Armored decks were a trade-off. Armor is heavy (obviously) and the deck is substantially above the waterline which means that in addition to the overall weight problem, armored decks also hamper stability.
The American answer to this conundrum at least when the Essex class was designed was to build carriers with lightweight wooden decks and use the weight savings to better armor the "strength deck" which was below the hangars. Thus, it was REALLY hard to put a bomb into a boiler room or magazine of an Essex (because the bomb needed to penetrate the unarmored flight deck, the unarmored deck between the two levels of hangars, and the armored strength deck. This is part of the reason that no Essex class carriers were EVER sunk in combat. It also meant that the American Essex Class carriers could carry a substantially larger air-wing.
During WWII the American Essex Class Carriers displaced about 36,000 tons at full load and carried around 100 aircraft of various types. The British Illustrious Class Carriers displaced about 23,000 tons and carried up to 57 aircraft. That isn't an entirely fair comparison because the Essex Class Carriers were larger but if you do the math, the Essex's carried approximately one aircraft for each 360 tons of displacement while the Illustrious's carried approximately one aircraft for each 403 tons of displacement. I also think that comparison shorts the unarmored US carriers a bit because wiki lists the Illustrious as carrying 36-57 planes while the Essex is listed as carrying 90-100. Later in the war I know the US started adding extra planes (mostly fighters and night-fighters for defense) so I think the US Carriers actually had a bigger disparity.
More planes made the carriers more potent because obviously the main defensive and offensive armament of a CV is the planes.
Armored deck carriers were also harder to fix when they did get damaged. US crews simply planked over the holes and kept fighting.
That's a great exposition, Medina. I agree with it completely.
I thought that you were going to tell WHY the Brits made the trade-offs that they did.
Another way of looking at it is the age-old weighing of offense vs. defense that we also see in tanks and aircraft. The U.S. carriers carried more offensive combat power whereas the British carriers were at least theoretically more survivable. There's an easily understood comparison between the Japanese A6M Zero fighter and the U.S. Navy's F4F Wildcat fighter. The Zero was all offense. It had long range, higher top speed, better maneuverability, and heavier armament than the Wildcat. But it was lightly constructed and didn't have self-sealing fuel tanks. The Wilcat, OTOH, was slower, less maneuverable, shorter ranged, and less heavily armed. But it was tough. It was built stronger and it had self-sealing fuel tanks.
As is often the case with trade-offs, the victor was often the side that made the most of its strengths while minimizing its weaknesses. The Japanese did that better at the start of the Pacific War, not really consciously, but because the Zero was a fighter pilots fighter plane. Fighter pilots want to dogfight, and that played right into the hands of the Zero pilots. Wildcat pilots had to learn to use tactics that would let them survive and win against the Zero that had all the better specs. They learned (as the Flying Tigers had in China) that American fighters could outdive Japanese fighters, so the way to attack them was to strike from above and dive away. Not only could they dive faster, but they could maneuver better at high speeds, because the Zero's controls were designed to maximize maneuverability at low-to-medium speeds--they got very stiff at high speeds. (The Flying Tigers had assumed that they were fighting Zeroes, but their opponents might have been Japanese Army fighters instead. It didn't matter; the principle was the same.)
Also, Cdr. John Thach developed the "beam defense" (a.k.a. "Thach Weave") tactic of flying with the flight leader and his wingman flying spread out, roughly abreast. If either were attacked from the rear, they would both towards each other, which would give the plane not being attacked a clear shot at the Zero chasing the other plane. That worked with larger flormations as well--one pair of Wildcats would fly abreast of another pair, and the same principle applied on a larger scale.
Finally, the Zero's heavier armament (2x23mm + 2x7.7mm vs. 4 or 6x.50-cal)) didn't really matter. The Wildcat could take a lot of punishment and keep flying. The Zero could take much less--in some case, a single burst of .50-caliber machine-gun fire.
