Hiyō-class aircraft carrier

The ships were ordered as the fast luxury passenger liners Izumo Maru and Kashiwara Maru by 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 24 knots (44 km/h; 28 mph) to save fuel, but the Navy wanted a maximum speed of no less than 25.5 knots (47.2 km/h; 29.3 mph) so they compromised by limiting the performance of the turbines to 80% of maximum power during peacetime.[1]

The ships had a length of about 219.32 meters (719 ft 7 in) overall. They had a beam of 26.7 meters (87 ft 7 in) and a draft of 8.15 meters (26 ft 9 in). They displaced 24,150 metric tons (23,770 long tons) at standard load.[2] Their crew ranged from 1,187 to 1,224 officers and enlisted men.[3]

Both ships were fitted with a pair of Mitsubishi-Curtis geared steam turbine sets with a total of 56,250 shaft horsepower (41,950 kW), each driving a 5.5-meter (18 ft) propeller. Steam was provided by six water-tube boilers; Jun'yō had Mitsubishi three-drum boilers that operated at a pressure of 40 kg/cm² (3,923 kPa; 569 psi) and temperature of 420 °C (788 °F) 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 Hiryū. The ships had a designed speed of 25.5 knots, but both exceeded that by small margins during their sea trials. They carried 4,100 metric tons (4,000 long tons) of fuel oil which gave them a range of 11,700 nautical miles (21,700 km; 13,500 mi) or more at 18 knots (33 km/h; 21 mph).[4]

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.[17]

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 Ryūjō. 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.[18]

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.[19] 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 Meredith. Their aircraft attacked and sank the destroyer. The next day, they found the small seaplane tender, McFarland, 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,[20] but Hiyō's machinery problems caused her to return to Truk. Some of her aircraft were transferred to her sister before she departed.[21]

In late October 1942, during the Guadalcanal Campaign, Jun'yō took part in the Battle of the Santa Cruz Islands. At this time, her air group consisted of 18 Zeros, 18 D3As and 9 B5Ns. Her aircraft made hits on the carrier Hornet, the battleship South Dakota and the light cruiser San Juan, but inflicted little substantial damage. A torpedo hit from one of her B5Ns, however, did force the Americans to abandon their effort to repair Hornet.[22] During this time, Hiyō's remaining aircraft flew to Rabaul on 23 October where they provided air cover for Japanese forces on Guadalcanal. A detachment from the air group was transferred to Buin, Papua New Guinea on 1 November and participated in the Naval Battle of Guadalcanal later in the month. Those aircraft that remained at Rabaul flew back to Truk by 11 November, but the Buin detachment was ferried back to Japan on 14 December.[23]

In mid-November 1942, Jun'yō was tasked to provide air cover for the convoy bringing reinforcements for the Japanese forces on Guadalcanal during the three-day-long Naval Battle of Guadalcanal. The ship's fighters were unable to do so; seven transports were sunk and the remaining four transports were damaged.[24] In December 1942 – January 1943, the carrier covered several convoys that brought reinforcements to Wewak, New Guinea and her air group was based there for several days to protect the forces there before returning to Truk on 20 January. The ship then covered the evacuation of forces from Guadalcanal through early February.[25]

Hiyō had returned to Japan in December and Jun'yō followed in February. Both ships returned to Truk in late March[19][21] and their air groups were detached from in early April to participate in Operation I-Go, a land-based aerial offensive against Allied bases in the Solomon Islands and New Guinea.[26] The ships returned to Japan in late May and sailed for Truk on 7 June,[19] but Hiyō was torpedoed that evening and forced to return to port for repairs.[21] Her fighters were flown to Truk by 15 July and assigned to the light carrier Ryūhō.[27] The ship was under repair at Yokosuka until 15 September.[21] Jun'yō's air group was deployed to Buin, Papua New Guinea on 2 July in response to the American invasion of Rendova Island on 30 June. Leaving her aircraft behind, the carrier returned to Japan in late July.[28]

Jun'yō ferried aircraft to Singapore in mid-August and troops and equipment to the Caroline Islands the following month. On 5 November 1943, she was hit by a torpedo, but the damage was light, other than the disabled rudder. The ship was under repair and refit until 29 February 1944 at Kure.[19] The air groups of both carriers were reconstituted at Singapore on 1 November.[26] The aircraft transferred to Truk on 1 December and then to Kavieng at the end of December before reaching Rabaul on 25 January 1944; the survivors were back at Truk on 20 February and the air group was disbanded.[26]

Hiyō departed Japan for Singapore on 24 November. She arrived on 3 December and was almost immediately assigned duties as an aircraft ferry until January when the ship returned to Japan.[21] In the meantime, the Japanese Navy had restructured its carrier air groups so that one air group was assigned to one carrier division and Air Group 652 was assigned to the 2nd Carrier Division with Hiyō, Jun'yō and Ryūhō on 1 March. The air group was last in priority to be rebuilt and only had 30 Model 21 Zeros, 13 Model 52 Zeros and 4 D3As on hand on 1 April of its authorized 81 fighters, 36 dive bombers and 27 torpedo bombers. The ships conducted training for their aircraft in the Inland Sea until 11 May when she sailed for Tawi-Tawi in the Philippines.[29] The new base was closer to the oil wells in Borneo on which the Navy relied and also to the Palau and western Caroline Islands where the Japanese expected the next American attack. However, 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.[30]

Battle of the Philippine Sea

Main article: Battle of the Philippine Sea

The Japanese fleet was en route to Guimaras Island in the central Philippines on 13 June, where they intended to practice carrier operations in an area better protected from submarines, when Vice Admiral Jisaburō Ozawa learned of the American attack on the Mariana Islands the previous day. Upon reaching Guimaras, the fleet refuelled and sortied into the Philippine Sea where they spotted Task Force 58 on 18 June. The Americans failed to locate Ozawa's ships that day and the Japanese turned south to maintain a constant distance between them and the American carriers as Ozawa had decided on launching his air strikes early the following morning. At this time, Air Group 652 consisted of 81 Zeros, 27 D3As, 9 Yokosuka D4Y "Judy" dive bombers and 18 Nakajima B6N "Jill" torpedo bombers, roughly evenly divided among the three ships. The three carriers launched multiple air strikes against the American ships, but generally failed to locate them and did not inflict any damage while losing most of their aircraft.[31]

At dusk, the Japanese turned away to the northwest to regroup and to refuel and the Americans turned west to close the distance. They discovered the retiring Japanese fleet during the afternoon of the following day and Vice Admiral Marc Mitscher ordered an air strike launched.[32] Hiyō was struck by two bombs, one of which detonated above the bridge and killed or wounded virtually everyone there. More seriously, the ship was struck by one torpedo dropped by a Grumman TBF Avenger from Belleau Wood. This knocked out the starboard engine room and started fires, but Hiyō was able to continue, albeit at a slower speed. Two hours later, a large explosion occurred when leaking gasoline vapor ignited and it knocked out all power on the ship. The fires raged out of control and Hiyō sank stern first[21] shortly afterwards at 16°20′N 132°32′E.[13] Roughly 1,000 men were rescued by her escorting destroyers, but 247 officers and enlisted men died aboard the carrier.[21]

Jun'yō at anchor at Sasebo, 1945

Jun'yō was hit by two bombs near her island. The ship was not badly damaged, but the damage did stop flight operations.[19] Air Group 652 claimed 2 Grumman F6F Hellcat fighters and 9 Grumman TBF Avenger torpedo bombers shot down, but lost 11 aircraft, plus another 3 that had to ditch. By the end of the battle, the air group only consisted of 11 A6M5s, 5 A6M2s and 1 B6N and it was disbanded on 10 July. Most of its remaining personnel were assigned to Air Group 653.[33]

After repairs at Kure, the ship remained in the Inland Sea without aircraft until 27 October when she was tasked to transport material to Borneo. On 3 November, she was attacked by the submarine Pintado, but her escorting destroyer, Akikaze, deliberately sacrificed herself by intercepting the torpedoes and sank with no survivors.[19] While returning from Manila, Jun'yō was attacked by the submarines Sea Devil, Plaice and Redfish early in the morning of 9 December 1944. She was hit by three torpedoes, but she was able to proceed on one engine. She reached Sasebo the following day and began repairs on 18 December.[19]

The repairs were abandoned in March 1945 for lack of materials and the ship was moved from the dock to Ebisu Bay, Sasebo on 1 April. Efforts to camouflage the ship began on 23 April and she was reclassified as a guard ship on 20 June. Jun'yō's armament was ordered removed on 5 August and the ship was surrendered to the Allies on 2 September. An American technical team evaluated the ship's condition on 8 October and deemed her a constructive total loss. Jun'yō was stricken from the Navy List on 30 November and scrapped between 1 June 1946 and 1 August 1947 by the Sasebo Ship Company.[13]

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