Japanese aircraft carrier Hiyō

The ship was ordered as the fast luxury passenger liner Izumo Maru by Nippon Yusen Kaisha (Japan Mail Steamship Company) in late 1938. In exchange for a 60 per cent subsidy of her building costs by the Navy Ministry, she was designed to be converted to an aircraft carrier in the event of war. The designs of Izumo Maru and her sister ship Kashiwara Maru were based on the German ocean liner SS Bremen, although they were only about half that ship's size at 27,700 gross register tons (GRT). If completed as designed, they would have been the largest ocean liners in Japan. The ships were designed to carry a total of 890 passengers; 220 first class, 120 second class and 550 third class.[2]

After her conversion, Hiyō had a length of 220 metres (721 ft 9 in) overall. She had a beam of 26.7 metres (87 ft 7 in) and a draught of 8.15 metres (26 ft 9 in). She displaced 24,150 tonnes (23,770 long tons) at standard load.[3] Her crew ranged from 1,187 to 1,224 officers and ratings.[4]

The ship was fitted with a pair of Mitsubishi-Curtis geared steam turbine sets with a total of 56,250 shaft horsepower (41,950 kW), each driving one propeller, using steam provided by six Kawasaki-LaMont boilers. Hiyō had a designed speed of 25.5 knots (47.2 km/h; 29.3 mph) and slightly exceeded that during sea trials. The ship carried enough fuel oil to give her a range of 11,700 nautical miles (21,700 km; 13,500 mi) at 18 knots (33 km/h; 21 mph).[5]

Flight deck arrangements

Aft view of the flight deck of her sister ship Jun'yō from the island, 19 October 1945

Hiyō's flight deck was 210.3 metres (690 ft) long and had a maximum width of 27.3 metres (89 ft 7 in). The ship was designed with two superimposed hangars, each approximately 153 metres (502 ft) long, 15 metres (49 ft) wide and 5 metres (16 ft) high. Each hangar could be subdivided by four fire curtains and they had fire fighting foam dispensers on each side. The hangars were served by two aircraft lifts.[6]

Her air group was initially intended to consist of 12 Mitsubishi A5M "Claude" fighters, plus four in storage, 18 Aichi D3A "Val" dive bombers, plus two in storage, and 18 Nakajima B5N "Kate" torpedo bombers. This was revised to substitute a dozen Mitsubishi A6M Zero fighters and three in storage for the A5Ms by the time the ship was commissioned in 1942. As a result of the lessons learnt from the Battle of Midway in June, the fighter complement was increased 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 congestion below decks.[7]

Hiyō was laid down on 30 November 1939 by Kawasaki on Slipway No. 4 at their shipyard in Kobe. She was yard number 660 and had the name Izumo Maru. The ship was purchased on 10 February 1941 by the Navy Ministry and she was temporarily referred to as No. 1002 Ship (Dai 1002 bankan) to keep her conversion secret. She was launched on 24 June 1941 and commissioned on 31 July 1942 with Captain Akitomo Beppu in command.[13]

The ship was assigned to the Second Carrier Division of the 1st Air Fleet after commissioning and became flagship of Rear Admiral Kakuji Kakuta on 12 August. After spending the next few months working up, Hiyō arrived at Truk, together with her sister Jun'yō, on 9 October, to begin operations against American forces in the Guadalcanal area as part of the 3rd Fleet.[14] On the night of 16 October, the two carriers were ordered to attack the American transports off Lunga Point, Guadalcanal, and they moved south to their launching point 180 nautical miles (330 km; 210 mi) north of Lunga. At 05:15 local each ship launched nine A6M Zeros and nine B5Ns. One of Jun'yō's B5Ns was forced to turn back with mechanical problems; the rest reached their objective and discovered two destroyers bombarding Japanese supply dumps on Guadalcanal around 07:20. Hiyō's aircraft attacked USS Aaron Ward seven minutes later without effect, and the American ship shot down one B5N and damaged another which was forced to make a crash landing. Jun'yō's eight B5Ns engaged USS Lardner and also failed to hit their target, not least because they were attacked by Marine Grumman F4F Wildcat fighters at 07:32. The Marine pilots shot down three B5Ns on their first pass and severely damaged another pair which were also forced to crash-land. The US fighters then shot down the three remaining bombers after they missed Lardner. The defending Zeros were able to shoot down only one Wildcat at the cost of one of Hiyō's fighters forced to crash land, although they claimed to have shot down thirteen Marine fighters.[14][15]

A fire in the ship's generator room occurred on 21 October and reduced her top speed to 16 knots (30 km/h; 18 mph) so Kakuta transferred his flag to Jun'yō while Hiyō returned to Truk for repairs. Three Zeros, one D3A and five B5Ns were also transferred to Jun'yō before she left. The remaining aircraft of her air group (16 Zeros and 17 D3As) were flown off for Rabaul, on the island of New Britain, on 23 October, from where the fighters escorted bombers attacking Guadalcanal the following day. A detachment from the air group was transferred to Buin, New Guinea, on 1 November and attacked American ships off Lunga Point on 11 November. Escorted by 18 Zeros from Hiyō and the 204th Naval Air Group, 9 D3As slightly damaged three cargo ships in exchange for 4 dive bombers shot down and another forced to crash land. The Zeros were able to ambush six Wildcats in the heavy cloud and shot down four while losing two of their own. That same day, those aircraft that remained at Rabaul flew back to Truk, but the Buin detachment remained there until 14 December when they were ferried back to Japan. Captain Michio Sumikawa relieved Beppu on 30 November.[14][16]

Hiyō spent November in Truk before returning to Japan in early December where she was rejoined by the rest of her air group. Aside from a brief refit at Kure from 26 February to 4 March 1943 that saw her anti-aircraft armament augmented and an additional radar installed, the ship was training in the Inland Sea until she sailed for Truk on 22 March.[14] Her air group consisted of 27 Zeros and 12 D3As and they were detached from Hiyō in early April to participate in Operation I-Go, a land-based aerial offensive against Allied bases in the Solomon Islands and New Guinea. On 7 April her aircraft formed most of the third wave of attacks on Guadalcanal. Escorted by 24 Zeros from Hiyō and another 6 from the light carrier Zuihō, the D3As attacked shipping in the Sealark Channel. The escorts claimed to have shot down three American aircraft for the loss of one Zero and three dive bombers.[17] Allied naval losses during the entire day included Aaron Ward, the oil tanker Kanawha, the minesweeper HMNZS Moa and damage to a transport and another tanker, although it is uncertain which aircraft sank or damaged each ship.[18]

A second series of attacks was made against Oro Bay, New Guinea, on 11 April. Jun'yō's 9 Zeros, together with Hiyō's 21 fighters, escorted the latter's D3As. One defending fighter was claimed for the loss of a single dive bomber. The following day, 17 of Hiyō's Zeros provided top cover for several waves of attacks on Port Moresby, New Guinea, her pilots claiming nine victories without losing any of their own. On 14 April, the Japanese attacked Milne Bay, New Guinea, with a large force escorted by 75 Zeros contributed by all the carriers involved. Hiyō's fighter pilots claimed to have shot down three Allied aircraft without loss and the bombers sank two transports. Her air group returned to Truk by 18 April to rejoin the ship.[19]

Jun'yō at anchor, late 1945

In response to the invasion of Attu Island on 11 May, the Second Carrier Division departed Truk, accompanied by three battleships and two heavy cruisers and reached Japan on 25 May. The Americans recaptured Attu before the fleet could depart to counter-attack. Now the flagship of the Second Carrier Division under Rear Admiral Munetaka Sakamaki, Hiyō departed Yokosuka on 7 June with Junyō en route for Truk. Later that evening, the ship was torpedoed by the submarine USS Trigger off Miyakejima. Hits in the starboard bow and boiler room knocked out all power but she managed to return to Japan the following day after restoring power.[14] Hiyō's fighters were flown to Truk by 15 July and assigned to the light carrier Ryūhō, as were Sakamaki and his staff. While being repaired at Yokosuka until 15 September, more 2.5 cm Type 96 AA guns were installed, and Sumikawa was relieved by Captain Tamotsu Furukawa on 1 September.[20][14] Two months later, Hiyō's air group was reconstituted in Singapore with 24 Zeros, 18 D3As and 9 B5Ns; the ship departed Japan for Singapore on 24 November.[20] She arrived on 3 December, loaded her air group and was almost immediately assigned duties as an aircraft ferry. On 9 December, Hiyō left Singapore en route for Truk with several deliveries on the way. The ship arrived there on 22 December and disembarked her aircraft before proceeding to Saipan to deliver more aircraft.[14][20] The air group was transferred to Kavieng and later Rabaul to provide air cover for Japanese operations there where the fighters claimed 80 victories in exchange for 12 losses.[21]

Hiyō returned to Japan on 1 January 1944 and Furukawa was relieved by Captain Toshiyuki Yokoi on 15 February. Her air group was reassigned to her on 2 March, albeit without aircraft. The Japanese Navy had restructured its carrier air groups so that one air group was assigned to one carrier division and the 652nd Naval Air Group was assigned to the Second Carrier Division with Hiyō, Jun'yō and Ryūhō.[14] The air group was the last to be rebuilt; it had only 30 Model 21 Zeros, 13 Model 52 Zeros and four D3As on hand on 1 April of its authorised 81 fighters, 36 dive bombers and 27 torpedo bombers. The ship conducted training for her aircraft in the Inland Sea until 11 May when she sailed for Tawi-Tawi in the Philippines.[22] The new base was closer to the oil wells in Borneo on which the Navy relied for fuel and also to the Palau and western Caroline Islands, where the Japanese expected the next American attack. The location lacked an airfield on which to train the green pilots, and American submarines were very active in the vicinity which restricted the ships to the anchorage.[23]

• Brown, J. D. (2009). Carrier Operations in World War II. Annapolis, Maryland: Naval Institute Press. ISBN 978-1-59114-108-2.
• Hata, Ikuhiko; Izawa, Yasuho & Shores, Christopher (2011). Japanese Naval Air Force Fighter Units and Their Aces 1932–1945. London: Grub Street. ISBN 978-1-906502-84-3.
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• Polmar, Norman & Genda, Minoru (2006). Aircraft Carriers: A History of Carrier Aviation and Its Influence on World Events: 1909–1945. Vol. I. Washington, D.C.: Potomac Books. ISBN 978-1-57488-663-4.
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• Tully, Anthony P. & Casse, Gilbert (2013). "IJN Hiyo: Tabular Record of Movement". Kido Butai. Combinedfleet.com. Retrieved 23 January 2018.

Wikimedia Commons has media related to Hiyō (ship, 1942).

• Hiyo in the World War II Database