Castle - class corvette The Castle - class corvettes were an updated version of the much more numerous Flower - class corvette s of the Royal Navy , and started appearing during late 1943 . They were equipped with radar as well as asdic . Design . The Admiralty had decided to cease Flower - class construction in favour of the larger River - class frigate s as the Flower - class had originally been intended for coastal escort work and were not entirely satisfactory for Atlantic convoy service . In particular , they were slow , poorly armed , and rolled badly in rough seas which quickly exhausted their crews . However , many shipyards were not large enough to build frigates . The Castle - class was designed to be built on small slipways for about half the overall effort of a Loch - class frigate . The Loch was a River built using the " American system " of prefabrication . Appearance was much like the later " long forecastle " variant of the Flowers and they were a little larger ( around 1 , 200 tons – about 200 tons more than the Flowers , and 40 ft ( 12 m ) longer ) . The most obvious difference was the lattice mainmast instead of the pole one fitted to the Flowers . There was also a more " square cut " look to the stern although it was still essentially a cruiser spoon type , this difference was only visible from abaft the beam . Armament was similar except that the depth charge fitment had been replaced by one for the Squid anti - submarine mortar . HMS " Hadleigh Castle " received the first production Squid mounting . Propulsion machinery was identical to the Flowers , and experienced officers felt that they were seriously under powered , having a tendency to turn into the wind despite everything the helmsman could do . The fact that Squid attacks required a fairly low speed ( compared to depth charge attacks ) only made matters worse . Most had been scrapped by the end of the 1950s , but a few survived a little longer as weather ships . However , the last was the Uruguayan " Montevideo " , originally and scrapped in 1975 . Most were operated by the Royal Navy , but twelve were assigned to the Royal Canadian Navy and one to the Royal Norwegian Navy . Three Castles were sunk through enemy action , and Castles participated in the sinking of seven U - boat s . Ships . Royal Canadian Navy . The following vessels were all originally built for the Royal Navy , but were transferred to the RCN on completion ( for details of builders and construction dates see under Royal Navy below ) . All their pennant numbers ( except " Hedingham Castle " , which was never completed ) , as well as their names , were changed when transferred . Royal Navy . The initial Castle was the " Allington Castle " , re - ordered on 9 December 1942 ( from the previous order placed for a Modified Flower - class corvette named " Amaryllis " ) ; another 13 vessels were ordered on 19 December , also under the 1942 War Programme . The remaining eighty - one ships were all ordered for the RN under the 1943 War Programme , of which thirty were completed . Fifty - one of these ships ( 15 from UK shipyards and 36 from Canadian shipyards ) which were cancelled late in 1943 are shown separately below . Fourteen ordered 19 January 1943 , of which 3 were cancelled : Sixteen ordered 23 January 1943 , of which five were cancelled : Five ordered 2 February 1943 : Three ordered 6 February 1943 : Two ordered 3 March 1943 , three ordered 4 May 1943 and two ordered 10 July 1943 were all cancelled , as were all thirty - six ordered from Canadian shipyards on 15 March 1943 . Cancelled . 15 ships ordered for the Royal Navy from UK shipyards as part of the 1943 Programme were all cancelled on 31 October 1943 : 36 ships were ordered on 15 March 1943 for the Royal Navy from Canadian shipyards for completion between May 1944 and June 1945 , but were all cancelled in December 1943 : Film appearance . The final third of the film " The Cruel Sea " is set on the Castle - class corvette " Saltash Castle " ( portrayed by ) . Post - war conversions . Three were converted to passenger / cargo ships for the Union Steamship Company of British Columbia , and were known as the " White Boats " ( see Twigg ) . They were operated from 1946 to 1958 , but were heavy on fuel and had limited cargo capacity , for example they could not carry cars in the hold .