The Prestonian-class ocean anti-submarine escort frigate was a class of 21 frigates that served with the Royal Canadian Navy from 1953–1967 and with the Royal Norwegian Navy from 1956–1977. They were converted from mothballed s of British design that had been placed in reserve following the end of the Second World War. The first vessel to be reactivated and undergo refit was which was recommissioned on 28 August 1953. The class did not use sequential pennant numbers. History During the Korean War, Canada committed to the North Atlantic Treaty Organization (NATO). It was believed at the time that there was a strategic threat to the shipping lanes supplying the European continent by the Soviet Union. This belief originated due to Soviet submarines becoming increasingly difficult to detect and identify, as a result of their updated technology. Vice-Admiral H.T.W. Grant, Chief of the Naval Staff of Canada, promised that Canada would contribute anti-submarine escort forces to combat the threat. Originally that meant updating only the existing fleet, however this policy was expanded when 21 decommissioned River-class frigates were converted to ocean escorts and recommissioned into the Royal Canadian Navy. The concept of anti-submarine warfare performed from a helicopter operating from the decks of escorts had been first proposed during the Second World War. Canada was the first nation to test a fully capable anti-submarine warfare helicopter flying from an escort. had a helicopter flight deck attached to her stern and performed sea trials from October to December 1956. These trials preceded the design of the destroyer helicopter carriers of the Royal Canadian Navy. In 1956, three frigates, , and were loaned to the Royal Norwegian Navy and renamed Troll, Draug and Garm respectively. They were purchased outright in 1959 and were the only export of the class. Three further River-class frigates, , and , were disarmed and transferred to the Department of Transport of Canada for use as weather ships, but were given Royal Canadian Navy pennant numbers and were subsequently considered as part of the class. Modifications The River-class frigate was a successful Canadian-built escort design from the Second World War, However, there was a requirement to update this design to meet the needs of the post-war Canadian Navy and to match the threat of the Soviet submarine force. The fo'c'sle deck was extended aft and was wall-sided. This extra space was primarily devoted to improved habitability. All accommodation throughout