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.
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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.