Black Swan - class sloop The " Black Swan " class and Modified " Black Swan " class were two classes of sloop of the Royal Navy and Royal Indian Navy . Twelve " Black Swans " were launched between 1939 and 1943 , including four for the Royal Indian Navy ; twenty - five Modified " Black Swans " were launched between 1942 and 1945 , including two for the Royal Indian Navy ; several other ships were cancelled ( see below ) . History . Like corvette s , sloops of that period were specialised convoy - defence vessels , except that sloops were larger , faster , possessed much superior anti - aircraft fire control via the Fuze Keeping Clock and a heavy armament of high angle 4 inch guns while retaining excellent anti - submarine capability . They were designed to have a longer range than a destroyer at the expense of a lower top speed , while remaining capable of outrunning surfaced Type VII and Type IX U - boat s . In World War II , " Black Swan " - class sloops sank 29 U - boats . The most famous sloop commander was Captain Frederic John Walker . His sloop became one of the most successful submarine hunters , taking part in the sinking of eleven U - boats . After the war , sloops continued in service with the Royal Navy , Egyptian Navy , Indian Navy , Pakistan Navy and the West German Navy . In April 1949 , was attacked on the Yangtze River by the Communist People ' s Liberation Army . Also , several " Black Swan " sloops fought in the Korean War . " Black Swan " class . Royal Navy The first two ships were built under the 1937 Programme , being ordered from Yarrow and Company , Scotstoun , on 1 March 1939 . The second pair was built under the 1939 Programme , being ordered from Furness Shipbuilding Company on 21 June 1939 . A further ten RN ships were ordered under the 1940 War Programme on 13 April 1940 ; however six of these ( the orders placed with White of Cowes , Thornycroft at Woolston , and Swan Hunter on Tyneside for two ships each ) were subsequently replaced by orders for an equal number of " Hunt " Class escort destroyers . There were incremental improvements as the building developed , and the " Woodcock " and " Wren " when completed were practically indistinguishable from the Modified " Black Swan " class . Royal Indian Navy Two ships were ordered under the 1939 Programme , the order being placed with Denny on 8 September 1939 . The second pair were ordered under the 1940 Programme , this order with Thornycroft being placed on 29 August 1940 . The first two were used as survey ships after the War . The second pair were transferred to the Pakistan Navy in 1948 . The third pair ( which were of the Modified " Black Swan " class – see below ) Modified " Black Swan " class . Royal Navy Fourteen sloops for the RN were in the 1940 Supplementary War Programme . The first two were ordered from Denny , Dunbarton , on 9 January 1941 , ten more were ordered on 27 March 1941 ( two each from Cammell Laird , Scotts , Thornycroft , Yarrow and John Brown ) , and a final pair from Fairfield , Govan , on 18 July 1941 . The contract with John Brown was transferred to Devonport Dockyard on 3 March 1942 , and then to Denny on 8 December 1942 . Another fourteen ships were authorised in the 1941 Programme , but the last three ships ( the names " Star " , " Steady " and " Trial " had been approved ) were not ordered under this programme . The first of the eleven actually ordered was contracted with Thornycroft on 3 December 1941 , with a further pair from Stephens , Linthouse , on 18 December . Eight more were ordered in 1942 , two on 11 February , two on 3 March ( originally from Portsmouth Dockyard ) , two on 12 August and two on 5 October . However the order for two sloops were ordered at Portsmouth was moved to Chatham Dockyard on 21 June 1943 , and they were laid down there , but were cancelled on 15 October 1945 . Two more sloops were authorised in the 1942 Programme ; the names would have been " Waterhen " and " Wryneck " but they were never ordered in that year ' s Programme . The 1944 Programme re - instated these two vessels , as well as the twelfth sloop authorised under the 1941 Programme , and now named as " Partridge " . These three ships were ordered on 9 October 1944 , but they were all cancelled on 15 October 1945 . These had been intended to be further modified and enlarged , with a beam of . Two further ships planned under the 1944 Programme would have been named " Woodpecker " ( ii ) and " Wild Swan " , but these were never ordered and the intention to build was dropped when the 1945 Programme was compiled . Royal Indian Navy Two ships for the Indian Navy were included in the 1941 Programme , the order being placed with Yarrow on 10 September 1941 . Construction dates are tabulated above . U - boat kills . Reassessments . During the war the " Starling " was credited , along with the sloops , , , and frigate , with sinking the " U - 482 " in the North Channel on 16 January 1945 . The British Admiralty withdrew this credit in a post - war reassessment .