Destroyer escort (DE) was the United States Navy mid-20th-century classification for a 20-knot (23 mph) warship designed with endurance to escort mid-ocean convoys of merchant marine ships. The Royal Navy and Commonwealth forces identified such warships as frigates, and that classification was widely accepted when the United States redesignated destroyer escorts as frigates (FF) in 1975. From circa 1954 until 1975 new-build US Navy ships designated as destroyer escorts (DE) were called ocean escorts. Destroyer escorts and frigates were mass-produced for World War II as a less expensive antisubmarine warfare alternative to fleet destroyers. Similar types of warships in other navies of the time included the 46 diesel-engined Kaibōkan of the Imperial Japanese Navy., 10 Kriegsmarine escort shi