The Castle-class corvette was an ocean going convoy escort developed by the United Kingdom during the Second World War. It was the follow-on to the Flower-class corvette, and designed to be built in shipyards that were producing the Flowers. The Castle-class was a general improvement over the smaller Flowers which were designed for coastal rather than open ocean use.

The Castle-class corvettes started appearing in service during late 1943.

In mid-1939, the Admiralty ordered 175 Flower-class corvettes for protecting shipping on the east coast of Britain. They were designed to be built in large numbers in small shipyards without propulsion and other components (mainly gearboxes, but also guns) then in short supply so not compete with other warships for construction. By the time the Flowers began entering service in late-1940, due to their long range they were required to undertake missions beyond coastal waters.^[2][3] The Flowers were unsuited for open-ocean escort missions in the North Atlantic, especially in poor weather; they lacked speed, endurance, and habitability^[3][4] but became the mainstay of the Mid-Ocean Escort Force protecting convoys crossing the Atlantic.

The Royal Navy recognized the limitations of the Flower and began designing an open-ocean escort in November 1940,^[5] which became the River-class frigate.^[6] The larger Rivers were too expensive to produce at the required rate and needed larger slipways. In response, the Loch-class frigate was developed in late-1942, which was developed from the River and designed for prefabricated construction.^[7] The Castle was developed from a proposal by Smith's Dock Company – who had designed the Flower-class – for a stretched Flower.^[8] The result was a scaled-down version of the Loch for shipyards^[9] that only built corvettes using mainly traditional methods.

The design was approved in May 1943^[8] and 96 Castles were ordered from yards in the UK and Canada. Fifteen British and all 37 Canadian ships were later cancelled;^[9] Canada receiving twelve British-built ships instead.^[8] The UK completed a further five as convoy rescue ships for its Merchant Navy. Four became weather ships after the war.^[9]

The Castle resembled later Flowers with an extended forecastle and mast behind the bridge, but was 135 tons heavier and 47 feet longer. The Admiralty Experiment Works at Haslar developed an improved hull form which, in combination with the increased length, made the Castle at least half a knot faster than the Flower despite using the same engine.^[8] The Castle also had a single-screw.^[10] A lattice mainmast was used instead of the pole version fitted to the Flowers.

Construction used mainly traditional methods with as much welding as possible. Scantling was based on the Flower, but lightened in some areas. The wireless office^[8] (the same as on the Loch^[9]), the radar office, and the lattice mast were installed as prefabricated units.^[8]

The Castle was armed with a Squid anti-submarine mortar, directed by Type 145 and 147B ASDIC.^[8] The Flower used the older Hedgehog mortar^[5] and could not be fitted with Squid.^[11] The first operational Type 147 and Squid were installed aboard HMS Hadleigh Castle in September 1943.^[12][8]

In place of the BL 4-inch Mk IX main gun, the Castles had the new QF 4-inch Mk XIX gun on a High-Angle/Low-Angle mounting which could be used against aircraft as well as surface targets such as submarines.

The Castle was criticized for being barely fast enough to fight German Type VII submarines and difficult to handle at low speed.^[8]

Fifteen 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.

The following 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-482 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.^[13] 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