Reconstruction
During her second reconstruction Kaga's two lower flight decks were converted into hangars and, along with the main flight deck, were extended to the bow. This increased the flight deck length to 248.55 meters (815 ft 5 in) and raised aircraft capacity to 90 (72 operational and 18 in storage). A third elevator forward, 11.5 by 12 meters (37 ft 9 in × 39 ft 4 in), serviced the extended hangars. Bomb and torpedo elevators were modified to deliver their munitions directly to the flight deck. Her arrester gear was replaced by a Japanese-designed Type 1 system. A small starboard island superstructure was also installed.
Her power plant was completely replaced as were her propellers. New Kampon multi-stage geared turbines were fitted that increased her power from 91,000 to 127,400 shp (67,859 to 95,002 kW) during trials. Each had a high-pressure, a low-pressure, and a cruising turbine coupled to a single shaft. Her boilers were replaced by eight improved oil-burning models of the Kampon Type B (Ro) with a working pressure of 22 kg/cm2 (2,157 kPa; 313 psi) at a temperature of 300 °C (572 °F). The hull was lengthened by 10.3 meters (33 ft 10 in) at the stern to reduce drag and she was given another torpedo bulge above the side armor abreast the upper part of the existing bulge to increase her beam and lower her center of gravity as a result of lessons learned from the Tomozuru Incident in early 1934. This raised her standard displacement significantly, from 26,900 to 38,200 long tons (27,332 to 38,813 t). The extra power and the extra displacement roughly offset each other and her speed increased by less than a knot, up to 28.34 knots (52.49 km/h; 32.61 mph) on trials. Her fuel storage was increased to 7,500 long tons (7,620 t) of fuel oil which increased her endurance to 10,000 nmi (18,520 km; 11,510 mi) at 16 knots (30 km/h; 18 mph). The lengthy funnel ducting was replaced by a single downturned starboard funnel modeled on that used by the Akagi with a water-cooling system for the exhaust gasses and a cover that could be raised to allow the exhaust gasses to escape if the ship developed a severe list and the mouth of the funnel touched the sea. The space freed up by the removal of the funnel ducts was divided into two decks and converted into living quarters for the expanded air group. The carrier's complement increased to 1708 crewmembers.
The two twin turrets on the middle flight deck were removed and four new 20 cm/50 3rd Year Type No. 1 guns in casemates were added forward. Her 12 cm anti-aircraft guns were replaced by eight 12.7-centimeter (5.0 in)/40 Type 89 guns in twin mounts. They fired 23.45-kilogram (51.7 lb) projectiles at a rate between 8 and 14 rounds per minute at a muzzle velocity of 700–725 m/s (2,300–2,380 ft/s); at 45°, this provided a maximum range of 14,800 meters (16,200 yd), and a maximum ceiling of 9,400 meters (30,800 ft). Their sponsons were raised one deck to allow them some measure of cross-deck fire. Eleven twin 25 mm Type 96 gun mounts were added, also on sponsons. They fired .25-kilogram (0.55 lb) projectiles at a muzzle velocity of 900 m/s (3,000 ft/s); at 50°, this provided a maximum range of 7,500 meters (8,202 yd), and an effective ceiling of 5,500 meters (18,000 ft). The maximum effective rate of fire was only between 110 and 120 rounds per minute due to the frequent need to change the fifteen-round magazines. Six 6.5-millimeter (0.26 in) Type 11 machine guns were also carried. Six Type 95 directors were fitted to control the new 25 mm guns, but Kaga retained her outdated Type 91 anti-aircraft directors.
Several major weaknesses in Kaga's design were not rectified. Kaga's aviation fuel tanks were incorporated directly into the structure of the carrier, meaning that shocks to the ship, such as those caused by bomb or shell hits, would be transmitted directly to the tanks, resulting in cracks or leaks. Also, the fully enclosed structure of the new hangar decks made fire suppression difficult, at least in part because fuel vapors could accumulate in the hangars. Adding to the danger was the requirement from the Japanese carrier doctrine that aircraft be serviced, fueled, and armed whenever possible on the hangar decks rather than on the flight deck. In addition, the carrier's hangar and flight decks carried little armor protection. Furthermore, there was no redundancy in the ship's fire-extinguishing systems. These weaknesses would later be crucial factors in the loss of the ship.
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