Before the war the Imperial Japanese Navy subsidised the construction of passenger liners that incorporated features that would enable them to be converted into aircraft carriers. Of the ten ships that were built six were actually converted to or completed as aircraft carriers along with the German Scharnhorst. However, only two of them were fast enough to work with the fleet and the others were used as aircraft transports and for training. The two ships of the Hiyo class were inferior to the Shokaku class fleet carriers of similar displacement and dimensions because they were slower, carried fewer aircraft and their hangars had less headroom.
I think these ships were more valuable to the Japanese as liners because they could be used as troop ships and dry cargo ships or tankers should have been converted in their place. The Americans and British did not convert any of their passenger liners to aircraft carriers for that very reason.
Therefore instead of the five "slow" ships (Taiyo, Unyo, Chuyo, Kaiyo and Shinyo (ex Schanhorst)) the Japanese convert five dry cargo ships as they can't spare the tankers. But for the Hiyo and Junyo I suggest something more ambitions, that is to build them as an additional pair of Shokaku class aircraft carriers. They are about the same displacement so there will be enough steel. They should be built in the same length of time as the real Hiyo and Junyo because they were repeat ships of an existing design. Hiyo was built by the same yard as Zuikaku. She was laid down 3 days after Zuikaku was launched, probably on the vacated slipway. They would still have the plans for Zuikaku and might still be "tooled up" to knock off another one at short notice. The stumbling blocks are manufacturing the more powerful machinery and time because the ships were taken over after they were laid down as liners so an earlier POD is required.
The Japanese also built naval auxiliaries that could be converted to aircraft carriers and five of them were so completed or converted. Two more were sunk before they could be converted. The five dry cargo ship conversions should consume less shipyard resources than the liner conversions because they would be simpler. That might release enough yard capacity to convert at least Mizuhio or Nisshin to light fleet carriers before they were sunk IOTL.
It is ironic that while the Japanese converted two bespoke seaplane carriers of the Chitose class to aircraft carriers while they were also converting the battleships Ise and Hyuga to seaplane carriers. The resources for the battleship conversions should have been used for something else. The Mizuhio and Nisshin had already been sunk, but they could have been used for more dry cargo ship conversions.
If another pair of Shokaku class fleet carriers can't be built instead of the Hiyo class my Plan B is to build 4 additional light fleet carriers based on the Soho and Chitose classes in their place. They were about half the displacement and carry half the aircraft, but were faster and their hangars had more headroom. Plan C is to build a pair of 25,000 ton fast tankers in their place. The high-speed allows them to make more trips and they are harder for submarines to sink. Plan D is to complete them as high-capacity troop ships for the invasion of Australia. Plan E is to complete them as aircraft carriers as OTL.
The only significant battles they took part in were Santa Cruz and the Philippine Sea. If Plans A and B were followed other ships carrying the same or more aircraft would be available. Therefore would they have been missed in Plans C and D?
I know this is like rearranging the deck chairs on the Titanic because the Japanese still loose the war. Like nearly all World War II what ifs it does not change history enough to make a significant different on its own.
I think these ships were more valuable to the Japanese as liners because they could be used as troop ships and dry cargo ships or tankers should have been converted in their place. The Americans and British did not convert any of their passenger liners to aircraft carriers for that very reason.
Therefore instead of the five "slow" ships (Taiyo, Unyo, Chuyo, Kaiyo and Shinyo (ex Schanhorst)) the Japanese convert five dry cargo ships as they can't spare the tankers. But for the Hiyo and Junyo I suggest something more ambitions, that is to build them as an additional pair of Shokaku class aircraft carriers. They are about the same displacement so there will be enough steel. They should be built in the same length of time as the real Hiyo and Junyo because they were repeat ships of an existing design. Hiyo was built by the same yard as Zuikaku. She was laid down 3 days after Zuikaku was launched, probably on the vacated slipway. They would still have the plans for Zuikaku and might still be "tooled up" to knock off another one at short notice. The stumbling blocks are manufacturing the more powerful machinery and time because the ships were taken over after they were laid down as liners so an earlier POD is required.
The Japanese also built naval auxiliaries that could be converted to aircraft carriers and five of them were so completed or converted. Two more were sunk before they could be converted. The five dry cargo ship conversions should consume less shipyard resources than the liner conversions because they would be simpler. That might release enough yard capacity to convert at least Mizuhio or Nisshin to light fleet carriers before they were sunk IOTL.
It is ironic that while the Japanese converted two bespoke seaplane carriers of the Chitose class to aircraft carriers while they were also converting the battleships Ise and Hyuga to seaplane carriers. The resources for the battleship conversions should have been used for something else. The Mizuhio and Nisshin had already been sunk, but they could have been used for more dry cargo ship conversions.
If another pair of Shokaku class fleet carriers can't be built instead of the Hiyo class my Plan B is to build 4 additional light fleet carriers based on the Soho and Chitose classes in their place. They were about half the displacement and carry half the aircraft, but were faster and their hangars had more headroom. Plan C is to build a pair of 25,000 ton fast tankers in their place. The high-speed allows them to make more trips and they are harder for submarines to sink. Plan D is to complete them as high-capacity troop ships for the invasion of Australia. Plan E is to complete them as aircraft carriers as OTL.
The only significant battles they took part in were Santa Cruz and the Philippine Sea. If Plans A and B were followed other ships carrying the same or more aircraft would be available. Therefore would they have been missed in Plans C and D?
I know this is like rearranging the deck chairs on the Titanic because the Japanese still loose the war. Like nearly all World War II what ifs it does not change history enough to make a significant different on its own.
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