The Admiralty had decided to cease Flower-class construction in favour of the larger River-class frigates 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-class frigate was similar to a River but built using the system of prefabrication.
The appearance of Castle-class corvettes 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 feet (12 m) longer).
The most obvious visual difference, was the lattice mainmast instead of the pole version 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.
The armament differed from the Flower class with the depth charge fitment replaced by one Squid anti-submarine mortar. Hadleigh Castle received the first production Squid mounting; the World War I-era medium-velocity surface-only BL 4-inch Mk IX main gun firing a 31-pound (14 kg) shell was replaced by the new low-velocity QF 4-inch Mk XIX gun on high-angle/low-angle mounting firing a heavier 35-pound (16 kg) shell, which added anti-aircraft capability to the existing capability against surface targets such as submarines.
The 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 of the Castle-class corvettes had been discarded by the end of the 1950s but a few survived a little longer as weather ships. The last Castle was the Uruguayan training ship Montevideo, originally HMS Rising Castle, which was scrapped in 1975.
Most were operated by the Royal Navy but twelve were transferred to the Royal Canadian Navy (RCN) before completion and one to the Royal Norwegian Navy. Three Castles were sunk through enemy action and Castles participated in the sinking of seven U-boats.
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.
HMCS Arnprior (K494) (ex-HMS Rising Castle)
HMCS Bowmanville (K493) (ex-HMS Nunney Castle), sold to Republic of China as cargo ship, but taken over by PLAN after end of Chinese Civil War and rearmed with Soviet guns, entering PLAN service as Guangzhou.
HMCS Copper Cliff (K495) (ex-HMS Hever Castle)
HMCS Hespeler (K489) (ex-HMS Guildford Castle) (later SS Chilcotin)
HMCS Humberstone (K497) (ex-HMS Norham Castle)
HMCS Huntsville (K499) (ex-HMS Wolvesey Castle)
HMCS Kincardine (K490) (ex-HMS Tamworth Castle)
HMCS Leaside (K492) (ex-HMS Walmer Castle, later SS Coquitlam II)
HMCS Orangeville (K491) (ex-HMS Hedingham Castle), sold to Republic of China as cargo ship, but taken over by ROCN on June 29, 1950 and rearmed with US guns, entering ROCN service as De An (德安)
HMCS Petrolia (K498) (ex-HMS Sherborne Castle)
HMCS St. Thomas (K488) (ex-HMS Sandgate Castle, later SS Camosun III)
HMCS Tillsonburg (K496) (ex-HMS Pembroke Castle), sold to Republic of China as cargo ship, but taken over by ROCN on June 29, 1950 and rearmed with US guns, entering ROCN service as Kao An (高安)
The initial Castle-class ship was 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:
Alnwick Castle, built by George Brown, at Greenock; begun 12 June 1943, launched 23 May 1944 and completed 11 November 1944. Paid off 1957 and broken up December 1958.
Barnard Castle, built by George Brown, at Greenock; begun 1943, launched 3 October 1944 and completed 1945 as convoy rescue ship Empire Shelter.
Flint Castle, built by Henry Robb, at Leith; begun 20 April 1943, launched 1 September 1943 and completed 31 December 1943. Paid off March 1956 and broken up 10 July 1958.
Guildford Castle, built by Henry Robb, at Leith; begun 25 May 1943, launched 13 November 1943 and completed 11 March 1944; to Canada as HMCS Hespeler, 1944. Sold for mercantile service 1946 (later SS Chilcotin)
Hedingham Castle, built by Henry Robb, at Leith; begun 23 July 1943, launched 26 January 1944 and completed 10 May 1944; to Canada as HMCS Orangeville, 1944. Sold for mercantile service 1947; to Republic of China Navy 1951 as Te An.
Knaresborough Castle, built by Blyth Dry Dock; begun 22 April 1943, launched 1 September 1943 and completed 5 April 1944. Paid off 1947 and broken up 16 March 1956.
Launceston Castle, built by Blyth Dry Dock; begun 27 May 1943, launched 27 November 1943 and completed 20 June 1944. Paid off 1947 and broken up 3 August 1959.
Sandgate Castle, built by Smiths Dock, at Middlesbrough; begun 23 June 1943, launched 28 December 1943 and completed 18 May 1944; to Canada as HMCS St. Thomas, 1944. Paid off 22 November 1945 and sold for mercantile service 1946 (later SS Camosun III).
Tamworth Castle built by Smiths Dock, at Middlesbrough; begun 25 August 1943, launched 26 January 1944 and completed 3 July 1944; to Canada as HMCS Kincardine. Paid off 17 February 1946 and sold for mercantile service 1946.
Walmer Castle, built by Smiths Dock, at Middlesbrough; begun 23 September 1943, launched 10 March 1944 and completed 5 September 1944; to Canada as HMCS Leaside. Paid off 16 November 1945 and sold for mercantile service 1946 (later SS Coquitlam II).
York Castle, built by Ferguson Brothers, at Port Glasgow; begun 1944, launched 20 September 1944 and completed February 1945 as convoy rescue ship SS Empire Comfort.
Sixteen ordered 23 January 1943, of which five were cancelled:
Hever Castle, built by Blyth Dry Dock; begun 29 June 1943, launched 24 February 1944 and completed 15 August 1944; to Canada as HMCS Copper Cliff, 1944. Sold for mercantile service 1947, then became Chinese (People's Liberation Army) 1949.
Leeds Castle, built by William Pickergill, at Sunderland; begun 22 April 1943, launched 12 October 1943 and completed 15 February 1944. Paid off November 1956 and broken up 5 June 1958.
Morpeth Castle, built by William Pickergill, at Sunderland; begun 23 June 1943, launched 26 November 1943 and completed 13 July 1944. Paid off 1946 and broken up 9 August 1960.
Nunney Castle, built by William Pickergill, at Sunderland; begun 12 August 1943, launched 26 January 1944 and completed 8 October 1944; to Canada as HMCS Bowmanville, 1944. Sold for mercantile service 1946, then became Chinese (People's Liberation Army) Kuang Chou 1949.
Oxford Castle, built by Harland and Wolff, at Belfast; begun 21 June 1943, launched 11 December 1943 and completed 10 March 1944. Paid off 1946 and broken up 6 September 1960.
Pevensey Castle, built by Harland and Wolff, at Belfast; begun 21 June 1943, launched 11 January 1944 and completed 10 June 1944. Paid off February 1946 and became weather ship Weather Monitor in 1959.
Rising Castle, built by Harland and Wolff, at Belfast; begun 21 June 1943, launched 8 February 1944 and completed 26 June 1944; to Canada as HMCS Arnprior, 1944. Paid off 14 March 1946 and transferred to Uruguay as Montevideo.
Scarborough Castle, built by Fleming & Ferguson, at Paisley; begun 1944, launched 8 September 1944 and completed January 1945 as convoy rescue ship (Empire Peacemaker)
Sherborne Castle, built by Harland and Wolff, at Belfast; begun 21 June 1943, launched 24 February 1944 and completed 14 July 1944; to Canada as HMCS Petrolia, 1944. Paid off 8 March 1946 and sold for mercantile service 1946.
Tintagel Castle, built by Ailsa, at Troon; begun 29 April 1943, launched 13 December 1943 and completed 7 April 1944. Paid off August 1956 and broken up June 1958.
Wolvesey Castle, built by Ailsa, at Troon; begun 1 June 1943, launched 24 February 1944 and completed 15 June 1944; to Canada as HMCS Huntsville, 1944. Paid off 15 February 1946 and sold for mercantile service 1947.
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.
HNoMS Tunsberg Castle – (ex-Shrewsbury Castle, lost on 12 December 1944).
15 ships ordered for the Royal Navy from UK shipyards as part of the 1943 Programme were all cancelled on 31 October 1943:
Caldecot Castle – ordered 19 January 1943 from John Brown & Company, Clydebank.
Dover Castle – ordered 19 January 1943 from A. & J. Inglis, Glasgow.
Dudley Castle – ordered 19 January 1943 from A. & J. Inglis, Glasgow.
Bere Castle – ordered 23 January 1943 from John Brown & Company, Clydebank.
Calshot Castle – ordered 23 January 1943 from John Brown & Company, Clydebank.
Monmouth Castle (originally to have been Peel Castle) – ordered 23 January 1943 from John Lewis & Sons, Aberdeen.
Rhuddlan Castle – ordered 23 January 1943 from John Crown & Sons, Sunderland.
Thornbury Castle – ordered 23 January 1943 from Ferguson Brothers, Port Glasgow.
Appleby Castle – ordered 3 March 1943 from Austin, at Sunderland.
Tonbridge Castle – ordered 3 March 1943 from Austin, at Sunderland.
Norwich Castle – ordered 4 May 1943 from John Brown & Company, Clydebank.
Oswestry Castle – ordered 4 May 1943 from John Crown & Sons, Sunderland.
Pendennis Castle – ordered 4 May 1943 from John Crown & Sons, Sunderland.
Alton Castle – ordered 10 July 1943 from Fleming & Ferguson, Paisley.
Warkworth Castle – ordered 10 July 1943 from Fleming & Ferguson, Paisley.
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:
Hurst Castle was sunk by U-483 northwest of Ireland on 1 September 1944.
HNoMS Tunsberg Castle was sunk by a mine near Båtsfjord, Norway on 12 December 1944.
Denbigh Castle was hit by a torpedo from U-992 in the Barents Sea on 13 February 1945. She was towed by Bluebell to the Kola Inlet but later capsized.
U-744 was sunk by Icarus, St. Catharines, Fennel, Chilliwack, Chaudiere, Gatineau and Kenilworth Castle on 6 March 1944
U-484 was sunk in the north-west of Ireland by Portchester Castle and Helmsdale on 9 September 1944
U-1200 was sunk south of Ireland by Pevensey Castle, Lancaster Castle, Portchester Castle and Kenilworth Castle on 11 November 1944
U-387 was sunk in the Barents Sea by Bamborough Castle on 9 December 1944
U-877 was sunk north-west of the Azores by St. Thomas on 27 December 1944
U-425 was sunk in the Barents Sea by Lark and Alnwick Castle on 17 February 1945
U-878 was sunk in the Bay of Biscay by Vanquisher and Tintagel Castle on 10 April 1945
The final third of the film The Cruel Sea is set on the Castle-class corvette Saltash Castle (portrayed by Portchester Castle).
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.
SS Camosun III — ex-HMCS St. Thomas, HMS Sandgate Castle
SS Chilcotin — ex-HMCS Hespeler, HMS Guildford Castle
SS Coquitlam II — ex-HMCS Leaside, HMS Walmer Castle
