<P> Offensive measures were less effective; efforts were made to use nets to find submerged U-boats, and explosive sweeps to destroy them, but these were largely failures . Attempts were also made to close routes like the Straits of Dover with boom nets and minefields, the so - called Dover Barrage; to lay minefields around U-boat bases, and station submarines on patrol to catch them leaving or entering port . These measures required a huge expenditure of effort and material, but met with little success . Just two U-boats were sunk by these measures in 1915 . </P> <P> At the beginning of this period the British Merchant Marine had a shipping fleet totaling of 21 million GRT . In six months of unrestricted submarine warfare U-boats sank ​ ⁄ million tons of Allied shipping, scarcely denting the British merchant fleet; Whilst new building, and additions from ships seized, had more than made up this loss . On the other hand, serious offence had been given to neutrals such as Norway and the Netherlands, and brought the United States to the brink of war . This failure, and the various restrictions imposed on the U-boat Arm in the Atlantic area largely brought the campaign there to a halt, although it continued with little hindrance in the Mediterranean and elsewhere, where there was less likelihood of offending neutrals . </P> <P> Given the ineffectiveness of early countermeasures, in 1917 Britain and in 1918 America adopted dazzle camouflage to attempt to reduce shipping losses to torpedoes . The results in both cases were inconclusive . </P> <P> The depth charge, or "dropping mine" as it was initially named, was first mooted in 1910, and developed into practicality when the British Royal Navy's Commander in Chief, Admiral of the Fleet Sir George Callaghan, requested its production in 1914 . Design work was carried out by Herbert Taylor at HMS Vernon Torpedo and Mine School in Portsmouth, England, and the first effective depth charge, the "Type D", became available in January 1916 . </P>

How did the allies overcome the problem of german u-boats