A Class submarines that were under construction in the closing stages of World War 2 were involved in a radical programme of modification in the early 1950s. The captured German Type XXI U-boats, advanced for their time, provided a lot of inspiration and enabled great strides forward in submarine design. The A Class had been primarily designed for war in the Pacific and carried more fuel to enable them to cover the vast distances in that theatre of war. Sixteen A Class boats were built with most of them constructed by Vickers Armstrongs at Barrow, although some were built at one or two other commercial yards and one at Chatham Dockyard. The A boats lasted a long time in British service, their subsequent substantial modernisations included removal of their guns and external torpedo tubes, streamlining of the hull, a larger conning tower and new sonars. They had increased battery power for underwater speed and endurance and were also fitted with the Snorkel breathing device, known as Snort in the Royal Navy.

In their final form, with a surface speed of 20 knots and a submerged speed of 8 or 9 knots, the A Class represented the archetypal post-war diesel-powered submarine. (Source: Submarine 1901-2001)