Japan’s Giant Aircraft Carrier
By Mike Bennighof, Ph.D.
November 2008

The loss of four aircraft carriers in the Battle of Midway in June 1942 shocked the Imperial Japanese Navy. In one battle, the bulk of its striking force had been destroyed, leaving only two fleet carriers available and both of them were under repair.

The Japanese would bring two converted passenger liners into service at about the same time, Junyo on 5 May 1942 and Hiyo on 31 July. Both served as fleet carriers, as had been intended from their requisition in August 1940, but were much less capable ships than the four big carriers lost at Midway.

The Imperial Navy needed more carriers quickly, and ordered conversion of a number of warships under construction. We looked at the battleship-carrier hybrids in an earlier installment. One of the warships slated for complete conversion to a carrier was the giant battleship Shinano.

Shinano had been projected as the third ship in the Yamato class: the largest ever built, armed with nine huge 18-inch guns. Yamato herself was laid down in November 1937, launched in August 1940 and commissioned in December 1941. Her sister Musashi was laid down in March 1938, launched in November 1940 and commissioned in August 1942.

The next pair of ships was ordered under the 1939 program, with Shinano laid down in May 1940 and the never-named Number 111 begun in the graving dock vacated by Yamato in November 1940. When war began in December 1941, construction halted on both new battleships, with Shinano about half finished and her sister 30 percent complete. Construction on both had been cancelled, but neither had been scrapped when the search for suitable carrier conversions began in the summer of 1942.

With construction already halted, Shinano was an obvious choice. She had been completed only to her armored deck, allowing reconstruction to begin immediately without the need to dismantle her superstructure. Work crews began building a hangar deck on the armored deck in the summer of 1942, before a set of carrier plans had even been produced.

Shinano as drawn by Shizuo Fukui.

Vice Admiral Keiji Fukuda, the Yamato class designer, and Vice Admiral Sei-ichi Iwamura, Chief of the Naval Technical Bureau, collaborated on the new carrier’s design. The huge, deep hull would give their new carrier very large magazines for aerial ordnance and vast storage capacity for aviation fuel — far larger reserves than those of any other Japanese carrier, and much more than her own air group could effectively use in a carrier engagement.

Therefore, the two admirals hit on a novel plan: Shinano would have no air group of her own. In a carrier battle, she would serve as a floating station to re-fuel and re-arm the planes of other carriers’ squadrons. This method, Fukuda and Iwamura argued, would multiply the squadrons’ effect. The naval headquarters staff disagreed and directed the designers to create a conventional carrier. Instead of complying, the two complained bitterly and loudly to all who would listen and many who did not, much like modern wargame designers, and finally obtained a compromise. Shinano would carry 47 of her own planes, and dedicate half her hangar space to servicing planes from other carriers. Incorporating the lessons of war, she had an open hangar with huge steel roll-down doors for stormy weather unlike the usual Japanese closed box.

When Shinano floated out of her dock at Yokosuka Navy Yard on 11 November 1944, she was the largest aircraft carrier ever built by far, and would remain so until the American nuclear-powered carrier Enterprise was commissioned in the early 1960s. At almost 72,000 tons, she retained much of her battleship protection, though the armor belt had been reduced from 16 inches to “only” eight. In addition to the armored hangar deck, she also had an armored flight deck three inches thick and coated with a non-flammable mixture of latex and sawdust. Wood fittings were minimized to reduce risk of fire, and her aviation fuel stores were located deep within the ship, well below the waterline. She carried her ammunition stores within the heavily-armored magazines installed for the 18-inch rifles she would have mounted as a battleship.

An armored bulkhead split the hangar deck in two; the front section would carry Shinano’s own air group, while the aft hangar would service other carriers’ planes. Without the ridiculous service mission, Shinano could have carried about 70 planes — fewer than Zuikaku and Shokaku, Japan’s alrgest pre-war carriers. In keeping with Japanese late-war practice, Shinano had a massive ant-aircraft battery with over 100 light guns, eight 5-inch dual-purpose weapons and several rocket launchers (whether all of these were fitted is not clear).

After some trials in Tokyo Bay, Shinano and four destroyers headed out for Matsumaya in the Inland Sea on 28 November, where it was hoped she would not be found by American carrier planes while she completed fitting out. At 2048 she was spotted by the American submarine Archerfish, which began stalking the unidentified big ship.

USS Archerfish sank the giant carrier.

Archerfish came in from the stern, usually a losing proposition for the submarine, but the carrier group gave up its speed advantage by zig-zagging sharply. By 0317 Archerfish had a firing solution at the optimum range of 1,400 yards, and fired all six of her bow torpedo tubes. Four torpedoes struck Shinano near the stern, and she began to settle in the water.

The big carrier remained afloat for almost exactly another seven hours before slipping under the waves and exploding. For some reason, the fitting of watertight doors at been left for Matsumaya, and the crew had not yet formed a damage control organization. Flooding proceded steadily with little interference from the crew.

In Leyte Gulf we’ve included two versions of Shinano, both battleship and carrier. As a battleship, she’s armed slightly differently than her two sisters, as the Japanese shortage of medium-caliber guns caused alterations in her planned secondary armament. Shinano is presented as complete (with watertight doors and a damage control crew, though the Japanese were never as good at this task as the Americans). Thus, while she is not the most capable Japanese aircraft carrier she is extraordinarily tough to sink, but like any carrier she can be put out of action fairly easily.

This piece originally appeared in July 2005.