Submarine Warfare of To-Day

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.

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 inshore. 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.

^{Sport and General}

^{A Captive Mine-laying Submarine}

^{U.C. 5 off Temple Pier, London.}

If, in the latter case, the patrol ship started her own engines and moved forward in the darkness to ascertain from whence the noise came, she gave away her presence to the hostile submarine, also fitted with listening appliances. Whereas if she remained still and waited for the enemy to approach, mines might be laid in the meantime across important fair-ways which it was her duty to guard.

German mine-laying submarines were designated U-C boats, and often these vessels would employ a ruse in order to lay their mines in safety. Sometimes a decoy would draw the patrols away on a fruitless chase while the mines were being launched from the tubes of another U-C boat. In one case a big armed steamer was attacked with torpedoes while mines were being laid across the line of advance by which a flotilla of warships would be likely to come out to her aid from a near-by base.

In these and other ways over 3000 mines were laid off the British coast in one year. There were also several raids by surface mine-layers, which succeeded in eluding the network of patrols in the fogs and snows which prevail in the North Sea during several months out of every twelve. The two most important of these were the cruises of the Wolfe and the Moewe. The former vessel left Germany during the November fogs of 1916, and, by skirting the Norwegian coast, succeeded in passing the British patrol flotillas. She carried 500 mines, and after crossing the North Sea in high latitudes, proceeded down the mid-Atlantic until off the Cape of Good Hope, where the first mine-field was laid. She then crossed the Indian Ocean, laying fields off Bombay and Colombo.

It was in these seas that she succeeded in capturing a British merchantman. Placing a German crew and a cargo of mines aboard, she sent the prize to lay a field off Aden, while she herself proceeded to New Zealand. In these far-distant waters another field was laid, and a few months later the last of her cargo was discharged off Singapore. From this time onward she became a commerce raider.

^{Fig. 23.—A typical German mine and sinker. A. The mine-casing containing about 300 lb. of high explosive, and the electric firing device which is put in force when the horns B are struck and bent by a passing ship. B. Horns, made of lead and easily bent if touched by a surface ship, but sufficiently rigid to resist blows by sea-water. C. Hydrostatic device, operated by the pressure of the water at a given depth, rendering the mine safe until submerged. D. Slings holding mine to mooring rope F. F. Mooring rope to reel in sinker. G. Reel of mooring wire, which unwinds when the mine floats to the surface. H. Iron supports held together (as in small left-hand diagram) by a band round the mine- casing. The mine goes overboard and sinks like this to the bottom. The band is then released by a special device, and the supports drop away, leaving the mine free to float to the surface (as in small right-hand diagram). I. A heavy iron sinker which acts as an anchor, holding the mine in one position.}

The Moewe left Germany in December, 1916, and crossed the North Sea amid heavy snow squalls. Proceeding into the North Atlantic, she awaited a favourable opportunity to approach the British coast. This came one wild January night with a rising gale and a haze of snow. All her mines, about 400 in number, were laid off the Scottish coast in the teeth of a nor’wester. Then, with the “jolly Roger at the fore,” she steamed out on to the wastes of sea lying between the New World and the Old.

^{Fig. 24.—Diagram illustrating the effect of tide on a moored mine. A vessel is approaching a mine D, moored to the bottom by a sinker H. The distance from the top of the horns of the mine to the surface of the sea is approximately 5 feet at low tide, and as the vessel’s draught is 7 feet she would strike the mine. If, however, the same vessel passed over the same mine a few hours later, at high tide, the level of the sea would have risen 5 feet, and the mine would then be 10 feet below the surface; in which case the ship would just pass over in safety. This is known as the “tide difficulty.” There is, in addition, the “dip” of the mine due to the strength of the tidal current. E and F show what is meant by the dip of a mine. It is the deflection from the vertical caused by the ebb and flow of the tide. It frequently causes a mine-field to be quite harmless to passing surface craft except during the period of slack water between tides.}

We now come to the mines themselves and the method of laying them both above and below the surface.

A good idea of the shape, size and general characteristics of these weapons will be obtained from the accompanying diagrams. On being discharged into the sea they automatically adjust themselves to float about ten feet below the surface (according to tide) and are anchored to the bottom by means of a wire mooring rope attached to a heavy sinker. To describe here the mechanical details of all the different types of German submarine mines would occupy many pages with uninteresting technical formul?. It is sufficient to say that they carried an explosive charge (200 to 400 lb. of T.N.T.) sufficient to blow to pieces vessels of several hundred tons and to seriously damage the largest warship. They were intended to float a few feet below the surface—being held down by the mooring rope—but, as there was no means of compensating for the rise and fall of the tide, many of them often showed their horns above the surface at low water and were immersed too deep to be of much use against any but the deepest draught ships at high tide. A reference to Fig. 24 will make this difficulty clear.

There was scarcely a ship afloat in the zone of operations which did not, during those years of storm, sight one or more of these hateful weapons with their horns showing above the surface. Motor launches were employed to scout for them during the hour before and the hour after low water. In this way many hundreds were discovered and destroyed almost as soon as they had been laid. One badly laid mine, which shows on the surface when the tide ebbs, will often give away a whole field of these otherwise invisible weapons, and the work of sweeping them up and destroying them is then rendered comparatively easy.

The effect of strong tides on a moored mine is considerable, and will render a field quite harmless for several hours out of every twenty-four. The reason for this is best described with the aid of a diagram.

It will be seen from the above that the mine will not remain vertically above its sinker when there is a tide, but will incline at an angle determined by the strength of the current, which, if considerable, will press the weapon down much deeper than the keel of any ship (see Fig. 24). When the tide turns the mine will first regain its true perpendicular position and then incline in the opposite direction, accommodating itself to the ebb and flow. From