Chapter 13

THE MINE BARRAGE

WHAT undoubtedly forms the most effective counter to unrestricted submarine warfare is the explosive mine barrage, as employed against the German U-boats in the North Sea and the Straits of Dover.

The practicability of these barrage systems depends, however, very largely upon the following factors:--(1) the geographical features of the area of operations; (2) the hydrographical peculiarities of the seas in which the mines have to be laid; (3) the number of properly equipped mine-laying vessels available; (4) an adequate and highly trained personnel; and (5) the mechanical skill and manufacturing power of the nation employing the system.

There are several forms of mine barrage. One is simply an elongated mine-field laid across a narrow sea to prevent the safe passage of hostile surface craft. In this case the mines are laid in the ordinary manner and at the ordinary depth below the surface. The anti-submarine barrage, however, consists of an enormous number of mines, laid _at a considerable depth below the surface_ and in such formation as to ensure that a submarine attempting to pass through the cordon _while submerged_ would inevitably collide with one or more of them.

With this latter form of barrage the surface of the sea is quite clear of mines and is comparatively safe for the unrestricted movement of a numerous patrol flotilla, which forms part of the system, the under-seas alone being made dangerous by the mines.

It will be apparent that if a hostile submarine base is enclosed by one or more of these barrages the under-water craft entering and leaving that base have the choice of travelling _submerged_ across the danger zone and thereby risking contact with the mines, or of performing the passage _on the surface_ and encountering the patrolling ships. In either case, the result is more likely than not to be the destruction of the submarine.

In most cases the exact position of the barrage would be unknown to the hostile submarines, which, even if running on the surface, would dive immediately on the approach of a patrol ship. The few lucky ones succeeding in getting safely through the cordon of deep-laid mines, or passing unnoticed the patrol of surface ships on their outward journey--as might be the case in fog--would have the same peril to face on the return to their base, and probably without the aid of thick weather. This double risk would probably have to be taken by every submarine in the active flotilla at least once a month, this being approximately the period they can remain at sea without replenishing supplies of fuel, torpedoes and food.

The object of the flotillas of shallow-draught patrol vessels operating in the vicinity of the deep mine barrier is twofold. Primarily their duty is to prevent the hostile submarines from running the blockade on the surface and, secondly, to prevent enemy surface craft from emerging from the base and sweeping clear a passage through the mine-field, or of laying counter-mines, which, on being exploded, would detonate some of the blockading deep-laid mines and so destroy a section of the barrier.

From this it will be apparent that a force of hostile submarines hemmed in in this way would run a double risk of losing a number of vessels on every occasion on which a sortie was made. This is what actually occurred to the German under-water flotillas in the years 1917-1919, and, in combination with the other methods employed by the Allied navies, was mainly responsible for the failure of the great under-sea offensive.

The only bases of the German navy being situated on the North Sea littoral, it was possible for the Allies to lay a vast mine barrier, stretching from the coast of Norway to the Scottish islands, and another smaller one across the Straits of Dover; also to concentrate in the vicinity of these two submarine "trench systems" a very numerous surface patrolling force, thus enclosing the thousands of square miles of sea forming what was sometimes boastfully referred to as the "German Ocean" in an almost impenetrable ring of steel and T.N.T.

Here let us consider the gigantic nature of the task that was successfully accomplished. The distance from the Norwegian coast to the Orkney Islands is approximately 600 miles. It was over this vast expanse of sea, bent at the eastern end so as to rest on the Heligoland Bight, that the system known as the "Northern Barrages" extended. No exact statistics of the actual number of mines used is at present available, but reckoning at the low rate of one mine to every 750 feet of sea, with five lines stretching from shore to shore, the number required would be 21,000 of these costly and difficult weapons. The number required annually to maintain such a barrage would also be very heavy, and it is safe to assume that _considerably_ over 50,000 mines were employed on the northern barrages alone. From this rough estimate some idea of the work of designing, manufacturing, testing, laying, renewing and watching this one field will be obtained.

There were, of course, in the actual barrage several mine-fields placed strategically, and probably a far greater number of weapons than that given in the above estimate was needed. There were also the smaller fields lying between the northern barrage and the one across the Straits of Dover. These were so placed as to catch hostile submarines operating off the east coast of England, or a surface raiding squadron, such as those which in the earlier years of the war bombarded certain British ports.

Finally, when victory had been achieved, there came the cold-blooded task of clearing these immense areas of sea, not only of German mines, laid haphazardly, but also of the thousands of British mines laid methodically and away from neutral traffic.

The English Channel barrage differed from the northern line in several important respects. Being so much shorter (31 miles against 680), it could more easily be made perfect. The swift-running tide, however, greatly increased the difficulty of laying effective mine-fields.

THE LIGHTED BARRAGE

This southern system consisted, on the surface, of a number of vessels specially built to ride out the heaviest gale at anchor. These were moored at intervals across the Straits of Dover, forming two lines from the English to the French coast. The first line extended from Folkestone to Cape Gris Nez, and the second line about seven miles to the westward of these points (see Fig. 32). Each vessel was fitted with powerful searchlights for use at night, and the dark spaces of sea between were patrolled by large numbers of armed craft.

By this means the only avenues by which hostile submarines could hope to pass on the surface through the barrage at night were the dark lanes of water between the lightships. It was these points which were closely guarded by strong patrol flotillas, whose duty it was to attack submarines attempting to get through and, with the aid of guns and depth charges, to force them to dive below the surface.

Here certain destruction awaited them on the submerged mine-fields. If, however, one line of defence was safely passed by a hostile submarine, there was another to be negotiated seven miles farther on, and once a submarine got between the two lines her chances of escape were indeed small, for whichever way she turned the surface would be covered with fast patrol craft and at night lighted by the rays of many searchlights, while the under-seas were almost impassable with mines.

If, however, notwithstanding these defensive systems, a submarine succeeded in passing through and getting to work on the lines of communication with the armies in France, there were hydrophone organisations and patrols all down the Channel from the lighted barrage to the Scilly Islands. By this means a U-boat would be seldom out of the hearing of these instruments for more than an hour or so at a time.

The success which attended the perfecting of this vast system was such that German submarines based on the Flanders coast gave up attempting to pass down the English Channel. They tried to go to and from their hunting grounds on the Atlantic trade routes round the north coast of Scotland. Here the great northern systems took their toll.

During the first nine months of the year 1918 the German submarine flotillas at Zeebrugge and Ostend lost thirty vessels, and no less than fifteen of these had, at the time of the signing of the Armistice, been discovered lying wrecked under the lighted barrage.