Castle class corvettes (1943)

We saw the Flower class corvettes in detail back in September 2022, and they were a very important part of the Battle of the Atlantic, freeing military yards for more important repairs and constructions while still being well adapted to their rough north Atlantic escort work. The base design was indeed a prewar whaler design already well tailored for these conditions. Overall 263 of these were manufactured, spread among almost 50 civilian yards between UK and Canada. This counted 44 Castle class corvettes. The latter were the pinnacle of ASW corvette design in WW2, going further away from the intial concept into a fully fledged military vessel, despite being still made in civilian yards. Of 95 planned, 44 were completed, 39 active for the RN, 51 cancelled, 12 for Canada, 3 lost. They saw service also with the Royal Norwegian navy and with China, included well after the war had ended with the PRC.

The ships concerned:

To the RN:
Allington Castle, Alnwick Castle, Alton Castle (cancelled), Amberley Castle, Appleby Castle (cancelled), Arnprior (ex-Rising Castle) to RCN, Avedon Castle (cancelled), Bamborough Castle, Barnard Castle, Barnwell Castle (cancelled), Beeston Castle (cancelled), Bere Castle (cancelled), Berkeley Castle, Bodiam Castle (cancelled), Bolton Castle (cancelled), Bowes Castle (cancelled), Bowmanville, Bramber Castle (cancelled), Bridgenorth Castle (cancelled), Brough Castle (cancelled), Caistor Castle, Caldecot Castle (cancelled), Calshot Castle (cancelled), Canterbury Castle (cancelled), Carew Castle (cancelled), Carisbrooke Castle, Chepstow Castle (cancelled), Chester Castle (cancelled), Christchurch Castle (cancelled), Clare Castle (cancelled), Clavering Castle (cancelled), Clitheroe Castle (cancelled), Clun Castle (cancelled), Colchester Castle (cancelled), Copper Cliff (ex-Hever Castle) to RCN, Corfe Castle (cancelled), Cornet Castle (cancelled), Cowes Castle (cancelled), Cowling Castle (cancelled), Criccieth Castle (cancelled), Cromer Castle (cancelled), Denbigh Castle, Divizes Castle (cancelled), Dhyfe Castle (cancelled), Dover Castle (cancelled), Dudley Castle (cancelled), Dumbarton Castle, Dunster Castle (cancelled), Egremont Castle (cancelled), Farnham Castle, Flint Castle, Fotheringay Castle (cancelled), Hadleigh Castle, Hedingham Castle, Helmsley Castle (cancelled), Hespeler (ex-Guilford Castle) to RCN, Humberstone (ex-Norham Castle, ex-Totnes Castle) to RCN, Huntsville (ex-Wolvesey Castle) to RCN, Hurst Castle, Kenilworth Castle, Kincardine (ex-Tamworth Castle) to RCN, Knaresborough Castle, Lancaster Castle, Launceston Castle, Leaside (ex-Walmer Castle) to RCN, Leeds Castle, Maiden Castle, Malling Castle (cancelled), Malmesbury Castle (cancelled), Monmouth Castle (cancelled), Morpeth Castle, Norwich Castle (cancelled), Oakham Castle, Orangeville (ex-Hedingham Castle) to RCN, Oswestry Castle (cancelled), Oxford Castle, Pendennis Castle (cancelled), Petrolia (ex-Sherborne Castle) to RCN, Pevensey Castle, Portchester Castle, Raby Castle (cancelled), Rayleigh Castle, Rhuddlan Castle (cancelled), Rushen Castle, St. Thomas (ex-Sandgate Castle) to RCN, Scarborough Castle, Shrewsbury Castle, Thornbury Castle (cancelled), Tillsonburg (ex-Pembroke Castle) to RCN, Tintagel Castle, Trematon Castle (cancelled), Tonbridge Castle(cancelled), Tutbury Castle(cancelled), Warkworth Castle (cancelled), Wigmore Castle (cancelled) York Castle (same).

To the RCN: HCMS Arnprior, Bowmanville, Copper Cliff, Hespeler, Humberstone, Huntsville, Kincardine, Leaside, Orangeville, Petrolia, St. Thomas, Tillsonburg.

Development

The last development of the “Flower” class with a much longer hull to cure for good the issues of habitability and fuel stowage. Same machinery exactly, and despite the heavier tonnage and longer hull, speed was preserved and radius of action greatly enchanced. The armament was also reinforced, with the same gun but far better ASW suite resting on the new Squid system and less on depht charges now considered obsolete. AA armament also took a boost with up to ten 20 mm Oerlikon guns.

In mid-1939, the Admiralty ordered 175 Flower-class corvettes to defend the east coast of Britain to be made in small shipyards without propulsion and other components such as gearboxes and any ordnance, then in short supply. By late-1940 it was realized already that longer range was required to undertake missions beyond coastal waters and these proved unsuited for open-ocean escort missions, North Atlantic and despite their initial caracteristics, they fared poorly in heavy weather proving too slow, and with degraded habitability. Despite being unpopular with crew, they but became the mainstay of the “Mid-Ocean Escort Force” on the British side of the operational area.

HMS Dumbarton castle (K388)

These limitations conducted to stury a new design by November 1940, the River-class frigate we already saw in 2023. They were however too complex expensive to produce in these small yards, especially at the required rate, and they needed larger slipways. The smaller Loch-class frigate were developed thus by late-1942, this time tailored for prefabricated construction so the same small yards only had to perform assembly. The Castle class next was developed from a proposal by Smith’s Dock Company which in between worked out the initial Flower-class into an improved model call the “modified flower” mostly built in Canada, and also created a “stretched Flower” which later became the Castle class when new name were assigned. For this, shortcuts were taken and they were designed essentially as scaled-down Loch class for small shipyards stuck to more traditional methods, so not assembling modules.

The design was approved in May 1943. 96 Castles were ordered from UK and Canada but many were cancelled. The “Castle” class was planned in the 1942 and 1943 programs with 59 ships on order, but curtailed to 46, and just 39 entering service with the additional HMS Empire Shelter, Empire Rest, Empire Lifeguard, Empire Peacemaker and Empire Comfort completed as convoy rescue ships operated under merchant flag outside the Royal Navy, armed with just two 3-in/40 guns and four 20mm Oerlikons.
15 British and all 37 Canadian ships were later cancelled with Canada havin instead twelve British-built ones transferred and a further five were completed as convoy rescue ships and four as weather ships postwar war. HMS Hurst Castle was sunk on the 1st September 1944 by U482 West of Ireland, HMS Denbigh Castle on 13 February 1945 by U992 in the Barents Sea. All were reclassified to frigates in 1947 and Berkeley Castle was docked after an accident on February 1953, never repaired and scrapped. Many also ended as Meteorological vessels with a single 40mm/60-mm Bofors gun and Squid, to be of use in wartime.

Design of the class

Castle class, deck arrangement plans

The Castle class somewhat were a mix between a Loch class and a modified late Flower class corvette, with extended forecastle and mast behind the bridge, and ended 135 tons heavier and 47 feet longer (14.3 meters) than the original. The Admiralty Works at Haslar which designed many ships in that era, created a model with an improved hull form tested in pool, which took better use of a an increased length for improving both speed and ride large waves with lesser roll and pirch. The more favourable lenght-width ratio made these at least half a knot faster, despite using the same engine as the original Flowers. They still were single-screw, which simplified cost and maintenance, but was a safety issue in case it broke down. A large wave could overturn such ship caught sideways. A lattice mainmast was another big difference in design, adopted from standard designed at the time in 1944. Its strnght was appreciated compared to the simple pole of the Flowers to support large and more powerful radar arrays and other systems.

Hull and general design

Construction still used mainly traditional methods, and still called for welding as much as possible. Scantling was based on the Flower albeit lightened in some areas whereas the the wireless office on board was provided from the same supplied that made it for the Loch class, whereas the radar office and lattice mast were already sent prefabricated and just assembled as single units. With that, they displaced 1,060 long tons (1,077 t) for an overall length of 252 ft (77 m), a beam of 37 ft (11 m) and draught of 10 ft (3 m). In the latter case, it was far less than the Flower class, they had more buoyancy and in general a taller freeboard, being far less “wet” compared to the Flowers, with habitability which was closer to a Loch class.

Powerplant

The Castle class repeated the same powerplant used already on the Flower class Corvettes. At its core, the basic whaler design did not needed two screws and so a single shaft VTE (Vertical, Triple expansion) fed by two admiralty 3-drum boilers were used to produced 2,750 hp. There were variations, since the Improved Flower class had a bit more power. The boilers were of the single-side fire-tube “Scottish” type, Oil-firing. This was enough for a top speed of 16.5 knots (30.6 km/h; 19.0 mph), half a knot improvement, helped also by the new forecastle shape and clipper bow and more refined water lines. The Castle class hwoever were criticized for being barely fast enough to followed surfaced German Type VII at night. They were completely lost by the late variants such as the VIIC/42 capable of 18.6 knots (34.4 km/h; 21.4 mph). In addition they were also difficult to handle at low speed.

Armament

The Castle class were still barebone in armament, with the same single shielded QF 4-inches general purpose gun as prior Flowers, located on a bandstand on the forcastle, to avoid excessive water spray. But this was completed with a Squid anti-submarine mortar installed in a superfiring position behind the banstandand, at the front of the bridge. The hull stretch allowed this important innovation. Hedgehogs/Squid were mandatory on the River class and the Squid was in 1944 the best evolution of the ASW mortar concept. Especially since it was directed by Type 145 and 147B ASDIC. It was jusged way superior to the Hedgehog mortar. This suite was completed by depht charges in standard racks at the stern and four throwers. HMS Hadleigh Castle was the first to mount the Squid/Type 147B ASDIC in September 1943.

QF 4-inch Mk XIX gun

Instead of the older BL 4-inch Mk IX main gun found on the Flower class, the Castles swapped it for the new QF 4-inch Mk XIX gun placed on High-Angle/Low-Angle mounting usabled with much better elevation both against aircraft and low surface targets such as surfaced submarines. This too was a lessons of the engagements made during the war, in which U-Boat surfaced between hunting escorts sometimes at very close range, to close to depress the gun enough. The AA guns were also too high in elevation to do anything while the Type VII’s much lower 8,8 cm and 20 mm FLAK could pepper at will their superstructure (see “greyhound” with tom hanks for illustration).
The Type XIX design started in 1938 but it was only introduced from 1941-42, finding use on the Bathurst class sloops, Castle class Corvettes and River/Loch class corvettes. Another advantage was the capability to fire starshells and illuminate the convoy to spot surfaced U-Boats.
Specs:
Barrel length 160 inches (4.064 m) bore (40 calibres) x 4-inch (101.6 mm)
Shell; Fixed QF HE, Starshell 35 pounds (16 kg) mv 396 mps(1,300 ft/s)
Breech: horizontal sliding-block, elevaiton -10° to +60°, max range 8,870 metres (9,700 yd) at +40°

Squid anti-submarine mortar

A twin Squid mortar arrangement on the superfiring position of the Loch class. It was a smaller, one mortar system on the Castle’s.

The 305mm/12-in Squid Mk 3 ASWRL was developed in 1942 as a replacement for the imprecise hedgehog, essentially a “spread and pray” type weapon creating a pattern supposed to fall on the enemy submarine’s position. Both both sonars and tacticvs went a long way in hard lessons learned in 1942 and thus a new type of ASW weapon was created, using a coupling with the best SONAR in development at the time with automated aiming, elevation and reload for rapid fire and unparalelled precision. The system created was so good as to be still around in 1977 in the RN and other navies.

The new system was ordered directly from the drawing board in 1942, managed by the Directorate of Miscellaneous Weapons Development and rushed into service in May 1943 on the destroyer leader HMS Ambuscade. The first polished, production was installed on HMS Hadleigh Castle, one of the first Castle class corvettes completed and on the next 70 frigates and corvettes. First successful use was by HMS Loch Killin on 31 July 1944, claiming U-333 and in total it was credited with 17 U-Boats in 50 attacks (2.9 to 1 ratio). Out of 5,174 depth charge attacks by comparion there were only 85.5 confirmed kills so a ratio of 60.5 to 1.

The Hedgehog had a ratio of 5.7 to 1. In 1959, the 195th Squid installation was made.
This was a caracteristic three-barrel 12-inch (305 mm) mortar, mounted in series and off-bore from each other to scatter bombs. A completely differnt system than tiny spigot mortars. The barrels were mounted in a frame allowinf a 90 degrees loadin angle and the projectiles were approaching that of a depht charge at 390 pounds (177 kg) comprising a 207-pound (94 kg) minol charge. The rest was full of propellant.

Squid installations were either at the stern (common postwar) or forward like Hedgehog on the Castle and Loch class vessels. They dropped into the sea with a sink rate of 43.5 ft/s (13.3 m/s) and clockwork time fuze to determine a set detonation depth. This data could be updated right up to any launch, and dive to explode down to max 900 feet (270 m).

They were automatically fired from sonar range recorder and created a pattern in triangle, c40 yards (37 m) wide at 275 yards (251 m) ahead of the ship. The installation being heavy, only post war ships had two sets of mortars. The fuses made sure these exploded some 25 feet (7.6 m) above and below the target. The pressure wave was enough to crush the pressure hull of any U-Boat, or postwar submarine for that matter. Post-war trials showed that by sheer accuracy, this ststem was nine times more effective than conventional depth charges, which led to their disappearance after 1950.

Last known used was in April 1977 by a Type 61 frigate, HMS Salisbury. Some survived on HMS Cavalier (D73), historic ships collection in Chatham, Kent and on HMCS Haida in Hamilton and at the naval museum in Malacca in Malaysia. It was retired in 1982 when the Östergötland-class destroyers were decommissioned.

AA Artillery

The basic defense was ensured by two single 20 mm mounted placed on a large bandstand across the rear amidship deck, behind the funnel. In time, more 20 mm Oerlikon mounts, notably these two were replaced by twin mounts, plus two single, making for six of them.

Depth Charges

The usual set comprised a single Depth Charge Racks (DCR) at the stern, offset to port with twenty depht charges ready for a full pattern drop. This was completed by four Mark II Depth Charges Throwers (DCT) on the poop deck, facing either side. Each carried five, with railing to the aft superstructure for a total storage of 40 more DCs. Reload was hand made using winches. Standard British type Mark VII (1939) 420 lbs. (191 kg) with a 290 lbs. (132 kg) TNT charge, sink Rate/Terminal Velocity of 9.9 fps (3.0 mps), max setting 300 feet (91 m) and later 500 feet (182 m).
Castle class corvettes were gradually equipped with the Mark X (1944) and Mark X*. The X**, not introduced in service in 1945 and benefiting from a great depth down to 1,500 feet (457 m).
The Mark X weighted 3,050 lbs. (1,383 kg) with an Explosive Charge of 2,000 lbs. (907 kg) TNT and a sink Rate of 6 fps (1.8 mps), 220 feet (67 m) max setting.

Sensors

Type 272 radar

Derived from the Type 271, using the CV35 reflex klystron, remote mount. More

Type 144Q sonar

Introduced in May 1943 for the late Hedgehog and Squid. The Q attachment was an additional Asdic set with separate transmitting and receiving equipment. The Q oscillator was mounted beneath the main oscillator, trained with it, projecting a fan shaped beam narrower in the horizontal but wide vertically to receive echoes at any angle up to 45 degrees below horizontal. This arrangement allowed angles up to 70 degrees from the bow. Range was selectable between 1000 and 2500 yards.
More and this video

Type 147B sonar

The 147B was ideally paired with the 144Q. This was a depth finding set with a sword shaped oscillator operating at 50 Kcs. and able to measure depths up to 1200 feet. It was mounted ahead of the main 144Q set, stored in a lifting tube within the hull when not in use, with angles whe deployed 65°H, 45°V capable of accurately track a target within 20 feet and integrated with Squid, forming together the most effective A/S weapons system of World War II.

HMS Hadleigh Castle (K335) from Conways.

Reconstruction by the author

The Canadian Castle

Unlile for the Flower class, especially the modified ones in 1942, nearly all Canadian built, production in Canada of 37 Castle class, that late into the war, was cancelled altogether. Instead,; the RCN obtained twelve British built Castle class corvettes, thus they were rigourously identical and no extra detail will be given on potential specifics, but if found. Here is an example career and brief fates of these twelve ships.

She was ordered on 23 January 1943, laid down at Harland and Wolff, Belfast (Yard number 1240) on 21 June 1943, launched on 8 February 1944 and completed on 26 June 1944. Transferred to the Royal Canadian Navy as HMCS Arnprior on 8 June 1944, before completion, in exchange for Algerine-class minesweepers. After training at Tobermory she was assigned to the Mid-Ocean Escort Force, escort group C-1 in Derry. She joined convoy ONM-249 on 19 August 1944 and took part in the Battle of the Atlantic until June 1945, refitted at St. John’s for two months and based at Halifax until decommissioned on 14 March 1946, sold to Uruguay as the TS Montevideo until 1975.

She was built at William Pickersgill & Sons Ltd, Sunderland, laid down on 12 August 1943, launched on 26 January 1944, transferred to the Royal Canadian Navy, Commissioned on 28 September 1944. Mid-Ocean Escort Force escort group C-4 until June 1945, based in Halifax until paid off 15 January 1946. Sold to ROCA as cargo ship, taken over by the PLAN after the end of the Civil War, rearmed with Soviet guns, served as Guangzhou until 1976.

Ordered 23 January 1943 at Blyth Shipbuilding and Dry Dock Company, she was laid down on 29 June 1943, launched on 24 February 1944 and transferred to the Royal Canadian Navy as Copper Cliff (Ontario), Commissioned on 25 February 1944. Assigned to the Mid-Ocean Escort Force (escort group C-6) as convoy escort in August 1944. Tested the Type 277 radar. In October serrved with group C-7 unyil June 1945, Halifax, and Esquimalt unyil 21 November, paid off, placed in reserve, sold in 1946 as mercantile Ta Lung, converted to cargo ship 1,305 tons GRT. Renamed Wan Lee in 1947, to PRC 1949, listed until 1968.

Ordered 19 January 1943 at Henry Robb Ltd, Leith, laid down 25 May 1943, launched 13 November 1943, transferred to the RCN as Hespeler, commissioned on 28 February 1944. Trained at Tobermory. Mid-Ocean Escort Force, escort group C-5 and took part in HX 300. 9 September 1944 south of Hebrides, with River class HCMS Dunver attacked bt Squid and sank U-484, previously spotted by a Short Sunderland from 423 Sqn RCAF. Refit Halifax and Liverpool, based in Esquimalt until PO reserve 15 November. Sold in 1946 as transport ship SS Chilcotin 1837t cargo, Capri, Vestar, lost 1966 in Sardinia (fire).

Ordered 19 December 1942 at A. & J. Inglis Ltd, Glasgow, laid down on 30 August 1943, launched on 12 April 1944, transferred as Humberstone; commissioned 6 September 1944. Mid-Ocean Escort Force, escort group C-8 until May 1945, sent to Esquimalt, PO 17 November 1945. 1946, sold for mercantile service as 1,304 tons GRT cargo Taiwei, Shanghai, sold 1947 Chang Chen, 1949, Tai Shan, King Kang, 1950 sold as Flying Dragon, San Blas, South Ocean (Korean flag) BU Hong Kong September 1959.

Ordered 23 January 1943 at Ailsa Shipbuilding Co. Ltd. in Troon she was laid down on 1 June 1943; launched on 24 February 1944, transferred to RCN as Huntsville, commissioned on 6 June 1944. Trained in Stornoway and integrated the Mid-Ocean Escort Force escort group C-5. In November she missed her group and instead acted as local escort, between St. John’s and New York City. She returtned to C5 in December and continued and until May 1945. Refitted in Halifax, completed in August, in reserve, paid off 15 February 1946, sold as mercantile 1,529 tons GRT as SS Wellington Kent.1951, Belle Isle II, badly damaged in collision 19 August 1960 off Trois-Rivières, Quebec, BU 1965–66.

Ordered 19 January 1943 at Henry Robb Ltd, Leith, laid down on 25 August 1943, launched 26 January 1944, transferred to the RCN, renamed Kincardine, commissioned 19 June 1944. Trained at Tobermory, Mid-Ocean Escort Force C-2 August 1944. Repaired at Smiths Dock, deployed as a local escort until 2 October. Last mission June 1945, TS in July attached to HMCS Cornwallis. Refitted at Liverpool, reserve at Halifax, PO 27 February 1946, sold to French Gvt September 1946 and resold to Morocco in 1947 as Saada, GRT 1,516 tons as fruit cargo based in Casablanca, BU La Spezia 15 September 1953.

Ordered 19 January 1943 at Smiths Dock Company, South Bank, laid down 23 September 1943, launched 10 March 1944, transferred RCN as Leaside, commissioned 21 August 1944. Later SS Coquitlam II, changed hands, BU 1979.

Laid down 2 November 1943 at John Brown, Launched 30 October 1944, reallocated to the Royal Canadian Navy before launching, renamed HMCS Orangeville, decom. August 1945. Sold to Republic of China as cargo ship, but taken over by the ROC Navy on 29 June 1950 and rearmed with US guns, entering ROCN service as De An (德安).

Petrolia was born as Sherborne Castle (Dorset) by Harland and Wolff, Belfast, laid down on 21 June 1943, launched on 24 February 1944, transferred, Commissioned on 29 June 1944, trained at Tobermory, versed to Mid-Ocean Escort Force C-3 July 1944. Last crossing in June 1945. August 1945 refit at Charlottetown, Prince Edward Island, reserve at Halifax in October. Sold for mercantile 8 March 1946, Castle Sg Co SA as “Maid of Athens”, converted to cargo ship 1,541 tons GRT, then Bombay, Bharat Line Ltd. as Bharat Laxmi. In service until May 1965, BU.

Ordered 19 January 1943, built at Smiths Dock Company, Southbank-on-Tees, Laid down on 23 June 1943, Launched on 28 December 1943, transferred to RCN as St. Thomas (Ontario), recommissioned 4 May 1944 under Commander Leslie Perman Denny, RCNR. Joined the Mid-Ocean Escort Force escort group C-3, escorted 13 convoys in the North Atlantic, credited with the sinking of U-877, 27 December 1944. Happened north-west of the Azores 1,000 kilometres (620 mi) off the coast of Newfoundland. She twice detected and used her Squid but apparently missed and begun to withdraw, when the damaged U-boat surfaced 4 km (2.5 mi) away. Instead of attacking she close in and rescued the whole crew as POW and ensured U-877 sank. She had a refit at Halifax in April 1945, sailed to Esquimalt in British Columbia in July until paid off 22 November 1945. Later sold to mercantile as SS Camosun III, Chilcotin (1958), Yukon Star (1958), discarded 1970, BU 1974.

Pembroke Castle (Wales) was laid down on 3 June 1943 by Ferguson Brothers Ltd., Port Glasgow, Launched 12 February 1944, transferred and recommissioned on 29 June 1944 to the Royal Canadian Navy and renamed Tillsonburg (southern Ontario) pennant K496. Trained at Stornoway, joined the Mid-Ocean Escort Force group C-3. Until the end of the war, final crossing mid-June 1945. Based at St. John’s, Sydney and Halifax for brief periods, paid off Halifax 15 February 1946. Sold to Republic of China as cargo ship, but taken over by ROCN on 29 June 1950 and rearmed with US guns, entering ROCN service as Kao An (高安). Taken over by Nationalist Chinese government 1 June 1950, renamed twice, Discarded 1963.