During the last nine months of the war, the number of ships lost to unrestricted German U-boat fire had dropped considerably. The introduction of the convoy system in June 1917, where merchant ships were accompanied by a British naval escort, had reduced the spike in the targeting of non-combat shipping. Instead of responding by concentrating their attacking force, the U-boats continued to operate singly. One attempt was made to use a U-boat convoy of six ships to bring down an Allied convoy. Although the U-boats sank three ships, two of their own were lost and it was decided that the loss ratio was too high. The Allies also increased their use of underwater mine barrages and aircraft assault, making it increasingly difficult for the German submarines to operate. In 1918 the gross tonnage loss had reduced by nearly two-thirds on the previous year, at 2.25 million.
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