Although Implacable and Indefatigable are usually described as the fifth and sixth units of the "Illustrious" Class, or even as improved Indomitables, the 1938 design of fleet carrier to which they were built was quite new. Apart from the designed standard displacement and beam, which were limited by treaty and dock-gate dimensions, only the well-proven external layout and main armament remained unchanged.
In response to a Naval Staff requirement for greater speed and increased aircraft complement, an extra turbine and shaft were installed and the lower hangar was extended forward so that it was 46 feet longer than that in Indomitable. The amour plating on the hangar sides was increased to 2", and the height of the lower hangar was lowered by 2 feet so that both hangars had an overhead clearance of only 14 feet.
The result was very cramped accommodation spaces, and a restriction on the type of aircraft that could be operated - the hangars were too low for Corsairs, and due to the unavailability of Hellcats the ships were compelled to carry short-range Seafires.
The main external differences were the much enlarged funnel and longer island.
Implacable was ordered in 1938 and Indefatigable a year later. Construction was halted in 1940 by order of Winston Churchill, and even after it was resumed little priority was given to their completion. Implacable took 5 years to build, and by the time she was completed her sister was fully operational and already at sea in combat.
After a short period of operations with the Home Fleet, both ships went to the Pacific, where their larger air groups were responsible for the majority of sorties flown by the carriers of the British Pacific Fleet.
Post war Implacable served as the Home Fleet’s deck-landing training ship, while Indefatigable had her hangars converted to classrooms and accommodation space and served as a boys’ training ship. Neither ship was significantly modernised, and both were only 10 years old when paid off.