Shinano Supercarriers for the IJN?

The Shinano was originally intended as the third Yamato class battleship, but was completed as an armored logistics ship following the loss of several Imperial Japanese Navy aircraft carriers throughout 1942. Although not intended for fleet carrier operations, the specifications are quite impressive. Reflecting its battleship origins, the Shinano had the heaviest armor ever carried by an aircraft carrier type ship, retaining a battleship style armored belt and adopting a British style armored flight deck with 75 mm (3 inches) of armor. The Shinano had a defensive air wing of 42 aircraft (with 5 reserve aircraft) and space for the carriage of 120 additional aircraft, could carry 720,000 liters (190,000 gallons) of aviation fuel. It was also massive, with a displacement on par with the Forrestal class and other early supercarriers of the United States Navy, and if post-war refits performed with other aircraft carriers are any indication, it may have become even larger after modernization with angled flight decks and other improvements.

I have two what ifs regarding the Shinano. First, what if all the Yamato class battleships had been built as Shinano class supercarriers from the beginning? Second, what kind of specifications might a post-war Shinano class modernization have had?

Delta Force said:

I have two what ifs regarding the Shinano. First, what if all the Yamato class battleships had been built as Shinano class supercarriers from the beginning? Second, what kind of specifications might a post-war Shinano class modernization have had?

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1. There's no particular reason for the Japanese to complete either the Yamato or Musashi as carriers. It would not fit into their overall plan for battling the United States.

2. Since the Shinano class would probably be expended in the Pacific atolls nuclear testing I doubt anyone would want to modernize radioactive hulks. Aside from that, I also doubt they would be seeing any service beyond 1950, if they had survived, with the growing size of modern attack aircraft.

Simply in terms of tonnage, two Yamatos yields

Simply in terms of tonnage, two Yamatos yields four Shokakus (and then some); using the completion/commissioning dates of Yamato and Musashi, that gives the IJN two more designed for the purpose fast carriers in the summer of 1942 and four more (in total) by the end of the year; essentially, replacing all four fleet carriers lost at Midway.

Did they have the aircraft and aircrew to effectively fill four more ~80 aircraft air groups? Actually, probably yes if they used some or all of the cadre that went to the less useful light fleet carrier conversions that were completed in 1942 (Shoho, Junyo, Hiyo, and Ryuho), plus some of the land-based IJNAF units equipped with carrier types...

Does having an extra Shokaku at Midway and/or four more for the Solomons make a difference?

Yeah, probably...to the extent the USN may actually hold off on any Pacific counteroffensive until the Essex and Independence class fast carriers start coming out by the division worth...

All sorts of ripples...

Best,

My understanding that the Shinano was not a fleet carrier in the usual sense but sort of a massive floating naval aircraft maintenance depot.

I agree with David that it is very unlikely the Japanese would build the first two Yamatos as carriers, but if they did, would this change their likely operational use from being depot ships? Three 70,000 massively armored carriers as the core of the Japanese carrier force by early 1944 would be interesting

David S Poepoe said:

1. There's no particular reason for the Japanese to complete either the Yamato or Musashi as carriers. It would not fit into their overall plan for battling the United States.

2. Since the Shinano class would probably be expended in the Pacific atolls nuclear testing I doubt anyone would want to modernize radioactive hulks. Aside from that, I also doubt they would be seeing any service beyond 1950, if they had survived, with the growing size of modern attack aircraft.

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I should have specified this is more of a technical what if, because there are the issues with having Japan's battleships completed as aircraft carriers, and with Japan avoiding World War II or the ships themselves surviving it.

I think there might also have been significant interest in the Yamato and/or Shinano class ships if any had survived the end of the war to be captured. The Iowa was designed to fight the Yamato, so they would want to see how they compare, and the Shinano would show what does and doesn't work on aircraft carriers that large. I wouldn't rule out a nuclear test, but I wouldn't rule out some post-war examination and perhaps even operational testing by the Commonwealth and/or United States, depending on how many ships are captured and under what circumstances.

David S Poepoe said:

1. There's no particular reason for the Japanese to complete either the Yamato or Musashi as carriers. It would not fit into their overall plan for battling the United States.

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More precisely, the Yamato and Musashi filled the niche role of super-battleship with giant heavy guns.

The Shinano though was as much of a (expensive) logistics ship as a viable carrier, so given the choice between the Shinano carrier or simply building more of the Shokaku class the latter carried a larger air wing, though with much less in the way of reserves (72 active and 12 in reserve compared to the 42 and 120 replacements of the Shinano). Particularly as during the early stages of the war the IJN still focused on the idea of the overwhelming strike that said a carrier battle will be won and lost within the first air attack. So the way to win was to go all in with as many aircraft and bombers into the air as possible as quick as possible to launch the overwhelming strike, an argument that favors cheaper unarmored fleet carriers.

Of course on the other hand, IJN doctrine was wrong and didn't exactly take into account what you were supposed to do if the first strike (completely) failed to take out the target, so that kind of bit them in the butt.

Delta Force said:

I think there might also have been significant interest in the Yamato and/or Shinano class ships if any had survived the end of the war to be captured. The Iowa was designed to fight the Yamato, so they would want to see how they compare, and the Shinano would show what does and doesn't work on aircraft carriers that large.

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I don't believe I have seen any credible source that states that Iowa class was designed to fight the Yamato.

Kome said:

The Shinano though was as much of a (expensive) logistics ship as a viable carrier, so given the choice between the Shinano carrier or simply building more of the Shokaku class the latter carried a larger air wing, though with much less in the way of reserves (72 active and 12 in reserve compared to the 42 and 120 replacements of the Shinano). Particularly as during the early stages of the war the IJN still focused on the idea of the overwhelming strike that said a carrier battle will be won and lost within the first air attack. So the way to win was to go all in with as many aircraft and bombers into the air as possible as quick as possible to launch the overwhelming strike, an argument that favors cheaper unarmored fleet carriers.

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I don't know how the reserve aircraft were carried on Shinano, but if the ship could operate as a large aircraft carrier, it would have a complement of around 144 active and 24 reserve aircraft, around twice a Shokaku. The major differences from an operational point of view would be that all those aircraft would be on a single ship, likely making flight operations interesting (it would be a massive number of aircraft for a land base, let alone an aircraft carrier). The ship would also be slower (27 knots for Shinano, 34 knots for Shokaku) but far more heavily armored.

David S Poepoe said:

I don't believe I have seen any credible source that states that Iowa class was designed to fight the Yamato.

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I might have mixed Iowa up with Montana. The Office of Naval Intelligence was never able to gather much in the way of specifics regarding the Yamato class, so there was much conjecture regarding their capabilities. Even now there is a lot of conjecture because the program was so secret and so many records were lost or destroyed after the war.

Delta Force said:

I don't know how the reserve aircraft were carried on Shinano, but if the ship could operate as a large aircraft carrier, it would have a complement of around 144 active and 24 reserve aircraft, around twice a Shokaku. The major differences from an operational point of view would be that all those aircraft would be on a single ship, likely making flight operations interesting (it would be a massive number of aircraft for a land base, let alone an aircraft carrier). The ship would also be slower (27 knots for Shinano, 34 knots for Shokaku) but far more heavily armored.

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That is something I failed to take into account. If the Shinano had been built from the ground up as an aircraft carrier rather than a battleship conversion, it might have been possible to increase its active aircraft capabilities to utilize its full potential.

In which case we would be looking at a Death Star-esque* monstrosity here. Limited only by Japan's capabilities to find enough pilots to staff the air wings of three(!) Shinano class carriers while still managing to maintain their elite piloting standards.

And if they could organize the ship well enough to prevent operations of taking off, refueling and landing from turning into a chaotic mess on the deck and in the hanger.

As for the OTL Shinano conversion though, the distinction between storage aircraft and active combat aircraft was pretty clear cut, so I don't know the layout of the carrier well enough to know if they could modify it to make full use of its aircraft.

*"The target area is a gap along the sides of the ship between the armor belt and the torpedo bulges. A precise hit by the torpedoes of a submarine will start a chain reaction that will sink the carrier"

Kome said:

As for the OTL Shinano conversion though, the distinction between storage aircraft and active combat aircraft was pretty clear cut, so I don't know the layout of the carrier well enough to know if they could modify it to make full use of its aircraft.

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That could be more a reflection of aircrew shortages than a physical inability or difficulty of operation. Of course, being designed from the keel up as a supercarrier would obviously allow the ships to be better designed for aircraft operations. That might allow even more aircraft to be carried, as well as innovative means of managing air operations.

Kome said:

*"The target area is a gap along the sides of the ship between the armor belt and the torpedo bulges. A precise hit by the torpedoes of a submarine will start a chain reaction that will sink the carrier"

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Which is almost exactly where she got hit on her first patrol.

Admiral Halsey said:

Which is almost exactly where she got hit on her first patrol.

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To further the irony, the reason why Shinano was so hastily ordered to leave Yokosuka and head to Kure for fitting out was because an attack was feared due to United States Navy reconnaissance aircraft being sighted in the area. Shinano was to receive firefighting and flood control systems at Kure, amongst other things.

Furthering the Star Wars analogies, Shinano pulled an Anakin, being sunk by a submarine while rushing to try to avoid being sunk by a submarine.

Japan could have completed two Shokakus for the cost of each completed Yamoto class battleship, but did it have enough shipyards to actually build them on a similar time table?

I want to learn said:

Japan could have completed two Shokakus for the cost of each completed Yamoto class battleship, but did it have enough shipyards to actually build them on a similar time table?

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A Shinano would have at least twice as much aviation capacity as a Shokaku, so there wouldn't really be a capacity loss.

Yes; the IJN had enough slipways to build at least four capital ships simultaneously

I want to learn said:

Japan could have completed two Shokakus for the cost of each completed Yamoto class battleship, but did it have enough shipyards to actually build them on a similar time table?

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Yes; the IJN had enough slipways to build at least four capital ships simultaneously - as witness, for example, the fact that Shokaku, Zuikaku, Yamato, and Musashi were all on the ways at the same time.

Operationally, designed for the purpose ships are always more efficient than conversions, and two hulls are better than one, in terms of spreading the risk of catastrophic losses...

Plus, even two Yamatos would cost more, in materials and shipyard time, than four Shokakus, so whatever is not used is an overall gain - more escorts or freighters or whatever.

Finally, although the US could afford both modern fast carriers and fast battleships, and the UK needed both because of the realities of their likely operating environments, the Japanese, French, and Italians really had to chose...none of the smaller naval powers had the resources to spare in the 1940s, unlike the US and UK.

Best,

Admiral Yamamoto was against the Yamato class, by the way. He was an advocate of carrier construction, though a Vice-Admiral at the time and assistant Navy Minister, he was overruled by the battleship lobby. Air-minded Admirals like Onishi and Ozawa were equally vociferous in their opposition.

*"The target area is a gap along the sides of the ship between the armor belt and the torpedo bulges. A precise hit by the torpedoes of a submarine will start a chain reaction that will sink the carrier"

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I think that the gap is also about the size of a wampus.

Shinano's first cruise wasn't a patrol. She was en route to Kure to complete fitting out and take on an air group-24 Zeroes and 24 B7A2 Grace attack aircraft.

Matt Wiser said:

Shinano's first cruise wasn't a patrol. She was en route to Kure to complete fitting out and take on an air group-24 Zeroes and 24 B7A2 Grace attack aircraft.

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Precisely. The Shinano and its crew hadn't achieved operational capability yet, so it would be unfair to judge the entire design on the basis of a worst case scenario under the worst circumstances imaginable (lack of damage control equipment and crew training).

Delta Force said:

Precisely. The Shinano and its crew hadn't achieved operational capability yet, so it would be unfair to judge the entire design on the basis of a worst case scenario under the worst circumstances imaginable (lack of damage control equipment and crew training).

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Though oddly, the other armored carrier (Taiho) deployed by the IJN was also sunk by a submarine torpedo. Which seems questionable, as a lot of resources spent on armor when they turned out to not be much more survivable in practice. Then again unarmored carriers Shokaku and Unryu were also sunk by torpedos, so it may just be coincidence.

Did the British armored carriers have better design protection against submarines? Or was it only the fact Allied ASW escort abilities were generations ahead of the IJN?