Illustrious-class aircraft carrier

HMS Illustrious
Class overview
Builders	Vickers-Armstrongs (3), Harland & Wolff (1)
Operators	Royal Navy
Preceded by	none
Succeeded by	Implacable
In commission	May 1940 - March 1968
Completed	Four
General characteristics
Displacement	as built; 23,207 tons standard 28,919 tons deep
Length	673 ft (205 m)
Beam	95 ft (29 m)
Draught	28 ft (8.5 m)
Propulsion	6 Admiralty 3-drum water-tube boilers Parsons geared steam turbines 111,000 shp on 3 shafts
Speed	30.5 knots (56.5 km/h)
Range	4,854 tons oil fuel, 11,000 nmi (20,000 km) at 14 knots (26 km/h)
Complement	817 (Naval) + 394 (Air Wing)
Armament	16 × QF 4.5 in Mk.III (114 mm L/45) in 8 × twin mounts BD Mk.II 48 × QF 2 pdr Mk.VIII (40 mm L/40) in 6 × octuple mounts Mk.VIII 21 × QF 40 mm Bofors in; 2 x quadruple mounts Mk.II 2 x twin mounts Mk.I 9 x single mounts Mk.III 45 × QF 20 mm Oerlikon in 45 × single mounts P Mk.III
Aircraft carried	as built 36 later up to 72

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. Implacable and Indefatigable both had two hangar levels, albeit with a limiting 14 feet (4 m) head room.

Design and concept

The Illustrious class was designed within the restrictions of the Second London Naval Treaty, which limited carrier size to an upper limit of 23,000 tons. They were different in conception to the Royal Navy's only modern carrier at the time, their predecessor HMS Ark Royal, and what may be described as their nearest American contemporaries, the Yorktown and Essex class carriers. The Illustrious class followed the Yorktown but preceded the Essex.

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, but the differences in aircraft capacity between these carriers and their USN counterparts is mostly due to the USN's operational doctrine, which allowed for a permanent deck park of aircraft to augment their hangar capacity. Illustrious's hangar was 85% as large as USS Enterprise's but Enterprise typically carried 50% of her aircraft capacity in her deck park. Indomitable's two hangars were actually larger than Enterprise's but she carried fewer aircraft because she did not have a large permanent deck park. In 1944/45 RN carriers began to carry a permanent deck park of similar size to their USN counterparts and this greatly increased their aircraft capacity.

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 similar to 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. The Illustrious class were fitted with four HACS controlled High Angle Director Towers, for fire control of her 4.5" guns.

Fate of the class

Illustrious and Formidable did not long survive WWII. Like their contemporary USS Enterprise they had fought a long and consuming war and were worn out; overhaul and renovation would have cost more than replacement. They were broken up for scrap in the 1950s. The last of the class, Victorious, had a long postwar career with a very expensive reconstruction to enable her to operate Cold War-era jet aircraft, and retired in 1968 after a fire. Indomitable suffered a hangar deck gasoline explosion in 1950 which caused severe damage; she lingered in reserve until after Queen Elizabeth II's Coronation Review and was then scrapped.

See also

Media related to Illustrious class aircraft carriers at Wikimedia Commons