The treaty compromise in Illustrious's design

Illustrious's protected hangar lay between the lifts, with 4.5-inch armour on its sides and ends, a 3-inch flight deck and a 4.5-inch waterline belt. Fitting that protection into roughly 23,000 tons required compromises, including three shafts rather than four. Her machinery produced 111,000 shaft horsepower, and the quoted 30 knots was a designed speed at deep load, which should not automatically be treated as an absolute maximum. The nominal 36-aircraft complement usually meant twenty-four torpedo-spotter-reconnaissance aircraft and twelve fighters. The larger aircraft combined torpedo attack with spotting for battleships and reconnaissance, so aircraft counts alone concealed differences in fleet utility. A larger carrier combining Ark Royal's capacity with Illustrious's protection would have been closer to the Royal Navy's preference. Treaty limits also constrained how many such hulls the navy could obtain.