Japanese logistics, purchase programs and war booty, reality and alternatives 1936-44

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A lot of British pilot manuals will say "100" octane and will only start saying 100/130 late in war, if then.
The British BoB fuel was 100/115-120, but since in the BoB they didn't have the test procedure or the PN scale you are not going to see it specified of spelled out. The British had 87 octane and they had 100 octane (really 100/115 or up) and 1941/42 they had 100/130 but they just called it 100. You are going to look for a very long time to see a fuel truck with a 100/130 sign/label. Once the British started using 100/150 fuel (which had more than one designation to start with) the 100/130 designation became more common (but not universal?)
to help differentiate.

The US on the other hand went from 87 to 90-91 and then to 100 (actually 100/100) and briefly to 100/125 ( how much was issued to actual squadrons/field is unknown) before getting to 100/130. Early US manuals are all over the place. Later ones will often list 100 octane when they mean 100/130 as the 100/100 had disappeared. They still had 91 or 91/96 for use trainers and transports. And the lower grades like 87 and less.

Pilots manuals are NOT a good indicator of the actual fuel being used without using the date/s and a bit of back ground knowledge.
 
re
Shortround6 said:
. . . The British BoB fuel was 100/115-120, but since in the BoB they didn't have the test procedure or the PN scale you are not going to see it specified of spelled out. . .
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from one of my posts here "Westland Welkin: Basis for a Whirlwind II?"

<start>
The UK Air Ministry began using the single-cylinder CFR test method as standard in 1935orB4. The British Air Ministry subsequently developed a standard single cylinder RMR (Rich Mixture Rating) procedure based on a Pegasus engine 8:1 CR cylinder (which was representative of the 'most severe service engine' in use at the time) in which fuel batches were checked against the various reference fuels. The new CFR test method was designated the British Air Ministry modified Motor Method. The Pegasus type cylinder used by the test engine was later changed to a Hercules cylinder (I have no idea why). Note that the abbreviation B.A.M. stands for British Air Ministry.

The designation B.A.M.100 (or BAM.100) that we often see used to reference pre- and early-war UK '100 octane' was actually the testing procedure designation that was to be used for proofing the fuel material batches, rather than the designation for the actual '100 octane' fuel material. The UK started using the term BAM.100 in January 1937 as a provisional designation for the new '100 octane' fuel.

Approximately 1000 tons of '100 octane' Avgas was trialed at RAF Sqn level beginning in June 1937, with the tests ending in September 1938. These tests gave the Air Ministry important information as to what appeared practical to achieve with the new fuel and what possible fuel blends allowed the desired improvements in engine performance.

The lean and rich mixture 100/130 PNs were set in September 1938 and remained the same for the rest of the war. (I believe Snowygrouch referenced this as well in one of his posts but I could not find it using Snowygrouch, as the search term is no longer usable due to his leaving the forum.) The BAM.100 testing procedure, which defined what method was to be used (in this case the British Air Ministry modified Motor Method for '100 octane') defined the PN (Performance Numbers) that had to be met by the actual fuel material. 72,000 tons of BAM.100 fuel were ordered in November 1938. First delivery of BAM.100 to England was in June 1939 (I think).

As to what fuel they actually used when testing the engines, all war-time engines were required by the Air Ministry DED/DERD to be tested using what they called a 'reference fuel' - ie a fuel that met the minimum standards set out in the test specification. There were different reference fuels for different grades - ie 73, 80, 87, 100, etc. The reference fuel used for testing BAM.100 had to result in PNs that matched or exceeded those of 100% iso-octane plus 4cc TEL per Impgal. So in theory at least, RR and the other UK engine manufacturers used the specified 100/130 PN reference fuels when they rated their engines.

After 1938 the fuel material blends were allowed to change, but they all had to achieve the 100/130 PN using the BAM.100 testing procedure. Before 1940, due to the fact that they had not settled on the specific blends achievable and allowed, there was no formal DED number assigned to the '100 octane' fuel material. (The designator DED.2475 may have been assigned to the '100 octane' fuel material sometime during the war but I am not sure.) In early-1940 responsibility for the fuel material specification was transferred from DED to RDE, and British '100 octane' fuel material meeting the BAM.100 test requirements became RDE/F/100.

When I was researching the pre- and war-time fuels I found it interesting that when the UK was paying cash for the fuel purchases (prior to the US entry into the war and Lend-Lease) the US refineries and government agencies often referred to British orders for '100 octane' by variations of the BAM100 designation - due to the British purchasing agencies requiring the US refineries to use the same testing procedures as the UK refineries (or testing procedures that gave the same results, subject to approval by the UK). There were a fair number of memos passed back and forth between the US government agencies involved and the refineries complaining about the difficulties in meeting the requirement. Part of the problem was the rapid expansion in production and the learning curves, and part of it was due the attempts to use less aromatics and reduce the expense.

Incidentally, when the tankers arrived in England (and overseas) they were met by specialists who tested the fuel. The Air Ministry also ordered various amounts of aromatics and TEL and had them shipped with the fuel material if necessary - the intent being to add aromatics and/or TEL if so needed to bring the fuel material upto standard. One or more of the refineries in England was also tasked with producing the aromatics specifically for this purpose.
<end>

in addition and to reiterate,

Basically, the British did have there test procedures already in service during the BoB (they were similar to the US 1-cylinder CFR method) and the US manufacturers were required to use the British procedure when orders for BAM100 were being filled - the UK even sent the specific 1-cylinder CFR engines required for the tests, along with instructions and instructors. If the '100 octane' used in the BoB was only 100/115-125 then it would have been because the shipments from the US were substandard (in which case, as mentioned in the older post I quoted above, the British would normally have corrected the materiel by adding the necessary chemicals, and/or there was no time to correct the materiel).

As far as I have been able to find, it was only after fuel shipments began under Lend-Lease that the 100/130 requirement was not met by the US manufacturers - due to the requirements being relaxed by the US government who then held the purse strings - since the UK was no longer paying for the materiel. The USAAF adopted 100/125 in May 1940 as their war-time standard and apparently the US government thought that should be good enough for the British (prior to May 1942 the minimum requirement for avgas destined for use in the US was 100/115). The British stuck to their guns, however, and demanded 100/130 and the US finally agreed to the 100/130 standard in late-1942.

NOTE that it should not be surprising the the British demanded 100/130 grade and stuck to it as their engine development programs were based on the pre-war decision to use 100/130 grade for their future engines.

The reasons for the different standards and the US problems with meeting the standards were several - including difficulty in producing the volumes necessary, the cost and difficulty meeting the standards in general and at the volumes necessary (apparently there was a significant increase in difficulty and hence cost when going from 100/125 to 100/130), and the need to expand existing facilities, build new facilities, train personnel, and monitor the production of said materiel.
 
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Easy tiger, no one's ranting here except you. I read your post, clearly despite your statements you STILL don't get it. Just because the Japanese knew about the Hudson doesn't mean they needed it or were gonna do that for the reasons that you even said in your post yet you STILL argue the Japanese should'a because magazine articles? Your presumption is failing you despite yourself. By the way, the Hudson entered RAF service in May 1939, the first one did not fly until very late 1938.
 
The history of aviation fuel in the 1930s and 40s is not well documented in a single source. It is often contradictory when comparing different sources. Some sources have different emphasis. S.D. Heron's book, "Development of Aviation Fuels" 1950, 115 pages, covers a lot of early development starting in 1915. But a lot of the book is also about the problems in production and the changes in both fuel blends and in production techniques/processes made to production. He wrote the book in the late 40s (published in 1950) and he makes little or no references to any fuel production that is not allied. The Book is often (always?) combined with "Development of Aircraft Engines" by Robert Schlaifer, assistant professor of business administration Harvard University.
The Subtitle of the two books is "Two studies of relations between government and business"

It is pretty much an Allied (more American) point of view.

However things can be drawn from it.
ThomasP said:
The lean and rich mixture 100/130 PNs were set in September 1938 and remained the same for the rest of the war.
Click to expand...
This statement may be suspect. S.D. Heron is generally credited with invention/development of the Performance Scale, or he claims to have done it and there are few (any?) claims that contradict that. There is also the difference between a man (and team) developing a method and getting it adopted by governments and industries in in at least 2 nations. Heron was British by birth and early work but by 1921 he was in the US. by 1934 he was technical director for aeronautical research for the Ethyl Corporation. In his books (there are at least two more) he doesn't say when he developed the PN scale (or I have forgotten it) but the quoted statement seems a little off. Or we are going by each other. Going by the old books the British were not specifying 100/130 PN numbers in the late 30s. They didn't have the PN scale in place or a way to test it. They were specifying that the 100 octane fuel contain at least 20% aromatics. They were trying for increased knock rating, they knew what they were trying to achieve. But they didn't have a way to measure it.
People were trying to use 100 iso-octane plus lead as a reference fuel but this was cumbersome and it had problems. Even a 1943 text book says that are a a number of ways of extrapolating the octane scale past 100 but each method gives different results. Most satisfactory was the 100 iso-octane plus lead but........


S-1 is 100% iso-octane. As seen, a little lead does a lot, once you go past 2cc improvement gets a lot less and past 4 ccs improvements get really small. Using even 4cc starts leading to engine problems with fouling and corrosion. Please note in the credit for the chart both the name Heron and the date of publication the chart was from. Granted there was often a delay in publications and we are talking about different countries but we have one source claiming that the British were specifying 100/130 limits two months before the claimed inventor of the PN scale was publishing a chart showing the increase of lead on 100 iso-octane. PN scale, according to Heron, came later.

In the "Development of Aviation Fuels" Heron does not give dates for different 100/130 fuels but he does claim there 3 different specifications, maybe there were more?
The first allowed 3cc of lead, this was changed to 4.0cc just after Pearl harbor and by late 1942 a specification allowing 4.6cc of lead was issued (it also allowed for changes in mixtures of aromatics). Both changes allowed for much greater production from existing feed stocks.
Heron also states that the British had a specification for 100/125 fuel in late 1941/early 1942. 125 reference fuel was simply 100 iso-octane with 1 cc of lead.

There was a huge shortage of test engines in the US and England in the early war years. Using full scale radial engine cylinders was expensive and cumbersome. Shipping untested aviation fuel to England to be tested in England was wasteful in that some of the refineries without appropriate test engines sent a fuel blend with more aromatics/alkylate than was needed to insure it the fuel passed acceptance tests in England. This was wasteful in raw resources.
The 100/125 specification did not last long before it was changed to 100/130 in both countries. But a post Pearl Harbor British specification for 100/125 does not line up well with a 1938 specification for 100/130. Perhaps Heron was wrong or misremembered?
Heron's goal with the Performance Number scale was to come up with a linear scale were each change in number was the same change in performance, which the octane scale is defiantly not. See chart, change from 70 octane (70% S-1 ) to 80 octane (80% S-I) is not even the same change as from 90 octane to 95 octane.
This was part of the problem with trying to use 100 iso-octane + X cc's of lead as a rating system.
The other problem was that all aromatics responded the same way to lead. Some increased significantly with addition of lead. Others didn't change much one way or the other.
Even the base stock fuels responded differently to lead.
 
re

and

Sorry, i was not clear. You are correct that the British were not using the term 100/130 PN as such in 1937-38, but the '100 octane' fuel materiel from late-1938 was required to pass the BAM.100 test, which was effectively what the later 100/130 grade specification was required to meet.

I cannot address the claim of the British having a '100 octane' specification with 100% iso-octane + 1cc of TEL as I have not run across that in any of the technical references for the British or in their contracts internally or with the US.

I cannot really comment on whether Heron was misremembering or not, as there is just far too much material out there that still is not available to the public and/or that has been lost to time. Hopefully the archives of the various nations will get off their asses and digitize more and more of the information they hold in trust, while it is still in a condition to be digitized.

PS
Found the post by Calum Douglas mentioning the quality of British '100 octane' fuel, here "Admiralty aircraft fuels and lubricants as of 1 April 1943."

One relevant quote: "There is essentially no difference between the 100 octane fuel the RAF was using in 1940 and "100/130" being used by the Allies until the end of the war, the 130 merely represents an agreed change in testing designation."

PSS
I believe 100% iso-octane + 1cc TEL/USgal should result in a 100/115 PN, not 100/125 ??
 
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another page from the 1943 text book by Fraas

A.S.T.M is American Society for Testing and Materials. Waukesha was given a contract for hundreds (to start?) of single cylinder test engines in 1941/42 (?) for both the US and Britain so the results would be as close as possible. It seems the US Army was being difficult
Again it is a 1943 book. What the British were using for test engines in 1930s or even 1940/41 are not mentioned. But the British ability to supply either Pegasus cylinders (and test engines with crankshafts and single rod) and/or Hercules cylinders in large numbers to US fuel refiners was limited.
Please note the small differences in test results using different methods.
In the top chart note that the British used a higher intake manifold temperature. There may have been other differences that were not noted.

Fuel specifications were a standard, not a formula or recipe. Often a minimum standard. There could be 100s of combinations of fuel components that could be blended to meet a particular standard. This varied by base stocks, availability of different base stocks and quantities of aromatics available at any given time.

As noted, this is for commercial fuel, not military. I will note that post war the max allowable lead was 3cc per gallon.
I will also note that when reading about lead in fuel we have to take into account that the US will give CCs per US gallon while the British will give CCs per Imp. Gal.

ThomasP said:
One relevant quote: "There is essentially no difference between the 100 octane fuel the RAF was using in 1940 and "100/130" being used by the Allies until the end of the war, the 130 merely represents an agreed change in testing designation."
Click to expand...
From an engine design view point this is correct. The change from 120 to 130 is not great. Some of the BoB fuel may have been 130 when measured under the new test procedures.
Some may have been 115. Problem is that you have to compare (on the same day) the fuel being tested to a reference fuel (actually several) and all the test will tell you is if the fuel being tested matches or exceeds the performance of the reference fuel. It may well exceed the refence fuel, but the test will NOT tell you by how much. If you have a max 100/130 refence fuel and you are testing a new batch you can say the new batch matches (or fails) and you can say the new fuel performs better but you can not say by how much.
The 100/130 fuel used later in the war used the same compounds it close to the same percentages and used lead (with a little more possible after 1942/43) but they didn't change things around like the Germans did with synthetics or really use things like triptane to boost performance numbers. Triptane was being tested in 1941 but it cost around $30 a gallon for test but refining triptane meant large changes in refineries. Toluene had a lot of potential as an additive but toluene was needed for TNT production.
So yes, there were no big changes from the BoB fuel to the later 100/130 fuel. The 100/145 fuel did have some changes, it wasn't just a matter of adding more lead
 
The British 1-cylinder CFR engines I refer to above were those used to test the British fuel orders before Lend-Lease took effect. The British test engine is described in the .pdf file attached below. As noted in the .pdf, it was originally a Pegasus cylinder and change to a Hercules cylinder later - both with 7.25:1 CR. Not noted in the article is that originally (in 1937 or 1938) the Pegasus test cylinder used a 8.0:1 CR (apparently there were British engines using this CR in service at the time?) as it was considered the most severe specification to be met in service use at the time.

NOTE that the B.A.M. 100 reference fuel and A.M.C.VII correlation fuel described in the .pdf are the base fuels without TEL. In the technical files I ran across, the performance of the test fuel with TEL that the production fuels would have to meet or exceed were referred to as B.A.M. 100/4 - ie the BAM100 reference fuel plus 4 cc of TEL per UKgal. (The actual term varied a bit, ie BAM. 100-4, BAM100/4, etc, or just BAM100 without any suffix indicating the number of ccs of TEL.)

B.A.M. 100/4 was the reference fuel for UK '100 octane' from late-1938, and one of the reference fuels (for the British at least) for the later 100/130 grade.

As you mention above, US and UK gallons were different in absolute volume and for the US measurements it would be listed as ~3.3 cc TEL per USgal (rounded up to 3.5 cc TEL per USgal?).

This is the best description I have been able to find of the pre/early-war and pre-Lend-Lease performance test method used by the UK. I annotated the source of the .pdf file at the top of the first page, but if goes away or something it is from:

"THE JOURNAL OF THE INSTITUTE OF PETROLEUM AND THE ROYAL AERONAUTICAL SOCIETY" Vol. 33 No. 280, dated 1947

Usually, the Journal is just listed as the "JOURNAL OF THE INSTITUTE OF PETROLEUM"
 

Attachments

  • Pre- & Early-war UK Method of Fuel Rating.pdf
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  • UK & US Knock Rating Method War-Time Development.pdf
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Is this TL involving naval construction as well? Of the many facets, couple that stand out to me are one, smaller Yamatos at about 45,000-50,000 tons with 9x41cm guns and 29-30kt speed, which might mean all 4 actually being finished and crucially a lot less stress on the shipyards building them, no need for extensive renovation and enlargement work AND the huge new drydocks built at Yokosuka and Sasebo can be made much smaller so they are ready earlier and consume less resources and workforce.

The second one is building or converting as many carries as reasonably possible, for instance 2 Unryus instead of the large and inneficient Junyos, more Unryus instead of the Ibuki cruisers, and the shadow carrier conversion program should ideally be started earlier so more CVs, CVEs an CVLs are ready by late 1941 and into 1942.

As to engines among others yes Nakajima should kill the Mamoru with fire and instead work on a 18 cylinder pre-Ha-219 engine, basically 18 cylinder version of the Ha-5 series. This will give them a 1850HP engine in 1942, excellent for carrier and land based bombers, and with MW boost, for fighters as well. They should also kill the useless Ha-20 too before the war, focusing in getting the Ha-25 and Ha-41 working asap (Ha-41 was way too late in 1941, should have been ready in 1939, while the Ha-109 should have been ready in 1941).

Regarding carrier fighters, build the A7M instead of the J2M, around the aforementioned pre-Ha-219 engine as it would be the only engine small enough and powerful enough in 1942 to power the A7M.

On the DB licence, either they get it earlier and get the proper tooling from Germany so they can produce them with acceptable tolerances and crucially acceptable reliability, or not bother at all and keep building the tried and tested radials (basically Kinseis instead of the Aichi and Kawasaki DBs). IF they can get DBs earlier we might get interesting side-effects like the Ki-28 and Ki-32 powered by japanese DB-600 versions (this instead of the obsolete Ha-9) with a big jump in speed presumably (which might mean Ki-28 adopted too alongside Ki-27, as a fast interceptor- and could even mount a motor cannon), Ki-45 designed around DB-600 from the start which at least might mean easily meeting the speed spec if nothing else, later versions switching to japanese DB-601s etc.

For the navy, if the Atsuta is reliable it might mean the D4Y entering service earlier even if just as a recce, and having the Atsuta used on other platforms like the J1N which could certainly use the jump in speed.

Like always, lots of possible permutations, the above barely scratching the surface.
 
Yay! I found the other article I was looking for. This one is also an excerpt from the Journal of the Institute of Petroleum, but is from 1951 (I annotated the volume and issue at the top of the first page).

This article describes the war-time development of the knock rating system as developed by the UK and US. I have added the ,pdf file to my post#147 upthread.

Sorry for being a bit scattered with this stuff, but when I first ran across this stuff I was just doing the research for my own amusement, I had not yet developed any intention of posting the information online.

PS I find this article confusing in some aspects re the various ratings.
 
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Some of this doesn't stand up well.
mack8 said:
smaller Yamatos at about 45,000-50,000 tons with 9x41cm guns and 29-30kt speed,
Click to expand...
While these sound more powerful than the KGV's and US North Carolina's the British and the US are building their new battleships in quantity.
Japanese were betting on quality and not quantity as they knew they could not out build the British and Americans.
The 4th Yamato (No 111) was laid down after the Iowa and New Jersey. The US already had 6 modern 16in BBs under construction.
Even if you could turn 3 Yamato's into four 45,000-50,000ton ships the Japanese are still behind in numbers.
mack8 said:
2 Unryus instead of the large and inneficient Junyos
Click to expand...
The Junyo's were converted passenger liner hulls. With the passenger liner machinery, 2 shafts, 6 boilers and 52,250hp each. The Unryu's used 4 shafts, 8 boilers and 152,000hp per ship.
mack8 said:
more Unryus instead of the Ibuki cruisers
Click to expand...
While two Ibuki's were planned only one was laid down. As built as a light carrier they used 1/2 the intended power plant. Full power plant would have been roughly the same as an Unryu. But the hull was roughly 2/3s the displacement.
With 3 of the 6th planed Unryu's launched but not completed (and 3rd completed ship, Katsuragi not completing until Oct 1944) the problem was not the number of hulls intended for carriers. The problem was Japan's industrial capacity to complete hulls, machinery in a timely fashion.
Spreading out the Japanese effort into even more hulls may not get you any more carriers in service any sooner.
 

They will need to streamline their pilot training to increase throughput.
 
Japamese empire & it's Navy had no means to have it all - BBs, CVs, CAs, CLs, DDs, submarines, air service, escort/anti-submarine force - in good numbers and in good 'quality'. In order to have something of any of that more and/or better, something else will need to take a cut. Especially concerning that Japan had every intention to battle it against the established powers, and over what is basically half of Earth's surface.

Taking the immense distances involved into account, the air service needs to be prioritized, meaning that aircraft carriers, embarked air groups and land-based aircraft are now 'capital' stuff. In return, that means that other 1st line assets should've been tailored to protect the aircraft carriers, and to act as support for the expected far-flung air bases and naval stations.

So I'd go even lower wrt. the battleships, making them to be similar to the Dunkuerque and Richelieu, ie. 2 x 3 x 16in guns, all-forward, with a lot of place devoted to the 100-127mm multi-purpose guns and the 40mm AA guns, and ~30 kt speed.

Make a series of light cruisers to be the 'bodyguard ships' for the CVs, task being to deal with air and submarine attacks.

Modify the Ise and Hyuga into the full-blown aircraft carriers before 1942?
 

Building smaller battleships does not necessarily mean that you can build more in the timespan under discussion.

A Yamato took about 4 years to build. That is competitive with battleships built in other coutries at the same time (once a country becomes involved n war all sorts of other factors come into play to accelerate / reduce build times). First pair of KGV (35,000 tons) in Britain about 4 years. North Carolinas (35,000 tons) 3-3.5 years. Richelieu (35,000 tons) 4.5 years, First two Littorios (40,000 tons) 5.5 years (they had delays shortly after being laid down while the design was modified). You need to look at what other related industries can produce. Armour of the required thicknesses. Guns and turrets. You are still talking about needing the same number of turrets and guns for your smaller vessels that have to come out of the same industry.


Turning to the carriers, the two shadow programmes ran alongside construction of the fleet carriers.

From auxiliary ships - Zuiho converted 1940, Shoho 1941, Ryuho 1942, Chitose & Chiyoda 1943.
From merchant ships (whose construction had originally been subsidised by the Japanese Govt from 1930) - Taiyo converted Feb-Aug 1941, Unyo Nov 1941-May 1942, Chuyo July-Nov 1942. Shinyo (from the German liner Scharnhorst trapped in Japan in 1939 and purchased by the Japanese Govt in July 1942) converted Sept 1942- Dec 1943. Kaiyo (acquired Dec 1942) converted Dec 1942-Nov 1943. The other 5 subsidised ships were lost in other roles before conversion could begin.

To carrry out more conversions earlier means identifying the shipyard space to allow it to take place.

Hiyos / Unryus
The Hiyos were started as merchant liners subsidised as suitable for carrier conversion and laid down in March & Nov 1939. Acquired by the IJN unofficially in Oct 1940 (officially Feb 1941). So their construction was well advanced by the time the Pacific war broke out allowing their early completion in May and July 1942.

The Unryu, leadship of her class was planned under the Aug 1941 Rapid Naval Armament Supplement Programme. That was a Programme for 293 vessels that emerged AFTER Japan had decided that war was inevitable. Her sister ships were not ordered until Aug 1942, after Midway. The design incorporated lessons gained therefrom. Only 3, Unryu, Amagi & Katsuragi could be laid down in 1942 and ultimately were the only ones to complete in late 1944, too late to be effective. The design turned out as a significantly modified Hiryu. So Junyo & Hiyo were completed before any of the Unryus could even be laid down.

But the Unryus were meant as a Mass Production Medium Sized Carrier. The true successor to the Shokakus completed in 1941 was the Taiho ordered as part of the 1939 4th Naval Armaments Supplement Programme but with pressure of work in the yards its design couldn't be finalised until just before it was able to be laid down in July 1941. More were planned in 1942 but the Unryus took priority.

Ibuki
The two Ibukis were planned under the same Programme as the Unryu. The IJN had been looking for two more heavy cruisers since 1937 but the industrial capacity was not there to build them until 1942. Ibuki was laid down in April 1942, and suspended on 30 June 1942 after Midway. On 30 July 1942, due to her relatively advanced state of hull construction it was decided to restart work to clear the slip for construction of a carrier (most probably Aso of the Unryu class). She was launched on 21 May 1943. But it wasn't until Aug 1943 that it was decided to complete her as a carrier, after several other proposals had been considered & dismissed. The other ship (Warship No 301) having only been laid down on 1 June 1942 and when the stop order came at the end of the month was scrapped on the slip.

Ibuki was able to be laid down so quickly because the IJN accepted the existing Suzuya design with minimum changes. Reworking the Hiryu design to get an Unryu involved more extensive modifications to the design.

Japan faced three fundamental problems:-

1. Until the big carrier battles of 1942, it was still very much a battleship centric world. Carriers were a support act.
2. Japanese industrial capacity was simply not large enough to allow the laying down of more warships than it did, when it did. At the same time it was trying to build up its merchant fleet and capacity had to be allocated to that end. And merchant yards couldn't necessarily just switch to warship building. By way of example of the problems faced, Zuikaku, Hiyo & Taiho were all built in succession on the same slipway. Each had to launched before the next could be laid down. Design work on Taiho was at a very early stage when Izumo Maru (aka Hiyo) was laid down in 1939.
3. It knew that if it showed its hand in building / converting any type of ship and even allowing for its exceptionally tight security, if the US got wind of its plans they could easily outbuild Japan. Indeed the US 1940 Two Ocean Navy Act demonstrated that, and in large part began to trigger the implementation of the planned conversion programme.

And as noted above Japan needed to also significantly expand it naval aircraft production and pilot training programmes to meet the demand being generated by having more carriers.
 
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