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HMS Illustrious
United Kingdom
Builders:	Vickers-Armstrong (2), Harland & Wolff (1)
Operators:	Royal Navy
Preceding class:	none
Following class:	Implacable
Commissioned:	May 1940
Decommissioned:	March 1968
Ships in Class
Ships in class:	3
General Characteristics
Displacement:	29,500 tons (as built)
Length:	673 ft (205 m)
Beam:	95 ft (29 m)
Draught:	28 ft (8.5 m)
Propulsion and power:	3 Parsons geared turbines 6 Admiralty 3-drum boilers 111,000 shp, 3 shafts
Speed:	30.5 knots
Complement:	2,200
Armament:	16 × 4.5 inch (8 × 2) 48 × 2 pdr (6 × 8) 21 × 40 mm AA (2 × 4, 2 × 2, 9 × 1) 45 × 20 mm AA (45 × 1)
Aircraft complement:	as built 36

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. They were laid down in the late 1930s as part of the rearmament of the Royal Navy in response to the 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 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 three vessels: HMS Illustrious (R87), HMS Formidable (R67) and HMS Victorious (R38). Three further similar ships were built as the war progressed, to modified designs in order that they could carry larger air wings. HMS Indomitable (R92) had a second half-length hangar deck below the main hangar deck and the two ships of the Implacable-class, HMS Implacable (R86) and HMS Indefatigable (R10) had two hangar levels, albeit with a limiting 14 feet head room.

The Illustrious-class was different in conception to what may be described as their nearest American contemporaries, the Essex-class. Where the latter emphasised large air groups as the primary means of defence, the Illustrious-class relied much more on anti-aircraft armaments and an armoured deck for survival, at the expense of reduced aircraft capacity. This resulted in a fundamental difference in the designs. American carriers had their armoured deck on the hangar deck, which became the strength deck. Thus, their flight deck and hangar was part of the superstructure and was unprotected even against light bombs. However, it could be made larger and could hence carry more aircraft. In the British ships, the armour was carried at the flight deck level - which became the strength deck - and there was an and armoured box-like hangar that was an integral part of the ship's structure. This armour scheme was in theory designed to keep enemy bombs out of the hangar completely. However, to make this possible at a given displacement, it was necessary to significantly reduce the size and head room 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 were also increased by using outriggers and deck parks. The original design was for 36 aircraft, but eventually the vessels would operate with a complement of up to 72 aircraft. However, the smaller overhead height of the hangars (16 feet (4.92 m)) in the upper hangars - and 14 feet (4.3 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 feet (6.15 m) in Saratoga). This restricted operations with the larger late-war aircraft such as the Vought Corsair.

The flight deck had an armoured thickness of 6 inches that was designed to resist 500 pound (226 kilogram) bombs, but not the heavier 500 kilogram (1,100 pound) bombs that they faced in combat from German dive bombers. However, the armoured deck was able to withstand larger bombs which did not hit the deck at a perpendicular angle. Later in the war, it was also found that bombs which penetrated and detonated inside the armoured hangar could cause deformation to the ship's structure (as the hangar was part of the ships structure, unlike American ships).

The utility of the armoured carrier concept is debatable. Essex-class ships survived horrendous damage in the Pacific war, especially USS Franklin and USS Bunker Hill. Their larger air groups proved to be invaluable defending the ships as they allowed threats to be intercepted at a greater distance from the carrier itself. Conversely, the armoured flight deck enabled the British carriers to resume flight operations often with just one hour's delay after an aerial or Kamikaze attack. Even though no US full-sized aircraft carriers were sunk by Kamikaze and attacks, many had their flight decks damaged beyond immediate repair, necessitating that the ships be pulled out of action (except for three, however - Enterprise, Bunker Hill and Franklin - these were generally repaired and returned to service within 72 hours of being hit). Franklin suffered two hits from armour-piercing bombs, one of which penetrated her armoured deck; little evidence suggests that an Illustrious-class ship would have fared any differently in this situation.

It is pertinent to note that the Royal Navy's air wing, the Fleet Air Arm, often had to make do with inadequate aircraft. This was in part due to pre-WWII arrangements whereby the Royal Air Force controlled the aircraft and flying of the Royal Navy, which in turn influenced the Navy's thinking when it came to carrier designs. British carriers were also designed primarily for the European theatre, where they often would operate within reach of land-based aircraft, and even local air superiority could not always be assumed. After the war, Indomitable, which was a modified version of this class (modified with two hangars instead of one), suffered an internal fire and gasoline explosion that was much smaller and more contained in scope than the bomb hits suffered by Franklin, and was an operational total loss from it. She was patched with concrete for Queen Elizabeth's Coronation Naval Review in 1953, and afterward returned to unmaintained reserve and was scrapped two years later.

Features of the Illustrious-class were incorporated into postwar American aircraft carrier designs. The Forrestal-class had its strength deck at the flight deck level ("deck armour" as such was considered obsolete, based on the experiences of carrier operations in World War II; armouring was limited to splinter protection only), but they retained the large airwing size of the Essexes by greatly increasing the ships' dimensions. A full discussion of the Forrestal class and its design process can be found in that article.

Illustrious and Formidable did not long survive WWII. Like their contemporary, USS Enterprise, they had fought a long and consuming war and they were worn out - overhaul and renovation would have cost more than building replacements. They were broken up for scrap in the 1950s. However, Victorious had a long postwar career, undergoing a very expensive reconstruction to enable her to operate Cold War-era jet aircraft. Victorious, the last of the class, was finally retired in 1968 after a fire.

• Flight deck#Armored decks

Illustrious-class aircraft carrier

Illustrious | Formidable | Victorious

Preceded by: single ship designs - Followed by: Implacable class

List of aircraft carriers of the Royal Navy