Were The Japanese Ocean Liners Converted To Aircraft Carriers A Waste Of Resources?

Just a quick reply, actually imo they would have slowed Kurita as they were something like 2-3kt slower than Nagato. If one is to give Kurita some air cover then perhaps better to fill Junyo and maybe even Ryuho (if we talk about OTL) with as many fighters and ASW aircraft as possible, and going further fill Ise/Hyuga with as many floatplane fighters as possible and send them with Kurita, not Ozawa (again, they only slowed him at 25kt). The carriers will be prime targets and will be sunk, but that might save Shinano, so the japanese might have THREE extra battleships available off Samar!

Problem is though, the aircraft intended for Junyo/Ryuho were sent and lost in Taiwan before the Leyte battle. Though i'm sure they could scrap together two or three dozens of Zeros and a dozen of floatplane fighters and a dozen B5N2s from somewhere if they would have thought that way, hell they could get 20-30 aircraft from Ozawa.

Kurita's force was already not that fast anymore, as Nagato was limmited to 21 knots at best at Samar, following bomb damage two days before. Normally she was good for some 24/25 knots under good conditions, just a bit less than a Yamato, which under the best warconditions was good for 27 knots.

More logical for aircover would have been land based fightercover, as the additional ships would make logistics more problematic for the already fuel starved IJN. By putting more aircraft in the Philippine regions, more scattered out on more airfields, would make more impact than a few slow aircraft carriers. Historically the Airfields were heavily bombed, but the Japanese had made things easy, by droning most of them on a few larger airbases, rather than scattering them all around.
 
Just a quick reply, actually imo they would have slowed Kurita as they were something like 2-3kt slower than Nagato. If one is to give Kurita some air cover then perhaps better to fill Junyo and maybe even Ryuho (if we talk about OTL) with as many fighters and ASW aircraft as possible, and going further fill Ise/Hyuga with as many floatplane fighters as possible and send them with Kurita, not Ozawa (again, they only slowed him at 25kt). The carriers will be prime targets and will be sunk, but that might save Shinano, so the japanese might have THREE extra battleships available off Samar!

Problem is though, the aircraft intended for Junyo/Ryuho were sent and lost in Taiwan before the Leyte battle. Though i'm sure they could scrap together two or three dozens of Zeros and a dozen of floatplane fighters and a dozen B5N2s from somewhere if they would have thought that way, hell they could get 20-30 aircraft from Ozawa.

I saw that when I looked them up in my copy of Conways. However, I think it also strengthens my argument that the converted liners fell between two stools - too slow to work with their fleet, but too fast for second line roles.

I like the idea of using Junyo and Ryuho instead of the Taiyos. They had the same combined capacity, 81 aircraft. Is it true that the aircraft for Unryu and Amagi were also lost in Taiwan as well?
 
Come to think of this better, i think HMS Warspite has a good point, indeed Kurita was in range of japanese land based aircraft (for the battle at least) so it does make more sense to provide that cover from land bases (if available), on the other hand, probably ASW aircraft patrols would be more difficult to carry our from land, so a carrier devoted to such (say Ryuho) would still be useful.

Back to Junyo, and perhaps on a more OT note, they have sent Zuikaku, a full fledged formidable fleet carrier with the decoy force and to sure sinking, perhaps they should have sent Junyo and/or Ryuho instead, sparing Zuikaku. Why not use a lesser carrier if available for such one way mission? They could have kept it back to form the nucleus of a new carrier division together with the Unryus and Shinano (not that it would have happened, but that's beside the point)

Anyway as far as the airgroups for Unryu and Amagi are concerned i do seem to recall about them perhaps being indeed sent to Taiwan, but there's nothing on the CF website in the respective TROMs, and haven't found anything definite in Hata/Izawa so far. I'll check Shores/Hata too later.

PS: In the previous pots, i meant of course Musashi, not Shinano being spared sinking.
 
IOTL the Japanese naval program of 1939 included the Tahio (which wasn't laid down until 1941) and the subsidised liners which were completed as Hiyo and Junyo. With hindsight I think they should have used the money to buy 3 Shokakus to be completed PDQ. Apart from the financial and shipbuilding capacity constraints I think the reason why the did what they did in 1939 was fear of US retaliation. IOTL the Americans replied to the Japanese ordering Shokaku and Zuikaku in 1937 by authorising Hornet and Essex in 1938. Unfortunately for the Japanese the cautious building program of 1939 turned out to be an own goal because the Americans ordered another 10 aircraft carriers in their FY1940 program as part of their "Two Ocean Navy" programme anyway.

I think that if the Japanese had ordered 3 fleet carriers in 1939 instead of Tahio and the 2 liners the American retaliation would have been limited to laying Essex down in April 1940 as a repeat Yorktown instead of April 1941 as the first Essex for completion at the end of 1941 instead of the end of 1942.

However, in 1939 the Japanese couldn't to know that the Americans would begin a massive program of naval expansion in 1940. Furthermore an extra Yorktown in 1942 is worth 2 Essex in 1943 as has already been established by another thread. Therefore the Japanese might have done even worse in 1942 had Hiyo and Junyo been built as aircraft carriers from the start.

N.B. although CV9 Essex wasn't formally ordered until FY1940 (along with CVs 10-19) she and Hornet (ordered FY 1939) were authorised as part of the Naval Act of 1938.

The fear of retaliation may have been a reason for the "caution" perhaps, although me i'm not so sure about the americans' ability to actually know what the japanese were building. Contemporary US assessments of such are quite far of the mark, and in fact they were overestimating the japanese construction and still built the ships we know in OTL. I don't think it makes much difference if Junyo/Hiyo are ordered as flull fledged carriers, perhaps the americans would suspect something is being built but they wouldn't know for sure whether they're carriers or "battlecruisers" etc. Remember the Chichibu/Kadekuru story.

As for CV-9, if it's available in 1942 it will conceivably be counterbalanced by the two extra IJN fleet carriers (instead of the Junyos) with much better characteristics and larger airgroups, though of course it's very important WHEN in 1942 does CV-9 enter operations, if available for Midway then it will be of course bad news for IJN.
 
As for CV-9, if it's available in 1942 it will conceivably be counterbalanced by the two extra IJN fleet carriers (instead of the Junyos) with much better characteristics and larger airgroups, though of course it's very important WHEN in 1942 does CV-9 enter operations, if available for Midway then it will be of course bad news for IJN.

The real Essex was laid down in April 1941 and completed at the end of 1942. If laid down in April 1940 and completed at the end of 1941 as I propose then its available to take part in the Coral Sea. Then its 3 US carriers against 3 Japanese. Lexington and Yorktown survive unscathed while Shokaku and possibly Zuikaku would be sunk.

Then if Midway still happens the Americans have 5 aircraft carriers (Enterprise, Essex, Hornet, Lexington and Yorktown) plus Saratoga on its way from the West Coast. Meanwhile the Japanese still have the 4 aircraft carriers of OTL because Junyo is still up in the Aleutians.

OTOH they could put some B-25s on it and sent it off to bomb Tokyo with the Hornet. That means that Shokaku and Zuikaku still survive the Coral Sea, but the Americans still have one extra aircraft carrier at Midway instead of 2 so the Yorktown probably survives.
 
The real Essex was laid down in April 1941 and completed at the end of 1942. If laid down in April 1940 and completed at the end of 1941 as I propose then its available to take part in the Coral Sea. Then its 3 US carriers against 3 Japanese. Lexington and Yorktown survive unscathed while Shokaku and possibly Zuikaku would be sunk.

Then if Midway still happens the Americans have 5 aircraft carriers (Enterprise, Essex, Hornet, Lexington and Yorktown) plus Saratoga on its way from the West Coast. Meanwhile the Japanese still have the 4 aircraft carriers of OTL because Junyo is still up in the Aleutians.

OTOH they could put some B-25s on it and sent it off to bomb Tokyo with the Hornet. That means that Shokaku and Zuikaku still survive the Coral Sea, but the Americans still have one extra aircraft carrier at Midway instead of 2 so the Yorktown probably survives.


Carriers completed is not the same as carriers ready for war. All warships, especially the more complex ones with aircraft, need time to train and work up as a single unit. So when a carrier is completed in end 1941, it will be ready as an operational unit a year later under normal conditions. So exclude both Coral Sea and Midway for this unit, as these battles will come too early, unless you are prepared to accept its loss, due to tis incompetence as a fighting unit.
 
Carriers completed is not the same as carriers ready for war. All warships, especially the more complex ones with aircraft, need time to train and work up as a single unit. So when a carrier is completed in end 1941, it will be ready as an operational unit a year later under normal conditions. So exclude both Coral Sea and Midway for this unit, as these battles will come too early, unless you are prepared to accept its loss, due to tis incompetence as a fighting unit.

I disagree.

Hornet was completed at the end of October 1941 and performed very competently in the Doolittle Raid in the middle of April 1942 and at Midway 6 weeks later. Junyo wasn't completed until the beginning of May 1942 and still took part in the Midway campaign. She and the Hiyo which was completed in June fought in the Solomons Campaign in the second half of 1942. According to its Wikipaedia entry the real Essex was operational by May 1943 which suggests it would have been ready for the Coral Sea and Midway if it had been built exactly one year earlier as a Yorktown class ship.

However, for the sake of argument I will bring the laying down date forward to September 1939 and the completion date to the end of October 1941 so she has 2 extra months to work up. Those were the building dates for Hornet ordered in FY1939 IOTL and are realistic for Essex if she was also ordered as a Yorktown class carrier in FY1939 too.
 
I disagree.

Hornet was completed at the end of October 1941 and performed very competently in the Doolittle Raid in the middle of April 1942 and at Midway 6 weeks later. Junyo wasn't completed until the beginning of May 1942 and still took part in the Midway campaign. She and the Hiyo which was completed in June fought in the Solomons Campaign in the second half of 1942. According to its Wikipaedia entry the real Essex was operational by May 1943 which suggests it would have been ready for the Coral Sea and Midway if it had been built exactly one year earlier as a Yorktown class ship.

However, for the sake of argument I will bring the laying down date forward to September 1939 and the completion date to the end of October 1941 so she has 2 extra months to work up. Those were the building dates for Hornet ordered in FY1939 IOTL and are realistic for Essex if she was also ordered as a Yorktown class carrier in FY1939 too.

I partly agree, as long as you mention USS Hornet was only used as a flying off platform and nothing more. Her own airgroup was still not fully combat ready at teh time, which forced Nimitz to allocate USS Enterprise in the taskforce as well, just to have one carrier in a combat ready status for escort duties. Even at Midway, Hornet's airgroup showed signs of not beeing fully trained. Simmilarly Junyo's participation in the Aleut Campaign was only marginal, with only half an airgroup available, which also showed signs of not being fully ready, given her marginal results in the airstrike on Dutch Harbor.

The main problem is you can send a not fully trained aircraft carrier to the frontline, if you want, but it always will be a risk to do so.
 
The real Essex was laid down in April 1941 and completed at the end of 1942. If laid down in April 1940 and completed at the end of 1941 as I propose then its available to take part in the Coral Sea. Then its 3 US carriers against 3 Japanese. Lexington and Yorktown survive unscathed while Shokaku and possibly Zuikaku would be sunk.

Then if Midway still happens the Americans have 5 aircraft carriers (Enterprise, Essex, Hornet, Lexington and Yorktown) plus Saratoga on its way from the West Coast. Meanwhile the Japanese still have the 4 aircraft carriers of OTL because Junyo is still up in the Aleutians.

OTOH they could put some B-25s on it and sent it off to bomb Tokyo with the Hornet. That means that Shokaku and Zuikaku still survive the Coral Sea, but the Americans still have one extra aircraft carrier at Midway instead of 2 so the Yorktown probably survives.

If CV-9 is laid in 1940, imo it's not a given that it will be finished by end 1941, mainly because the war greatly accelerated OTL Essex construction. So perhaps the ATL CV-9 would take about as much to build as Hornet. Could indeed possibly be just in time for Midway with a cobbled togther and green air group. Though to my mind i imagine CV-9 taking the place of Wasp in the Pacific in time for Guadalcanal, with the Wasp remaining in the Atlantic for a bit longer, but probably it too being sent to the Pacific after the likely Guadalcanal losses.

I might have asked this before (and sorry if i don't remember the reply), but as even the US has a finite number of slips, instead of what other ship the ATL CV-9 is laid in April 1940? What was built before the OTL Essex on that slip?

Anyway, we digress considerably, i'm sure all of this can be better discussed in an appropriate topic about an earlier CV-9. Back to the issue of the possible US reaction to Junyo and Hiyo being ordered as Shokakus from the start, i made the case that the very poor US intelligence on japanese shipbuilding of the time could preclude that. I found some interesting details of what the US knew about IJN in those years before WW2:
http://www.hpu.edu/CHSS/History/Gra...n_Martin_COMPLETED_MADMS_THESIS_JULY_2009.pdf
http://www.ibiblio.org/hyperwar/USA/ref/FM/PDFs/FM30-58.PDF

Also, fairly recently i read somewhere the message from i think 1939 of someone from the US embassy in Japan to the US government about or including details about IJN shipbuilding plans (or what they believed those plans are), as such things go, i stumbled upon it looking for something else, and now i can't find the bloody thing! If anyone knows what i'm talking about, would appreciate a link, thanks.
 
If CV-9 is laid in 1940, imo it's not a given that it will be finished by end 1941, mainly because the war greatly accelerated OTL Essex construction. So perhaps the ATL CV-9 would take about as much to build as Hornet. Could indeed possibly be just in time for Midway with a cobbled togther and green air group. Though to my mind i imagine CV-9 taking the place of Wasp in the Pacific in time for Guadalcanal, with the Wasp remaining in the Atlantic for a bit longer, but probably it too being sent to the Pacific after the likely Guadalcanal losses.

I might have asked this before (and sorry if i don't remember the reply), but as even the US has a finite number of slips, instead of what other ship the ATL CV-9 is laid in April 1940? What was built before the OTL Essex on that slip?

Anyway, we digress considerably, i'm sure all of this can be better discussed in an appropriate topic about an earlier CV-9. Back to the issue of the possible US reaction to Junyo and Hiyo being ordered as Shokakus from the start, i made the case that the very poor US intelligence on japanese shipbuilding of the time could preclude that. I found some interesting details of what the US knew about IJN in those years before WW2:
http://www.hpu.edu/CHSS/History/Gra...n_Martin_COMPLETED_MADMS_THESIS_JULY_2009.pdf
http://www.ibiblio.org/hyperwar/USA/ref/FM/PDFs/FM30-58.PDF

Also, fairly recently i read somewhere the message from i think 1939 of someone from the US embassy in Japan to the US government about or including details about IJN shipbuilding plans (or what they believed those plans are), as such things go, i stumbled upon it looking for something else, and now i can't find the bloody thing! If anyone knows what i'm talking about, would appreciate a link, thanks.

I only had a skim through those documents, but very interesting.

The December 1941 estimate on Japanese naval strength gives them five 40,000 tons battleships (3 complete and 2 under construction) when they actually had 4 Yamatos under construction. I wonder if they had mistaken the fifth 40,000 ton battleships and 3 pocket battleships for the Yamato, 2 Super Yamatos and 2 battle cruisers planned in the 1942 Programme.

The Americans might have arrived at a total of 3 Zuikaku class (the document lists them as a separate class from Shokaku) by mistaking Takasago for Tahio ordered in 1939 and the unnamed vessel for Unryu ordered in 1941. I don't know where they could have got the third Soryu (Koryu) from. They've also underestimated the displacements for all the carriers. The Zuikakus are 20,000 tons and Shokaku 15,000 tons, Soryus 10,050 tons, the converted capital ships 26,900 tons and the Ryujo as 7,600. The displacements for the Soryus and the converted capital ships are those quoted in Jane's Fighting Ships 1939 and in the case of the capital ship conversions it's the displacement before they were rebuilt, but the illustrations are of these ships after they were rebuilt.

They seem to have got the numbers of cruisers and destroyers about right. I didn't check their characteristics.
 
Regarding ship identification, at Coral Sea for instance they identified (both through intelligence and through spotting) Shoho as the "Ryukaku", a fleet carrier able to carry 84 aircraft. Indeed one argument i think was made and can be made that many of the conversions finished or under construction were misidentified and their characteristics either badly inflated or deflated (f.e. that "Takasago" might be "Hayataka" i.e. Junyo, that name resulting perhaps from wrongly reading the kana). Still no idea how they came to believe Japan was building BCs, maybe they badly misidentified the Aganos? You mentioned about 1939 Jane's, do you have it available, and do they list there any of those fictious CVs, BBs, "pocket-battlehips" etc.?

So all in all, like i said earlier, imo if they lay two Shokakus instead of Junyos in 1939, the US would hardly be better informed than they were OTL.

Getting back a bit to CV-9, any idea why it hasn't been laid down about the same time as Hornet (1939) and to same design, was it lack of available slips, or something else? Afterall, CV-9 was ordered in 1938 but only laid in April 1941 (which is remarcably late, considering that many take potshots at the ineficiency of japanese planning and yards, well it kinda looks the same here!).
 
It is my reading that the Japanese passenger liner program was part of their 'shadow navy' scheme to obey -at least on paper- the rules laid down in the Washington and London treaties while keeping a reserve of 'civil' ships that could be converted to Navy aircraft carriers in a short time... at least in half the time it would take to build a new carrier from scratch.

The design of the -conversion kit carriers- of course had to make some concessions so the craft would never be more then second-rate ships. However they would mostly be used against second-rate enemies anyhow and they would free up the main battle carriers from auxiliary support duties. Mostly however they would be a propaganda tool, basically doubling Japans number of carriers - at least on paper- and giving it a stronger hand at the negotiating table.

Of course the main flow in this scheme was that after Pearl Harbor none of the Western powers would go for anything less then the total surrender of Japan, so there would be no negotiations and therefore no propaganda coup. But otherwise then that the 'shadow navy' program made perfect sense in a 1930's world.
 
Regarding ship identification, at Coral Sea for instance they identified (both through intelligence and through spotting) Shoho as the "Ryukaku", a fleet carrier able to carry 84 aircraft. Indeed one argument i think was made and can be made that many of the conversions finished or under construction were misidentified and their characteristics either badly inflated or deflated (f.e. that "Takasago" might be "Hayataka" i.e. Junyo, that name resulting perhaps from wrongly reading the kana). Still no idea how they came to believe Japan was building BCs, maybe they badly misidentified the Aganos? You mentioned about 1939 Jane's, do you have it available, and do they list there any of those fictious CVs, BBs, "pocket-battlehips" etc.?

So all in all, like i said earlier, imo if they lay two Shokakus instead of Junyos in 1939, the US would hardly be better informed than they were OTL.

I have a very fragile ex-Library copy. The only aircraft carriers it lists are the Hosho, Kaga, Akagi, Ryujo, Hryu, Soryu and Shokaku, which is listed as a member of the Soryu class. Zuikaku and the phantom ship are not mentioned. However, it does list the 1934, 1937 and 1939 Japanese Naval Laws and says that one or 2 carriers were to be built under the 1937 Law. One was Shokaku and the other was Zuikaku.

Jane's 1939 also says that the Japanese were building 4 battleships estimated to displace 40,000 tons, armed with eight or nine 16" guns with a speed of around 30 knots. However, here is no mention of the phantom battle cruisers.

I'm not convinced that another pair of Zuikakus could have been laid down instead of Hiyo and Junyo without the Americans noticing. That is because the Japanese Parliament would have to pass a law allowing them to be built. In 1937 the Diet passed a law that allowed the building of Yamato, Musashi, Shokaku and Zuikaku. In reply the US Congress passed the Navy Act of 1938 which among other things allowed the USN 175,000 tons of aircraft carriers instead of 135,000 tons. It used the extra 40,000 tons to build Yorktown and Essex.

If the Japanese Diet had authorised 4 aircraft carriers in 1937 instead of 2 it is very likely that Congress would reply by authorising 80,000 tons of aircraft carriers in 1938. My guess is that they would use it to order a 4th Yorktown to be completed in 1941 and a second Essex to be completed in 1943.
 
Getting back a bit to CV-9, any idea why it hasn't been laid down about the same time as Hornet (1939) and to same design, was it lack of available slips, or something else? Afterall, CV-9 was ordered in 1938 but only laid in April 1941 (which is remarcably late, considering that many take potshots at the ineficiency of japanese planning and yards, well it kinda looks the same here!).

AFAIK the USA had the shipbuilding resources to lay down 2 aircraft carriers in the second half of 1939 and complete them in the second half of 1941. In fact I'll stick my neck out and say that they could probably have built 4 in that period if Congress would pay for them. They could probably have built the aircraft and trained the aircrew for their air groups by the end of 1941 too.

AFAIK the USN wanted to build CV8 and CV9 to a new design, but it wasn't ready. Therefore CV8 was built as a repeat of Yorktown as an emergency measure while the second ship would not be built until the new design was ready. That turned out to be 1940 when 11 Essex class (CV9-19) were ordered as part of the Two Ocean Navy programme.
 
I'm not convinced that another pair of Zuikakus could have been laid down instead of Hiyo and Junyo without the Americans noticing. That is because the Japanese Parliament would have to pass a law allowing them to be built. In 1937 the Diet passed a law that allowed the building of Yamato, Musashi, Shokaku and Zuikaku. In reply the US Congress passed the Navy Act of 1938 which among other things allowed the USN 175,000 tons of aircraft carriers instead of 135,000 tons. It used the extra 40,000 tons to build Yorktown and Essex.

If the Japanese Diet had authorised 4 aircraft carriers in 1937 instead of 2 it is very likely that Congress would reply by authorising 80,000 tons of aircraft carriers in 1938. My guess is that they would use it to order a 4th Yorktown to be completed in 1941 and a second Essex to be completed in 1943.

That's an interesting argument for the Junyo and Hiyo - they could be slipped in under the radar. Two converted carriers vs. zero US carriers in response is better than 2 for 2.
 
Altogether the Japanese built 10 liners that were designed to be converted to aircraft carriers in an emergency.

According to Guy Robbins in The Aircraft Carrier Story 1908-45, the 3 Asama Maru class liners of 16,675 G.R.T. and 20.7 knots were subsidised by the Ministry of Transport. They were commissioned 1929-30. The Navy wanted to convert them to auxiliary aircraft carriers (XCV), but they were sunk before the shipyard capacity to do it became available. The 3 ships were sunk in the following order: Tatsuta Maru, 09/02/1943; Chichibu Maru 28/04/1943; and Asama Maru 01/11/1944.

Once more according to Robbins, when the USA began the construction of large merchant ships, Japan responded with a law to subsidise first class merchant ships. In its first phase twelve liners were built five of which were designed for conversion to aircraft carriers. These were the Nitta Maru and Kashiwara Maru classes.

3 Nitta Maru class of 17,830 G.R.T. and 22.2 knots were subsidised for the Japanese Mail Steamship Company's (NYK) European service in 1931 (but another sources I checked that against say that NYK decided to build them in 1937, which is more plausible as Nitta Maru and Yawata Maru were laid down in 1938). Kasuga Maru was laid down on 06/01/1940, launched on 19/09/1940, taken over by the IJN in May 1941 when she was still fitting out and completed on 15/09/41 as the XCV Taiyo. Nitta Maru (completed as a liner on 23/03/1940) and Yawata Maru (completed as a liner on 31/07/1940) was requisitioned by the Navy in February 1941 and in October 1941 respectively. Yawatta Maru was immediately converted into the XCV Unyo (25/11/1941 to 31/05/1942) and Nitta Maru served as a troopship before being converted into the XCV Chuyo (20/08/1942 to 25/11/1942). Chuyo the last conversion had the shortest career as an aircraft carrier because she was sunk on 04/12/1943, Unyo was sunk on 16/09/1944 and Taiyo on 18/08/1944.

2 Argentina Maru of 12,755 G.R.T. and 21.5 knots were built of the Osaka Ship Company's (OSK) South America service in 1932 (but Wikipaedia says they were ordered in 1938, which is more plausible because the Combined Fleet website says both ships were completed in 1939 and Argentina Maru was launched on 31/05/1939). Both were requisitioned by the IJN in May 1941 and used as troop ships, for example they were both employed as transports during the Battle of Midway. Argentina Maru was converted to an XCV between December 1942 and 23/11/1943. She survived the war and was broken up in 1948. Her sister ship Brazil Maru was sunk on 05/08/1942 before she could be converted, according to the Combined Fleet website she was on her way to Yokosuka for her conversion.

The bonus ship was the German liner Scharnhorst of 18,184 G.R.T. and 22.0 knots, which was converted into the XCV Shinyo. She was purchased in February 1942 and her conversion was completed on 15/12/1943. Her career as an aircraft carrier lasted less than a year because she was sunk on 14/11/1944.

According to Wikipaedia the 2 Kashiwara Maru class liners of 27,500 tons and 25.5 knots were ordered in late 1938. They were laid down in 1939. They were officially purchased in February 1941, but unofficially in October 1940. Both ships were launched in June 1941 and completed in the middle of 1942. Although they carried double the number of aircraft as the other XCV and were fast enough to operate with the fleet their hangars were only 16 feet high. On the other hand the Ark Royal and IIRC the first 4 armoured carriers had hangars that were 16 feet high and the hangars on the Implacable class were only 14 feet high. Hiyo was sunk on 20/06/1944 during the Battle of the Philippine Sea and Junyo was damaged. According to Wikipaedia, after repairs at Kure, Junyo remained in the Inland Sea without aircraft until 27 October when she was tasked to transport material to Borneo. (The Battle of Leyte Gulf was from 23rd to 26th October so she probably didn't take part because she was still under repair and/or lack of aircraft). She survived two unsuccessful attacks by US submarines before being torpedoed on 09/12/1944 and repairs were abandoned in May 1945 for lack of materials, but survived the war and was scrapped in 1946.

What I find interesting about the above...

is that the 7 ships of the Nitta Maru, Argentina Maru and Kashiwara Maru classes were all ordered after 31st December 1936. That is after the aircraft carrier tonnage quotas imposed by the Washington Treaty had been abolished. Therefore the Japanese could have built 7 proper aircraft carriers instead of them (say 2 Shokakus and 5 Unryu class) but it's likely that the Americans would have built more aircraft carriers in retaliation. They might have brought the whole Two Ocean Navy programme forward 2 years.

Based on what has been discussed so far I now think that the IJN should have given an even bigger subsidy to the shipping lines that ordered the Nitta Maru and Argentina Maru classes to order faster ships that could work with the fleet like the Kashiwara Maru class. Preferably they would have been additional ships of that class, but the slipways probably weren't big enough. Furthermore the IJN should also have converted them to aircraft carriers sooner so that as many as possible would be in service by the end of 1941 to deliver the heaviest possible knock out blow.

Therefore Argentina Maru, Brazil Maru and Nitta Maru should have been converted to an XCV immediately after they were requisitioned in the first half of 1941. In the case of Nitta Maru that might bring her commissioning as the Chuyo forward from the end of November 1942 to the end of May 1941. Although Argentina Maru's conversion IOTL took nearly a year I think she and Brazil Maru could have been converted in 3-4 months like Nitta Maru and Yawata Maru IOTL which would mean they would be ready by the end of 1941 too. The earlier conversions of Nitta Maru and Yawatta Maru would have allowed 2 more auxiliaries or 2 of the older fast liners to be converted in 1942.

So instead of 10 aircraft carriers at the end of 1941 (6 fleet carriers, Ryujo, Zuiho, Hosho and one slow XCV) they would have 14, that is the 6 fleet carriers, Ryujo, Zuiho and Hosho - plus 5 XCV type aircraft carriers operating 135 aircraft, which, were fast enough to operate with the fleet for the battles of 1942.
 
As a follow on to the last post Hiyo and Junyo as built displaced about 1,500 tons less than Shokaku and Zuikaku. They were about 100ft shorter but had 2 feet more beam. They were also considerably slower at 25.5 knots compared to 34.2 knots for the Shokakus because their machinery produced 56,250 shp on 2 shafts compared to 160,000 shp on 4 shafts.

If the Japanese Government had paid a bigger subsidy for the building and operation of these ships provided their maximum speed was at least 28.5 knots, how much bigger would the ships be?

I'm not a naval architect, but my guess is that they would need at least 50% more horse power, which would in turn require a larger hull and possibly 4 shafts. The longer and beamier hull would give the ships a longer flight deck and hangar which might also have more headroom.

Would similar increases in horsepower be needed to make the Kaiyo and Taiyo classes capable of 28.5 knots?
 
Hi NOMYSYRRUC,

Very interesting research about the japanese XCVs, thanks for that.

As to what you posted on the 14th...

I'm not convinced that another pair of Zuikakus could have been laid down instead of Hiyo and Junyo without the Americans noticing. That is because the Japanese Parliament would have to pass a law allowing them to be built. In 1937 the Diet passed a law that allowed the building of Yamato, Musashi, Shokaku and Zuikaku. In reply the US Congress passed the Navy Act of 1938 which among other things allowed the USN 175,000 tons of aircraft carriers instead of 135,000 tons. It used the extra 40,000 tons to build Yorktown and Essex.

If the Japanese Diet had authorised 4 aircraft carriers in 1937 instead of 2 it is very likely that Congress would reply by authorising 80,000 tons of aircraft carriers in 1938. My guess is that they would use it to order a 4th Yorktown to be completed in 1941 and a second Essex to be completed in 1943.
Indeed it is possible that they might not go unnoticed IF the japanese made enough shipbuilding details public, but it appears to me that the actual details of the shipbuilding plans were secret, the americans might have known that the Diet voted for this and that shipbuilding plan, but wouldn't know the exact particulars, i.e. numbers of ships of each type, let alone characteristics. See the example you posted, they apparently thought the japanese approved 4 BB and 1 or 2 CV in 1937 (am i reading that correctly?)

The japanese passed a suplementary shipbuilding program in 1938, it's possible they might approve the extra Shokakus then, perhaps masked as other types of vessels (like they did with the Yamatos, masked by fictious DDs and subs).

Not saying that it wouldn't be possible for americans to catch wind whatsoever, it is of course a possibility, but i'm just toying with possibility that they wouldn't.
 
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As a follow on to the last post Hiyo and Junyo as built displaced about 1,500 tons less than Shokaku and Zuikaku. They were about 100ft shorter but had 2 feet more beam. They were also considerably slower at 25.5 knots compared to 34.2 knots for the Shokakus because their machinery produced 56,250 shp on 2 shafts compared to 160,000 shp on 4 shafts.

If the Japanese Government had paid a bigger subsidy for the building and operation of these ships provided their maximum speed was at least 28.5 knots, how much bigger would the ships be?

I'm not a naval architect, but my guess is that they would need at least 50% more horse power, which would in turn require a larger hull and possibly 4 shafts. The longer and beamier hull would give the ships a longer flight deck and hangar which might also have more headroom.

Would similar increases in horsepower be needed to make the Kaiyo and Taiyo classes capable of 28.5 knots?

I did indeed toyed before with getting better Junyos. Like i mentioned another time, from what i can read the commercial machinery was very heavy for the power (four times heavier compared to Hiryu type machinery, again not sure if in abolute terms or just P/W ratio). I would think though it would be rather eye catching to build passenger ships with military turbines, on the other hand if they DID and somehow hide it, well, things would be getting quite interesting. I would love to hear from people in the know comparing military and civilian turbines of the day in fuel efficiency, P/W ratios and other aspects, were the civilian ones much more fuel efficient while being a lot heavier? Would putting military turbines in make these liners fuel hungry significantly shorter range hogs, which of course would ruin any sort of planned profitability (in hindsight of course, that would be an insignificant detail, they hardly gave any service as liners)?

IF what i can gather about these military and civilian turbines is fairly accurate, you can perhaps have the Taiyos (were they narrower and shorter compared to the Junyos i take it? need to check that) with destroyer type turbines (52,000 HP on two shafts) with a speed perhaps equal to that of Junyo (25 kts plus?), while you can have the Junyos with half-cruiser type machinery (76,000HP-80,000HP on two shafts) for maybe 27.5+ kts. Junyos are fairly good for aircraft capacity, but the Taiyos could use more (somehow).

Getting back to what you said, getting all these XCVs even more tailored for the CV role would help, but of course would be interesting to know the capacities of the slips they were built on and if they could have been made larger/faster.

Btw, one other thing to check is the initial machinery power for all the XCVs before the Junyos.
 
Finally in the OP you posted this among others NOMYSYRRUC
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.
May i ask which are the dry cargo ships you mention, would be interesting to see their characteristics. Thanks.

And one other thought in reply to the OP, converting Chitose, Chiyoda and Nisshin first instead of the Taiyos will of course give IJN 3 much more useful CVLs (about same aircraft capacity but could work with Kido Butai no problem) in the same timeframe. The XCVs can be bumped back to 1943.
 
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