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

Page 302 of Jane's Fighting Ships 1939 edited by Raymond V. Blackman

Second Fleet Replenishment Law, 1933 (First Law completed at the end of 1937)

2 cruisers of 8,500 tons [Chikuma and Tone]
2 aircraft carriers of 10,000 tons [Hiryu and Soryu]
14 destroyers
16 torpedo boats [these were not MTBs, but small destroyers and only 8 were built]
4 submarines
6 submarine chasers
1 submarine depot ship [Soho and Zuiho]
1 repair ship [Akashi]
2 oilers
3 aircraft tenders [Chitose, Chiodya and Mizuho]

The Second Replenishment Law also includes the strengthening of the Fleet Air Arm by a further 8 squadrons between1934-37, giving a total of 39 squadrons in 1937. [By contrast RAF Expansion Scheme A of 1934 increased the FAA by 54 aircraft (the equivalent of 4½ squadrons) to the equivalent of 16½ by 1939.]

Third Fleet Replenishment Law, 1937

4 battleships, 1 or 2 aircraft carriers, and a number of cruisers, destroyers and submarines. To be completed by the end of 1942.

Fourth Fleet Replenishment Law, 1939

To include the expenditure of Yen 1,700,000,000 (of which Yen 1,200,000,000 will be on new construction) up to the end of 1945.

According to the Wikipaedia (and Conway's) the Second Fleet Replenishment Law was passed in 1934 and 48 ships were built instead of 51 in the Jane's article. The differences are a second submarine depot ship was built, 4 submarine chasers instead of 6 and no oilers.

In the battleships section it says that the 4 ships were laid down at Kure, Yokusaka, Kawasaki and Mitsubishi 1938-39. The aircraft carriers section does not mention Zuikaku at all. It listed Shokaku's launching date as 2nd June 1939, which to paraphrase Ken Bruce is, "one day out." In the cruisers section it says that 7 light cruisers of 7,000 tons were suspected to be under construction. 2 Katori class were actually built under the 1937 Law followed by 1 Katori, 4 Agano and 2 Oyodo class in the 1939 Law. In the destroyers section that 12 Kagero class were building when 15 were ordered under the 1937 Law. I didn't check the submarines.

So Jane's estimate of the battleships is an overestimate of 100%. Yamato was laid down at Kure in 1937, Musashi at Mitsubishi in 1938 and the battleship laid down at Kawasaki may have been mistaken for the Zuikaku laid down there in 1938. Where they go the Yokusaka ship from I don't know. They underestimated the number of aircraft carriers under construction by 50%, but were more accurate with the cruisers and destroyers.
 
Last edited:
Finally in the OP you posted this among others NOMYSYRRUC
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.

Ask away, but I can't give a very good answer because I hadn't thought about it in that detail. The best answer I can give is whatever the nearest Japanese ship to the American C-3 cargo ship was.

Converting as many of the auxiliaries as possible to aircraft carriers in 1941 (instead of building faster liners from 1937 and converting as many of those as possible by the end of 1941) is much simpler. As you wrote they were all more useful ships as their maximum speeds were about 3 knots faster than the fastest liner conversions (Hiyo and Junyo), it would be cheaper because the shipping lines don't have to be paid a bigger subsidy and it doesn't reduce the number of troop ships. They would actually have more if 2 more auxiliaries were converted in place of Taiyo and Unyo.

So I think the argument has swung back to my opening statement that apart from Hiyo and Junyo the liner conversions weren't worth the effort. As you wrote the Japanese should have converted all the auxiliaries first and then the smaller liners. That is unless they can build another 5 large liners in place of the 5 smaller ships.
 
So I think the argument has swung back to my opening statement that apart from Hiyo and Junyo the liner conversions weren't worth the effort. As you wrote the Japanese should have converted all the auxiliaries first and then the smaller liners. That is unless they can build another 5 large liners in place of the 5 smaller ships.

The Japanese have to get as many aircraft to the fight as the Americans can with the Essex Class. So instead of fewer but faster Junyos (which pushes them up the expense curve and makes them less appealing), what about more but slower escort carriers intended for frontline combat (18kt). Yes, 18kt is not as good as 28kt, but bringing 25 more aircraft to the party each time is better than having 28kt.
 
Auxiliaries Built Under The Shadow Programme

Ryuho ordered as the submarine depot ship Tagei in the 1931 Fleet Replenishment Law and built at Yokusaka.
Laid Down 12/04/1933 Launched 16/11/1933 Completed 01/03/1934. Converted to an aircraft carrier from 12/12/1941 to 28/11/1942 and renamed Ryuho.

To my knowledge her only action was the Philippine Sea and spent most of her career as an aircraft carrier as a transport and for flying training. She survived the war.

Soho ordered as the submarine depot ship Tsurugisaki in the 1934 Fleet Replenishment Law and built at Yokusaka.

Laid Down 03/12/1934 Launched 01/06/1935 Completed 15/01/1939. Converted to an aircraft carrier from 01/01/1941 to 26/01/1942 and renamed Soho.

She was sunk during the Battle of the Coral Sea on 07/05/1942.

Zuiho ordered as the submarine depot ship Takasaki in the 1934 Fleet Replenishment Law and built at Yokusaka.

Laid Down 20/06/1935 Launched 19/06/1936 Completed 17/10/1940 as the aircraft carrier Zuiho.

She was sunk during the Battle of Leyte Gulf on 25/10/1944.

Mizuhio ordered as a seaplane tender (Type B) in the 1934 Fleet Replenishment Law and built at Kawasaki.

Laid Down 01/05/1937 Launched 06/05/1938 Completed 25/02/1939.

She was sunk on 02/05/1942 before she could be converted into an aircraft carrier.

Chitose ordered as a seaplane tender (Type A) in the 1934 Fleet Replenishment Law and built at Kure.

Laid Down 26/11/1934 Launched 29/11/1936 Completed 25/07/1938. Converted to an aircraft carrier from 26/01/1943 to 01/01/1944.

She was sunk during the Battle of Leyte Gulf on 25/10/1944.

Chiyoda ordered as a seaplane tender (Type A) in the 1934 Fleet Replenishment Law and built at Kure.

Laid Down 14/12/1936 Launched 19/11/1937 Completed 15/12/1938. Converted to an aircraft carrier 08/01/1943 to 31/10/1943.

She was sunk during the Battle of Leyte Gulf on 25/10/1944.

Nisshin ordered as a seaplane tender (Type A) in the 1937 Fleet Replenishment Law and built at Kure

Laid Down 02/11/1938 Launched 30/11/1939 Completed 27/02/1942.

She was sunk on 22/07/1943 before she could be converted into an aircraft carrier.

Summary

7 built of which
1 completed as CV
4 converted to CV
2 sunk before they could be converted.
 
Last edited:
Finally in the OP you posted this among others NOMYSYRRUC
May I ask which are the dry cargo ships you mention, would be interesting to see their characteristics. Thanks.
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.
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.
Ask away, but I can't give a very good answer because I hadn't thought about it in that detail. The best answer I can give is whatever the nearest Japanese ship to the American C-3 cargo ship was.
Now I have thought about it some more they would be based type 1TL and 2TL tankers, that is the tanker conversions that the Imperial Japanese Army (yes the Army, not the Navy) planned in 1944.

The specifications of the 4 type 1TL conversions was between the American Bougue and Sangamon classes of escort carrier, which are as follows and come from Conway's.

Standard displacement:

11,800 tons - Type 1TL
9,393 tons - Bogue
10,494 tons - Sangamon

Dimensions:

502ft wl, 526ft oa x 65ft 7in x 29 ft - Type 1TL

465ft wl, 495ft 8in oa x 69ft 6in wl, 111ft 6in max x 23ft 3in full load - Bougue

525ft wl, 553ft oa x 75 ft wl, 105ft 2in max x 30 ft 7in full load - Sangamon

Machinery:

1-shaft steam turbines, 2 boilers, 8,600 shp = 18.0 knots - Type 1TL.
1-shaft steam turbines, 2 boilers, 8,500 shp = 16.5 knots - Bouge
2-shaft steam turbines, 4 boilers, 13,500 shp = 18.0 knots - Sangamon.

Flight deck:

508ft x 75ft (one lift) for the Type 1TL,
440ft x 82ft (2 lifts) Bougue
495ft x 75ft 6in (2 lifts) for the Sangamon

Aircraft:

12 Type 1TL
28 Bougue class
31 Sangamon

The Sangamons and their successors the Commencent Bay class retained their original tanker configurations. In the case of the Sangamon they could carry 12,876 tons of oil as cargo (which could be used as fuel); in 1945 they were rated for 4,780 tons of fuel, plus ballast, for 23,900 nautical miles at 15 knots. The 3 books I checked didn't say that the Japanese Type 1TL conversions didn't retain their original tanker configurations, but it didn't say that they didn't either. In the latter case 4 Type 1TL conversions in 1941 wouldn't reduce the capacity of the Japanese tanker fleet. The Conway's article on these ships also says that the Japanese army planned a Type 3TL conversion, but I didn't find it in the two other sources that I checked.

The Imperial Japanese Army also planned to convert a pair of Type 2TL tankers of 10,100 G.R.T, with a single-shaft geared turbine and 2 boilers producing 4,500 shp for 15 knots. They were to be fitted with a 410 x 75 flight deck upon which the 8 aircraft would be stowed because there was no hangar and no lift. These ships are analogues of the tankers the British converted to Merchant Aircraft Carriers. They displaced between 7,999 and 9,249 G.R.T., a single shaft powered by a diesel engine of 3,300 to 4,000 bhp for 11 to 13 knots. The flight deck had an effective length of 450 feet. 3 aircraft were carried, but they had no lift or hangar so they had to be stowed on deck. The British MACs also retained their cargo carrying capacity too. Unfortunately the sources for the Japanese Type 2TL conversions don't say if they retained their original tanker configurations either, but they also don't say that they didn't. In the latter case 2 Type 2TL conversions in 1941 wouldn't reduce the capacity of the Japanese tanker fleet either.

As to the size of the tanker fleet, Appendix VII of the British official history on the Control Of Merchant Shipping And The Demands Of War is a statement of world tonnage in 1939 by flag of steam and motor vessels of 1,600 tons and over. According to that the total Japanese had 1,054 of those types of vessels with a displacement of 5,030,00 G.R.T. and 7,545,000 D.W.T., including 47 tankers of 430,000 G.R.T. and 645,000 D.W.T.

The British official history of the War Against Japan had an appendix about the Japanese economy. It says this about the Japanese merchant fleet.

During the 1930s Japan had improved and enlarged her merchant fleet by replacing slow and out-of-date vessels with fast new ones. By 1940 the merchant fleet consisted of over 700 ocean going freighters, 132 passenger-cargo vessels, off which 49 were capable of speeds of 15 knots or over, and 49 ocean-going tankers. The total tonnage of this fleet on the outbreak of war in December 1941 was approximately 6 million and the percentage of imports carried in her own vessels had risen from 54 in 1937 to 65% in 1941.
Therefore converting 6 of the 49 tankers available in 1941 would reduce Japan's oil carrying capacity by 12½ percent they didn't retain their original tanker configurations. On the other hand the remainder's chances of survival would increase considerably if the Japanese had organised an effective convoy system built around the tanker conversions at the end of 1941.

I think 4 Type 1TL and 1 Type 2TL conversions in 1941 (or whatever the equivalents available at the time were) combined with more conversions of seaplane tenders and submarine tender into light fleet carriers in 1941-42 would have been better than the conversions of the smaller liners, which would have been of more value if they had remained as troop ships.
 
Another type of Japanese warship that was a waste of resources were the 12 small destroyers classed as torpedo boats. 4 were ordered under the 1931 Fleet Replenishment Law and 16 were ordered (but only 8 were built) under the 1934 Law. Even the Japanese though so because they cancelled 8 of the 16 ships ordered under the 1934 law in favour of 8 submarine chasers, which M.J. Whitley in Destroyers of World War II called, "A wise, if inadequate decision." According to him the 12 ships were mainly employed on escort duty.

The last 8 ships of the Otori class displaced 840 tons. They carried an armament of three 4.7" guns and three 21" torpedo tubes. Machinery was 2-shaft geared turbines, 2 boilers and 19,000 shp for 30.5 knots. The wartime Matsu class destroyer escorts displaced 1,262 tons, standard. They carried a similar armament of three 5" guns and four 24" torpedo tubes. Machinery was exactly the same and their maximum speed was 27.8 knots.

They were built to exploit a clause in the 1930 London Treaty that allowed unlimited construction of small destroyers displacing less than 600 tons. Therefore the Japanese could not have built 20 Matsu's instead of the torpedo boats. However, there was another clause in the Treaty that allowed unlimited construction of warships displacing less than 2,000 tons and a maximum speed less that 20 knots with an armament that could not exceed four 6" guns and no torpedoes. The Royal Navy used this clause to build its sloops and the Kingfisher class patrol vessels.

I would like the Japanese to use that clause to build 40 Type A escorts under the 1931 and 1934 programmes. They displaced 860 tons, standard with an armament of three 4.7" guns and 2-shaft diesels producing 4,200 shp for a maximum speed of 19.7 knots. In the real world the Japanese eventually ordered 960 escorts of Types A, B, C and D between 1937 and 1944 as follows: 4 - 1937 Fleet Replenishment Programme; 30 - 1941 Supplementary Programme; 4 - 1942 Fleet Replenishment Programme; 34 - Modified 1942 Fleet Replenishment Programme; 521 - 1943 Supplementary Programme; and 367 - 1944 Supplementary Programme. Hindsight tells us that they should have built as many as possible under the 1941 and 1942 programmes.

The Japanese also converted about a dozen old destroyers to what they called patrol vessels by removing the torpedo tubes and removing one boiler, which reduced their maximum speed to 18 knots. Their nearest British equivalents were the V&W class destroyers converted to long range escorts. However, they scrapped 5 second class destroyers of their 850 ton type (which were built at the end of World War One) in 1932 to comply with the terms of the 1930 London Treaty. They could have been converted to patrol vessels. 2 more were converted to minesweepers in 1930 and scrapped in 1938; 2 retained as destroyers were discarded in 1939 and 5 more were converted to tenders in 1940. It might have been possible to convert these 14 destroyers to patrol vessels too and augment the IJN's pool of potential convoy escorts.
 
NOMISYRRUC great posts, especially about the japanese MACs which is exactly what i was thinking to bring up. It appears their biggest fault was lack of arresting gear which preventead the use of normal carrier aircraft (i read the respective army conversion were slated to use K5Y trainers for IJN and Ki-76 and Ka-go autogyros for IJA). It also appear they did have one elevator (but have to recheck if both IJA and IJN conversions had it). I have read a bit about the british MACs, some if not all were even slower that the japanese ones, yet they operated Swordfishes (and i think fighters too) alright, becasue thy had arresting gear. Also, the american CVEs, which were not any faster than these MACs could operate things like TBFs(!). Curious, were they exclusively catapulted, or they actually took off normally?

So the japanese could have transformed their MACs into significantly more useful ships by adding arresting gear and be able to operate at least things like old B4Y and D1A biplanes and possibly A6Ms, if not the B5N monoplanes. Even more so if they would have done the same (transform into MACs) with most if not all the 1TL and 2TL tankers built during the war, for a relatively small reduction in cargo capacity they would have had vastly more numerous and better suited ASW aircraft over their convoys. That ought to account for something, like increasing proportionaly the US submarine losses/decreasing to the same amount japanese merchant loses.
 
Last edited:
Back to the CVEs, been looking to see how can they be made any faster, and it looks like even if all would be modified to use destroyer type engines, like f.e. Kaiyo which did 23kt (another source says 23.8kt) on 52,000HP, which is puzzling considering the power is almost the same as Junyo, but still 2kt slower on a much smaller ship (was the hull shape that much worse?). So probably the Taiyos (25,200HP) and the Shinyo (26,000HP) can't be made faster than 23.5-24kt max even if the power is doubled it seems, and to do that it will take even longer to modify them than it did. The only tactical improvement would be the ability to properly accompany the slow battle line (Fuso, Ise, Nagato classes at 24-25kt), otherwise they would be even more expensive and overbuilt for the aircraft transport and ASW roles than they were OTL.
 
Last edited:
Back to the CVEs, been looking to see how can they be made any faster, and it looks like even if all would be modified to use destroyer type engines, like f.e. Kaiyo which did 23kt (another source says 23.8kt) on 52,000HP, which is puzzling considering the power is almost the same as Junyo, but still 2kt slower on a much smaller ship (was the hull shape that much worse?). So probably the Taiyos (25,200HP) and the Shinyo (26,000HP) can't be made faster than 23.5-24kt max even if the power is doubled it seems, and to do that it will take even longer to modify then than it did. The only tactical improvement would be the ability to properly accompany the slow battle line (Fuso, Ise, Nagato classes at 24-25kt)...
Ability to provide fighter and ASW cover for the slow battle line would not be a bad thing as it would release the faster carriers for independent operations. We have already discussed this with the posts about using the liner conversions to screen the battleship forces during the Battle of Leyte Gulf. During that part of the discussion it was suggested that they could have been given land based air cover from the Philippines. That was the plan, but Task Force 38 had already wiped them out. Also the British Colossus class light fleet carrier began as a fighter carrier to escort the battleships after the battleship carrier concept had been discredited.
...otherwise they would be even more expensive and overbuilt for the aircraft transport and ASW roles than they were OTL.
Agreed. Furthermore these improvements would consume money and shipbuilding resources that the Japanese might not have in the first place. That means they would have to do less of something else in compensation. I think the only justification for trying to make them faster is if the result was 5 extra Hiyo class - More power requires a bigger hull, a longer and wider flight deck and possibly a double hangar. Otherwise I think the best course of action for the Japanese would be to keep them as troop ships and convert more naval auxiliaries into light fleet carriers.

All but one of the naval auxiliaries was ordered when the 1930 London Naval Treaty was in force. The exception was Nisshin built under the 1937 Fleet Replenishment Law. A few more were included in the 1942 Fleet Replenishment Law, but appear to have been cancelled and replaced by some of the 15 Unryu class ordered in the Modified 1942 Law. If the Japanese did have some shipyard space that was unused between 1937 and 1942 (which is very unlikely) they should have used it to build more of naval auxiliaries, more tankers and more escort vessels, but not necessarily in that order.
 
NOMISYRRUC great posts, especially about the japanese MACs which is exactly what i was thinking to bring up. It appears their biggest fault was lack of arresting gear which preventead the use of normal carrier aircraft (i read the respective army conversion were slated to use K5Y trainers for IJN and Ki-76 and Ka-go autogyros for IJA). It also appear they did have one elevator (but have to recheck if both IJA and IJN conversions had it). I have read a bit about the british MACs, some if not all were even slower that the japanese ones, yet they operated Swordfishes (and i think fighters too) alright, becasue thy had arresting gear. Also, the american CVEs, which were not any faster than these MACs could operate things like TBFs(!). Curious, were they exclusively catapulted, or they actually took off normally?

So the japanese could have transformed their MACs into significantly more useful ships by adding arresting gear and be able to operate at least things like old B4Y and D1A biplanes and possibly A6Ms, if not the B5N monoplanes. Even more so if they would have done the same (transform into MACs) with most if not all the 1TL and 2TL tankers built during the war, for a relatively small reduction in cargo capacity they would have had vastly more numerous and better suited ASW aircraft over their convoys. That ought to account for something, like increasing proportionaly the US submarine losses/decreasing to the same amount japanese merchant loses.

The only part I can comment on is the part I put in bold because I simply don't know about the rest. The short response is what convoys? They were few, small and weakly protected. I found an article about the Japanese convoy system such as it was on the US Naval Institute website and might do a précis about it in a later post and based on that another one about what the Japanese should have done.
 
Back to the CVEs, been looking to see how can they be made any faster, and it looks like even if all would be modified to use destroyer type engines, like f.e. Kaiyo which did 23kt (another source says 23.8kt) on 52,000HP, which is puzzling considering the power is almost the same as Junyo, but still 2kt slower on a much smaller ship (was the hull shape that much worse?). So probably the Taiyos (25,200HP) and the Shinyo (26,000HP) can't be made faster than 23.5-24kt max even if the power is doubled it seems, and to do that it will take even longer to modify them than it did. The only tactical improvement would be the ability to properly accompany the slow battle line (Fuso, Ise, Nagato classes at 24-25kt), otherwise they would be even more expensive and overbuilt for the aircraft transport and ASW roles than they were OTL.

I put the characteristics of the Japanese aircraft carriers into a spreadsheet and although the Junyo class compares unfavourably against the Shokaku class it doesn't do so badly against the Hiryu. The Junyo class had a flight deck of 690 ft x 89.5 ft, which isn't much shorter for than the Hiryu's 711.5 ft x 88.5 ft. Aircraft capacity was 54 in the Junyo class, 63 in Hiryu and 54 in the smaller Soryu. However, the Junyo class is considerably slower at 25.5 knots against 34.3 knots for Hiryu. Also the Junyo's hangars have less headroom, which is surprising as it has a much better length to beam ratio than Hiryu (8:1 v 10:1). The cause of that might have been that she had a larger island than was normal for a Japanese aircraft carrier, which was in part because she was the first Japanese aircraft carrier with a conventional funnel.

Taiyo, the best of the small liner conversions was similar to the naval auxiliary conversions. Their waterline length at 569ft 11in was considerably shorter than the 610ft of Chitose and 660ft 9in of Soho; Taiyo's beam at 73ft 10in is greater than the 68ft 3in of Chitose and 59ft 12in. Despite having a much shorter hull her flight deck at 564ft 3in only 26 feet shorter than the converted auxiliaries (590ft Chitose and 590ft 6in Soho, but Zuiho had hers extended to 631ft 11in in 1944). They carried about the same number of aircraft, i.e. 27 for Taiyo and 30 for the auxiliaries.

However, Taiyo's Achilles heel was that she was too slow for fleet work because her 4 boilers produced 25,200 shp and drove 2 shafts for a maximum speed of 21.0 knots. Chitose had the same number of boilers, but they produced 44,000 shp and were augmented by diesel engines producing 12,800 bhp, which allowed her 2 shafts to propel her at a maximum speed of 28.9 knots. Soho had 4 boilers producing 52,000 shp on 2 shafts for 28.0 knots.

The prototype auxiliary aircraft carrier, the submarine tender Ryuho ordered in the First Fleet Replenishment Programme of 1931, was actually the largest. It's waterline length was 689ft and her beam was 64ft 3in, but as she had the same machinery as Soho and Zuiho she was 1.5 knots slower. On the other hand when converted her flight deck was 607 ft long and this was increased to 650ft in 1944.

Therefore (subject to financial and shipbuilding limitations) it seems to me that the 5 auxiliaries built in the 1934 Fleet Replenishment Programme should have had 50% greater displacement (effectively making them improved Ryuhos) and 50% more powerful machinery to maintain their speed. The seaplane tender version would carry 36-48 seaplanes instead of 24. The submarine tender version would similarly be able to handle 50 to 100% more submarines at a time.

When they were converted to aircraft carriers the larger hull would have allowed a longer and wider flight deck. Plus, and this is the most important part, a double hangar (with more headroom than the Junyos). The longer hull and second hangar doubled their groups to 54-63 aircraft. The result would effectively be a slow Soryu, but still fast enough for fleet work.

Only one fast auxiliary (Nisshin) was included in the 1937 Fleet Replenishment Program. This might be because the Japanese aircraft carrier fleet was no longer limited to a maximum of 81,000 tons. However, they did lay down 4 Katori class training cruisers 1938-41. With hindsight the Japanese would have been better off building 4 additional Nisshins class in their place. If more Nisshins were too expensive and/or the slipways were too short then the 4 Type 1TL carriers I have proposed should have been built in their place.
 
Most interesting red as usual. As you mention about larger auxiliaries, obvioulsy they would be more expensive and possibly they might built fewer of them, since they also have larger seaplane capacities. Also they introduced new technologies like diesels or all welded hulls to IJN, so in a way they were also convenient testbeds.

But still to achieve what you suggest (bigger hulls with more hangar space) i was thinking they could design them so that when eventually they are converted to CV they become sort-of like a Ryujo done right. In other words, two shaft half-cruiser machinery (say Taigei designed around same machinery as Ryujo with is half Myoko type machinery if i'm not mistaken at 65,000HP, while the subsequent ones like Tsurugisakis, Chitoses etc. designed around half-Mogami machinery of 76,000 HP) and double hangars which if it increasew the aircraft capacity to even just Junyo levels at 53 (48 operational), it would be a great leap. Soryu was rated at 71 aircraft initially of which 57 operational, so it may be hard to match that unless they literally design these tenders along Soryu type hulls which actually gets us back to obtaining something similar to the G18 project of 1944 but 10 years earlier. Hmm, that is actully quite interesting to ponder.

But anyway, this will be a lot more expensive unless like i said they don't built 7 auxiliaries but say only 5, but on the other hand if they have couple of larger (15,000 tons or so) of 28kt Zuihos able to carry 48 or even 54 operational aircraft in 1942, to be followed by the similarly capable Ryuho and the Chitoses, well that would be trouble for USN. But they could pay for this by building smaller 45,000 tons Yamatos (which is something i'm getting increasingly fond of ATL wise), and still get Nisshin and Mizuho too that way, even if same size as OTL (and preferbaly Mizuho also a 28kt capable ship).

So you can get the almost-fleet-carriers Zuiho, Shoho, Ryuho, Chitose, Chiyoda at about 15,000-16,000 tons standard, 28kt, 48-54 operational aircraft and if they ever get to convert them, Mizuho and Nisshin also of 28kt but 10,000 tons or so and able to carry 30 operational aircraft, and possibly no less than four 45,000 tons Yamatos, still formidable but being smaller they could get finished quicker bumping up Shinano and No.111 as much as a year perhaps (so they are operational in 1944).

Oh and as for the Katoris, subject to limitations of the slips they were built on, if possible i would definitely have Aganos build instead of them or even better, the Agano based CLAA design, and have some of the old CLs converted to training ships.
 
Last edited:
So you can get the almost-fleet-carriers Zuiho, Shoho, Ryuho, Chitose, Chiyoda at about 15,000-16,000 tons standard, 28kt, 48-54 operational aircraft and if they ever get to convert them, Mizuho and Nisshin also of 28kt but 10,000 tons or so and able to carry 30 operational aircraft.

The Zuiho and Shoho seem like a winning design to me. Economical but capable of fleet operations. Kind of a B version of the Independence class.
 
Most interesting red as usual. As you mention about larger auxiliaries, obvioulsy they would be more expensive and possibly they might built fewer of them, since they also have larger seaplane capacities. Also they introduced new technologies like diesels or all welded hulls to IJN, so in a way they were also convenient testbeds.

But still to achieve what you suggest (bigger hulls with more hangar space) i was thinking they could design them so that when eventually they are converted to CV they become sort-of like a Ryujo done right. In other words, two shaft half-cruiser machinery (say Taigei designed around same machinery as Ryujo with is half Myoko type machinery if i'm not mistaken at 65,000HP, while the subsequent ones like Tsurugisakis, Chitoses etc. designed around half-Mogami machinery of 76,000 HP) and double hangars which if it increasew the aircraft capacity to even just Junyo levels at 53 (48 operational), it would be a great leap. Soryu was rated at 71 aircraft initially of which 57 operational, so it may be hard to match that unless they literally design these tenders along Soryu type hulls which actually gets us back to obtaining something similar to the G18 project of 1944 but 10 years earlier. Hmm, that is actully quite interesting to ponder.

But anyway, this will be a lot more expensive unless like i said they don't built 7 auxiliaries but say only 5, but on the other hand if they have couple of larger (15,000 tons or so) of 28kt Zuihos able to carry 48 or even 54 operational aircraft in 1942, to be followed by the similarly capable Ryuho and the Chitoses, well that would be trouble for USN. But they could pay for this by building smaller 45,000tons Yamatos (which is something i'm getting increasingly fond of ATL wise), and still get Nisshin and Mizuho too that way, even if same size as OTL (and preferbaly Mizuho also a 28kt capable ship).

So you can get the almost-fleet-carriers Zuiho, Shoho, Ryuho, Chitose, Chiyoda at about 15,000-16,000 tons standard, 28kt, 48-54 operational aircraft and if they ever get to convert them, Mizuho and Nisshin also of 28kt but 10,000 tons or so and able to carry 30 operational aircraft.


Perhaps in theory this might have been nice, but there is a severe restriction, namely the number of aircraft and aviators available at the time. An aircraft carreir is only as usefull as its aircraft are, so having a restricted number of aircraft and aviators will render it less efficient.

In the OTL the IJN Always was short of skilled aviators to maximise the available landingplaforms in the fleet. All Kido-Butai and other carriers rarely, if ever had an aircraftcomplement reaching their maximum capacity. Since the Shokaku and Taiho classes had the largest capacity, this was not reached in wartime, due to shortages in both aircraft and aviators. (Both Shokaku and Taiho had a capacity of 84, with 72 operational, Kaga actualy had a theoretical larger number (90 / 72), though calculated by older, smaller aircraft when reconstructed.)
 
Obviously, to have this ATL working you gotta have the IJN train proportionally more pilots for these larger CVs.

As For Shokaus and Taiho etc. actually at Pearl Harbour apparently all japanese CVs carried the maximum they could operate (Kaga and Shokakus 72, Akagi 66 or 69, Soryus 57, plus spares). Shokakus again carried 72 each at Eastern Solomons. There was definitely a shortage at Coral Sea and Midway though.

As for Taiho, because of her armoured deck she was rated at 53 new and larger generation aircraft (A7M, B7A) which equates to over 60 older ones and in fact apparently she carried 65 or 66 (before the pre-battle accidents) A6M, D4Y, B6N and couple D3As at Marianas. I know the figures that can be found on the www, but the 66 figure i think it is much more accurate being that it's coming from knowledgeable researchers into the matter.
 
Back to the auxiliaries (sub and seaplane tenders), given that OTL they all seem to hover close to the 10,000 tons standard mark, makes me wonder if there were limitations in the Washington or London treaties as to the maximum displacement of such auxiliaries?
 
Back to the auxiliaries (sub and seaplane tenders), given that OTL they all seem to hover close to the 10,000 tons standard mark, makes me wonder if there were limitations in the Washington or London treaties as to the maximum displacement of such auxiliaries?

Not as far as I know, but if you send me a PM with your email address I'll email my copy to you to check.
 
Obviously, to have this ATL working you gotta have the IJN train proportionally more pilots for these larger CVs.

As For Shokaus and Taiho etc. actually at Pearl Harbour apparently all japanese CVs carried the maximum they could operate (Kaga and Shokakus 72, Akagi 66 or 69, Soryus 57, plus spares). Shokakus again carried 72 each at Eastern Solomons. There was definitely a shortage at Coral Sea and Midway though.

As for Taiho, because of her armoured deck she was rated at 53 new and larger generation aircraft (A7M, B7A) which equates to over 60 older ones and in fact apparently she carried 65 or 66 (before the pre-battle accidents) A6M, D4Y, B6N and couple D3As at Marianas. I know the figures that can be found on the www, but the 66 figure i think it is much more accurate being that it's coming from knowledgeable researchers into the matter.

For Taiho the figure of 53 was the actual number carried during her only warmission. Her actual capacity was identical to the Shokaku's though, which she was essentially evolved from, though occasionally a lower figure is stated, idnicating larger aircraft, that eventually never went into production. (twin engined torpedobombers for instance, that never went through the paper study phase.)
 
I stand by what i said, all reliable sources especially japanese list Taiho able to carry 53 aircraft, while Taiho-kai design could carry 60 (A7M, B7A, C6N types), including deck-park. Because of the armoured deck they could only fit a single level hangar, not two like on Shokaku. At Marianas Taiho's initial airgroup was 22 A6M, 22 D4Y, 18 B6N and 3 D3A, but i have to double check my notes as there may be one or two less or more aircraft of each type. Nine were already lost in accidents before the battle though, leaving 56 according to wiki.
 
I stand by what i said, all reliable sources especially japanese list Taiho able to carry 53 aircraft, while Taiho-kai design could carry 60 (A7M, B7A, C6N types), including deck-park. Because of the armoured deck they could only fit a single level hangar, not two like on Shokaku. At Marianas Taiho's initial airgroup was 22 A6M, 22 D4Y, 18 B6N and 3 D3A, but i have to double check my notes as there may be one or two less or more aircraft of each type. Nine were already lost in accidents before the battle though, leaving 56 according to wiki.

Taihō's upper and lower hangars were approximately 150 m (492 ft 2 in) long and 5 m (16 ft 5 in) high. The upper hangar was 1 m (3 ft 3 in) wider than the lower. Fighters were normally stowed in the middle and forward sections of the upper hangar and were raised to flight deck level using the bow elevator to facilitate more rapid handling. Dive bombers occupied the remaining upper hangar spaces with torpedo bombers stowed in the lower hangar. With greater all-up weights and longer take-off runs than the fighters, these planes were brought up to the flight deck using the aft elevator where they could then be spotted as far astern as possible

The Shōkaku-class carriers' 242.2-meter (794 ft 7 in) flight deck had a maximum width of 29 meters and overhung the superstructure at both ends, supported by pillars. Ten transverse arrestor wires were installed on the flight deck that could stop a 4,000-kilogram (8,800 lb) aircraft. If the aircraft missed those, it could be stopped by one of three crash barricades. Although space and weight were allocated for two aircraft catapults, their development was not completed before the Shōkaku-class ships were sunk. The ships were designed with two superimposed hangars; the upper hangar was about 200 meters (656 ft 2 in) long and had a width that varied between 18.5 and 24 meters (60 ft 8 in and 78 ft 9 in). It had a height of 4.85 meters (15 ft 11 in) while the lower hangar was 4.7 meters (15 ft 5 in) high and only usable by fighters. The lower hangar was about 20 meters (65 ft 7 in) shorter than the upper one and its width ranged from 17.5 to 20 meters (57 ft 5 in to 65 ft 7 in). Together they had a total area of 5,545 square meters (59,690 sq ft).[6] Each hangar could be subdivided by five or six fire curtains and they were fitted with fire fighting foam dispensers on each side. The lower hangar was also fitted with a carbon dioxide fire suppression system. Each subdivision was provided with a pair of enclosed and armored stations to control the fire curtains and fire fighting equipment.

The Shōkaku-class carriers were initially intended to have an air group of 96, including 24 aircraft in reserve. These were envisioned as 12 Mitsubishi A5M ("Claude") monoplane fighters, 24 Aichi D1A2 ("Susie") Type 96 dive bombers, 24 Mitsubishi B5M ("Mabel") Type 97 No. 2 torpedo bombers, and 12 Nakajima C3N Type 97 reconnaissance aircraft.[10] All of these aircraft were either superseded by larger, more modern aircraft or cancelled while the ships were being built, so the air group was revised to consist of 18 Mitsubishi A6M Zero fighters, 27 Aichi D3A ("Val") dive bombers, and 27 Nakajima B5N ("Kate") torpedo bombers.[11] In addition, the ship carried 2 Zeros, 5 "Vals", and 5 "Kates" as spares for a total of 84 aircraft.

Taihō's planned air complement varied considerably throughout her design and construction. Initially, it was envisioned she would carry 126 aircraft (with 30 of these in reserve). Later, this was pared down to 64, raised again to 78 and finally reduced to 53. One reason for the discrepancy in numbers was (in sharp contrast to the United States) the Imperial Japanese Navy's lack of insistence that its carrier planes have the smallest possible folded wingspan (many designs' folded only near the tips, while the wings of the Yokosuka D4Y Suisei dive-bomber did not fold at all). Her aircraft capacity was also changed based on previous wartime experience and the fact that Taihō was expected to carry larger newer-model carrier planes still under development at the time of her construction: 24 Mitsubishi A7M2 Reppu "Sam" fighters, 25 Aichi B7A2 Ryusei "Grace" torpedo bombers and four Nakajima C6N1 Saiun "Myrt" reconnaissance planes. As none of these types were available at the time of her commissioning, Taihō went to sea with older-model aircraft.
 
Top