Convoy

Convoys had been known for centuries, and from 1914 they protected troop transports, for example from Australia and New Zealand. Merchant ships, however, long sailed alone: the Admiralty believed that a mass of ships would be an easy target, that masters could not keep station and that there were too few escorts. Faced with record losses in spring 1917, it agreed to trials; the first convoy from Gibraltar arrived without loss in May.

In June the Admiralty approved the system, and from July convoys sailed regularly from North America, Gibraltar and Dakar. Cruisers escorted them across the ocean and destroyers took over in the approaches to the British Isles. A submarine found a group harder to locate than single ships, and by attacking it exposed itself to the escort. Losses in convoy proved small, and in 1918 the U-boat threat clearly declined.