What Can Japan Do To Improve The Performance Of Its WW2 Submarine Fleet?

I'm suggesting they time it for no later than February of 1943 to avoid the monsoon season. Perhaps earlier yet, end-December 1942, and not even give the British a chance to mount that offensive into the Arakan. Keep them continuously off balance. The strategy will be that of Operation U-Go, just mounted a year-plus in advance. India was not well defended in late 1942-early 1943.

As for that last item, I don't know if it necessarily holds 100 percent true. Commonwealth Spitfire V pilots were STILL attempting to engage A6Ms and Ki-43s in turning battles (with predictable results) as late as March 1943.
Ceylon may be a good choice as the locals my be ready to rebel against the British.
 
A human aspect that is overlooked is that IJN crews fatigued quicker than their USN and RN peers due to the more austere conditions aboard IJN subs.
IJN submariners were arguably more used to hardship than Allied or even Kriegsmarine crews, but yes - the fact that they were doesn't mean such conditions should have been imposed.

Another human aspect that goes overlooked is that IJN submariners largely lacked the aggressiveness that their American counterparts displayed. Men like Takakazu Kinashi were not the norm.
 
A human aspect that is overlooked is that IJN crews fatigued quicker than their USN and RN peers due to the more austere conditions aboard IJN subs.
Conditions on Japanese submarines were terrible, especial in the tropical heat where condensation was ubiquitous and heat rash was a common problem. The Germans were a little better off, and the British were better still, with more fresh water, better food and better trained crews. The Italians also had poor living conditions. American F Boats had air-conditioning and enough fresh water for the men to stay clean shaven actually take showers and have clothes washing machines. They had refrigerators and small freezers for ice cream.

The A/C wasn't just a luxury it protected electronic and electrical systems, kept mold under control, and protected the general health and moral of the crew. I've read that late in the war British crews got to visit U.S. Boats and were amazed by the better living conditions aboard. But life for no one was ever easy on a submarine. during the late Cold War my ship ended up spending half its time chasing Soviet boats. Even then their conventional boats were falling apart, and their nukes were poisoning their crews.
 
One of my pet ideas for the Japanese to take Ceylon in the spring of 1942 when it was lightly garrisoned. With 6 long range Japanese subs and a few German Type IX & XIV U-Boats joining them they could isolate India, and threaten the convoy routes to Suez, and Basra. The Allied position in India & the Middle East would be seriously threatened, and the major Soviet supply line though Iran imperiled. The problem was the IJA didn't want to do it until it was too late.
It's 5300 km from Tokyo to Singapore, another 2600 km from Singapore to Ceylon, and is attacking Soviet flagged ships directly-something they refrained from in the Bering straights historically. It's nearly twice the distance of Hawaii from Tokyo, they simply can't support any sustained invasion which would require carriers to be parked nearby for weeks.

The Japanese Empire didn't care a bit about the Axis, firing at Soviet flagged ships in the Indian Ocean is a quick way to more Soviet arms in China.
 
It's 5300 km from Tokyo to Singapore, another 2600 km from Singapore to Ceylon, and is attacking Soviet flagged ships directly-something they refrained from in the Bering straights historically. It's nearly twice the distance of Hawaii from Tokyo, they simply can't support any sustained invasion which would require carriers to be parked nearby for weeks.

The Japanese Empire didn't care a bit about the Axis, firing at Soviet flagged ships in the Indian Ocean is a quick way to more Soviet arms in China.
They wouldn't need to park carriers off Ceylon for weeks. Once they captured a few airfields land-based aircraft would fly in from the Andaman Islands. The IJN doesn't have to torpedo Soviet flagged ships in the Indian Ocean. They can let the German U-Boats do that. Besides would a Soviet flagged ship know it was torpedoed by a Japanese or German submarine?
 
They wouldn't need to park carriers off Ceylon for weeks. Once they captured a few airfields land-based aircraft would fly in from the Andaman Islands. The IJN doesn't have to torpedo Soviet flagged ships in the Indian Ocean. They can let the German U-Boats do that. Besides would a Soviet flagged ship know it was torpedoed by a Japanese or German submarine?
The Russian ships were employed on the Pacific Route. Only 1 was sunk by a IJN sub, six others were sunk by USN subs :/
 
It's 5300 km from Tokyo to Singapore, another 2600 km from Singapore to Ceylon, and is attacking Soviet flagged ships directly-something they refrained from in the Bering straights historically. It's nearly twice the distance of Hawaii from Tokyo, they simply can't support any sustained invasion which would require carriers to be parked nearby for weeks.

The Japanese Empire didn't care a bit about the Axis, firing at Soviet flagged ships in the Indian Ocean is a quick way to more Soviet arms in China.

They wouldn't need to park carriers off Ceylon for weeks. Once they captured a few airfields land-based aircraft would fly in from the Andaman Islands. The IJN doesn't have to torpedo Soviet flagged ships in the Indian Ocean. They can let the German U-Boats do that. Besides would a Soviet flagged ship know it was torpedoed by a Japanese or German submarine?
This is why I proposed Invasion India and a blockade of Ceylon. The supply line for a Ceylon invasion is admittedly a bit lengthy. But why count the distance from Tokyo to Singapore? The troops required to invade Ceylon can embark from Burma and/or the Andamans after coming there overland from China. All those Kwantung Army divisions just sitting around up north doing nothing but growing fat and idle could use some busy work. And Kido Butai is more than capable of operating at length out of South East Asia with Singapore as its primary anchorage.

But in all fairness, this kind of major move into the Indian Ocean area cannot take place after a Midway debacle. The only way it can work is if Midway, and perhaps even Coral Sea, doesn't happen. And MOST ideally, only if Japan doesn't involve America.
 
And be very very clear, Japan built good subs
In terms of offensive capability yes. New Junsens were fast and well-armed. Defensively, however, they weren't so good. Shallow test depth was a problem. Compared to u-boats, especially, IJN subs couldn't dive very deep--only about 2-300 feet. From what I've read that, and rather poor mauverability, made IJN subs "easy prey for astute ASW forces."
 
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IJN subs were a serious threat from the beginning to the end of the war.
Generally they weren't much of a threat or very effective after 1942. Occasional kills like Liscombe Bay and Indianapolis weren't much compared to the slaughter IJN subs endured. The Marianas campaign was egregious. A single US vessel, the England, picked off around half a dozen IJN subs, and those were far from the only losses in June '44.
 
Even if the IJN gets better at anti merchant sub tactics I don't think it's going to matter, the context is just far to different than say the Germans in the North Atlantic

The German sub fleet had a small island just off the coast of their main operating bases to try and isolate. The IJN has a far far harder job to do here. To do any thing remotely analogous to the allies they will have to cover an area larger are over far greater distances.

And to make all this worse Japanese just can't make and support that larger sub fleet, and it will be operating in the teeth of the USN which had far, far greater resources so it will suffer from attrition

Plus sinking merchant ships to try an eke out some kind of win in a prolonged resource war is not Japan's plan because they know they lose that war it no matter what for reasons outside of sub-warfare anyway

We look at the Decisive battle doctrine and see it's unavoidable flaws now, but it's the only one they have because it's the only one that has any chance of working simply because in comparison all others have no change of working. All they can do it hope they can do enough damage to the USN that the US decides Japan is not worth the bother and leaves them to it. Now we know that's not going to work, but at the time is was the only hope they had. So IJN subs will be used in that role



 
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And to make all this worse Japanese just can't make and support that larger sub fleet, and it will be operating in the teeth of the USN which had far, far greater resources so it will suffer from attrition
The most remunerative hunting ground for IJN subs was the Indian Ocean, where targets were plentiful and allied ASW relatively weak. IJN subs enjoyed a fair amount of success there down to early '44.
If the IJN had intended to focus on merchant ships, it would've built much smaller subs than the New Junsens, or more RO boats instead of I-boats. Assuming a POD of some years before the war, Japan could've had about twice as many subs, and better able to sink merchant ships as their smaller size made it possible to approach enemy vessels with less chance of being spotted at night, just like u-boats did. (Indeed the Germans had a low opinion of big I-boats and gave Japan at least one IX u-boat to copy to better hunt merchantmen.)
Even with twice as many boats, Japan would've stood no chance of sinking enough ships, by itself. But what if, in 1942, when losses to u-boats gravely concerned the allies, IJN subs increased tonnage losses by 50% or so?
 
The most remunerative hunting ground for IJN subs was the Indian Ocean, where targets were plentiful and allied ASW relatively weak. IJN subs enjoyed a fair amount of success there down to early '44.
If the IJN had intended to focus on merchant ships, it would've built much smaller subs than the New Junsens, or more RO boats instead of I-boats. Assuming a POD of some years before the war, Japan could've had about twice as many subs, and better able to sink merchant ships as their smaller size made it possible to approach enemy vessels with less chance of being spotted at night, just like u-boats did. (Indeed the Germans had a low opinion of big I-boats and gave Japan at least one IX u-boat to copy to better hunt merchantmen.)
Even with twice as many boats, Japan would've stood no chance of sinking enough ships, by itself. But what if, in 1942, when losses to u-boats gravely concerned the allies, IJN subs increased tonnage losses by 50% or so?
Compared to the N.Atlantic the IJN subs just can't sink enough to be a grave concern to anything but the most limited and local context. And what does operating in the Indian ocean actually get them in terms of wining the war?

The Imperial Japanese Navy's doctrine of fleet warfare (guerre d'escadre) resulted in its submarines seldom posing a threat to allied merchant convoys and shipping lanes to the degree that the Kriegsmarine's U-boats did as they pursued commerce raiding against Allied and neutral merchant ships. During the war, IJN submarines did sink about 1 million tons (GRT) of merchant shipping (184 ships) in the Pacific; by contrast U.S. Navy submarines sank 5.2 million tons (1,314 ships) in the same period,[5] while U-boats of Nazi Germany's Kriegsmarine, the IJN's Axis partner, sank 14.3 million tons (2,840 ships) in the Atlantic and other oceans.


You could triple the amount the IJN subs sank and it's still just not very much on the scale of WW2 especially when you consider either side's ability to replace losses

Plus as I said the wallie resources in this area just mean they re-adjust resources and go after them, and while those redirected resources have to come from somewhere that's also true for the IJN and the IJN is far more resource strapped than the wallies here, especially as they also have to maintain a huge surface fleet

Plus the Indian ocean is a long way from the Japanese home yards, so they will have to operate from captured territory. This is not problem in itself (the KM operated from France happily enough), but here even those captured bases are also a long way and over water from home yards. It just basically a harder operating context for Japanese Subs..

Just looking at the Indian Ocean it almost twice the size of the North Atlantic,
 
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You could triple the amount the IJN subs sank and it's still just not very much on the scale of WW2 especially when you consider either side's ability to replace losses

If the IJN sub force was twice the size of OTL and designed for anti-commerce operations, triple is probably an underestimate. If the force bagged 2 million extra tons in the Indian Ocean (and off the east coast of Australia etc) in '42, that, coupled with the huge losses inflicted by u-boats (and some Italian subs) that year, could've meant serious problems for the allies before either their shipbuilding or ASW expansion was really underway.

Plus the Indian ocean is a long way from the Japanese home yards, so they will have to operate from captured territory. This is not problem in itself (the KM operated from France happily enough), but here even those captured bases are also a long way and over water from home yards. It just basically a harder operating context for Japanese Subs..

Japan sent many subs there, albeit in conjunction with its battle fleet, in April 1942. History shows that, despite its size, the Indian Ocean was a relatively safe and remunerative hunting ground.
 
If the IJN sub force was twice the size of OTL and designed for anti-commerce operations, triple is probably an underestimate. If the force bagged 2 million extra tons in the Indian Ocean (and off the east coast of Australia etc) in '42, that, coupled with the huge losses inflicted by u-boats (and some Italian subs) that year, could've meant serious problems for the allies before either their shipbuilding or ASW expansion was really underway.

and where does that 2m figure come from?

this is one of those perfect scenarios where the IJN doubles it's production and changes it sub tactics all while the allies do nothing and you get a perfect storm of lost allied shipping in the Indian ocean

And even ignoring all that this requires the Japanese for some reason deciding to ignore their OTL plan and go all in in killing allied merchant shipping in the Indian ocean, why? How does that get them a win here?

For a counter example if the KM can force the UK to bow out that not only removes the UK from the war, it also removes the UK as a base for any possible US invasion of Europe and makes it far less likely the US will even declare war against Germany and Italy. That's a reason to do a thing.


Japan sent many subs there, albeit in conjunction with its battle fleet, in April 1942. History shows that, despite its size, the Indian Ocean was a relatively safe and remunerative hunting ground.
In conjunction with a surface fleet is kind of key, So now you have to keep a surface fleet here

Also you keep making this point but how much merchant shipping did Japanese subs actually hit in the Indian ocean

also it was just in conjunction with a Japanese Surface fleet but also with the KM subs at Penang
 
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Ideas that I have come up with:

1. Get rid of the Army's submarine program and only have a naval submarine program.
2. Get rid of the supply submarines and aircraft carrier submarines and focus entirely on normal attack submarines.
3. Focus submarine strategy on attacking merchant shipping instead of attacking warships.

Any other ideas?
Sure, use the labor and material wasted on Yamato, Musashi and Shinano for sub production instead.
 
One of my pet ideas for the Japanese to take Ceylon in the spring of 1942 when it was lightly garrisoned.
One problem with Ceylon is its numerous land leeches. Even if the allies didn't fire a shot taking Ceylon could cost the Japanese a lot of blood, lol.
 
Consider another point. In 1942 the IJN submarines sent the Saratoga to the dry-docks twice with successful torpedo attacks. Once off Hawaii in January 1942, and again in the SE pacific in September. They destroyed the Wasp and sank the Yorktown. This was about the same success rate as the IJN carrier fleet. In 1943-44 the subs sank the escort carrier Liscombe Bay and sent two US fleet carriers back to dry dock with torpedo hits. The IJN carrier fleet scored 0 those two years.
Which 2 US fleet carriers were hit by IJN subs in 1943-44? Or you may perhaps refer to any of Lexington, Independence and Intrepid which were torpedoed by aircraft in that period?
 
Much better to make an extra five or six Shokaku-class.
In an ideal world hell yes, but since this is about subs, if the OP permits we can do the whole rob Peter to pay Paul thing to at least get more subs.

So my take is smaller Yamatos, so 20,000 tons x3 steel is used for more subs, that would be what, 20 good size subs? Then Unryus instead of Junyos so another 2x 7000 tons of steel saved for another say 6 good subs. And not building the No.111 so whatever steel already used go to subs, i've read it was 30% completed so that would make another roughly 20,000 tons of steel, so say another 8 decent subs.

And as impressive as they were, perhaps the I-400s shouldn't be built, but built twice as many smaller subs?

Overall, we get to roughly another 40 subs for IJN before and during the war, a significant increase. Perhaps even more can be had if more shuffling is done than i postulated above.

Can we give them a bit more luck as well? With more subs available and not doing the whole minisub attack fiasco, perhaps the IJN subs can hit and sink Enterprise outside Pearl Harbour in 1941. And maybe sink Saratoga outright either in February or August 1942. And with more subs, maybe some more of them can be send to Midway, maybe forming some additional scouting lines, so maybe at least they'll get to detect Yorktown if nothing else and alert Nagumo?
Could they sink North Carolina as well (that was one incredibly lucky shot, but if the IJN had more subs in the area because they have more to begin with in the first place, who knows?)

Later on from 1943 onwards, it is indeed a question of better tactics/better use for the IJN subs, a domain i'm not familiar with unfortunately. It's quite bewildering that from the huge mass of US fleet carriers and other large combatants flooding the Pacific they haven't hit even one that i can think of from 1943 to 1945 (not counting the already mentioned CVE and CA), despite the good success they had against USN CVs and some other large combatants in 1942.

So with more subs, and better tactics/use let's say they manage to triple the kill count over OTL, so 3 million tons instead of 1 million tons sunk, including several large combatants (carriers, BBs cruisers etc.) damaged or sunk between 1943-45. Maybe not enough to affect the overall war outcome, but enough to affect it's progress tactically.
 
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