1917 was the great year of submarine war: the Germans straining every nerve to kill off all the ships that went to or from the Allied ports, the Allies trying their best to kill off all the submarines. The Mediterranean was bad, the North Atlantic was worse, the west coasts of the British Islands worst of all. The American Navy came in and did splendid service off the south coast of Ireland, in the Bay of Biscay, and along the North Atlantic seaways between French and British and American ports. More and more destroyers were put into service, aided by "chasers"--very much smaller vessels with only one gun and a few men, but so cheap and easily built that they could be turned out in swarms to help in worrying the submarines to death. The "scooters" and "Porte's babies," as we saw in Chapter XXIV, were, however, even better than these swarming "chasers."
The enormous steel nets were also used more than ever. You can fancy what they were like by thinking of a gigantic fishing-net many miles long, with armed steamers instead of floats. In the entrances to some harbours there were sea-gates made by swinging open a bit of the net by means of its steamers to let traffic go through, and then swinging it back again. The mine-fields were made bigger than ever; it was then that the vast one, mostly laid by the Americans, was begun from the Orkneys to Norway. Mines were also laid by British submarines and by daring fast surface mine-layers round Heligoland and other places off the German coast. In this way the waters in which submarines could work were made narrower and narrower and were better and better guarded.
But more and more submarines were launched, and they still sneaked out to sea along the Dutch and Norwegian coasts where the Navy could not stop them because they used to slink through "territorial waters," that is, within three miles of the coast, where the sea belonged to the nearest country, just the same as the land. The Navy, however, had lines of patrols always on the watch from the Orkneys to the Shetlands, on to Iceland, over to Norway, and north to the Arctic ice. The narrow waters of the English Channel were watched by the famous Dover Patrol under Sir Roger Keyes. From Folkestone to Cap Griz Nez in France there was an unbroken line of the strongest searchlights on vessels anchored to ride out the biggest gales. Seven miles west was another line. Between were hundreds of patrol boats always ready, night or day, to fire at anything on the surface or to drop depth charges on anything that dived. A depth charge is a sort of mine that can be set to go off at a certain depth, say thirty to sixty feet down, when it makes a sea-quake that knocks the submarine out of gear and sinks it, even if it does not actually hit it. Besides all these guards on the surface there were nets and mines underneath. That is why the British army in France never had its line of communication with England cut for one single day all through the war.