The Comparison of armoured to unarmoured flight deck designs is often made between some of the aircraft carrier designs of the Royal Navy and the United States Navy. The two navies followed differing philosophies in the use of armour on the carrier flight decks. The two classes most easily compared, and which differ the most, are the RN's Illustrious class, and their nearest USN contemporaries, the Yorktown and Essex classes. The Illustrious class followed the Yorktown but preceded the Essex.
|Comparative armour thickness of some World War II era aircraft carriers|
|Flight deck||Main deck||Side belt|
|Yorktown class (19,875 tons)||n/a||1.5 in (38 mm)||2.5 – 4 in (64 – 101 mm)|
|Illustrious class (23,000 tons)||3 in (76 mm)||2.5 in (64 mm)||4.5 in (114 mm)|
|Essex class (27,200 tons)||n/a||1.5 in (38 mm)||2.5 – 4 in (64 – 101 mm)|
|Shōkaku class (25,600 tons)||n/a||3.9 in over machinery (99 mm)||1.8 in (46 mm)|
US and earlier Japanese carriers had their armour placed at the hangar deck, essentially treating the hangar spaces and flight deck as superstructure. A bomb that struck the flight deck would likely penetrate and explode in the hangar deck, but the armour there would still protect the ships innards — including the engine spaces and fuel storage. The flight deck could also possibly detonate light bombs prematurely, which would reduce the chance of them going through the hangar deck. Such a design allowed for larger, open-sided hangar bays (improving ventilation) and the installation of deck-edge elevators. Most carriers with hangar deck armour also usually had wooden flight decks which were easy to repair. However, wooden flight decks were particularly vulnerable to burning, so after refuelling, fuel lines in the deck would be purged with exhaust gasses to reduce this danger.
British naval historian D.K. Brown put the practical difference between American and British design philosophies in no uncertain terms: "More fighters would have been better protection than armour." The benefits of flight deck armour proved chiefly ironic in nature: Fewer aircraft meant a lower priority to attack than the more heavily-armed American carriers and less ammunition and aviation fuel meant less kindling in the event of a fire. US carriers and their fighters shot down more than 1,900 suicide aircraft during Operation Kikusui (the last and largest Kamikaze attack in the Okinawa campaign), versus a mere 75 for the British, yet both forces suffered the same number of serious hits (four) .
The kamikaze threat overall was not serious, as many kamikaze strikes missed the deck armour entirely, or bounced off the decks of both British or American carriers. In both cases, most kamikazes either struck glancing blows that did only superficial damage that was fixed within minutes or hours, or missed entirely, due to the poor training and poorer flight experience of their pilots. The majority of kamikazes that did inflict harm caused no more damage than they would have against a smaller ships. After a successful kamikaze hit, the British were able to clear the flight deck and resume flight operations in just hours, while their American counterparts took a few days or even months. The U.S.N liaison officer on HMS Indefatigable commented: "When a kamikaze hits a U.S. carrier it means 6 months of repair at Pearl [Harbor]. When a kamikaze hits a Limey carrier it’s just a case of "Sweepers, man your brooms."”
What was not discovered until late in the war was that the kamikaze impacts proved to have a long term effect on the structural integrity of British carriers and their post war life was shortened. HMS Formidable (R67) was an excellent example of this; while she weathered a severe kamikaze hit in 1945 which cratered her deck armour, the hit caused severe internal structural damage and permanently warped the hull (damage worsened in a postwar aircraft-handling accident wherein a Vought Corsair rolled off a lift and raked the hangar deck with 20mm cannon fire, causing a severe fire); she was written off during the 1947 survey of the postwar fleet as beyond economical repair and lingered in reserve until 1956 before being towed off to the breakers. Two of her sisters fared just as poorly. Illustrious suffered a similar battering, especially off of Malta in 1941 when hit by German dive bombers and after the war was limited to 22 knots (41 km/h) because of accumulated war damage (her centerline shaft was permanently disabled due to accumulated wartime damage); she spent five years as a training and trials carrier (1948–53) and was disposed of in 1954. Indomitable was completely refit to like-new condition, only to suffer a severe gasoline explosion onboard, which warped her hull. The explosion occurred on the hangar deck, and while severe, would have been shrugged off by an Essex-class carrier, several of which returned to service after far worse explosions. Indomitable was patched with concrete for Queen Elizabeth II's Coronation Review, then immediately afterwards towed to the breakers.
By contrast, it was found after the war, the lower deck armour of American carriers made certain that the bombs and kamikaze craft which did hit, tended to do their damage outside of the ship's structure. American carriers of the Essex class also survived some of the worst kamikaze hits of the war, albeit with high casualties. USS Franklin was struck by two 250/550 kg/lbs armour-piercing bombs, one of which penetrated her armoured hangar floor and set off ammunition, killing 724 personnel. USS Bunker Hill was severely damaged by pair of kamikaze hits which killed 346 men. As most of the damage was outside of the ship's structure, and combined with higher quality construction of US shipyards (while the British were forced to rush the construction of the ships to get them combat ready, the numerous and capacious American yards on the East and West Coasts allowed the US Navy to build carriers at a more leisurely pace individually while producing ships collectively at a furious rate), this ensured that even significantly damaged carriers could be economically restored to good condition, as the Franklin and Bunker Hill were successfully repaired though they saw no postwar service.
The only carrier lost to a deck hit was the American Independence-class light carrier, USS Princeton (CVL-23). Indeed, many light and escort carriers were unarmoured, with no protection on the hangar or flight deck, and thus they fared poorly against deck hits.
The supercarriers of the postwar era, starting with the Forrestal class — nearly 200 feet longer and 40 feet beamier than their World War II counterparts — would eventually be forced to move the strength deck up to the flight deck level as a result of their great size; a shallow hull of those dimensions became too impractical to continue. The issue of protection had no influence on the change; the Forrestal class had 1.5" of flight deck armour, but the subsequent Kitty Hawk and all following classes do not have armoured flight decks (deck armour is of little to no use against modern anti-ship missiles).
As before, however, the USN continued to design its ships for large air groups, continuing to reason that the best defence was a good offence.