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 pa.s.sing the British patrol flotillas. She carried 500 mines, and after crossing the North Sea in high lat.i.tudes, 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.

[Ill.u.s.tration: 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 pa.s.sing 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.

[Ill.u.s.tration: FIG. 24.--Diagram ill.u.s.trating 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 pa.s.sed 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 pa.s.s 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 pa.s.sing 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 anch.o.r.ed 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 formulae. 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 pa.s.sing 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 pa.s.sed 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 the oft-repeated a.s.surances 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.

The mines were stored in the stern compartment of the submarine, between guide-rails fitted with rollers. They were in two rows and moved easily on the well-greased wheels. The loading was accomplished through water-tight hatchways in the deck above. In order to expel these mines from the interior of the submarine when travelling under the surface each weapon had to be moved into a short expulsion tube or chamber, the inner cap of which was closed when a mine was inside, and the outer or sea-cap opened. A supply of compressed air was then admitted into the back of the tube and the mine forced out into the open sea, in the same way as a torpedo is now expelled from a submerged tube.

Before another mine could be launched the sea-cap had to be closed, the water blown from the tube, the inner cap opened and a second mine placed ready in the chamber. This, however, did not end the difficulty of laying mines from submarines. The increase in the buoyancy of the boat, due to the loss of weight as each mine was discharged into the sea, had to be instantly and automatically compensated by the admission of quant.i.ties of sea-water of equal weight into special tanks, hitherto empty, situated below the mine-tubes. If this had been neglected the submarine would have come quickly to the surface, stern uppermost, owing to the lightening of the hull by the expulsion therefrom of some fifteen weapons weighing many hundreds of pounds each.

When the mine was clear of the submarine it sank to the bottom, owing to the weight of the sinker or anchor. After a short immersion, however, a special device enabled the top half, containing the charge of explosive and the contact firing horns, to part company with the heavy lower half, composed of the iron sinker and the reel of mooring wire. The explosive section then floated up towards the surface, unwinding the wire from the sinker.

Each mine being set, before discharge, to a certain prearranged depth (obtained by the captain of the U-C boat either by sounding wires or from special charts showing the depth of water in feet), the weapon could not rise quite up to the surface, being checked in its ascent, when ten feet from the top, by the mooring wire refusing to unwind farther.

This may sound a little involved, but a careful study of the accompanying diagrams will make the various movements of the mine and its sinker, after leaving the submarine, quite clear to the lay reader.

There were also other types of mines employed. Some were fitted with an automatic device which was actuated by the pressure of the water at a set depth. These weapons could be expelled from submarines without the necessity of knowing and adjusting the depth at which they were to float below the surface. A mine of this pattern rose up, after discharge from the tube, until the pressure of water on its casing was reduced to 4 1/2 lb. per square inch (the pressure which obtains at a depth of ten feet below the surface[8]), and there the weapon stopped, waiting patiently for its prey.

Another kind of mine was of the floating variety--tabooed by The Hague Convention--which drifted along under the surface with no moorings to hold it in one position.

Now that the reader is familiar with the mines themselves and the actual methods of laying them, we can pa.s.s on to a brief review of the German mine-laying policy during the Great War.

The submarine offensive reached its maximum intensity in 1916-1917, during which period no less than 7000 mines were destroyed by the British navy alone.[9] Of this number about 2000 were drifting when discovered. There was, with one small exception, no portion of the coast of the United Kingdom which was not mined at least once during those eventful _two_ years, the unmined area being undoubtedly left clear to facilitate a raid or invasion. About 200 minesweeping vessels were blown up or seriously damaged, but the losses among the Mercantile Marine were kept down to less than 300 ships out of the 5000 sailings which, on an average, took place weekly.

The heavy losses inflicted on the enemy's submarine fleets in 1917 marked the turning of the tide, and from that date onwards there was a steady but sure reduction in the number of mines laid.

During the first twelve months of the intensified submarine war the Germans concentrated their mine-laying on the food routes from the United States, the sea communications of the Grand Fleet off the east coast of Scotland and the line of supply to France. Then, when they commenced to realise the impossibility of starving the sea-girt island, and the weight of the ever-increasing British armies began to tell in the land war, the submarine policy changed to conform with the general strategy of the High Command, and the troop convoy bases and routes were the objects of special attack.

The arrival in Europe of the advance guard of the United States army caused another change in the submarine strategy. From that time onwards the Atlantic routes a.s.sumed a fresh importance and became the major zone of operations.

In the first year of the war the U-C boats discharged their cargoes of mines as soon as they could reach their respective areas of operation.

The mines were usually laid close together in one field, frequently situated off some prominent headland, or at a point where trade routes converged. Then the enemy learned to respect the British minesweeping and patrol organisation, and endeavoured to lay their "sea-gulls' eggs"

in waters which had been recently swept, or where sweeping forces appeared to be weak in numbers.

When this failed they played their last card, scattering the mines in twos and threes over wide areas of sea. To meet this new mode of attack large numbers of shallow-draught M.L.'s were employed to scout for the mines at low water.

It was about this time that the great Allied mine barriers across the entrances and exits to and from the North Sea were completed and the losses among the U and U-C boats became heavy. A rapid abatement in the submarine offensive soon became apparent, and utter failure was only a matter of time.

FOOTNOTES:

[8] The question of water pressures and many other problems of submarine engineering relating to under-water fighting are fully treated in _Submarine Engineering of To-day_, by the Author.

[9] A few of the 7000 were British mines no longer required in the positions in which they had been laid.

CHAPTER XII

THE MYSTERIES OF MINESWEEPING EXPLAINED

THE task which confronted the naval minesweeping organisations in the years succeeding 4th August 1914 was an appalling one. Any square yard of sea around the 1500 miles of coast-line of the British Isles might be mined at any moment of any day or night. There were, in addition, the widely scattered fields laid by surface raiders like the _Wolfe_ and the _Moewe_, which, as described in a previous chapter, extended their operations to the uttermost ends of the earth. A wonderfully efficient patrol of the danger zones had its effect in reducing the number of submarine mine-layers available to the enemy and in rendering both difficult and hazardous the successful execution of their work, but neither a predominant and subsequently victorious fleet, nor an equally skilful and alert patrol, could guarantee the immunity of any considerable area of sea from mines.

The Germans laid many thousands of these deadly and invisible weapons in the 140,000 square miles of sea around the British Isles _alone_ in the face of over 2000 warships. To search for these patches of death in the wastes of water may well be likened to exploring for the proverbial "needle in a haystack." Yet the sweepers, whose sole duty it was to fill this breach in the gigantic system of Allied naval defence, explored daily and almost hourly, for over four years, the vast ocean depths, discovering and destroying some 7000 German mines, with a loss of 200 vessels of their number. The result of this silent victory over one of the greatest perils that ever threatened the Sea Empire was that some 5000 food, munition and troop ships were able to enter and leave the ports of the United Kingdom _weekly_ with a remarkably small percentage of loss from a peril which might easily have proved disastrous to the entire Allied cause.

This, then, in broad outline, was the task which confronted this section of the naval service, and its successful accomplishment forged a big link in the steel chain encompa.s.sing the glorious victory.

Before pa.s.sing on to describe the ships and the appliances used it is first necessary to give a more detailed account of the operations generally included under the heading of minesweeping. As it was impossible to tell exactly where mines would be laid from day to day, an immense area of sea had to be covered by what was known as _exploratory sweeping_. This consisted of many units of ships emerging from the different anti-submarine bases almost every day throughout the year and proceeding to allotted areas of water, where they commenced sweeping north, south, east or west, in an endeavour to discover if the areas in question were safe for mercantile traffic. If no mines were discovered that particular area would be reported safe, but if only one of these weapons was cut from its mooring by a sweep-wire the area would be closed to merchant ships until the sea around was definitely cleared of the hidden danger. This system of open and closed areas entailed an enormous amount of efficient administrative staff work apart from the actual sweeping, and its success was partly dependent upon the vigilance of the patrols employed to divert shipping from dangerous patches of sea.