Submarine Warfare of To-day Part 5

In addition to all these types of anti-submarine craft there were, forming part of the auxiliary fleet, over 300 ships, mostly trawlers and drifters, engaged in maintaining the great lines of boom defences, closing vast stretches of sheltered waters frequented by the battle fleets, and a considerable number of examination ships, staffed by interpreter officers, whose duty it was to examine all neutral shipping pa.s.sing through the 10,000 miles of the blockade.

These, then, were the ships of the new navy, and their formation into flotillas, or units, was usually accomplished by grouping four or five vessels of similar type together under the command of the senior officer afloat--mostly a lieutenant R.N.R. or R.N.V.R. In the case of minesweepers the unit nearly always consisted of an even number of ships, because their work was carried out in pairs, and with M.L.'s it usually consisted of five boats, as this was the number required for the intricate tactical work of submarine chasing.

There were, of course, units from the United States, French, j.a.panese, Italian and Brazilian navies, in addition to the formidable British armada.

The auxiliary units were all based on one or other of the fifty odd war stations which encompa.s.sed not only the coasts of Great Britain and Ireland, but also the littoral of every land in our world-wide Empire.

The numbers given here do not include the local fleets of purely colonial naval bases, nor the large flotillas of destroyers and "P"

boats operating in home and foreign waters in conjunction with the auxiliary navy. If these were incorporated the anti-submarine fleets would be almost doubled.

Now that the reader is familiar with the _raison d'etre_ of the new navy, the personnel, the ships and their formation into fleets, the scope and limitations of their activity, and of the losses they sustained, the way is clear for a description of the curious weapons used, the mysteries of anti-submarine warfare, and the bases themselves before entering the zone of war and seeing something of the actual work of the auxiliary navy.

FOOTNOTE:

[3] _Yachting Monthly_ and _R.N.V.R. Magazine_, August, 1917.

CHAPTER V

THE HYDROPHONE AND THE DEPTH CHARGE

OF all the weapons used in the anti-submarine war the two most important were the _hydrophone_ and the _depth charge_. They were employed in conjunction with each other and comprised the surface warship's princ.i.p.al means of offence against submarines operating beneath the surface.

The hydrophone resembles a delicate telephone. It is so constructed that when the instrument is lowered over the side of a ship into the sea any noise, such as the movement of a submarine's propellers, can be heard on deck by an operator listening at an ordinary telephone receiver connected to the submerged microphone by an electrified wire.

There were many different types of hydrophone in use during the Great War. So important was this instrument for the work of submarine hunting that money was spent in millions, and a corps of naval and civil experts were engaged for several years, bringing it to a state of efficiency.

Each type introduced into the Service was an improvement on its predecessor, and there were different patterns for the use of almost each cla.s.s of vessel. The fast destroyer required a different instrument to the slow-moving trawler. The motor launch could only employ successfully a totally different type to the submarine, and, to add to the difficulties, the German submarines themselves were generously supplied with similar instruments. The games of "hide-and-seek" played on and under the seas with the aid of this wonderful little instrument would have been distinctly amusing had men's lives--and often those of women and children--not been dependent upon the issue.

The portable hydrophone, used by some of the smaller and slower vessels of the auxiliary fleet, consisted of a microphone, or delicate mechanical ear, carefully guarded by metal discs from accidental damage, and connected to ear-pieces or ordinary telephone receivers by an electric wire which pa.s.sed through a battery. Where the wire came in contact with the sea water it was heavily insulated and lightly armoured.

When it was required to use this instrument the vessel was stopped and the microphone lowered overboard to a depth of about 20 feet. This was the distance down from the surface at which submarine noises could be heard most distinctly. The operator on deck or in the cabin then adjusted the ear-pieces and sat listening for any noises coming through the water. Although the sea is a far better conductor than air, the range at which sounds could be heard varied considerably. On a calm day or night the noise of a ship's propellers could frequently be distinguished at from five to seven miles; whereas on a rough day, with the sea splashing and the wind roaring, it was often difficult to hear anything beyond half-a-mile.

[Ill.u.s.tration: FIG. 6.--Diagram showing essential parts of a portable hydrophone. _A._ Head and ear pieces, by means of which a trained listener hears submarine sounds. _B._ Flexible leads to enable an officer to verify reports from listener. _C._ Battery box, containing spare set of cells. _D._ Terminals. _E._ Terminals of spare cells. _F._ Flexible armoured electric cable which is lowered over side of ship.

_G._ Metal case protecting the microphone _H_. _H._ Microphone or delicate receiver of submarine sounds, which is submerged (when required, but not when ship is moving) to a depth of about 18 feet, as in small diagram. The sound is detected by the microphone and transmitted up the cable _F_ and wires _B_ to the ear-pieces _A_.]

In fine weather a submarine could usually be heard at a distance of about two or three miles. There were, however, many microscopic noises of the under-seas which were picked up and magnified by this type of hydrophone. They were called "water noises," and often made it extremely difficult to differentiate between them and the sound of a moving submarine at a great distance. Later types were not so p.r.o.ne to these disturbing influences.

To describe here the different natural and artificial noises heard on a portable hydrophone is extremely difficult. One general statement can, however, be made. It is the noise caused by the rapidly revolving propellers of both surface ships and submarines that is the guiding factor in the work of detection by submarine sound. A destroyer travelling at full speed on a calm sea, when heard on a hydrophone resembles the roar of a gigantic dynamo. The sound does not alter as the distance between the _stationary_ listening ship and the _fast-moving_ warship increases or decreases; it continues to be a roar or low hum, according to distance, until it fades out of hearing altogether. The same statement applies also to a slow-moving cargo steamer, only in this case the _single_ propeller is revolving very much slower, and, when listening on a hydrophone about two or three miles distant, each successive beat of the engines can be distinctly heard.

The simple movement of a vessel's hull through the water cannot be heard on a hydrophone. Therefore for detecting the presence in the vicinity of a _sailing_ ship at night or in a thick fog this instrument is quite useless. The same drawback applies also to the location of a floating derelict or iceberg, and restricts the use of the hydrophone to faithfully reporting the presence of power-driven ships or special sound signals at a range of a few miles.

[Ill.u.s.tration: FIG. 7.--An improved directional hydrophone fitted through keel of motor launch. The tube _B_, at the lower extremity of which is the microphone, can be raised or lowered from _C_, the cabin of the M.L. This instrument is so arranged that the direction from which the submarine sound is coming can be simply and quickly ascertained.]

A German submarine heard at a range of about a mile on a calm night presents a curious sound which almost defies description. Its princ.i.p.al const.i.tuent consists of a "clankety clank! clankety clank!" at first barely distinguishable from the low swish of the water past the face of the submerged microphone, then louder when the sound has been distinguished and the human ear is on the alert. But when this sound was heard in war there was little time for a.n.a.lysing or noting. It was the call to action. The microphone was hauled to the surface and the chase began, a halt being made every half-mile or so for a further period of listening on the hydrophone. If the sound was louder the commander of the pursuing vessel knew that he was on the right track, and if the sound came up from the sea more indistinct the course was changed and a run of a mile made in the opposite direction, when the vessel was again stopped and the instrument dropped overboard.

Should this manoeuvre have placed the surface ship in close proximity to the submarine, one or more _depth charges_ were released, and if the explosion of these damaged the comparatively delicate hull or machinery of the under-water craft, she had either to rise to the surface and fight for her life with her two powerful deck guns, or, if badly damaged, sink helplessly to the bottom, emitting oil in large quant.i.ties from her crushed tanks.

Before entering upon a description of the depth charge, however, there is more to say of the hydrophone, which has played such an important part in the defeat of the U-boats.

When the advantages of this instrument had been fully demonstrated in the stern trial of war, successful efforts were made to improve upon the original crude appliances. The "water noises" were reduced and, greatest improvement of all, the hydrophone was made "directional." By this is meant that when a sound was heard its approximate direction north, south, east or west of the listening ship could be more or less accurately determined. What this improvement meant to a vessel hunting a submarine in a vast stretch of sea will be easily realised. When the sound came up the wires from the submerged microphone the operator had simply to turn a small handle in order to determine from which direction the noise was coming.

If, for example, the sound was first heard away to the east, the instrument was turned to another quarter of the compa.s.s. Then, if the noise was plainer, the instrument was turned again until the sound decreased in intensity. In this way the line of maximum sound was obtained, and this showed the direction from the listening ship in which the U-boat was operating.

[Ill.u.s.tration: FIG. 8.--Plan showing how microphones or ears _B_ are fitted in a submarine _A_ to enable it to detect the approach of surface craft.]

With the perfection of this invention the hydrophone section of the naval service came into being. Special courses in the detection of submarine sounds were inst.i.tuted for officers and also for seamen listeners. The actual movements of a submarine under water at varying distances from a hydrophone were recorded by a phonograph, and records made so that the sounds might be reproduced at will for the education of the ear. Surgeons with aural experience estimated the physical efficiency in this respect of would-be volunteers for the hydrophone-listening service, and vessels were formed into special hydrophone flotillas, whose duties consisted of listening in long lines for submarines and when a discovery was made attacking them in the most approved tactical formation, with the aid of depth charges and guns.

A considerable measure of success attended these arrangements, and the author spent many cold hours listening at night for the sound of the wily submarine. On more than one occasion an exciting chase resulted.

It must, however, be pointed out that there is one great drawback to the successful use of the hydrophone. It exists in the necessity for the listening ship to stop before the hydrophone is hoisted outboard, it being quite impossible to hear anything beyond the roar of the engines of the carrying ship so long as they are in motion. Furthermore, all progress through the water must have ceased and the listening ship have become stationary before artificial sounds, such as the propellers of a submarine, can be distinguished from the natural noises of the sea water.

Now it will at once be apparent that not only does a stationary ship offer a splendid target for under-water attack, but also it allows a somewhat humorous game of hide-and-seek to be played between a hunting vessel and a hunted submarine.

Nearly all U-boats were fitted with a number of hydrophones and therefore were as well able to receive timely warning of an approaching surface ship as the surface ship was of the presence of the submarine.

But the surface ship had the advantage of speed.

The result of all this was that when a German submarine heard a surface vessel approaching she dived to the bottom, if the water was not too deep or the sea-bed too rocky. Then shutting off her engines she listened. The surface ship, mystified by the sudden cessation of the noise she had been pursuing, also waited, and this stagnation sometimes lasted for hours. Then if the surface ship moved, as she was often compelled to do in order to avoid drifting with the tide away from the locality, the submarine moved also, and the one that stopped her engines first detected the other, but could not catch up to her again without deafening her own listening appliance. In which case the next move would probably be in favour of her opponent.

All of this is, perhaps, a little complicated, but a moment's pause for reflection will make this curious situation clear to the reader. And so the game went on, with decisive advantage to neither the surface ship nor the submarine. Darkness usually intervened and put an end to further manoeuvring, frequently allowing the submarine to escape.

A case of this kind occurred to a vessel, of a certain hydrophone flotilla, commanded by the author. For over four hours the U-boat eluded the pursuing surface ships by moving only when they moved and stopping when they too had stopped, darkness and a rising sea eventually favouring the escape of the submarine, which, a few hours later, was able to attack (unsuccessfully) a big surface ship less than thirty miles distant from the scene.

Nevertheless the hydrophone is a submarine instrument with a brilliant future. It has already been improved out of all resemblance to its original self, and more will undoubtedly follow. It is, however, purely an appliance for the detection of submarines when cruising beneath the surface, and not a weapon for their destruction. It should also be remembered that any improvement made in the efficiency of the hydrophone will benefit not only the surface ship, but also the submarine, for it cannot be supposed that under-water craft will be left without these wonderful submarine ears when their surface destroyers are equipped with them.

The alliance between the hydrophone and the depth charge is a natural one. The former instrument enables the surface ship to discover, first, the presence of a submarine in the vicinity, and, secondly, its approximate position. At this point its utility _temporarily_ ceases and that of the depth charge begins. When a surface ship is hot on the track of a moving submarine she endeavours to attain a position directly over the top of her quarry, or even a little ahead, and then releases one or more depth charges according to whether the chance of a hit is good or only poor.

From this it will be apparent that whereas the hydrophone is the instrument used for the initial detection of the submarine, and afterwards for enabling the surface ship to get to close quarters with her submerged adversary, it is the depth charge with which the attack is actually made.

This weapon is really a powerful submarine bomb. It consists of several hundred pounds of very high explosive encased in a steel sh.e.l.l, with a special firing device which can quickly be set so that the charge explodes at almost any depth below the surface after being released from the above-water vessel.

The methods in use during the war for its release from the decks of surface ships were very diverse, the most usual being for a number of these weapons to be fitted on slides and held in place by wire slings which could be released by simply pulling out a greased pin or bolt.

When the depth charge rolled off the stern of the surface ship it sank to the "set depth" and then exploded like a submarine mine. The result was a shattering effect exerted through the water for several hundred feet around. If the submarine was close to the explosion her comparatively thin plates were nearly always stove-in. When she was over a hundred feet away, however, the rivets holding her plates together were often loosened, and the resulting leak frequently compelled her to come to the surface, where she could be destroyed by gun-fire.

[Ill.u.s.tration: DROPPING DEPTH CHARGES

_British Official Photograph_]

It often happened, however, that neither one nor the other of these things occurred, but that the submarine's delicate electrical machinery was thrown out of order by the violence of a depth-charge explosion, even when a considerable distance away. With the electric engines used for submerged propulsion no longer available, and possibly the interior of the vessel in darkness, there were only two courses open.

She could either rise to the surface and endeavour to fight it out with the aid of her powerful deck guns, or else sink to the bottom and trust to luck that other depth charges would not be dropped close enough to seriously damage her hull. In the open sea, however, the latter chance was denied because of the depth of water. Three hundred feet may be taken as the greatest depth to which an ordinarily constructed fighting submarine can safely descend without running a grave risk of having her plates crushed in by the great water pressure. Even at this depth the weight on every square foot of hull surface exceeds 8 3/4 tons.

If the damaged submarine rose to the surface the guns of her pursuers were ready and could generally be relied upon to place her at least _hors de combat_ before the hatches of the under-water vessel could be opened and her own guns brought into action.

[Ill.u.s.tration: FIG. 9.--Diagram showing how depth charges are carried on the stern of a motor launch. _AA._ Depth charges, each containing 300 lb. of high explosive. _B._ Hydrostatic device by means of which the charge can be made to explode when it has sunk from the surface to a depth of 40 or 80 feet, and by which it is rendered comparatively safe while on deck. _C._ Slings holding charges in place on inclined platform. _D._ Greased bolts which, on being pulled out, allow wire slings to fly free and depth charge to roll into the sea. Depth charges can only be released from vessels under way, otherwise the explosion which occurs a few seconds after release damages surface vessel.]