could be subdivided by four fire curtains and they had fire fighting foam dispensers on each side. The hangars were served by two aircraft lifts. Her air group was initially intended to consist of 12 Mitsubishi A5M "Claude" fighters, plus four in storage, 18 Aichi D3A "Val" dive bombers, plus two in storage, and 18 Nakajima B5N "Kate" torpedo bombers. This was revised to substitute a dozen Mitsubishi A6M Zero fighters and three in storage for the A5Ms by the time the ship was commissioned in 1942. As a result of the lessons learnt from the Battle of Midway in June, the fighter complement was increased to 21 Zeros and the B5Ns were reduced to 9. By the end of the year, 6 more Zeros replaced an equal number of D3As, giving totals of 27 A6Ms, 12 D3As and 9 B5Ns. Although it was possible to fit all these aircraft into the hangars, 8 or 9 were usually stored on the flight deck to reduce congestion below decks. Armour, armament and sensors As a conversion from an ocean liner, it was not possible to add much armour, although the ship had a double hull. Two plates of Ducol steel, each thick, protected the sides of the ship's machinery spaces. The ship's aviation gasoline tanks and magazines were protected by one layer of Ducol steel. Her machinery spaces were further subdivided by transverse and longitudinal bulkheads to limit any flooding. The ship's primary armament consisted of a dozen Type 89 dual-purpose guns in twin mounts on sponsons along the sides of the hull. Hiyō was also initially equipped with eight triple mounts for 2.5 cm Type 96 light AA guns alongside the flight deck. This was the standard Japanese light AA gun during World War II but it suffered from severe design shortcomings that rendered it a largely ineffective weapon. According to historian Mark Stille, the twin and triple mounts "lacked sufficient speed in train or elevation; the gun sights were unable to handle fast targets; the gun exhibited excessive vibration; the magazine was too small, and, finally, the gun produced excessive muzzle blast". In early 1943, four more triple mounts were added and another four triple mounts were added late in the year. Two of these last four mounts were mounted on the stern and the others were placed in front of and behind the island. A dozen single mounts