Submarine Warfare of To-Day

^{adjacent squares of sea would be covered daily by one or more ships of each unit. The southern half of the area would be patrolled in the same way. The “Outer Beat” is shown by the arrows C, and the “Inner Beat” by the arrows D. The points +F show the possible positions of armed patrols acting independently of any unit or flotilla.}

Subsequently a further message might come to the naval centre that the same submarine had been chasing a merchantman in square “D,” “E” or “F” in the adjoining area. A concentration of fast ships, such as destroyers, M.L.’s or coastal motor boats, could then be made so as to intercept the raider or enclose her in a circle while other vessels hunted her down.

In a like manner important convoys coming down the coast, or entering a danger zone from the open sea, could be met by a local flotilla and escorted to a rendezvous with a flotilla from the adjoining area. In this way they were passed through the submarine and mine infested seas to and from their harbour terminus.

Almost the same methods were employed in dealing with the thousands of German mines. But to describe that part of anti-submarine warfare here would be to encroach on the subject of a succeeding chapter.

Patrols

The method of patrolling the areas and squares of sea was comparatively simple, though the same cannot be said of the actual work. The lines of patrol were called “beats,” and there was usually an “inner” and an “outer” beat for each unit or flotilla of ships. If when a ship (or a unit) reached her allotted square, from which the line of patrol extended, she elected to proceed on the inner beat, she would generally accomplish the return journey to the point of departure on the outer beat, thus covering her respective zone of patrol, but leaving the exact route to the discretion of the commanding officer. In this way no hostile submarine with a knowledge of the system could be sure of when or where a patrol ship would be met. In the same way it was left to the commander of a flotilla to either divide his ships into pairs, single units, or to maintain them as a homogeneous fleet, so that any combination of hostile submarines could not be made which would be sure of being able to attack a single patrol. Such an enemy combination might encounter a single ship, but it might also walk into the arms of a whole flotilla; or it might attack a single ship only to find itself surrounded by a following fleet.

^{Fig. 22.—Diagram illustrating the operations of a hydrophone flotilla composed of armed motor launches. Each vessel is given a number, and the flotilla proceeds in line-abreast along the course shown by the dotted lines. Each vessel is one mile from the other, and the whole line stops by signal at the point marked with a cross. Hydrophones are put in operation, and after a period of listening the flotilla continues on its course, as no submarine sounds are heard. The flotilla turns to head south, and a stop is again made to listen on the hydrophones. This time the sound of a hostile submarine is heard by vessel No. 1, bearing S.W. This report is confirmed by vessel No. 2 hearing the same sound, bearing a few degrees farther W. The two bearings A and B are then drawn on a chart, and the point where the two lines cross is the approximate position of the invisible submarine. The attack with depth charges is then ordered.}

The beats which were most distant from the base were given to the largest ships. This was done because it was often impossible for the more distant patrols to reach a place of shelter before one of the fierce gales which swept the northern seas was upon them. Trawlers, large steam yachts and converted merchantmen were usually employed on squares more than one hundred miles distant from a harbour of refuge, while motor launches kept watch and ward on the seas closer inshore.

The duration of patrols varied according to their position. Some lasted three weeks and others only a few days or hours. When the ships returned to their base after a spell at sea they were given a corresponding “rest” in harbour. A three weeks’ patrol meant several days’ “stand-off,” while a two or three days’ patrol entitled the ship to twenty-four hours in the comparative comfort of a harbour.

It must not be imagined, however, that a stand-off meant entire idleness or thorough rest. There were duties to perform which robbed it of much that it was intended to give. Ships had to be coaled, provisioned, painted or repaired. Engines had to be overhauled, sentries posted ashore, a guard to be furnished, and every day one ship in each unit that was in harbour had to be manned and in readiness for emergencies.

Hydrophone Flotillas

We now come to the actual methods employed by surface craft when attacking submarines. Although, as previously stated, much was left to individual initiative, there were, nevertheless, certain recognised methods.

Taking as an example the operations of a hydrophone flotilla of armed motor launches, the number of vessels forming the unit was usually five. When out scouting for the enemy they proceeded in line-abreast for about one sea mile, then stopped their engines and listened on their hydrophones for the noise of a submarine cruising in the vicinity. If nothing was heard the mile-long line of miniature warships advanced another mile and again stopped to listen. This man?uvre was repeated until one or other of the ships heard the familiar sound of a U-boat. Nothing might be visible on the surface of the sea, but if this was the case and the noise came up from the ocean depths over the electrified wires of the detector, it was conclusive proof that a submarine was in the near vicinity.

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, man?uvring 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 principally of either independent attacks with the aid of hydrophones and depth charges, or, more frequently, the assumption of an innocent air in order to induce the submarine to open the attack at close range.

^{Thornycroft & Co., Ltd.}

^{The Result of a Direct Hit}

A photograph left by the Germans in Ostend showing a coastal motor boat washed ashore after the great raid.

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