In 1923, the Admiralty prepared a 10-year building programme which included an aircraft carrier and 300 aircraft for the Fleet Air Arm. The economic downturn following the First World War caused it to be postponed. In 1930, the Director of Naval Construction, Sir Arthur Johns, began to update the plans for the carrier by incorporating recently developed technology. His aim was to increase the number of aircraft carried by shortening the landing and take-off distances of aircraft using arrestor gear and compressed steam catapults respectively, which would make more deck space available for storage and aircraft preparation. Along with the inclusion of two hangar decks, this allowed Ark Royal to carry up to 72 aircraft, although the development of larger and heavier aircraft during the carrier's construction meant that the actual number carried was between 50 and 60. Ark Royal featured an enclosed hangar design where the fight deck was the 'strength deck' and was strongly built with .75in (19mm) thick Ducol steel plating. The two hangar decks were thus enclosed within the hull girder, which also gave splinter protection to the hangars. The machinery spaces were protected by 4.5-inch (11.4 cm) belt armour. Three lifts moved aircraft between the hangars and the flight deck.
s flight deck overhangs the stern. Her unusual height above the waterline is visible in comparison with the tugboat.
Another feature was the length and height of the flight deck. At 800 feet (240 m), the flight deck was 118 feet (36 m) longer than the keel; the latter dictated by the length of Royal Navy drydocks in Gibraltar and Malta. Due to the twin hangar decks, the flight deck rose to 66 feet (20 m) above the waterline.
The Washington and London Naval treaties had restricted warship tonnage for a number of nations after the end of the great war and were both to expire by the end of 1936.[a] With a potential naval arms race developing between Britain, Japan and Italy, the British government sought a second treaty, which included limiting the maximum displacement of an aircraft carrier to 22,000 long tons (22,000 t). Ark Royal would have to fit this anticipated limit; to conserve weight, armour plating was limited to the belt, engine rooms, and magazines, while welding instead of riveting 65% of the hull saved 500 long tons (510 t). Installation of an armoured flight deck was not possible, as the weight would have placed Ark Royal above the proposed limit, while reducing her endurance and stability. The ship was designed with a three layer side protection system based upon a void-liquid-void scheme very similar to that used on the King George V-class battleships, and was designed to protect against torpedoes with up to a 750-pound (340 kg) warhead.
The ship was fitted with six boilers, which powered three Parsons geared turbines. The turbines were connected via three driveshafts to three propellers 16 feet (4.9 m) in diameter, to produce a maximum theoretical speed of 30 knots (56 km/h; 35 mph). Speed was important, as with catapults and arrestor gear, Ark Royal would have to turn into the wind to launch and recover aircraft. To avoid endangering other ships with the frequent course changes associated with flight operations, Ark Royal would have to break away from accompanying ships, and catch up on completion. Additionally, as the carrier was not armed for ship-to-ship combat, speed was her main protection against enemy warships.