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

Full conversion requires a lot of shipyard time. You need to spend many months tearing the ship down to level you need to redo a lot of the lower spaces. They you need the same amount of time as a new hull needs to build back up from that low level. Is it quicker to just build a new hull or tear down the old one and modify the lower spaces (magazines, fuel storage, etc)?

Perhaps I am biased. Back in the 1970s and 80s there was a real 'craze' for refurbishing fire trucks to 'save money'. My chief not only bought into it, he wrote at least one magazine article for "Fire Engineering magazine". For a small dept we wound up with more than our fair share of white elephants.

One of his 'refurbs' consisted of not repowering an existing fire truck but taking the existing engine and transmission out the old truck along with the 100 ft areal ladder system (turntable, hydraulics and stabilizer jacks) from one company and dropping those parts into a new chassis and body from another company. The engine was in good shape but the arial was old, out dated design (from the 1950s?) that was soon not even rated by insurance companies. The truck was overweight for the axles and tires when it showed up, let alone after we filled the compartments with tools. The Company that had built the ladder system didn't want to know about it after the new company mounted it on their chassis. The new chassis company didn't didn't want to know about the ladder as they hadn't built it. We had an orphan as far as repairs, warranty work. We were stuck with this thing for many years after that chief retired.
I could suck up another page (or 2) with some of the other refurb projects we were stuck with. The whole refurb craze died out when they figured out that they weren't getting the value for the money. If a fire truck was supposed to last 25 years and you refurbed it after 15 years you didn't get another 25 years after that. You were lucky you got a few more years. They should have been putting money aside to just pay for new trucks on a regular basis.

Most people today have an unrealistic idea of typical WW2 carrier task force operating speeds.

Generally they were in the region of 15-25 knots, lower if attached to a convoy. USN practice for island strikes in the 1942-44 period was for an overnight run in to the launch area at 25 knots. In 1944-45 when Task Groups lingered in support of landings the whole group would run at c15 knots and increase by a few knots for TG launches. If a carrier needed to operate her aircraft out of sequence with the TG she would adopt a speed to allow launch / recovery as quickly as possible while remaining within the rings of escorts. The BPF while operating off the Sakishima Gunto in April / May 1945 was generally operating at 17-21 knots (in the first two weeks of April TF57 speed couldn't exceed 24 knots due to Illustrious operating with only 2 props).

TG speed was also affected by the ships operating with the carriers and the need to keep the whole group together, or at least relatively close to each other, for defence. So operating with most battleships would slow things down. The operating area also played a part. The South Pacific seems to have enjoyed calmer winds that most other areas.

At Philippine Sea Taiho & Shokaku were operating at 25-26 knots when torpedoed. (Taiho was hit just a few minutes after the launch of her strike, and was not slowed by the hit).

The Junyos were perfectly capable of operating the B5N & B6N torpedo bombers as well as both the D3A and D4Y dive bombers. The light carriers however generally did not operate dive bombers, due to their relatively small air groups with the emphasis on fighters. The next generation B7A dive/torpedo bomber was designed to operate from the Taiho & later carrier designs and would probably have been beyond the capablities of the earlier carriers unless the IJN had succeeded in getting a working catapult to sea on its carriers. Otherwise they would have had to have relied on RATOG.

The IJN CVE spent most of their time as aircraft transports.

When designing the Colossus / Majestic classes the RN concluded that 25 knots was adequate for the role they envisaged for them in 1942 but raised it to only 28 knots in the 1943 Centaur class. Nothing like the 30-32 knots of the various armoured carrier classes or the 33 knots preferred by the USN, or even faster speeds favoured on the whole by the Japanese for their fleet carriers.

Thumpalumpacus said:

They will need to streamline their pilot training to increase throughput.

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Yes, as I mentioned back in the age of the dinosaurs on page 1 of the thread!

If we're happy with 25 kn carriers, I'd like to (re)toot my horn and propose carriers powered by low speed diesel engines!

tomo pauk said:

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?

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Even if we ignore the treaties, to which Japan was still a signatory when these decisions would have to be made(?), as mentioned before ~1942 nobody was really sure that carriers were the new kingmakers of naval warfare. If you were in charge of naval strategy and procurement before that and you suggested to stop investing in battleships and go all-in on carriers, you'd be sacked for recklessly endangering your nations naval power!

Now, if we postulate that the IJN has the foresight to see the future power of carrier aviation, or at least are ready to gamble their naval might on that possibility, sure, just build carriers (and yes, produce enough pilots and aircraft to fill those decks, including accounting for attrition!). Keep the old BB's (Ises, Kongos, Nagatos, ...) as escorts for carrier task groups, shore bombardment, and similar secondary duties. But don't build more of them, and don't consume valuable dockyard space/time to try to convert them to some kind of carriers either.

EwenS said:

Most people today have an unrealistic idea of typical WW2 carrier task force operating speeds.

Generally they were in the region of 15-25 knots, lower if attached to a convoy. USN practice for island strikes in the 1942-44 period was for an overnight run in to the launch area at 25 knots. In 1944-45 when Task Groups lingered in support of landings the whole group would run at c15 knots and increase by a few knots for TG launches. If a carrier needed to operate her aircraft out of sequence with the TG she would adopt a speed to allow launch / recovery as quickly as possible while remaining within the rings of escorts. The BPF while operating off the Sakishima Gunto in April / May 1945 was generally operating at 17-21 knots (in the first two weeks of April TF57 speed couldn't exceed 24 knots due to Illustrious operating with only 2 props).

TG speed was also affected by the ships operating with the carriers and the need to keep the whole group together, or at least relatively close to each other, for defence. So operating with most battleships would slow things down. The operating area also played a part. The South Pacific seems to have enjoyed calmer winds that most other areas.

At Philippine Sea Taiho & Shokaku were operating at 25-26 knots when torpedoed. (Taiho was hit just a few minutes after the launch of her strike, and was not slowed by the hit).

The Junyos were perfectly capable of operating the B5N & B6N torpedo bombers as well as both the D3A and D4Y dive bombers. The light carriers however generally did not operate dive bombers, due to their relatively small air groups with the emphasis on fighters. The next generation B7A dive/torpedo bomber was designed to operate from the Taiho & later carrier designs and would probably have been beyond the capablities of the earlier carriers unless the IJN had succeeded in getting a working catapult to sea on its carriers. Otherwise they would have had to have relied on RATOG.

The IJN CVE spent most of their time as aircraft transports.

When designing the Colossus / Majestic classes the RN concluded that 25 knots was adequate for the role they envisaged for them in 1942 but raised it to only 28 knots in the 1943 Centaur class. Nothing like the 30-32 knots of the various armoured carrier classes or the 33 knots preferred by the USN, or even faster speeds favoured on the whole by the Japanese for their fleet carriers.

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Where carrier speeds mattered is turning into the wind and/or returning to the task force. Also, Judys used RATO on smaller carriers to help with taking off. I think the Jill did, too.

z42 said:

Even if we ignore the treaties, to which Japan was still a signatory when these decisions would have to be made(?), as mentioned before ~1942 nobody was really sure that carriers were the new kingmakers of naval warfare. If you were in charge of naval strategy and procurement before that and you suggested to stop investing in battleships and go all-in on carriers, you'd be sacked for recklessly endangering your nations naval power!

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As it can be read in the post quoted, I've not suggested that investment in the BBs is stopped, but just that the investment is smaller.

tomo pauk said:

As it can be read in the post quoted, I've not suggested that investment in the BBs is stopped, but just that the investment is smaller.

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I'm not sure such a halfway position is that useful. You either have a battlefleet that has some chance of beating the enemy battlefleet, or you don't. If you don't, use your money for something else than building a fleet that will certainly be roflstomped by the enemy fleet.

z42 said:

I'm not sure such a halfway position is that useful. You either have a battlefleet that has some chance of beating the enemy battlefleet, or you don't. If you don't, use your money for something else than building a fleet that will certainly be roflstomped by the enemy fleet.

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There is certainly merit in what you are suggesting.

Among the reasons I was suggesting making these BB lights is the obsolescence of the existing Japanese BBs so some replacement might be prudent to have, the CVs might use the bigger support ships (against enemy surface units that might come close to the CVs because of fog of war, as well against aircraft), and having three of them might be worth more than having two of the big ones if it comes to blows.

Thumpalumpacus said:

Where carrier speeds mattered is turning into the wind and/or returning to the task force. Also, Judys used RATO on smaller carriers to help with taking off. I think the Jill did, too.

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This is where you run into the problems of which period and which navy you are discussing.

You seem to have missed the point I was making about certainly US carrier operations from the spring of 1943 when the operating manual was rewritten. The RN adopted these procedures in the BPF in 1945.

Where carrier task groups were operating as a unit (3-4 carriers per group) with all the carriers operating aircraft, the whole group would adjust its speed to allow launch and landing of aircraft. Only if an individual carrier within the group needed to operate aircraft out with the TG operating cycle would it reposition as far downwind as possible while remaining with the ring of escorts. It would then run into the wind at whatever speed was needed to launch / land its aircraft. It would then return to its allocated position in the group ASAP. At no point would it leave the safety of the outer ring of escorts.

But even in earlier periods a carrier couldn't afford to travel too fast and away from its escorts, lest it be attacked.

The objective seems to have been to generate c30 knots minimum wind over the deck. No natural wind conditions were rare.

z42 said:

Keeping on the topic of Japanese engines, what about diesel engines for ships? In particular, low speed two stroke ones, having several advantages (which is why they have dominated oceanic shipping post war) :

• Ability to use the same cheap bunker fuel that steamships use.
• No need for an expensive and heavy reduction gearing nor reversing gearbox as the engine itself is reversible. Thus the engine is directly connected to the prop shaft.
• Very high thermal efficiency, about 40% can be expected in the WWII time frame. Including at part load. This could be a significant advantage, as a large fraction of the Japanese oil supply was used as fuel oil for the IJN.
• With a piston ported design, as was common for two stroke diesels of the time, no valves and valve gear needed.
• If using crankcase scavenging, no need for a blower either.
• No risk for a steam explosion if the ship is hit.

There are of course also a number of significant disadvantages for this kind of engine, particularly for warships:

• Poor power density. Contemporary low speed diesels have about half the power density of medium speed diesels. In the WWII time frame medium speed diesels were somewhat worse than steam plants (the Deutschland class, for instance), but newer diesels at the time being somewhat better. OTOH without reduction gearing, separate cruising and reversing turbines etc, the low speed diesel can claw back some of that disadvantage. Further the much better fuel efficiency allows carrying much less fuel than a comparable steamship.
• Height of the engine. And not only the engine itself, there needs to be space above the engine for piston and conrod replacement. For a gunship, this probably means a bump in the citadel roof above the machine room spaces. OTOH the machinery spaces are probably shorter than for an equivalent steam plant, enabling a shorter citadel.
• Availability of large enough engines. The largest pre war diesel I'm aware of is the 22500 hp B&W 2000. With a four shaft ship, that might be enough for roughly a cruiser sized ship. OTOH with a concerted effort bigger ones might be designed.

Historically, the Japanese experience with diesels for high speed warships wasn't a happy one. They were seriously considering diesels for warships, e.g. several of the design studies leading up to the Yamato class were diesel powered, in part and total. But experience with a ship using the selected engine (sub tender Takei(sp?)) was very bad, and instead an all-steam design was chosen. Somewhere I read that at the time they were chasing the double acting diesel chimera, also resulting in a number of submarines that had to eventually be reengined with lower power single acting engines.

In 1928 Mitsui licensed the B&W marine diesel portfolio, in the interwar years the market leader in that segment. So it seems they could have built decent enough diesels, at least had they avoided the double acting chimera.

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Diesels were a rapidly advancing technology in the 1920s-30s-40s. Even a few years made difference.
Ship powerplants also often built to different standards. Destroyer powerplants (steam) were built to a much higher to weight ratio than battleship powerplants. There are a number of reasons for this.
DD powerplants are expected to break down more often.
DD powerplants are expected to get a lot more maintenance than larger ships.
DD powerplants are a lot easier to replace, if needed, than larger ships. Many destroyers only have the top deck over much of the machinery spaces. Cruiser or battleship needs holes cut through 3-5 decks.
RN expected around 20% of their destroyers to be out of service at any given time for powerplant maintenance. That was a 1930s peacetime planning estimate. Having 20% of you battleships out of service at any given time?

Geared Turbines are low profile.

Extreme steam turbine, one boiler, two turbines, 8000hp. Not as good as the German high speed diesels but for the allies.......you needed about 6 Packard gasoline engines to equal it.
Problem for diesels is scaling up. 40-60,000hp for destroyers gets very difficult.
Graf Spee had 54,000hp, an Arethusa (and later the Dido's) had 62-64,000hp.
The Hipper had 132,000hp.

A 12 year old thread on diesels.

There was often a gap between what was promised and what was delivered in both German Diesels and German Steam power plants.
And comparing high speed diesels from different years is only slightly easier than comparing aircraft engines from different years.
Problem with large ships is the 3-4 year construction time.

The IJN built a number of diesel powered (or part diesel powered) auxiliaries inter war, some as part of their shadow carrier programme. However part of the carrier conversion process saw the diesels replaced with geared turbines in most.

Sub depot ships
Taigei - 4 diesels on 2 shafts generating 14,000shp for 20 knots. Her problems went beyond the power plant. Converted to carrier Ryuho in 1942 and given geared turbines.
Tsurigisaki & Takasaki - 8 diesels on 2 two shafts generating 56,000shp for 29 knots. Converted to carriers Shoho & Zuiho in 1940 & 1941 respectively and given geared turbines.

Seaplane carrier / midget sub carrier
Chitose & Chiyoda - built with mixed power plant. Two shafts each driven by a 6,800shp (6,400shp in reality) diesel and a geared turbine of 22,000shp. Total 56,800shp for a max speed of 29 knots. Due to lack of diesel power a cruising turbine was fitted to the turbine set to allow them to cruise at 16 knots on combination of diesel and cruising turbine.

Diesels were No 11 Type 10 two stroke 10cylinder units.

They retained the machinery layout on conversion to carriers in 1943.

Seaplane carriers
Mizuho - 2x4cylinder diesels generating 15,200shp for 22 knots.
Nisshin - 2x6cylinder diesels generating 47,000shp for 28 knots.
Akitsushima - 4xdiesel generating 8,000shp for 19knots

The escort carrier Kaiyo (ex Argentina Maru) also had her original diesel machinery replaced by geared turbines which had the effect of raising her speed by 2 knots to 23.8 knots, making her the fastest IJN escort carrier.

Admiral Koga Mineichi was possibly the most boring admiral of the IJN. He managed to reach the top without making many enemies or achieving anything too notable. Koga was promoted to Vice-Admiral and placed in command of the Training Fleet on 1st December 1936. The Training Fleet had for some time operated the ancient armoured cruisers Yakumo Imperial Cruisers, Iwate Imperial Cruisers and despite being damaged Asama Japanese Cruisers. Koga historically used his influence to persuade the IJN to order a class of four training cruisers, the Katori Class Katori-class cruiser - Wikipedia, which were built quite slowly if very cheaply.

So let us assume that an Alien Space Bat decides to offer some minor help to the IJN when Koga takes command. It persuades (forces?) Koga to order a squadron of rather different ships and uses its powers to cause the IJN to accept the increased cost. Katori is built as a Japanese copy of a Colossus Class aircraft carrier with a slight increase in length to allow it to carry half of an Agano Class cruiser's machinery giving 50,000 shp rather than the 40,000 shp of Colossus. As well as allowing the cadets to learn about aircraft handling, part of the hangar could also become a classroom if desired.

In addition, two Matsu Class destroyers Matsu-class destroyer - Wikipedia are built to enable the cadets to learn how to use guns and torpedo tubes and two of the Shimushu class (ideally redesigned to make them easier to produce as in the later classes) Shimushu-class escort ship - Wikipedia to introduce the cadets to Diesel engines and to practice anti-submarine tactics with the Matsus. The last ship of the new Training Fleet is an early Ashizuri Ashizuri Class to allow the cadets to learn the mysteries of underway resupply of a carrier with aviation gasoline, bombs, torpedoes and other supplies.

The planned cost of a single Colossus, £1.8 million, is about the same as all four Katori Class (assuming that the last, Kashihara, had been completed). However, the Colossus Class carriers ended up costing around £2.3 million each, possibly because of wartime inflation. Thus we are spending more on the Training Fleet than the IJN planned. However, we might also be setting up some interesting production lines, especially if the carrier can be built in Japan nearly as fast as Colossus was built for the RN. The other consequence is that the staff of the Training Fleet learn useful things whilst baffling their cadets.

cherry blossom said:

Admiral Koga Mineichi was possibly the most boring admiral of the IJN. He managed to reach the top without making many enemies or achieving anything too notable. Koga was promoted to Vice-Admiral and placed in command of the Training Fleet on 1st December 1936. The Training Fleet had for some time operated the ancient armoured cruisers Yakumo Imperial Cruisers, Iwate Imperial Cruisers and despite being damaged Asama Japanese Cruisers. Koga historically used his influence to persuade the IJN to order a class of four training cruisers, the Katori Class Katori-class cruiser - Wikipedia, which were built quite slowly if very cheaply.

So let us assume that an Alien Space Bat decides to offer some minor help to the IJN when Koga takes command. It persuades (forces?) Koga to order a squadron of rather different ships and uses its powers to cause the IJN to accept the increased cost. Katori is built as a Japanese copy of a Colossus Class aircraft carrier with a slight increase in length to allow it to carry half of an Agano Class cruiser's machinery giving 50,000 shp rather than the 40,000 shp of Colossus. As well as allowing the cadets to learn about aircraft handling, part of the hangar could also become a classroom if desired.

In addition, two Matsu Class destroyers Matsu-class destroyer - Wikipedia are built to enable the cadets to learn how to use guns and torpedo tubes and two of the Shimushu class (ideally redesigned to make them easier to produce as in the later classes) Shimushu-class escort ship - Wikipedia to introduce the cadets to Diesel engines and to practice anti-submarine tactics with the Matsus. The last ship of the new Training Fleet is an early Ashizuri Ashizuri Class to allow the cadets to learn the mysteries of underway resupply of a carrier with aviation gasoline, bombs, torpedoes and other supplies.

The planned cost of a single Colossus, £1.8 million, is about the same as all four Katori Class (assuming that the last, Kashihara, had been completed). However, the Colossus Class carriers ended up costing around £2.3 million each, possibly because of wartime inflation. Thus we are spending more on the Training Fleet than the IJN planned. However, we might also be setting up some interesting production lines, especially if the carrier can be built in Japan nearly as fast as Colossus was built for the RN. The other consequence is that the staff of the Training Fleet learn useful things whilst baffling their cadets.

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The Japanese could have done a lot to improve training right before Pearl Harbor and in 1942/43. But the main things that needed to changed were attitude, doctrine.
The US navy managed to use the two famous Great Lakes paddle wheel carriers to help train up to 17,000 pilots and other personnel. One (or both?) were coal fired.
They managed to train the number of pilots/crewmen despite operating on Lake Michigan which froze during the winter.
Japan could have used the Inland sea and a similar, cheap conversion to help with training.
Japan was not only country to neglect ASW. What is needed is not special training ships but a change in attitude and special school (shore base) to teach both crews and captains how to engage subs. Old DDs and other patrol boats may not be ideal but they can work/substitute. The school can also provide faster feed back concerning sensors and tactics. What works and what doesn't. Japanese navy was claiming up to 10 times the number sub of kills they were really getting. Just 3-5 old subs acting as clock work mice might have quickly shown them that this rate of kills to number of engagements was way off.

If I may pivot back to aircraft, Japan's aircraft cannons have an interesting dichotomy.
The Type 99 mark 2 hit reasonably hard (Its HEF shells seemed to pack a punch) and was decently accurate but fired very slowly and could not be synchronized.
Meanwhile the Ho-5 had a blistering fire rate, could be synchronized relatively easily and was highly reliable but was one of the weakest 20 mm cannons in the world and had very poor muzzle velocity. The weaknesses of one were the strengths of the other, and vice-versa.
The IJN attempted to fix the low fire rate of the Type 99 with the Model 5 (sources are scarce, Wiki says between 670~750 rpm). Meanwhile the IJA attempted to fix the low power of the Ho-5 through their own quasi-Minengeschoß designated Ma 202 - 8.7 g of incendiary with 3.2g of PETN, designed to explode on contact then let the incendiary do its job better. I'm unsure if the Ma 202 had a different muzzle velocity, but it likely wouldn't be much different than the ~700 m/s of the HEFI and AP shells.

I'm unsure which direction to go with here; either standardize one of the two cannons for aircraft or license a different, better cannon for standardization.

Japanese have had even worse problems with their fighters' firepower before these two guns are introduced. The Army 1-engined fighters were without any 20mm cannons for the 1st half of the Japanese-Anglo-American war, while the Zero was with the cannon that had the low ammo count per cannon, and the 1st Type 99 was with just 600 m/s.
The adopted anti-tank cannons were never installed in the Army 1-engined fighters - while not great, installing a pair of these on a 1-engined fighter would've still brought a good firepower against the sturdy Western aircraft, especially the 4-engined types.

Several things the Japanese can do before they bomb Pearl Harbor:
- to adopt the Mine shell for their cannons ASAP (would've give the Type 99 Mk1 the 700 m/s MV; I consider 700 m/s as a reasonably good MV)
- early adoption of belt feed for the Mk1
- big drum ASAP for the Mk1
- both Navy and Army adopt the Mk2 (Oerlikon FFL) ASAP
- speed up the Oerlikons

Let's say that 3 of these suggestions are actually accepted and proceeded with. For the Navy, that would've gave them less need for the Zeros to land and rearm, with major consequences during the battles of 1942, especially at Midway. For the Army, that means they actually have cannon-armed fighters between 1941 and 43, again meaning that hits are transformed into kills much more reliably.
Once engine power is up (1500 HP and more), 4 light cannons are not too heavy to lug around for a fighter.

More adventurous suggestions:
- neck-out the Army 20mm gun to 23mm
- a 25mm gun, with lower MV than the 25mm AA gun but with greater RoF, lighter and more compact
- adopt the MK 108 but with lighter & shorter shell for the greater MV

TM06 said:

I'm unsure which direction to go with here; either standardize one of the two cannons for aircraft or license a different, better cannon for standardization.

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The problem here is license when.
What was available in 1938?
1939?
1940?
1941?
and available from whom?
Japan was getting increasingly isolated from the world in the 1930s due to it's war in China. It's ability to access foreign weapons was getting more and more restricted.
The Japanese did develop quite a number of weapons of their own. They managed to get them to function which is the main objective early in development (see US 37mm gun which had problems).

TM06 said:

The Type 99 mark 2 hit reasonably hard (Its HEF shells seemed to pack a punch) and was decently accurate but fired very slowly and could not be synchronized.
Meanwhile the Ho-5 had a blistering fire rate, could be synchronized relatively easily and was highly reliable but was one of the weakest 20 mm cannons in the world and had very poor muzzle velocity. The weaknesses of one were the strengths of the other, and vice-versa.

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Problem here is timing. The type 99 mk II is late, coming into service in late 42/early 43. It takes time ( a lot of it) to replace the majority of front line aircraft, 6 months to a year.
The Ho-5 being the weakest 20mm requires a 2nd look. Since it was also a rather light cannon and it's ammo was light it's power to weight ratio was good.
The blistering rate of fire soon vanished. Lack of chrome and nickel for high quality steel alloy parts soon required a drop in rate of fire to 700-750rpm. Which is still pretty good.
The rate of fire does mean that the weight of shell/s delivered is still pretty good. Even it's 'normal' HE shell (79 grams) held 4g Cyclonite + 3.7g of incendiary which put's payload higher than than the Soviet 20mm gun/s. (6.1g?)
Compared to the 10g HE in the type 99 shells it is a around 75% but the HEI shell in the type 99s held 5g TNT + 3g incendiary so the explosive/incendiary effect was closer.

Basic problem for the Japanese Army (especially) and Navy was not enough guns in the majority of their aircraft for bomber busting. Allied (American) bombers got larger during the war (B-25sin 1942, B-24s showing in at least some numbers in 1943, B-29s show up late) but using Ki-43s as bomber busters for way, way too long shows the problem.

TM06 said:

The IJN attempted to fix the low fire rate of the Type 99 with the Model 5 (sources are scarce, Wiki says between 670~750 rpm)

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According to one source the type 99 model 5 production was 10 prototype and 50 pre-production by July 1945. It is basically a non-starter. An extra 200 guns in July of 1945 doesn't change anything.
The Navy was trying to shift to four 20mm guns in the land based interceptors with slower firing guns but basically these didn't show up in even scant numbers in service squadrons until 1944, too late. The Raiden also shows a problem, with 621 planes built 260 (?) had two long barrel 20mm guns and two short barrel. Trajectories/times of flight very hard to synchronize at any but the shortest ranges and all of these planes were built in 1944. Later 1944 production got four 20 mm guns of the same type. The Early J2Ms had a single long barrel gun in each wing. Were they trying to keep the weight down or were there production/supply problems?

tomo pauk said:

Japanese have had even worse problems with their fighters' firepower before these two guns are introduced. The Army 1-engined fighters were without any 20mm cannons for the 1st half of the Japanese-Anglo-American war, while the Zero was with the cannon that had the low ammo count per cannon, and the 1st Type 99 was with just 600 m/s.

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Very true, Japanese army was slow even to get 12.7mm guns into aircraft. And two 7.7mm and two 12.7mm guns was not state of the art in late 1942 or 1943. Especially the Japanese 7.7mm gun and the Japanese 12.7mm guns.

tomo pauk said:

The adopted anti-tank cannons were never installed in the Army 1-engined fighters - while not great, installing a pair of these on a 1-engined fighter would've still brought a good firepower against the sturdy Western aircraft, especially the 4-engined types.

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The Ho-3 was obsolete, it was more a contemporary of the French HS-7 & 9 guns. Very similar ballistic performance, weight and rate of fire (also drum feed). Basic problem for further development is the 400rpm rate of fire (peak?). A Spitfire VB of mid 1941 could deliver the same weight of fire as three Ho-3s and with 120 rounds in the drums compared to the 150 in the three Ho-3s the total number of shells was 80%. Japanese needed to both speed up the rate of fire significantly and change the feed system. Or get the smaller, lighter, Ho-5 into production a lot earlier.

tomo pauk said:

Several things the Japanese can do before they bomb Pearl Harbor:
- to adopt the Mine shell for their cannons ASAP (would've give the Type 99 Mk1 the 700 m/s MV; I consider 700 m/s as a reasonably good MV)
- early adoption of belt feed for the Mk1
- big drum ASAP for the Mk1
- both Navy and Army adopt the Mk2 (Oerlikon FFL) ASAP
- speed up the Oerlikons

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-German mine shell is only of benefit if the country in question can actually manufacture them in quantity at an acceptable scrap rate. Japanese might have gone for a lighter conventional shell if higher MV was really desired.
-easier was simply using a larger magazine/drum. Oerlikon was offering 45, 60, 75 and 100 round drums in the 1930s. Perhaps the larger ones had problems? or didn't work well when flipped on their sides?
- see above
-Japanese army was fooling around with a few Oerlikon derived guns in 1930s. The type 94 flex 20 x 99mm. 15 round drum/easy fix. 43kg, a harder fix. 380rpm, an even harder fix.
People were trying to improve the rate of fire for the Oerlikon 20mm guns during the 1930s, they made made progress ( they started at around 300rpm) but without more details we don't know why it took so long. Please remember that Oerlikon guns are blowbacks and they are rather dependent on the quality of the cartridge case. If the quality of the casings are not good enough they occasionally blow out with high pressure gas coming back through the action and/or the casing/parts getting stuck in the barrel. Getting a gun to fire fast is really easy, getting to fire fast and do it reliably and without be dangerous to both the gun itself and the crew is a harder. Different nations may have had different safety standards.

tomo pauk said:

Once engine power is up (1500 HP and more), 4 light cannons are not too heavy to lug around for a fighter.

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Very true, But how light are we talking? and how much ammo?
and depending on the gun, what does it take to go from drum to belt? Hispano gun specs often do not include the belt feed device 8-10.5kg? but then a 60 round drum was around 10-14kg(?)

tomo pauk said:

Japanese have had even worse problems with their fighters' firepower before these two guns are introduced. The Army 1-engined fighters were without any 20mm cannons for the 1st half of the Japanese-Anglo-American war, while the Zero was with the cannon that had the low ammo count per cannon, and the 1st Type 99 was with just 600 m/s.
The adopted anti-tank cannons were never installed in the Army 1-engined fighters - while not great, installing a pair of these on a 1-engined fighter would've still brought a good firepower against the sturdy Western aircraft, especially the 4-engined types.

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I'd argue that the IJA single-engined fighters would've been fine had they added another pair of 12.7 mm guns earlier, the Ho-103 was a reasonably capable HMG with good fire rate, decent muzzle velocity and reasonable one-second-burst-mass when they introduced the Ma 102 shell. Most of the aircraft the IJA were facing at that point in the war didn't seem to warrant the installation of cannons much earlier.
Being proactive is a boon though, so testing the installation of the Ho-1 or especially the Ho-3 isn't a bad call.

tomo pauk said:

Several things the Japanese can do before they bomb Pearl Harbor:
- to adopt the Mine shell for their cannons ASAP (would've give the Type 99 Mk1 the 700 m/s MV; I consider 700 m/s as a reasonably good MV)
- early adoption of belt feed for the Mk1
- big drum ASAP for the Mk1
- both Navy and Army adopt the Mk2 (Oerlikon FFL) ASAP
- speed up the Oerlikons

Let's say that 3 of these suggestions are actually accepted and proceeded with. For the Navy, that would've gave them less need for the Zeros to land and rearm, with major consequences during the battles of 1942, especially at Midway. For the Army, that means they actually have cannon-armed fighters between 1941 and 43, again meaning that hits are transformed into kills much more reliably.
Once engine power is up (1500 HP and more), 4 light cannons are not too heavy to lug around for a fighter.

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Those seem to be the most future-proof options.
IMO, the Mark 1 has a much lower ceiling than the Mark 2, so it might be worthwhile to ditch the Mark 1 for fighters earlier than was done historically - both guns were produced around the same time but the IJN preferred the lighter Mark 1 (a little political corruption also helped on that front).
A faster-firing Mark 2 with a mine shell showing up by late 1940 to early 1941 would significantly improve their chances.

tomo pauk said:

More adventurous suggestions:
- neck-out the Army 20mm gun to 23mm
- a 25mm gun, with lower MV than the 25mm AA gun but with greater RoF, lighter and more compact
- adopt the MK 108 but with lighter & shorter shell for the greater MV

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For what it's worth, the army did scale up the Ho-3/5 system to create the 37 mm Ho-203 and Ho-204 respectively, the latter of which was arguably the best 37 mm guns of their class (BK 3.7, 37 mm M4, NS-37, Vickers S). There's also the Ho-155 which was a Ho-5 scaled up to 30 mm, which was also a very capable and surprisingly lightweight cannon for its class.
Both upscaling attempts being successful points to a 23 mm gun being quite reasonable as well, and it would be the one out of those 3 suggestions I would hedge my bets on.

Shortround6 said:

Problem here is timing. The type 99 mk II is late, coming into service in late 42/early 43. It takes time ( a lot of it) to replace the majority of front line aircraft, 6 months to a year.
The Ho-5 being the weakest 20mm requires a 2nd look. Since it was also a rather light cannon and it's ammo was light it's power to weight ratio was good.
The blistering rate of fire soon vanished. Lack of chrome and nickel for high quality steel alloy parts soon required a drop in rate of fire to 700-750rpm. Which is still pretty good.
The rate of fire does mean that the weight of shell/s delivered is still pretty good. Even it's 'normal' HE shell (79 grams) held 4g Cyclonite + 3.7g of incendiary which put's payload higher than than the Soviet 20mm gun/s. (6.1g?)
Compared to the 10g HE in the type 99 shells it is a around 75% but the HEI shell in the type 99s held 5g TNT + 3g incendiary so the explosive/incendiary effect was closer.

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As mentioned earlier, the two guns were created around the same time, the Mark 2 was only introduced that late due to IJN preference and political corruption. Stamping that out makes the Mark 2 appear much earlier than it did historically.

The Ho-5 isn't the weakest 20 mm by P/W yes, but in terms of one-second-burst-mass it fell a bit short, and dropped further once they cranked down the fire rate. Comparing it favourably to the Russian 20 mm shells is also damning with faint praise, the Russian shells were probably the worst in the world (?) but their cannons - especially the B-20 - were solidly better.

Shortround6 said:

According to one source the type 99 model 5 production was 10 prototype and 50 pre-production by July 1945. It is basically a non-starter. An extra 200 guns in July of 1945 doesn't change anything.

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If Wiki is right and the main change of the Mod 5 is the stronger buffer springs, I'd imagine it would be relatively easy to introduce that earlier in the timeline, no? The fire rate doesn't need to be stratospheric, but 480~500 rpm is unnaceptable for a 20 mm.

Shortround6 said:

The Ho-3 was obsolete, it was more a contemporary of the French HS-7 & 9 guns. Very similar ballistic performance, weight and rate of fire (also drum feed). Basic problem for further development is the 400rpm rate of fire (peak?). A Spitfire VB of mid 1941 could deliver the same weight of fire as three Ho-3s and with 120 rounds in the drums compared to the 150 in the three Ho-3s the total number of shells was 80%. Japanese needed to both speed up the rate of fire significantly and change the feed system. Or get the smaller, lighter, Ho-5 into production a lot earlier.

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See it from the Japanese PoV. A pair of Ho-3 cannons (plus perhaps a pair of LMGs for the feel-good feeling) is far cry vs. even the heaviest firepower the Ki-43 had. Even the Ki-43 and Ki-61 were initially with just 2 LMGs and 2 HMGs.
I have nothing against the better and more refined weapon, just that this is available historically, and can do the job until these better weapons arrive.

Shortround6 said:

-German mine shell is only of benefit if the country in question can actually manufacture them in quantity at an acceptable scrap rate. Japanese might have gone for a lighter conventional shell if higher MV was really desired.

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Good call on the 'classic' lighter shell for better MV.

Shortround6 said:

Very true, But how light are we talking? and how much ammo?
and depending on the gun, what does it take to go from drum to belt? Hispano gun specs often do not include the belt feed device 8-10.5kg? but then a 60 round drum was around 10-14kg(?)

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What does it take? Inspiration supported by money and resources.
Japanese-produced guns were generally pretty light, from 30 to 50 kg.
The more the ammo the better, favoring the belt feed system.