Cornwall displaced 9850LT at standard load and 13520LT at deep load. The ship had an overall length of 630feet, a beam of 68feet and a draught of 20feet. She was powered by Parsons geared steam turbines, driving four shafts, which developed a total of 80000shp and gave a maximum speed of 31.5kn. Steam for the turbines was provided by eight Admiralty 3-drum boilers. Cornwall carried a maximum of 3425LT of fuel oil that gave her a range of 13300nmi at . The ship's complement was 784 officers and men.[1]
The ship mounted eight 50-calibre 8-inch (203 mm) guns in four twin gun turrets. Her secondary armament consisted of four QF 4inches Mk V anti-aircraft (AA) guns in single mounts. Cornwall mounted four single 2-pounder (40 mm) light AA guns ("pom-poms"). The ship carried two quadruple torpedo tube above-water mounts for 21inches torpedoes.[2]
Cornwall was only lightly protected with little more than a single inch of plating protecting vital machinery. Her magazines were the exception and were protected by 2inches-4.375inchesin (-in) of armour. Space and weight was reserved for one aircraft catapult and its seaplane, but they were not fitted until after she was completed.[3]
Cornwall, the fifth ship of her name to serve in the Royal Navy,[4] was named after the eponymous county. The ship was laid down at Devonport Dockyard on 9 October 1924 and was launched on 11 March 1926. Completed on 6 December 1927, she was assigned to the 5th Cruiser Squadron (CS) on the China Station and spent the bulk of the interbellum period there.[5] In 1929–30 she received a High-Angle Control System, used to direct her anti-aircraft guns, and a catapult was fitted the following year. Two quadruple Vickers .50-calibre (12.7 mm) Mark III machine guns were added in 1934.[6]
In July 1936, Cornwall returned home to begin a major refit, which included a 4.5inches Krupp cemented armour belt abreast the engine and boiler rooms as well as the dynamo room and the fire control transmitting station. This belt extended 6feet down from the lower deck. Four inches of armour were also added to protect the sides of the boiler room fan compartments. A hangar for her aircraft was added and a new, more powerful catapult was installed. The ship's director was moved to the roof of the hangar and a new power-operated director-control tower was installed in its original location. Her single four-inch AA guns were replaced with twin-gun mounts for Mark XVI guns of the same calibre. Two octuple-barrel 2-pounder mounts were added abreast the searchlight tower and the original 2-pounder guns were removed. The changes raised the ship's displacement by 107LT and cost an estimated £215,000.[7] After the refit was completed in December 1937, the ship was assigned to the 2nd Cruiser Squadron before rejoining the 5th CS in 1939.[8]
On 5 October 1939, a month after the start of World War II, she was assigned to Force I to hunt for German commerce raiders in the Indian Ocean and spent most of the rest of the year there. Cornwall was then transferred to the South Atlantic for convoy escort duties.[8] On 13 September 1940, the ship rendezvoused with a convoy that was carrying troops intended to capture Dakar from the Vichy French, but was detached to intercept the Vichy French light cruiser that was escorting an oil tanker to Libreville, French Equatorial Africa, five days later and forced them to return to Casablanca in French Morocco. She then returned to the Indian Ocean and sank the German commerce raider on 8 May 1941. Cornwall rescued 3 officers, 57 ratings and 22 prisoners after the battle.[9]
After the start of the Pacific War on 7 December, the ship began escorting convoys across the Indian Ocean,[8] examples being Convoy JS.1 from Colombo, Ceylon, to the Dutch East Indies in late January–early February 1942[10] and the troop Convoy MS. 5 to Australia in early March. Later that month, she was assigned to the fast Force A of the Eastern Fleet.[11] On 2 April, Cornwall and her half sister,, were detached from the fleet, Dorsetshire to resume an interrupted refit and Cornwall to escort convoy SU-4 (composed of the U.S. Army transport and Australian transport) to Australia and the aircraft carrier to Trincomalee in Ceylon for repairs. On 4 April, the Japanese fleet was spotted and the two cruisers left harbour and, after a hurried refuelling at sea, set out for Addu Atoll shortly after midnight. The following day, the two cruisers were sighted by a spotter plane from the about 200 miles (370 km) south-west of Ceylon.[12]
As part of the engagement known as the Easter Sunday Raid, a wave of Aichi D3A dive bombers took off from three Japanese carriers to attack Cornwall and Dorsetshire, 320km (200miles) south-west of Ceylon, and sank the two ships. British losses were 424 men killed; 1,122 survivors spent thirty hours in the water before being rescued by the light cruiser and two destroyers.[13]