The two were built for the Imperial Japanese Navy (IJN) during World War II. Both ships of the class, and , were originally laid down as luxury passenger liners before being acquired by the IJN for conversion to aircraft carriers in 1941. Jun'yō was the first of the sister ships to be completed in May 1942 and the ship participated in the invasion of the Aleutian Islands the following month. Both ships participated in several battles during the Guadalcanal Campaign in late 1942. Their aircraft were disembarked several times and used from land bases in a number of battles in the South West Pacific.
Hiyō was torpedoed in June 1943 and Jun'yō in November; both ships spent about three months under repair. They spent most of the time after their repairs training and ferrying aircraft before returning to combat. Hiyō was sunk by a gasoline vapor explosion caused by an American aerial torpedo hit during the Battle of the Philippine Sea in mid-1944 while Jun'yō was damaged by several bombs. Lacking aircraft, she was used as a transport in late 1944 and was torpedoed in December. The ship was under repair until March 1945 when the repairs were deemed uneconomical. Jun'yō was then effectively hulked for the rest of the war. After the surrender of Japan in September, the Americans deemed the ship not worth her repair costs; she was broken up in 1946–1947.
Design and description The ships were ordered as the fast luxury as respectively Kashiwara Maru and Izumo Maru, by the Nippon Yusen Kaisha (Japan Mail Steamship Company-NYK) in late 1938. In exchange for a 60% subsidy of their building costs by the Navy Ministry, they were designed to be converted to aircraft carriers. To facilitate this process, they were fitted with a double hull, additional fuel oil capacity, provisions for the fitting of additional transverse and longitudinal bulkheads, installation of a longitudinal bulkhead to separate the turbine rooms, a strengthened main deck, more height between decks, rearrangement of the superstructure and passenger accommodations to facilitate the installation of aircraft elevators and hangars, more space for additional wiring, installation of a bulbous bow and the addition of aviation gasoline storage tanks fore and aft of the machinery spaces. NYK was only interested in a maximum speed of to save fuel, but the Navy wanted a maximum speed of no less than so they compromised by limiting the performance of the turbines to 80% of maximum power during peacetime.Lengerer & Rehm-Takahara, pp. 15, 17
The ships had a length of about overall. They had a beam of and a draft of . They displaced at standard load.Lengerer & Rehm-Takahara, p. 107 Their crew ranged from 1,187 to 1,224 officers and enlisted men.Jentschura, Jung & Mickel, p. 52
Both ships were fitted with a pair of Mitsubishi-Curtis geared steam turbine sets with a total of , each driving a propeller. Steam was provided by six water-tube boilers; Jun'yō had Mitsubishi three-drum boilers that operated at a pressure of and temperature of while Hiyō had Kawasaki-LaMont boilers. Their machinery, designed for merchant service, was over four times heavier than that of the purpose-built aircraft carrier . The ships had a designed speed of 25.5 knots, but both exceeded that by small margins during their sea trials. They carried of fuel oil which gave them a range of or more at .Lengerer & Rehm-Takahara, pp. 189–190
Flight deck arrangements The flight deck was long and had a maximum width of .Lengerer & Rehm-Takahara, pp. 108–109 A large island was fitted on the starboard side that, for the first time in a Japanese carrier, was integrated with the ship's funnel. This was angled 26° outwards to help keep its exhaust from interfering with flight operations.Lengerer & Rehm-Takahara, pp. 113–114 The ships were designed with two superimposed hangars, each approximately long, wide and high. Each hangar could be subdivided by four fire curtains and they were fitted with fire fighting foam dispensers on each side. The hangars were served by two square elevators with rounded corners, on each side. The elevators had a maximum capacity of and took 15 seconds to go from the lower hangar to the flight deck. The ships were fitted with electrically operated Kure type model 4 arresting gear with nine cables. They also mounted two Type 3 crash barricades. No aircraft catapult was fitted. The ships mounted a crane on the port side of the flight deck, just aft of the rear elevator. When collapsed, it was flush with the flight deck.Lengerer & Rehm-Takahara, pp. 108–114
Their air group was originally intended to consist of 12 Mitsubishi A5M "Claude" fighters, plus 4 in storage, 18 Aichi D3A "Val" dive bombers, with an additional 2 in reserve, and 18 Nakajima B5N "Kate" torpedo bombers. This was revised to substitute a dozen Mitsubishi A6M Zero fighters, plus 3 in storage, for the A5Ms by the time the ships were commissioned in 1942. As a result of the lessons learned from the Battle of Midway in June, the ships' fighter complement was strengthened 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 crowding below decks.Lengerer & Rehm-Takahara, p. 111
Armor, armament and sensors As a conversion from an ocean liner, it was not possible to add much armor, although the ships had a double hull. Two plates of Ducol steel, each thick, protected the sides of the ships' machinery spaces. Their aviation gasoline tanks and magazines were protected by one layer of Ducol steel. In addition, their machinery spaces were further subdivided by transverse and longitudinal bulkheads to limit any flooding.Lengerer & Rehm-Takahara, p. 188
The primary armament consisted of a dozen 40-caliber 12.7 cm Type 89 anti-aircraft (AA) guns in twin mounts on sponsons along the sides of the hull.Sturton, p. 183 They fired projectiles at a rate between 8 and 14 rounds per minute at a muzzle velocity of ; at 45°, this provided a maximum range of , and a maximum ceiling of .Campbell, pp. 192–193 The ships were also initially equipped with eight triple 25 mm Type 96 light AA guns alongside the flight deck. They fired projectiles at a muzzle velocity of ; this provided a maximum range of , and an effective ceiling of at +85°. 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.Campbell, p. 200
In mid-1943, four more triple mounts were added and another four triple mounts in late 1943–early 1944. 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 were also added, some of which were portable and could be mounted on tie-down points on the flight deck. After the Battle of the Philippine Sea in June 1944, Jun'yōs anti-aircraft armament was reinforced with three more triple mounts, two twin mounts and 18 single mounts for the 25 mm Type 96 gun. These guns were supplemented by six 28-round AA rocket launchers. Each rocket weighed and had a maximum velocity of . Their maximum range was .Campbell, p. 216 In October 1944, Jun'yō had a total of 91 25 mm barrels; 57 in 19 triple mounts, four in two twin mounts, and 30 single mounts.Lengerer & Rehm-Takahara, p. 193
Two Type 94 high-angle fire-control directors, one on each side of the ship, were fitted to control the Type 89 guns. Each director mounted a rangefinder. When Jun'yō first commissioned only the rangefinders were fitted and the directors were added later. Four Type 95 directors controlled the 25 mm guns and another pair were added in early 1943. Early warning was provided by two Type 2, Mark 2, Model 1 air search radars. The first of these was mounted on the top of the island in mid- to late 1942 on each ship, and the other was added during 1943. This latter system was fitted on the port side of the hull, outboard of the rear elevator.Lengerer & Rehm-Takahara, pp. 188–189, 193 A smaller Type 3, Mark 1, Model 3 air search radar was added in 1944 on Jun'yō.Stille, p. 23
Ships + Construction data Ship Kanji Original name Builder Laid down Launched Commissioned Fate Izumo Maru () Kawasaki Heavy Industries, Kobe 30 November 193924 June 194131 July 1942Sunk, 21 June 1944, during the Battle of the Philippine Sea Kashiwara Maru () Mitsubishi Heavy Industries, Nagasaki 20 March 193926 June 19413 May 1942Scrapped, 1946–1947
Service history The ships were purchased on 10 February 1941 by the Navy Ministry for the price of ¥48,346,000 and their armament and aircraft cost an additional ¥27,800,000. The cost to convert the two ships was budgeted at ¥38,073,000, for a grand total of ¥114,219,000. Kashiwara Maru and Izumo Maru were temporarily referred to as No. 1001 Ship (Dai 1001 bankan) and No. 1002 Ship respectively to keep their conversions secret. Jun'yō was initially classified as an auxiliary aircraft carrier (Tokusetsu kokubokan), but following the loss of four Japanese fleet carriers in the Battle of Midway, she was redesignated as a regular carrier (Kokubokan) in July; Hiyō, completed after the loss of the carriers, received that designation from the beginning.Lengerer & Rehm-Takahara, pp. 106–107
Despite being launched several days after Hiyō, Jun'yō was the first of the pair to be commissioned in May 1942. She was assigned to the Fourth Carrier Division of the 1st Air Fleet, together with . The ship was tasked to support the invasion of the Aleutian Islands, an operation designed to coincide with the attack on Midway. Jun'yō carried 18 A6M2 Zeros and 18 D3As for this operation. The ship launched her first airstrike at dawn on 3 June against Dutch Harbor on Unalaska Island. She accomplished little during this operation, losing five aircraft to all causes, and her own aircraft only shot down five American aircraft.Brown, pp. 145–148
Upon arrival at Truk on 9 October, the two sisters were assigned to the Second Carrier Division to begin operations against American forces in the Guadalcanal area as part of the 3rd Fleet.Tully 1999 On 15 October, the two carriers reached the vicinity of Malaita Island in the Solomon Islands and their aircraft discovered a resupply convoy for Guadalcanal that was escorted by the destroyer . Their aircraft attacked and sank the destroyer. The next day, they found the small seaplane tender, , in Lunga Roads offloading avgas into barges. Dive bombers from the sisters blew the ship's stern off, but failed to sink McFarland. The two carriers were intended to play a prominent role in the Japanese effort to retake Guadalcanal Island and were assigned to the Advance Force for this operation. Their aircraft were supposed to provide air cover after the Japanese night attack that retook Henderson Field and then they were to be flown ashore,Brown, pp. 178–179 but Hiyōs machinery problems caused her to return to Truk. Some of her aircraft were transferred to her sister before she departed.Tully 2006