Although the convoy was the crucial innovation, the danger faded away for other reasons as well. The establishment of a more effective mine barrier both across the English Channel and between northern Scotland and Norway hindered the passage of U-boats. In an important advance in technology, the ineffective mines Britain used during the first years of the war had now been replaced with more deadly varieties. Under First Lord of the Admiralty Sir Eric Geddes, a dynamic businessman who had organized the railroad system in France, British shipbuilding led the Allied side in replacing lost ships. By March 1918, the number of newly constructed vessels exceeded the continuing losses to the U-boats. Advances in technology produced mixed results. Efforts to locate submarines using sound-detecting devices ("hydrophones") proved unsuccessful. It happened that these primitive instruments worked best only when all ships in the area had stopped, a deadly move to make when U-boats were present. Depth charges, however, became steadily more potent weapons against the U-boat, and the imaginative use of the airplane grew in significance. Aerial patrols did not sink large numbers of U-boats, but they forced German sub commanders to remain submerged—meaning that the submarines had to travel at slower speeds—to avoid aerial attack. During the last months of the war, the Germans concentrated their operations in coastal waters, and here aerial patrols played an even larger part in harassing the submarine. By the beginning of 1918, under the leadership of the new commander at Dover, Admiral Roger Keyes, a more effective combination of mines, nets, and bright nighttime lighting hindered U-boat traffic through the Dover Strait. Large U-boats coming from German ports found it almost impossible to pass the new barrier and were forced to take the lengthy northern route around Scotland to reach the open Atlantic. These vessels had a cruising time of only about a month. The additional six to ten days required to take the northern route slashed the time they were available to attack Allied shipping lanes. For smaller U-boats stationed in Flanders, sailing through the Channel became far more dangerous than heretofore. By midsummer 1918, it had become impossible. An even more ambitious plan was put into effect to block the northern route between Scotland and Norway. Promoted and carried out largely by the United States Navy, this mine and net barrier was in place only in the last months of the war. It probably made the route more dangerous for the large U-boats that used it, although it could not stop the underwater traffic. British intelligence, centered on the famous Room 40 at the Admiralty, intercepted and interpreted German radio traffic, allowing Admiralty officials to learn the approximate location of U-boats. Convoys, whose escorts all had radios, could be warned and rerouted. The Allies were able to block the effort to terrorize neutrals. Britain had potent tools to use in dealing with the Scandinavian countries, for example. If they wished to trade anywhere on the globe, they needed coaling facilities that only Britain could provide. Even in the face of huge losses in early 1917, these neutral countries continued to carry goods to Britain. Britain's new Ministry of Food acted to make the production and distribution of food more efficient. Farmers were encouraged to put their land back into the production of cereal grains (as opposed to less productive use for pasture). Government bureaucrats rationed the food available, and they saw to it that grain once used for beer now became flour for bread. They also made sure that shipping became more efficient under the threat of the U-boat. Available vessels now concentrated on carrying foodstuffs across the relatively short North Atlantic passage instead of sailing to Australia or Argentina for the same kind of cargo. Direct attacks on the Flanders bases turned out to be the least useful of all possibilities. Encouraged by the navy, but also impelled by the need to take pressure off the mutinous French army. General Sir Douglas Haig launched his 1917 Flanders campaign, aimed, in part, at retaking the coasts holding U-boat bases. It was a bloody failure that never came close to its objectives. Aerial attacks and long-distance shelling of bases like Ostend and Zeebrugge proved ineffective. In April 1918, a daring and memorable direct attack was made by an amphibious force of sailors and marines commanded by Keyes. It aimed at blocking the U-boats' passage from Zeebrugge and Ostend to the open sea by sinking British ships in the area's narrow coastal channels. Despite the bravery of the force involved, the British attack put in place only a partial and temporary barrier. Its main effect was to raise British morale, at home and on the western front, during the grim days of the Germans' spring offensive. In the end, Britain and its allies had a range of resources upon which to draw in defeating the submarine. The imminent nature of the crisis and the clear-cut danger that failure meant losing the war in a measurable period of time assured that all those resources would be employed.