Reconstruction
Akagi's modernization involved far less work than that of Kaga, but took three times as long due to financial difficulties related to the Great Depression. The ship's three flight decks were judged too small to handle the larger and heavier aircraft then coming into service. As a result, the middle and lower flight decks were eliminated in favor of two enclosed hangar decks that extended almost the full length of the ship. The upper and middle hangar areas' total space increased to about 93,000 square feet (8,600 m2); the lower hangar remained the same size. The upper flight deck was extended to the bow, increasing its length to 249.17 meters (817 ft 6 in) and raising aircraft capacity to 86 (61 operational and 25 in storage). A third elevator midships, 11.8 by 13 meters (38 ft 9 in × 42 ft 8 in) in size, was added. Her arrester gear was replaced by a Japanese-designed, hydraulic, Type 1 system with 9 wires. The modernization added an island superstructure on the port side of the ship, which was an unusual arrangement; the only other carrier to share this feature was a contemporary, the Hiryū. The port side was chosen as an experiment to see if that side was better for flight operations by moving the island away from the ship's exhaust outlets. The new flight deck inclined slightly fore and aft from a point about three-eighths of the way aft.
Akagi's speed was already satisfactory and the only changes to her machinery were the replacement of the mixed coal/oil-fired boilers with modern oil-fired units and the improvement of the ventilation arrangements. Although the engine horsepower increased from 131,200 to 133,000, her speed declined slightly from 32.5 to 31.2 knots (60.2 to 57.8 km/h; 37.4 to 35.9 mph) on trials because of the increase in her displacement to 41,300 long tons (42,000 t). Her bunkerage was increased to 7,500 long tons (7,600 t) of fuel oil which increased her endurance to 10,000 nautical miles (18,520 km; 11,510 mi) at 16 knots (30 km/h; 18 mph). The rear vertical funnel was changed to match the forward funnel and incorporated into the same casing.
The two twin turrets on the middle flight deck were removed and fourteen twin 25 mm (1 in) Type 96 gun mounts were added 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 m (8,200 yd), and an effective ceiling of 5,500 m (18,000 ft). The maximum effective rate of fire was only between 110–120 rounds per minute due to the frequent need to change the 15-round magazines. Six Type 95 directors were fitted to control the new 25 mm guns and two new Type 94 anti-aircraft directors replaced the outdated Type 91s. After the modernization Akagi carried one Type 89 director for the 20 cm (7.9 in) guns; it is uncertain how many were carried before then. The ship's crew increased to 2,000 after the reconstruction.
The ship's anti-aircraft guns were grouped amidships and placed relatively low on the hull. Thus, the guns could not be brought to bear directly forward or aft. Also, the island blocked the forward arcs of the port battery. As a result, the ship was vulnerable to attack by dive bombers. The ship's 12 cm 45-caliber 10th Year Type guns were scheduled to be replaced by more modern 12.7 cm (5.0 in)/40 Type 89 gun mounts in 1942. The anti-aircraft sponsons were to be raised one deck to allow them some measure of cross-deck fire as was done during Kaga's modernization. Subsequent events, however, prevented the ship from surviving combat long enough for the upgrade to take place.
Several major weaknesses in Akagi's design were not rectified. Akagi'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 firefighting difficult, at least in part because fuel vapors could accumulate in the hangars. Adding to the danger was the requirement of 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.
Read more about this topic: Japanese Aircraft Carrier Akagi