Submarine Warfare of To-Day

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 man?uvring, 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 shell, 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.

^{British Official Photograph}

^{Dropping Depth Charges}

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? 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.

^{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.}

In shallow water where there was a fairly smooth bottom it generally happened that a submarine damaged by depth charges elected to sink to the sea-bed and trust to luck. This was also frequently resorted to as a means of eluding pursuit even when the U-boat was not damaged by the first few charges dropped. It was then that the hydrophones carried by the surface ships were again brought into use to ascertain if the submarine was still under way. When no sound was heard those on the surface knew that “Fritz” was lying doggo, or else that he had escaped. If a number of ships were available a few waited over the spot where it was considered the U-boat was lying, while the others scoured the surrounding seas in circles trying to pick up the sound of the runaway’s engines if she had escaped in the melee. When nothing further was heard they returned to the scene and set about the work of systematically bombing the surrounding sea-bed.

^{Fig. 10.—Diagram illustrating a depth charge attack on a submerged submarine. A. Motor launch, which has dropped a depth charge to destroy a submarine B travelling at a depth of 90 feet below the surface. C is the depth charge sinking as the M.L. steams away from the danger area. D is the point (80 feet below the surface) at which it will explode, and E indicates the danger area for the submarine B.}

As many as one hundred depth charges were dropped in quite a small area of sea and yet a submarine known to have been lying “doggo” in the locality was not damaged. In cases such as this other means, which will be described in a succeeding chapter, were then resorted to.