, manoeuvring into a special formation which enabled them to cover about half a square mile of sea with the explosive force of their collective depth charges.

When the attack had been completed all vessels engaged resumed their stations and waited with quick-firing guns ready in case the monster should rise from the deep to make a dying effort to destroy her pursuers.

The tactical methods of anti-submarine attack were, of course, numerous, and they varied according to the speed of the surface ships engaged.

What was possible of accomplishment by fast-moving coastal motor boats or the larger-sized M.L.'s proved impracticable for the more heavily armed but slow-moving trawlers and drifters. The tactics of these latter craft were often of the simplest character, and consisted princ.i.p.ally of either independent attacks with the aid of hydrophones and depth charges, or, more frequently, the a.s.sumption of an innocent air in order to induce the submarine to open the attack at close range.

[Ill.u.s.tration: THE RESULT OF A DIRECT HIT

A photograph left by the Germans in Ostend showing a coastal motor boat washed ash.o.r.e after the great raid.

_Th.o.r.n.ycroft & Co., Ltd._]

In many respects this proved the most effective method of anti-submarine warfare. Not only did it frequently cause the under-water craft to rise to the surface and commence the attack by gun-fire, in order not to expend a valuable torpedo on what appeared to be an unarmed and helpless ship, but it also produced a _moral_ effect throughout the German submarine flotillas.

When a few U-boats had been either sunk or damaged in this way the news that every Allied ship was heavily armed circulated among the enemy personnel, and they became very nervous of attacking in any position except totally submerged. This meant the loss of at least one torpedo, out of from five to ten carried, for every attack made, whether successful or unsuccessful, and the latter were predominant.

It soon became apparent that either they must risk surface attacks and so save their torpedoes, or else curtail their cruises to meet the rapid expenditure of their only submarine weapon. This does not, of course, cover the activities of under-water mine-layers, whose nefarious purpose consisted simply of laying their mines wherever they appeared most likely to catch Allied shipping. These craft were usually armed with torpedoes as well as mines, to enable them to continue the work of destruction when the cargo of the latter had been safely laid. In this way the problem of combating the German submarine offensive resolved itself into two parts, one being to checkmate the commerce raider and the other the mine-layer. With the second of these difficulties we shall deal in a later chapter.

Many merchantmen, both Allied and neutral, owed their escape to this camouflage warfare, which was brought to a high pitch of perfection and daring in the now famous mystery ships.

What may be said to form the second method of anti-submarine warfare was the decoy or camouflage system. Of primary importance in this category were the mystery ships already described, but there were also innumerable other _ruses de guerre_ which increased its efficiency.

To describe one of these will enable the reader to draw on his own imagination for the remainder. A vessel was steaming in from the Atlantic and was about a hundred miles from the Cornish coast when she was attacked by a submarine above water. The surface ship was heavily armed, but instead of using her weapons at once she sent out frantic wireless signals for a.s.sistance. Every few minutes the call went far and wide in plain Morse.

The sh.e.l.ls from the submarine splashed into the sea around, but none struck the target for some minutes. Had the surface ship desired, she could in all probability have avoided the under-water craft by using her superior speed, but instead she dropped back, allowing the submarine to catch up to her, and the sh.e.l.ls began to burst unpleasantly close.

Still the frantic wireless calls went forth. First the simple message: "I am being attacked by a large German submarine." Then the vehemence increased to: "I am being heavily sh.e.l.led." A few minutes elapsed and then the call: "Help. Submarine gaining on me." And finally: "Abandoning ship."

At this point the submarine was close astern and the liner slowing down preparatory to lowering her life-boats. The sh.e.l.ls were damaging her superstructure, but a heavy swell interfered with the German marksmanship. Then came the surprise. A life-boat on the liner's p.o.o.p was hoisted clear of the deck and from under its cover there appeared the lean grey muzzle of a 4.7-inch gun. A few sharp blasts of cordite and the submarine sagged and disappeared.

The captain of the liner had noticed when first attacked that the submarine was fitted with wireless and the calls sent out by him were in _plain Morse code_. On the strength of these the German commander had saved his torpedoes but lost his ship.

Another form of anti-submarine tactics was the employment of indicator and mined nets around an apparently disabled ship, or in lines across narrow channels known to be used by German submarines on their way to and from their bases. This method has, however, received full mention in other chapters.

What may be termed the third system of anti-submarine warfare was the use of extensive mine barriers, specially laid to catch submarines attempting to pa.s.s through them under water. The surface of the sea was patrolled by shallow-draft vessels and the under-seas guarded by mines.

If a submarine was sighted in the vicinity of one of the mine barriers already described she was attacked and forced to submerge herself in order to escape destruction from the guns of the pursuing surface flotilla. From that moment her fate was sealed. By cautious manoeuvring and using to full advantage their great superiority of speed (20-40 knots against 6-10 knots) the surface ships were able to head their quarry into the mine-field. Usually the submarine dived deep in order to throw her pursuers off the track, and all unconscious of the deep-laid mines in thousands she plunged to her doom--a heavy rumble, followed by an upheaval of the surface, and the chase was over.

This method, when carried out on the vast and scientifically sound principle described in a previous chapter, offers the best possible antidote to the submarine. Its employment in the Great European War placed the seal of complete success on the Allied anti-submarine offensive. It should, however, be remembered that comparatively narrow seas and a restricted zone of major operations made possible of accomplishment with some hundreds of thousands of mines (average cost, 400) what would in many cases and in many seas have been quite impracticable with as many millions of these difficult weapons.

The employment of submarines against submarines also forms a method of under-sea warfare which gives considerable scope for both daring and resource. It is of course quite impossible for one of these vessels when totally submerged to fight another in the same blind condition. But with just the small periscopic tube--or eye of the submarine--projecting above the surface, one craft can scout and watch for another to rise to the surface, thinking no enemy is near, in order to replenish her air supply for breathing or for recharging the electric storage batteries which supply the current for submerged propulsion.

When such a position obtains the submarine which comes unknowingly to the surface stands a grave danger of being torpedoed by her opponent.

This actually occurred to at least one German U-boat during the Great War.

One or more submarines can also be employed around a slow-moving decoy ship. In this case they would have the advantage of being invisible until the actual moment of attack. The result of such a manoeuvre would be either a gun duel on the surface or the torpedoing of the attacking submarine by one or other vessel of the decoy's submerged escort.

It was a ruse of this kind which achieved success in the North Sea during the early stages of the war. A trawler was employed to tow a submarine by a submerged hawser. This mode of progress was adopted to enable the submarine to economise the strictly limited supply of electricity carried for under-water propulsion.

The trawler then cruised very slowly about, dragging the submarine under the surface behind her. In order to divert any suspicion which might have been aroused by her slow speed she was rigged so as to give the impression that a net was being towed, and the area of operations chosen was well-known fishing-ground.

In this curious way days were spent before the desired consummation was reached. Then a large U-boat came boldly to the surface and opened fire.

Instantly the submarine astern of the trawler was released from the tow rope and forged ahead under her own electric engines. The commander of the surface decoy stopped his ship and commenced lowering the small life-boat carried. This was done in order to distract the attention of the Germans from the tiny periscope which was planing through the water to the attack.

A sh.e.l.l struck the trawler, carrying away her funnel, but did no other damage, and a few seconds later the water around the U-boat rose up in a vast upheaval of white. The plan had succeeded, and when the air cleared of the smoke from the trawler's damaged stack there was nothing afloat on the surface of the sea around--except an ever-widening patch of oil and bubbles.

A few minutes later the thin grey line of the British submarine rose above the swell some five hundred yards distant from the scene of her triumph.

Another means by which one subaqueous fleet can attack another is by laying mines in the seas around the enemy base.

These simple methods formed what may be termed the backbone of the widespread anti-submarine operations during the Great War, but with the experience gained and the brains of almost every nation focussed on the problem of providing an effective counterblast to the under-water warship, there can be little doubt that in the next great naval conflict new and more scientific means of attacking these pests of the sea will have been perfected, though what degree of success they will attain in the stern trial of war the future alone can tell.[7]

FOOTNOTES:

[6] Greenwich mean time.

[7] For a careful study of the effect of the submarine on the old theories of sea power see _Submarines and Sea Power_, by Charles Domville-Fife (Messrs George Bell & Sons, Ltd., London, and Messrs Lippincotts, New York.).

CHAPTER XI

THE MYSTERIES OF GERMAN MINE-LAYING EXPLAINED

TO those unversed in modern war it may have appeared strange that, although the Allied navies held command of the sea from the opening of the Great War in 1914 to the signature of Peace in 1919, the Germans were nevertheless able to lay several thousand mines every year off the coasts of England, France and even the most distant colonies and dominions. It often occurred that harbour entrances and narrow fair-ways were repeatedly mined, notwithstanding a vigilant day-and-night watch from the bridges, look-outs and decks of many patrol ships cruising or listening in the vicinity.

The explanation is that the mines were laid by large submarines capable of approaching the coast, laying their deadly cargo from specially constructed stern tubes and retreating to comparative safety far out in the broad ocean, without rising more than momentarily to the surface for the purpose of observation.

[Ill.u.s.tration: A CAPTIVE MINE-LAYING SUBMARINE

U.C. 5 off Temple Pier, London.

_Sport and General_]

This, it may be said, did not absolve the ships listening on their hydrophones, who should have been able to detect the approach of a submarine from the sound of her engines. During the first year of war the hydrophone was a very imperfect instrument, and although the sound might be heard it was quite impossible to tell from what direction it was coming. Later on, when the listening appliances had been greatly improved, there still remained two detrimental factors. The noise of breakers beating against rocks, sands or other obstructions destroyed much of the value of these instruments when used close insh.o.r.e. On dark and rough nights the roar of wind and sea and the lurching of the vessel rendered subaqueous sounds extremely difficult to detect; and in a fair-way or channel used by surface shipping it was frequently impossible, even in fine but dark weather, to tell if the sound coming up from the sea emanated from a surface ship or a submarine.