Larger numbers of escorts became available, both as a result of American building programmes and the release of escorts committed to the North African landings during November and December 1942. In particular, destroyer escorts (DEs) (similar British ships were known as "frigates") were designed, which could be built more economically than expensive fleet destroyers and were better designed for mid-ocean anti-submarine warfare than corvettes, which, although maneuverable and seaworthy, were too short, slow, and inadequately armed to match the DEs. Not only would there be sufficient numbers of escorts to securely protect convoys, they could also form hunter-killer groups (often centered on escort carriers) to aggressively hunt U-boats.