The Castle-class corvette was a stretched version of the preceding Flower class, enlarged to improve seakeeping and to accommodate modern weapons. The ships displaced 1010LT at standard load and 1510LT at deep load. They had an overall length of 252feet, a beam of 36feet and a deep draught of 14feet. They were powered by a pair of triple-expansion steam engines, each driving one propeller shaft using steam provided by two Admiralty three-drum boilers. The engines developed a total of 2880ihp and gave a maximum speed of 16.5kn. The Castles carried enough fuel oil to give them a range of 6500nmi at . The ships' complement was 99 officers and ratings.[1]
The Castle-class ships were equipped with a single QF 4adj=onNaNadj=on Mk XVI gun forward, but their primary weapon was their single three-barrel Squid anti-submarine mortar. This was backed up by one depth charge rail and two throwers for 15 depth charges. The ships were fitted with two twin and a pair of single mounts for 20mm Oerlikon light AA guns.[2] Provision was made for a further four single mounts if needed. They were equipped with Type 145Q and Type 147B ASDIC sets to detect submarines by reflections from sound waves beamed into the water. A Type 277 search radar and a HF/DF radio direction finder rounded out the Castles' sensor suite.[3]
Alnwick Castle was laid down by George Brown & Co. at their shipyard at Greenock on 12 June 1943 and launched on 23 May 1944. She was completed on 11 November and served as a convoy escort until the end of the Second World War in May 1945.
On 17 February 1945, Alnwick Castle, Lt. Cdr. H.A. Stonehouse R.N.R., and used depth charges to sink German submarine U-425 near Murmansk.
Alnwick Castle was placed in reserve on 25 May and was sold for scrap in 1958.[4] She arrived at Gateshead in December to be broken up.[5]