The Illustrious class was a class of aircraft carrier of the Royal Navy that were some of the most important British warships in World War II (WWII). They were laid down in the late 1930s as part of the rearmament of British forces in response to the emerging threats of Nazi Germany, Fascist Italy and Imperial Japan.
Each of these ships played a prominent part in the battles of WWII. Victorious took part in the chase of the German battleship Bismarck, Illustrious and Formidable played prominent parts in the battles in the Mediterranean during 1940 and 1941 and all three took part in the large actions of the British Pacific Fleet in 1945.
The Illustrious class comprised four vessels: HM Ships Illustrious, Formidable and Victorious. Indomitable was built to a slightly modified design with a second half-length hangar deck below the main hangar deck. The following two ships of the Implacable class were also built to modified designs in order that they could carry larger air wings. and , had two hangar levels, albeit with a limiting head room.
Where other designs emphasised large air groups as the primary means of defence, the Illustrious class relied on their anti-aircraft armament and the passive defence provided by an armoured flight deck for survival; resulting in a reduced aircraft complement. Other carriers had armour carried on lower decks (e.g the hangar deck or main deck); the unprotected flight deck and the hangar below it formed part of the superstructure, and were unprotected against even small bombs. However, the hangar could be made larger and thus more aircraft could be carried.
In the Illustrious class, armour was carried at the flight deck level—which became the strength deck—and formed an armoured box-like hangar that was an integral part of the ship's structure. However, to make this possible without increasing the displacement it was necessary to significantly reduce the size and headroom of the hangar. The later three vessels, Indomitable, Indefatigable and Implacable, had re-designed two-level hangars which enabled them to carry larger air groups than the original design. The size of the air wings was also increased by using outriggers and deck parks. The original design was for 36 aircraft, but eventually the vessels operated with a complement of up to 72 aircraft. However, the smaller overhead height of the hangars (16 ft (4.88 m) in the upper hangars and 14 ft (4.27 m) in the later ships with lower hangars) compared unfavourably to the 17 feet 3 inches (5.3 m) of the Essex class, 17 ft 6 inches (5.38 m) in Enterprise and 20 ft (6.10 m) in Saratoga. This restricted operations with larger aircraft designs, particularly post-war.
This armour scheme was designed to withstand 1,000 pound bombs (and heavier bombs which struck at an angle); in the Home and Mediterranean theatres it was likely that the carriers would operate within the range of shore-based aircraft. The flight deck had an armoured thickness of 3 inches, closed by 4.5-inch sides and bulkheads. There were 3-inch strakes on either side extending from the box sides to the top edge of the main side belt, which was of 4.5 inches. The main belt protected the machinery, petrol stowage, magazines and aerial weapon stores. The lifts were placed outside the hangar, at either end, with access through sliding armoured doors in the end bulkheads.
Later in the war it was found that bombs which penetrated and detonated inside the armoured hangar could cause structural deformation, as the latter was an integral part of the ship's structure.
The armament was the same as the Ark Royal, with twin 4.5 inch countersunk turrets arranged on the points of a quadrant. As the ship's own firepower, rather than its aircraft, was relied upon for protection, the guns were mounted sufficiently high so that they could fire across the decks; de-fuelled aircraft would be stowed in the hangar for protection during aerial attack.