The stunning loss at Midway of four carriers, two-thirds of the First Air Fleet, shocked Japanese naval authorities into a radical change in construction policy. The battle had not only proved positively that the carrier was the new prime weapon of sea warfare, it had also left Japan sadly bereft of that same crucial weapon. New carriers were authorised to join those already building, but it would be months or years before any of these appeared. Shipyards were already overcrowded with vessels needing repairs, and soon the pinch of material and labour shortages would make itself felt. One stopgap expedient that could offer aircraft platforms relatively quickly was the conversion of vessels already afloat or on the stocks, and such proposals were advanced even before the end of June 1942.
The desperation with which the IJN strove to rebuild its carrier force after the Midway disaster is demonstrated by how hastily the conversion programme was conceived. By the end of August 1942, the Naval Technical Department (Kaigun Kanseihonbu), vigorously prodded by the Naval Aviation Department (Kaigun Kokuhonbu), had drawn up plans for the conversion of the incomplete super-battleship Shinano, third unit of the Yamato class, into a flight-deck carrier. Other ships targeted for conversion included the incomplete cruiser Ibuki, two of the big seaplane tenders, seven merchant ships and three existing warships that would become hybrids: the cruiser Mogami and battleships Ise and Hyuga.
Hyuga and Ise as reconstructed during 1943, with aircraft deck and other aviation facilities aft, catapults amidships and retaining four 14in turrets.
The after superstructure and mainmast were retained. Parallel to them were two 85ft catapults, one on each beam, raised on tall pedestals to the level of the aircraft deck. The catapults overlapped no 4 turret and the muzzles of the no 3 turret's guns, thus severely restricting the training arcs of both.
Secondary armament was radically revised by removal of all sixteen 5.5in casemate guns (a number already reduced from the original twenty during reconstruction in the 1930s) and their replacement by eight more 5in AA guns in twin mounts, bringing the heavy AA battery up to a respectable sixteen barrels. Light AA armament was increased to fifty-seven 25mm guns in triple mounts.
Conversion of Ise began at Kure on 15 March 1943 and was completed on 8 October; Hyuga was converted at Sasebo between 1 August and 30 November 1943.
The redesign of the Ises involved the removal of considerable topweight; the two after turrets weighed 864 tons each, and their barbettes represented a reduction of an additional 800 tons or so. Japanese naval architects had become particularly skittish about stability as a result of prewar problems, which had culminated in the capsizing of the torpedo boat Tomodzuru in 1934; they were consequently worried that the loss of so much topweight on the Ises would increase the metacentric height to a point where rolling would become too rapid, an undesirable quality both for aircraft operations and good gunnery. Consequently, an 8in layer of concrete was added to the aircraft deck. This apparently had the desired effect, for the total displacement of the ships was reduced by only about 600 tons and draught by 6in.
Ise and Hyuga have often been referred to as 'seaplane carriers', but this was not the designers' intention. The twenty-two machines were originally to be Yokosuka D4Y Suisei (Comet) dive bomber aeroplanes (Allied code name Judy). The aircraft's design was based on a German Heinkel He 118V4 imported in 1938. It began life as a high-speed carrier-based reconnaissance aeroplane, as the IJN had finally come to the conclusion that its carriers needed a few aerial scouts of their own. It was developed, with several variants, into a dive bomber to replace the increasingly obsolescent Aichi D3A, the mainstay dive bomber since the start of the war.
Most references give the complements of the hybrid battleships as twelve Suiseis and ten Zuiuns, but Admiral Matsuda told American questioners during a postwar interrogation that there were eleven of each type per ship, half carried in the hangar and half on deck, with a mix of types in both places.
Design: The Shinano was laid down as a battleship of the Yamato class but converted, beginning in mid-1942, into an aircraft carrier. The original plan was to deploy the Shinano as a replenishment and support ship for carrier task forces but this was modified to include an operational air group of 40–50 aircraft in addition to large numbers of replenishment machines for other carriers. There was a single open hangar 550 feet long built over the existing battleship hull and supporting a 3.1-inch armored flight deck served by two elevators. A greatly enlarged iteration of the Taiho’s island structure was fitted.
Service: The Shinano was completed for trials on November 19, 1944, but never commissioned. While in transit from Yokosuka to Kure for final fitting out it was struck by four torpedoes fired by the submarine Archerfish on November 29, 1944. The watertight doors for its very extensive internal subdivision and much of the pump machinery were yet to be installed so it sank within seven hours due to uncontrolled flooding.
Hybrid: In addition to conversion of the incomplete battleship Shinano into a carrier, the IJN studied conversion schemes for all ten of the older battleships: the four ex-battlecruisers of the Kongo class, and the two-ship Fuso, Ise and Nagato classes. All had been completed between 1913 and 1921, and all had undergone at least one major reconstruction and several extensive refits. None except the Kongos had fired a shot in anger since the beginning of the war, and with the sudden new dominance of the carrier some officers doubted that they ever would.
The battleship conversions would have been radical: all superstructure, main batteries and casemated secondary guns would be replaced by full-length flight decks, island superstructures, offset funnels and a battery composed exclusively of AA guns. Aircraft capacity of each was estimated at about fifty-four.
The Kongos, with their 30.5 knot speed, would have been the best bet for such remodelling. They were the only battleship hulls that could keep pace with existing or future carriers. However, that speed made them valuable as the only big-gun carrier escorts, a role they had played since the Pearl Harbor operation, and it was probably that unique quality that eliminated them from consideration.
Speed, or the lack of it, was the Achilles' heel of the other battleships. Although their modernisations had raised their average speed to about 25 knots, this was still too slow for carrier operations. And in the case of the Nagatos, there was reluctance to deprive the fleet of their 16in guns, second only to the 18.1 in guns of the Yamatos in range and hitting power. There was still a lingering, if remote, chance that a battleline action could come about.
After considerable discussion, conversion of Shinano was authorised but the rest of the project foundered, principally on the factors of time and resources. It was estimated that full conversion of the older ships could take up to twenty-four months, and the navy needed carriers sooner than that. Moreover, the enormous scope of the work would divert labour and material from completion of carriers already building and postpone the laying down of others.