The captain of the liner had noticed when first attacked that the submarine was fitted with wireless and the calls sent out by him were in plain Morse code. On the strength of these the German commander had saved his torpedoes but lost his ship.
Another form of anti-submarine tactics was the employment of indicator and mined nets around an apparently disabled ship, or in lines across narrow channels known to be used by German submarines on their way to and from their bases. This method has, however, received full mention in other chapters.
What may be termed the third system of anti-submarine warfare was the use of extensive mine barriers, specially laid to catch submarines attempting to pass through them under water. The surface of the sea was patrolled by shallow-draft vessels and the under-seas guarded by mines. If a submarine was sighted in the vicinity of one of the mine barriers already described she was attacked and forced to submerge herself in order to escape destruction from the guns of the pursuing surface flotilla. From that moment her fate was sealed. By cautious manœuvring and using to full advantage their great superiority of speed (20-40 knots against 6-10 knots) the surface ships were able to head their quarry into the mine-field. Usually the submarine dived deep in order to throw her pursuers off the track, and all unconscious of the deep-laid mines in thousands she plunged to her doom—a heavy rumble, followed by an upheaval of the surface, and the chase was over.
This method, when carried out on the vast and scientifically sound principle described in a previous chapter, offers the best possible antidote to the submarine. Its employment in the Great European War placed the seal of complete success on the Allied anti-submarine offensive. It should, however, be remembered that comparatively narrow seas and a restricted zone of major operations made possible of accomplishment with some hundreds of thousands of mines (average cost, £400) what would in many cases and in many seas have been quite impracticable with as many millions of these difficult weapons.
The employment of submarines against submarines also forms a method of under-sea warfare which gives considerable scope for both daring and resource. It is of course quite impossible for one of these vessels when totally submerged to fight another in the same blind condition. But with just the small periscopic tube—or eye of the submarine—projecting above the surface, one craft can scout and watch for another to rise to the surface, thinking no enemy is near, in order to replenish her air supply for breathing or for recharging the electric storage batteries which supply the current for submerged propulsion.
When such a position obtains the submarine which comes unknowingly to the surface stands a grave danger of being torpedoed by her opponent. This actually occurred to at least one German U-boat during the Great War.
One or more submarines can also be employed around a slow-moving decoy ship. In this case they would have the advantage of being invisible until the actual moment of attack. The result of such a manœuvre would be either a gun duel on the surface or the torpedoing of the attacking submarine by one or other vessel of the decoy’s submerged escort.
It was a ruse of this kind which achieved success in the North Sea during the early stages of the war. A trawler was employed to tow a submarine by a submerged hawser. This mode of progress was adopted to enable the submarine to economise the strictly limited supply of electricity carried for under-water propulsion.
The trawler then cruised very slowly about, dragging the submarine under the surface behind her. In order to divert any suspicion which might have been aroused by her slow speed she was rigged so as to give the impression that a net was being towed, and the area of operations chosen was well-known fishing-ground.
In this curious way days were spent before the desired consummation was reached. Then a large U-boat came boldly to the surface and opened fire. Instantly the submarine astern of the trawler was released from the tow rope and forged ahead under her own electric engines. The commander of the surface decoy stopped his ship and commenced lowering the small life-boat carried. This was done in order to distract the attention of the Germans from the tiny periscope which was planing through the water to the attack.
A shell struck the trawler, carrying away her funnel, but did no other damage, and a few seconds later the water around the U-boat rose up in a vast upheaval of white. The plan had succeeded, and when the air cleared of the smoke from the trawler’s damaged stack there was nothing afloat on the surface of the sea around—except an ever-widening patch of oil and bubbles.
A few minutes later the thin grey line of the British submarine rose above the swell some five hundred yards distant from the scene of her triumph.
Another means by which one subaqueous fleet can attack another is by laying mines in the seas around the enemy base.
These simple methods formed what may be termed the backbone of the widespread anti-submarine operations during the Great War, but with the experience gained and the brains of almost every nation focussed on the problem of providing an effective counterblast to the under-water warship, there can be little doubt that in the next great naval conflict new and more scientific means of attacking these pests of the sea will have been perfected, though what degree of success they will attain in the stern trial of war the future alone can tell.7
CHAPTER XI
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 wa
ys 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 formulæ. 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 this it will be apparent that in places where there is a strong current or tide a mine-field is only dangerous to passing ships of shallow or medium draft for a few hours (during slack water) out of the twenty-four. Between the ebb and the flow of a tide there is a short period when the water is almost still. Then the movement begins to set in from the opposite direction and gradually gains in speed until about one hour before high or low tide. This period of what is known as “slack water” varies considerably in different places and different weather conditions, but plays an important part in all minesweeping operations.
In this way many a ship has passed over a mine-field all unconscious of the fate which would have befallen her had she traversed the same area of sea an hour or so earlier or later.
Mines which break adrift, or are laid without moorings of any kind, are called floating mines. The latter are a direct violation of International Law, as they cannot be recovered when once they have been laid, and become a danger to neutral as well as to enemy shipping. The laws of civilised warfare also require even a moored mine to be fitted with some mechanical device which renders it safe when once it has broken adrift from the wire and heavy sinker which holds it in a stated position. The reason for this humanitarian rule is that neutrals can be warned not to approach a given area of sea in which there are moored mines, but if these weapons break adrift—as they frequently do in heavy weather—and float all over the oceans, they would seriously endanger the lives and property of neutral states unless something were done to render them innocuous.
The total disregard of all the laws and customs of civilised warfare by the Germans in 1914-1919 has now been so well established that it seems almost unnecessary to give yet another instance of this callousness. In the case about to be quoted, however, there is, as the reader will observe, an almost superlative cunning.
Any cursory examination of a German moored mine will show that there is a device fitted ostensibly to ensure the weapon becoming safe when it breaks adrift from its moorings and thus complying with The Hague Convention. For several months after the outbreak of war it puzzled many minesweeping officers and men why, with this device fitted, every German floating or drifting mine was dangerous. A few, relying on these weapons being safe when adrift, had endeavoured to salve one and had paid for the experiment with the lives of themselves and their comrades. This caused every mine, whether moored or adrift, to be regarded by seamen as dangerous, notwithstanding th
e oft-repeated assurances that German mines fulfilled all International requirements in this respect. Then a mine which had broken away from its moorings was successfully salved, in face of the great danger involved, and the truth came out.
A device was fitted to render it safe, but, with truly Hunnish ingenuity, the metal out of which an essential part of this appliance was made was quite unable to bear the strain imposed by its work, and, to make doubly sure, another part was half filed through. The result was that, instead of rendering the mine safe when torn from its moorings by rough seas, the essential parts broke and left the mine fully alive.
Any discovery such as this—only made at the great risk of salving a live mine—could be easily explained away by German diplomacy as faulty workmanship in a particular weapon, reliance being placed on the fact that not many mines could be salved in this way without heavy loss of life; but numbers were recovered in spite of the dangers and extraordinary difficulties of such operations, and the guilt was for ever established in the minds of those who sail the seas.
Little need be said here regarding the method of laying mines from surface ships like the Wolfe and Moewe. The weapons were arranged to run along the decks on railway lines and roll off the stern, or through a large port-hole, into the sea as the vessel steamed along.
With submarine mine-layers or U-C boats the method was, however, much more complicated and needs full description. Each vessel was fitted with large expulsion tubes in the stern and carried some eighteen to twenty mines. These weapons, although similar in their internal mechanism to the ordinary mine, were specially designed for expulsion from submerged tubes or chambers.
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