Type VIIs versus escort mass

Germany built roughly 700 Type VII submarines, but their limited submerged speed and endurance made convoy interception and pursuit difficult. Britain and its allies completed or converted large numbers of Flower- and Castle-class corvettes, trawlers, destroyers, Black Swan-class sloops, and old American four-stackers. Their escort inventory drew on purpose-built warships, conversions, requisitioned civilian hulls, and transferred ships. U-boats sought merchant ships, while escorts were easier to build, less attractive torpedo targets, and replaceable through several production streams.