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Good point, and absolutely true, as little could be done against armor piercing bombs, but against kamikazi's where speed of the bomb/plane reached maybe 400 mph, and the impact spread throughout the entire aircraft, rather then a focal point of the nose of a bomb/shell led to saving the carrier as opposed to penetrating it, and causing fires in below decks...by late in the Pacific, the chances of a high level bomber dropping an ordinance and hitting a carrier from high altitude was slim....yet any fire that did start below deck was multiplied by armored decking containing heat and fumes adding to the problem...my previous response was mainly directed to lead or point to Canadiancitizen as to maybe not having heard about the decks of British carriers being armored
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The only reason the armored flight deck proved useful against Kamikazes was that these had little penetrating power. Where an enemy made more conventional attacks using heavy bombs the British armoring system proved very vulnerable and actually amplified damage greatly.
After the crippling of the Illustrious in the Mediterrainian the RN issued recommendations that carriers with armored flight decks under bombing attack lower one of their elevators to allow bomb blasts to vent from the hanger bay should the (and likely to be) flight deck be penetrated.
The other problem with the armored flight deck on a carrier the size of British ones was that it limited the size of the air wing. Weight restrictions were severe with that much mass so high in the ship. Metacentric heights were harder to obtain where there was little risk of capsizing in the event of uneven damage to the ship.
The US solution appeared to be much better. The flight deck was made from thin plate or armor plate about 1/2" to 3/4" thick. This was enough to generally set off most bomb's fuzes. The actual surface was then planked over with wood as were many weather decks on ships of the period.
The hanger bay was an open superstructure that allowed plenty of ventilation particularly of gasoline fumes, a big danger on the ship. The hanger deck, or the deck below that, was typically about 3 or 4" thick and constituted the main armored deck of the ship. This was deemed sufficent to stop most bombs from penetrating to the main spaces and magazines particularly when combined with the burster deck of the flight deck.
For the British armored carriers a flight deck penetration could mean the death of the ship. An explosion in the confined hanger bay (the sides are armored too) would cause massive damage there. A longer fuzed bomb once through the flight deck faced no further serious armor to stop it from reaching magazines or machinery spaces.
As I pointed out earlier, Kamikazes represent a special case. They had no ability to penetrate armor as such. Instead, they typically squashed themselves like bugs on a windshield when they struck armored portions of ships. Their danger came from the massive fuel fires they created. Armor generally protected the ship from this danger too.
As such, they represent a poor argument for armored flight decks on carriers.
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These two posts are essentially correct in detail but miss an important point as to why armored flight decks were particularly disadvantageous on carriers. The British armored flight deck carriers had to be kept within certain displacement ranges because of considerations of repair facilities and ports which could not handle very large ships. There is some evidence, too, that the British were careful to keep their carriers smaller than they could have built them because of manning shortages. In any case, because they wanted armored flight decks which carried considerable weight penalties very high up in the ship's structure, the designers decided to make the armored flight deck the "strength" deck of th ship. this avoided adding more weight by making the hangar deck the strength deck, as it was in US carriers. Thus the flight deck, keel, and frames of the British carriers formed a sort of box girder which held the ship together and counteracted the various stresses which were naturally encountered in any sea-going vessel.
Where the problem comes in is, if there is a serious fire anywhere below the flight deck, it will apply massive amounts of heat to one side of the girder. If enough heat is applied for a sufficient period of time, that side of the girder will deform or warp, any structures (frames) attached to the warped girder will be pulled out of symmetrical alignment, causing the keel and indeed, the entire hull to "wrack" or be twisted out of shape. It would be as if a giant had grabbed the hull and twisted each end in opposite directions; the result is the same as in a car frame that has been involved in a serious accident, everything is out of alignment, nothing works properly, and extreme wear on tires, wheels, drive shaft, etc., is the consequence. With a car, it's usually possible to strip it down to the frame and use a frame alignment device to twist it back into the proper shape. Unfortunately, with a carrier, there is no machine big enough, or powerful enough, to twist the hull back into the proper shape.
Formidable, Indomitable, and Illustrious all suffered hangar deck fires that wracked their hulls so badly that in one case the center propeller shaft froze up and would not turn. The armored flight decks were no guarantee against hangar fires due to penetration by AP bombs, splinters, or even burning gasoline dripping from crashed kamikazes which had not penetrated the flight deck. Hangar fires that Essex class carriers would have shrugged off because of their hangar decks being the strength deck, actually warped the ships girder on the British armored carriers so badly that they were essentially constructive total losses.
The Midway-class, the lead ship of which was commissioned in 1945, did have an armored flight deck, but because the US could afford to build this class without regard to displacement restrictions the weight of the flight deck did not require that it be the strength deck. The hangar deck in the Midway class was the strength deck and the hangar sides were open so that, in the event of a fire or explosion, the heat and force would be vented rather than deforming the ship's girder. If you look at pictures of the Midway, however, you will note that the flight deck appears closer to the water than in the Essex class; it was actually the same height as in the Essex class but the greater length creates that appearance. However, the designers did not add the additional deck which could have been possible without the weight of the Armored flight deck. For this reason the Midways were considered to be rather "wet" in a seaway.
The Japanese also built an armored flight deck carrier, the Taiho, during WW II. At the Battle of the Philippine Sea, the Taiho took a single submarine torpedo which fractured fuel lines and caused leaks in the fuel storage tanks. The Taiho had been fueled with unrefined Tarrakan crude oil which emitted volatile vapors. These vapors were inadvertently spread throughout the ship by inexperienced damage control crews. Eventually, they encountered a spark and exploded. The force of this explosion could not be vented through open hangar sides and instead, blew out the sides and bottom of the hull, sinking the ship in minutes.