The operations of Allied submarines also greatly diminished the value of the "cruiser" submarines which Germany began to construct in 1917. These great subsurface vessels were introduced as an "answer" to the convoy system. The adoption of the convoy, as I have already explained, made it ineffective for the Germans to hunt far out at sea. Until the Allies had put this plan into operation, the relatively small German U-boats could go two or three hundred miles into the Atlantic and pick off almost at will the merchant ships, which were then proceeding alone and unescorted. But now the destroyers went out to a point two or three hundred miles from the British coast, formed a protecting screen around the convoy, and escorted the grouped ships into restricted waters. The result of this was to drive the submarines into these coastal waters; here again, however, they had their difficulties with destroyers, subchasers, submarines, and other patrol craft. It will be recalled that no destroyer escort was provided for the merchant convoys on their way across the Atlantic; the Allies simply did not have the destroyers for this purpose. The Germans could not send surface raiders to attack these convoys in mid-ocean, first, because their surface warships could not escape from their ports in sufficient numbers to accomplish any decisive results, and, secondly, because Allied surface warships accompanied every convoy to protect them against any such attack. There was only one way in which the Germans could attack the convoys in mid-ocean. A fleet of great ocean-going submarines, which could keep the sea for two or three months, might conceivably destroy the whole convoy system at a blow. The scheme was so obvious that Germany in the summer of 1917 began building ships of this type. They were about 300 feet long, displaced about 3,000 tons, carried fuel and supplies enough to maintain themselves for three or four months from their base, and, besides torpedoes, had six-inch guns that could outrange a destroyer. By the time the armistice was signed Germany had built about twenty of these ships. But they possessed little offensive value against merchantmen. The Allied submarines and destroyers kept them from operating in the submarine zone. They are so difficult to manoeuvre that not only could they not afford to remain in the neighbourhood of our anti-submarine craft, but they were not successful in attacking merchant vessels. They never risked torpedoing a convoy, and rarely even a single vessel, but captured a number by means of their superior gunfire. These huge "cruiser submarines," which aroused such fear in the civilian mind when the news of their existence first found its way into print, proved to be the least harmful of any of the German types.