Silverstone, p. 329 was the name ship of her class of two aircraft carriers of the Imperial Japanese Navy (IJN). Originally planned as the ocean liner in 1939, she was purchased by the Navy Ministry in 1941 for conversion to an aircraft carrier. Completed shortly after the Battle of Midway in June 1942, she participated in the Guadalcanal campaign, but missed the Battle of the Santa Cruz Islands in October because of an electrical generator fire.

The carrier's aircraft were disembarked several times and used from land bases in battles in the South West Pacific. Hiyō was torpedoed in mid-1943 and spent three months under repair. She spent most of the next six months training and ferrying aircraft before returning to combat. She was sunk by a gasoline-vapour explosion caused by an American torpedo hit during the Battle of the Philippine Sea on 20 June 1944 with the loss of 247 officers and ratings, about a fifth of her complement.

Design and description 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 Navy Ministry subsidy of 60% of her building costs, 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 , although they were only about half that ship's size at . 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.Lengerer & Rehm-Takahara, pp. 17, 19, 106

After her conversion, Hiyō had a length of overall. She had a beam of and a draught of . She displaced at standard load.Lengerer & Rehm-Takahara, p. 107 Her crew ranged from 1,187 to 1,224 officers and ratings.Jentschura, Jung & Mickel, p. 52

The ship was fitted with a pair of Mitsubishi-Curtis geared steam turbine sets with a total of , each driving one propeller, using steam provided by six Kawasaki-LaMont boilers. Hiyō had a designed speed of and slightly exceeded that during sea trials. The ship carried enough fuel oil to give her a range of at .Lengerer & Rehm-Takahara, pp. 189–190

Flight deck arrangements

Hiyōs flight deck was long and had a maximum width of . The ship was designed with two superimposed hangars, each approximately long, wide and 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.Lengerer & Rehm-Takahara, pp. 108–114

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.Lengerer & Rehm-Takahara, p. 111

Armour, armament and sensors As a conversion from an ocean liner, it was not possible to add much armour, although the ship had a double hull. Two plates of Ducol steel, each thick, protected the sides of the ship's machinery spaces. The ship's aviation gasoline tanks and magazines were protected by one layer of Ducol steel. Her machinery spaces were further subdivided by transverse and longitudinal bulkheads to limit any flooding.Lengerer & Rehm-Takahara, p. 188

The ship's primary armament consisted of a dozen Type 89 dual-purpose guns in twin mounts on sponsons along the sides of the hull. Hiyō was also initially equipped with eight triple mounts for 2.5 cm Type 96 light AA guns alongside the flight deck. This was the standard Japanese light AA gun during World War II, but it suffered from severe design shortcomings which rendered it a largely ineffective weapon. According to historian Mark Stille, the twin and triple mounts "lacked sufficient speed in train or elevation; the gun sights were unable to handle fast targets; the gun exhibited excessive vibration; the magazine was too small, and, finally, the gun produced excessive muzzle blast".Stille, p. 8 In early 1943, four more triple mounts were added, and another four triple mounts were added late in the year. 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.Lengerer & Rehm-Takahara, pp. 188, 193

Two Type 94 high-angle fire-control directors, one on each side of the ship, were fitted to control the Type 89 guns. Four Type 95 directors controlled the 2.5 cm guns, and another pair were added in early 1943. Early warning was provided by two Type 2, Mark 2, Model 1 early-warning radars. The first of these was mounted on the top of the island shortly before she was completed in July 1942 and the other was added later in the year. This latter system was fitted on the port side of the hull, outboard of the aft lift.Lengerer & Rehm-Takahara, pp. 188–189, 193 A smaller Type 3, Mark 1, Model 3 early-warning radar was added in 1944.Stille, p. 23

Construction and career 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.Lengerer & Rehm-Takahara, pp. 17, 106–107

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.Tully & Casse 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 north of Lunga. At 05:15 local each ship launched nine A6M Zeros and nine B5Ns. One of Jun'yōs 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. Hiyōs aircraft attacked 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'yōs eight B5Ns engaged 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 Hiyōs fighters forced to crash land, although they claimed to have shot down thirteen Marine fighters.Lundstrom, pp. 318–319

A fire in the ship's generator room occurred on 21 October and reduced her top speed to , 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.Hata, Izawa & Shores, pp. 43, 133; Lundstrom, p. 472

Hiyō spent November in Truk before returning in early December to Japan, 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. 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 , 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.Hata, Izawa & Shores, pp. 51–52 Allied naval losses during the entire day included Aaron Ward, the oil tanker , the minesweeper and damage to a transport and another tanker, although it is uncertain which aircraft sank or damaged each ship.Rohwer, p. 244

A second series of attacks was made against Oro Bay, New Guinea, on 11 April. Jun'yōs 9 Zeros, together with Hiyōs 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 Hiyōs 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. Hiyōs 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.Hata, Izawa & Shores, pp. 53–54; Rohwer, p. 244