Hōshō (Japanese: 鳳翔; meaning "Soaring Phoenix") was the world's first commissioned ship that was built as an aircraft carrier, and the first aircraft carrier of the Imperial Japanese Navy (IJN). Commissioned in 1922, the ship was used for testing carrier aircraft operations equipment, techniques, such as take-offs and landings, and carrier aircraft operational methods and tactics. The ship provided valuable lessons and experience for the IJN in early carrier air operations. Hōshō's superstructure and other obstructions to the flight deck were removed in 1924 on the advice of experienced aircrews.
Hōshō and her aircraft group participated in the Shanghai Incident in 1932 and in the opening stages of the Sino-Japanese War in late 1937. During those two conflicts, the carrier's aircraft supported Imperial Japanese Army ground operations and engaged in aerial combat with aircraft of the Nationalist Chinese Air Force. The small size of the ship and her assigned airgroups (usually around 15 aircraft) limited the effectiveness of her contributions to combat operations. As a result, the carrier was placed in reserve after her return to Japan from China and she became a training ship in 1939.
During World War II, Hōshō participated in the Battle of Midway in June 1942 in a secondary role. After the battle, the carrier resumed her training role in Japanese home waters for the duration of the conflict and survived the war with only minor damage from air attacks. She was surrendered to the Allies at the end of the war and used to repatriate Japanese troops until she was scrapped in 1946.
Construction of a seaplane carrier was authorized by the Japanese government in its "eight-six" fleet program of 1918. A planned sister ship, named Shokaku, was cancelled in 1922 before any construction started. Hōshō was the second warship, after the British HMS Hermes, to be built from the keel up as an aircraft carrier, but was launched and completed earlier than Hermes.
Hōshō was initially designed as a seaplane carrier like HMS Campania with a forward flying-off deck, 32 aircraft, four low-angle 14-centimeter (5.5 in) guns, and four anti-aircraft (AA) guns. The plan was revised after reports were received from Japanese observers with the Royal Navy in Europe about the desirability to be able to land aircraft on the ship. The new requirements were modeled on HMS Furious after she received her rear flight deck in 1918. The ship was to be capable of 30 knots (56 km/h; 35 mph) and fitted with a forward flight deck, island and funnels amidships, and a large hangar aft. Shortly thereafter based on observations of landing trials on Furious and HMS Argus, the world's first flush-decked aircraft carrier, Hōshō's flight deck design was revised in April 1919. The island was removed and the funnels were moved to one side to create an unobstructed, full-length flight deck, and the ship was reclassified as an aircraft carrier. The ship's hull was based on that of a large cruiser and she was given a small island. Her three funnels were mounted on the starboard side and swiveled to lie horizontal during flight operations. Hōshō's designed speed was reduced to 25 knots (46 km/h; 29 mph), based on British experiences during World War I.
Hōshō was completed with an overall length of 168.25 meters (552 ft). She had a beam of 17.98 meters (59 ft) and a mean draft of 6.17 meters (20 ft 3 in). The ship displaced 7,470 long tons (7,590 t) at standard load, and 9,494 long tons (9,646 t) at normal load. Her crew totaled 512 officers and men. The ship was almost completely unarmored.
Hōshō had two Parsons geared turbine sets with a total of 30,000 shaft horsepower (22,000 kW) driving two propeller shafts. Eight Kampon Type B water-tube boilers with a working pressure of 18.3 kg/cm2 (1,790 kPa; 260 psi) and a temperature of 138 °C (280 °F) provided steam to the turbines, although only four were oil-fired. The other four used a mix of oil and coal. The ship's designed speed was 25 knots, but she made 26.66 knots (49.37 km/h; 30.68 mph) from 31,117 shp (23,204 kW) on her sea trials on 30 November 1922. She carried 2,700 long tons (2,740 t) of fuel oil and 940 long tons (960 t) of coal, an extraordinary total for such a small ship, to give her a range of 8,680 nautical miles (16,080 km; 9,990 mi) at 12 knots (22 km/h; 14 mph).
To reduce rolling and increase stability for aircraft operations, a gyrostabilizer produced by the American Sperry Gyroscope Company was installed. The installation initially proved unreliable as the Japanese technicians were badly trained by Sperry, but eventually the system proved its worth as the technicians gained experience.
Hōshō's flight deck was 168.25 meters (552 ft) long and 22.62 meters (74 ft 3 in) wide. The forward end sloped down at an angle of −5° to help aircraft accelerate during takeoff. A small island was mounted well forward on the starboard side and contained the ship's bridge and air-operations control center. The island was fitted with a small tripod mast intended to carry the ship's fire-control system. Fifteen different types of arresting gear were evaluated before the British longitudinal wire system was adopted. Low landing speeds of the time meant that aircraft had little difficulty in stopping, but their light weight made them vulnerable to wind gusts that could blow them over the side of the carrier, and the longitudinal wires helped to prevent that. Forward of the island was a collapsible crane for loading aircraft into the forward hangar.
The flight deck, unlike those on Royal Navy carriers, was superimposed on the ship's hull rather than constructed as a strength deck supporting the carrier's hull structure. A system of lights and mirrors along the flight deck assisted pilots in landing on the carrier.
Hōshō was the only Japanese aircraft carrier with two hangars. The forward hangar was 67.2 by 9.5 meters (220 ft 6 in by 31 ft 2 in) and only one deck in height as it was intended to house nine small aircraft, such as fighters. The two-story rear hangar measured 16.5 by 14 meters (54 ft 2 in by 45 ft 11 in) at the forward end and 29.4 by 12 meters (96 ft 5 in by 39 ft 4 in) at the rear end. It was designed to house six large aircraft, such as torpedo bombers, as well as six reserve aircraft. Each hangar was served by an aircraft elevator. The forward elevator was 10.35 by 7.86 meters (34.0 by 25.8 ft) and the aft elevator measured 13.71 by 6.34 meters (45 ft 0 in by 20 ft 10 in).
Hōshō had a normal capacity of fifteen aircraft, subject to the limitations of her hangars. She was first commissioned with an air group of nine Mitsubishi 1MF (Type 10) fighters and three to six Mitsubishi B1M3 (Type 13) torpedo bombers. In 1928, the fighters were replaced by the A1N1 (Type 3). Three years later the air group consisted of Nakajima A2N (Type 90) fighters and Mitsubishi B2M (Type 89) torpedo bombers. In 1938 Nakajima A4N (Type 95) fighters and Yokosuka B3Y (Type 92) bombers flew from the ship. In 1940 the air group was modernized with Mitsubishi A5M (Type 96) "Claude" fighters and Yokosuka B4Y1 (Type 96) "Jean" bombers.
Hōshō was armed with four 50-caliber 14 cm/50 3rd Year Type guns, two on each side. The two forward guns had a firing arc of 150°, including straight ahead, while the rear guns could fire 120° on either side. They fired 38-kilogram (84 lb) projectiles at a rate of six to ten rounds per minute with a muzzle velocity of about 850 m/s (2,800 ft/s); at 35°, they had a maximum range of 19,750 m (21,600 yd). A heavy gun armament was provided for Hōshō; as carrier doctrine was just evolving at this time, the impracticability of carriers engaging in gun duels had not yet been realized. Her large flight deck and lack of armor made her a vulnerable target in surface battles.
A pair of 40-caliber 8 cm/40 3rd Year Type guns on retractable mounts provided Hōshō's only anti-aircraft defense. They were positioned on the flight deck, just forward of the rear elevator. These guns fired 5.67–5.99-kilogram (12.5–13.2 lb) projectiles at a muzzle velocity of about 680 m/s (2,231 ft/s); at 45°, this provided a maximum range of 10,800 meters (11,800 yd), and they had a maximum ceiling of 7,200 meters (23,600 ft) at 75° elevation. Their effective rate of fire was 13 to 20 rounds per minute.