was an aircraft carrier built for the Imperial Japanese Navy (IJN). Originally intended to be one of two s, Kaga was converted under the terms of the Washington Naval Treaty to an aircraft carrier as the replacement for the battlecruiser , which had been irreparably damaged during the 1923 Great Kantō earthquake. Kaga was rebuilt in 1933–1935, increasing her top speed, improving her exhaust systems, and adapting her flight decks to accommodate more modern, heavier aircraft.
The ship figured prominently in the development of the IJN's carrier striking force doctrine, which grouped carriers together to give greater mass and concentration to their air power. A revolutionary strategic concept at the time, the employment of the doctrine was crucial in enabling Japan to attain its initial strategic goals during the first six months of the Pacific War.
Kagas aircraft first supported Japanese troops in China during the Shanghai Incident of 1932 and participated in the Second Sino-Japanese War in the late 1930s. With other carriers, she took part in the attack on Pearl Harbor in December 1941 and the invasion of Rabaul in the Southwest Pacific in January 1942. The following month her aircraft participated in a combined carrier airstrike on Darwin, Australia, and helping secure the conquest of the Dutch East Indies by Japanese forces. She missed the Indian Ocean raid in April as she had to return to Japan for repairs after hitting a reef in February.
Following repairs, Kaga rejoined the 1st Air Fleet for the Battle of Midway in June 1942. After bombarding American forces on Midway Atoll, Kaga and three other IJN carriers were attacked by American aircraft from Midway and the carriers , , and . Dive bombers from Enterprise severely damaged Kaga; when it became obvious she could not be saved, she was scuttled by Japanese destroyers to prevent her from falling into enemy hands. The loss of Kaga and three other IJN carriers at Midway was a crucial setback for Japan, and contributed significantly to Japan's ultimate defeat. In 1999, debris from Kaga including a large section of her hull was located on the ocean floor northwest of Midway Island.Nauticos In 2019, discovered her wreck on the ocean floor.
Design and construction
Kaga was laid down as a , and was launched on 17 November 1921 at the Kawasaki Heavy Industries shipyard in Kobe. On 5 February 1922 both Tosa-class ships were canceled and scheduled to be scrapped under the terms of the Washington Naval Treaty.Friedman, p. 232; Jentschura, p. 35
The Treaty authorized conversion of two battleship or battlecruiser hulls into aircraft carriers of up to standard displacement. The incomplete battlecruisers Amagi and were initially selected, but the Great Kantō earthquake of 1923 damaged Amagis hull beyond economically feasible repair, and Kaga was selected as her replacement. The formal decision to convert Kaga to an aircraft carrier was issued 13 December 1923, but no work took place until 1925 as new plans were drafted and earthquake damage to the Yokosuka Naval Arsenal was repaired. She was officially commissioned on 31 March 1928, but this signified only the beginning of sea trials. She joined the Combined Fleet (Rengō Kantai) on 30 November 1929 as the IJN's third carrier to enter service, after (1922) and Akagi (1927).Lengerer, p. 128
Kaga was completed with a length of overall. She had a beam of and a draft at full load of . She displaced at standard load, and at full load, nearly less than her designed displacement as a battleship.Lengerer, p. 129 Her complement totaled 1,340 crewmembers.Jentschura, p. 42
Flight deck arrangements
Kaga, like Akagi, was completed with three superimposed flight decks, the only carriers ever to be designed so. The British carriers converted from "large light cruisers", , , and , each had two flight decks, but there is no evidence that the Japanese copied the British model. It is more likely that it was a case of the parallel designs converging in separate attempts to improve launch and recovery cycle flexibility by allowing simultaneous launch and recovery of aircraft.Brown, p. 2 Kagas main flight deck was long and wide,Peattie, p. 231 her middle flight deck was only about long and started in front of the bridge, and her lower flight deck was approximately long. The utility of her middle flight deck was questionable as it was so short that only some of the lightly loaded aircraft could use it, even in an era when the aircraft were much lighter and smaller than they were during World War II.Lengerer, pp. 130, 134 The ever-increasing growth in aircraft performance, size and weight during the 1930s meant that even the bottom flight deck was no longer able to accommodate the take-off roll required for the new generations of aircraft being fielded and it was plated over when the ship was modernized in the mid-1930s.Lengerer, pp. 130, 136 Kaga lacked an island until one was added during the modernization.Peattie, pp. 54–55
As completed, the ship had two main hangar decks and a third auxiliary hangar with a total capacity of 60 aircraft. The hangars opened onto the middle and lower flight decks to allow aircraft to take off directly from the hangars while landing operations were in progress on the main flight deck above. No catapults were fitted. Her forward aircraft elevator was offset to starboard and in size. Her aft elevator was on the centerline and . Her arresting gear was a French transverse system as used on their aircraft carrier and known as the Model Fju (Fju shiki) in the Japanese service. As originally completed, Kaga carried an air group of 28 Mitsubishi B1M3 torpedo bombers, 16 Nakajima A1N fighters and 16 Mitsubishi 2MR reconnaissance aircraft.Hata, p. 24, gives the carrier's initial aircraft complement as 12 Type 3 fighters (plus three crated spares), six reconnaissance aircraft (with two spares), and 18 torpedo bombers (six spares).
Armament and armor
Kaga was armed with ten 3rd Year Type guns, one twin-gun Type B turret on each side of the middle flight deck and six in casemates aft. They fired projectiles at a rate of three to six rounds per minute with a muzzle velocity of ; at 25°, they had a maximum range between . The Type B turrets were nominally capable of 70° elevation to provide additional anti-aircraft (AA) fire, but in practice the maximum elevation was only 55°. The slow rate of fire and the fixed 5° loading angle minimized any real anti-aircraft capability.Campbell, p. 187 This heavy gun armament was provided in case she was surprised by enemy cruisers and forced to give battle, but her large and vulnerable flight deck, hangars, and other features made her more of a target in any surface action than a fighting warship. Carrier doctrine was still evolving at this time and the impracticability of carriers engaging in gun duels had not yet been realized.Peattie, pp. 53, 55The United States Navy did much the same with the provision of four twin gun turrets on their carriers. See Gardiner and Grey, p. 110.
She was given an anti-aircraft armament of six twin 10th Year Type Model A2 gun mounts fitted on sponsons below the level of the funnels, where they could not fire across the flight deck, three mounts per side.Lengerer, p. 131 These guns fired projectiles at a muzzle velocity of ; at 45° this provided a maximum range of , and they had a maximum ceiling of at 75° elevation. Their effective rate of fire was 6 to 8 rounds per minute.Campbell, p. 194 She had two Type 89 directors to control her 20 cm guns and two Type 91 manually powered anti-aircraft directors (Kōshaki) to control her 12 cm guns.Parshall and Tully, pp. 140, 467 Kagas waterline armored belt was reduced from during her reconstruction and the upper part of her torpedo bulge was given of armor. Her deck armor was also reduced from .Lengerer, p. 134
Propulsion
When Kaga was being designed, the problem of how to deal with exhaust gases in carrier operations had not been resolved. The swiveling funnels of Hōshō had not proved successful and wind-tunnel testing had not provided an answer. As a result, Akagi and Kaga were given different exhaust systems to evaluate in real-world conditions. Kagas funnel gases were collected in a pair of long horizontal ducts which discharged at the rear of each side of the flight deck, in spite of predictions by a number of prominent naval architects that they would not keep the hot gases away from the flight deck. The predictions proved to be correct, not least because Kaga was slower than Akagi, which allowed the gases to rise and interfere with landing operations. Another drawback was that the heat of the gases made the crew's quarters located on the side of the ship by the funnels almost uninhabitable.
Kaga was completed with four Kawasaki Brown-Curtis geared steam turbines with a total of on four shafts. As a battleship her expected speed had been , but the reduction in displacement from allowed this to increase to , as demonstrated on her sea trials on 15 September 1928. She had twelve Kampon Type B (Ro) boilers with a working pressure of , although only eight were oil-fired. The other four used a mix of oil and coal. She carried of fuel oil and of coal to give her a range of at .
Early service and development of carrier doctrine
On 1 December 1931 Kaga was assigned as the flagship of the First Carrier Division under the command of Rear Admiral Takayoshi Katō.Hata, p. 24 The First Carrier Division, along with Hōshō, departed for Chinese waters on 29 January 1932 to support Imperial Japanese Army troops during the Shanghai Incident as part of the IJN's 3rd Fleet. The B1M3s carried by Kaga and Hōshō were the main bombers used during the brief combat over Shanghai.Peattie, p. 37
Kagas aircraft, operating from both the carrier and a temporary base at Kunda Airfield in Shanghai, flew missions in support of Japanese ground forces throughout February 1932. During one of these missions three of Kagas Nakajima A1N2 fighters, including one piloted by future ace Toshio Kuroiwa, escorting three Mitsubishi B1M3 torpedo bombers, scored the IJN's first air-to-air combat victory on 22 February when they shot down a Boeing P-12 flown by an American volunteer pilot.Peattie, pp. 50–51 Kaga returned to home waters upon the declaration of the cease-fire on 3 March and resumed fleet training with the rest of the Combined Fleet.
At this time, the IJN's developing carrier doctrine was still in its earliest stages. Kaga and the other carriers were initially given roles as tactical force multipliers supporting the fleet's battleships in the IJN's "decisive battle" doctrine. In this role, Kagas aircraft were to attack enemy battleships with bombs and torpedoes. Aerial strikes against enemy carriers were later, beginning around 1932–1933, deemed of equal importance in order to establish air superiority during the initial stages of battle. The essential component in this strategy was that the Japanese carrier aircraft must be able to strike first with a massed aerial attack. As a result, in fleet training exercises the carriers began to operate together in front of or with the main battle line. This revolutionary strategy emphasized maximum speed from both the carriers and the aircraft they carried as well as larger aircraft with greater range. Thus, longer flight decks on the carriers were required in order to handle the newer, heavier aircraft which were entering service.Peattie, pp. 72–76, 152; Stille, p. 13; Goldstein (Jisaburō Ozawa), pp. 76–78
Kaga was soon judged inferior to Akagi because of her slower speed, smaller flight deck ( shorter), and problematic funnel arrangement. Because of Kagas perceived limitations, she was given priority over Akagi for modernization.Peattie, pp. 55–56 Kaga was relegated to reserve status on 20 October 1933 to begin a second major reconstruction, with an official start date of 25 June 1934.Lengerer, p. 172
Reconstruction