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

Historically, if there was any part of the Japanese military that was underutilized during World War Two, it was its submarine fleet. It was overly diverse in submarine design and type, prioritized attacking military vessels instead of merchant shipping, had the usual IJA v IJN feud, and was just never a priority. Yet had it been a priority for the Japanese government, it could have potential for helping the Japanese war effort significantly (OBVIOUSLY NOT A WAR WINNING POD). Unlike the Atlantic, commerical shipping in the Indian and Pacific Ocean was not well protected. Not to mention the British reliance on India for the war effort and the US having to shipping millions of tons of supplies to the South Pacific Islands to keep the war effort going. So, what changes can we make in this atl world so that the Imperial Japanese submarine force can devastate the Allies in the Pacific Theater of the war (WITHOUT DECREASING PRODUCTION OF MILITARY EQUIPMENT IN OTHER AREAS).

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?
 
IJN submarine strategy was dictated by the obsession with the Decisive Battle doctrine. Their subs were part of the plan to whittle down the U.S. Fleet as they advanced across the Pacific. Getting the IJN to change their whole grand strategy wouldn't be easy. By temperament and doctrine their submarine force would feel ill-used if they were ordered to focus on merchant shipping. As it was, they did score some major success at a critical point of the war. In late 1942 They sank 2 aircraft carriers, seriously damaged a Battleship, and sank the CL Juneau. During the invasion of Makin Island, they sank a CVE. At the end of the war, they also sunk the CA Indianapolis. IJN subs were a serious threat from the beginning to the end of the war.

Throughout the war USN submarines had the primary mission of sinking IJN warships. If U.S. intel had any idea where IJN warships would be subs were sent to lurk off their ports to ambush them. USN subs spent a lot of their time doing recon of every kind, checking the weather, recovering downed aviators, and dropping off UDT's. Going after merchant shipping was almost something they did when they had nothing else to do. It's no wonder that even with rotten torpedoes they sank 156 IJN warships, or 1/3 of the Japanese Fleet.
 
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Unless they figure out some way to find and attack that merchant shipping, all of what you listed is just rearranging deck chairs on the Titanic. A survey of Japanese submarine operations leaves one with the inescapable conclusion that something was fundamentally wrong with Japanese submarine search tactics. Post-Pearl Harbor the Japanese posted multiple submarines around Oahu that accomplished basically nothing of note, and their submarine picket line at Midway was a comprehensive failure that is an underrated part of the disaster at Midway.

In fairness, a large part of this is simply that finding ships from a submarine is hard. Despite the advantages of excellent intelligence work, Wolfpack tactics, and predictable convoy routing, the Germans only even attacked one-third of inbound British convoys. Nonetheless, Parshall and Tully go into detail on the picket line at Midway and are very emphatic that it failed due to not being dense enough - the Americans just slipped right through the gaps. It’s noteworthy that the greatest Japanese submarine successes were when they knew where the fleet would be ahead of time (Yorktown at Midway, Juneau) or the Americans got careless with their patrol routes (the other Guadalcanal successes).

So it’s not just doctrine. It’s not just more submarines. They need a comprehensive rework of their submarine tactics, they need better intel of American shipping, they need better radio equipment, and they need to do this all prewar before the Americans manage to flood the seas with ASW escorts.
 
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Driftless

Donor
Maybe a few of the subs, capable of carrying cargo could have been useful - but it would have required a great deal of foresight, and co-operation with the IJA..... ASB, I know....

Could the IJN have anticipated the Island Hopping campaigns, or even a gradual reduction of their (very) extended perimeter would require being able to supply threatened garrisons when they've lost air superiority and even local naval control? Had they forseen how the war played out in reality, those cargo sub runs would be reserved mostly for high-priority loads of medicine, ammunition, and select food stuffs. etc.

The US got a little mileage of larger converted minelayer subs. I can imagine the RN might have gotten some use out of some cargo subs for worst days of Malta.
 
And the biggest problem is that you need hind-site to make these changes.
Japan KNEW for a fact that a long war was a war Japan could NOT win.
The type of submarine campaign the OP is suggesting is only useful in a long war.
So the OP wants Japan to put into place plans and build Subs solely for a war that it is 100% sure it will lose.

And be very very clear, Japan built good subs and had the best torpedo at the start of the war. Yes they could have concentrated on more traditional subs and used them in a more traditional anti merchant ship role however doing so will have ZERO effect on the outcome of the war.
As for trying yo have an effect on shipping around India.. That is a long way to support a large number of subs so they will be less effective due to travel times and distances.
Yes it would be a bit worse for GB and the US but in truth it will basically have no effect on the war.

So we are looking at a change that requires Japan to plan for a long war and thus build a bunch of subs and creat a doctrine. to help the fight a war they know they will lose.
And then if they do we still have negligible effect on the outcome.
 
Unless they figure out some way to find and attack that merchant shipping, all of what you listed is just rearranging deck chairs on the Titanic. A survey of Japanese submarine options leaves one with the inescapable conclusion that something was fundamentally wrong with Japanese submarine search tactics. Post-Pearl Harbor the Japanese posted multiple submarines around Oahu that accomplished basically nothing of note, and their submarine picket line at Midway was a comprehensive failure that is an underrated part of the disaster at Midway.

In fairness, a large part of this is simply that finding ships from a submarine is hard. Despite the advantages of excellent intelligence work, Wolfpack tactics, and predictable convoy routing, the Germans only even attacked one-third of inbound British convoys. Nonetheless, Parshall and Tully go into detail on the picket line at Midway and are very emphatic that it failed due to not being dense enough - the Americans just slipped right through the gaps. It’s noteworthy that the greatest Japanese submarine successes were when they knew where the fleet would be ahead of time (Yorktown at Midway, Juneau) or the Americans got careless with their patrol routes (the other Guadalcanal successes).

So it’s not just doctrine. It’s not just more submarines. They need a comprehensive rework of their submarine tactics, they need better intel of American shipping, they need better radio equipment, and they need to do this all prewar before the Americans manage to flood the seas with ASW escorts.
If based out of Singapore and Truk, could they not just camp somewhat near Indian and Australian ports?

If the infrastructure is built, could subs based from Marcus Island camp off the Canadian and US west coast?

Instead of finding ships in the middle of the ocean, just wait somewhat near the ports?
 
If the US and British were forced to use surface ships for convoy duty in the Pacific and Indian Oceans, how much would this take away from the resources needed to escort the carriers for engaging the Japanese surface fleet?
 
If based out of Singapore and Truk, could they not just camp somewhat near Indian and Australian ports?

If the infrastructure is built, could subs based from Marcus Island camp off the Canadian and US west coast?

Instead of finding ships in the middle of the ocean, just wait somewhat near the ports?
Infrastructure is not the problem. Japanese boats had the range to operate off the West Coast, and did.

The problem with sitting off port is that it leads to a lot of dead submarines, because that's the easiest place in the world to sanitize of submarines. There are also too many ports - five on the West Coast, plus Hawaii, plus six major Australian ports, and probably close to twenty major ports in India alone. Counting transit time and refits, the Japanese plain do not have enough submarines even before losses bite.

As for "somewhat near the ports", ask the Germans how easy it was to find convoys in the Western Approaches. Hint: that was their main hunting ground and where most of that "attacked only a third of convoys" data was made there. This with vastly superior submarine tactics.

If the US and British were forced to use surface ships for convoy duty in the Pacific and Indian Oceans, how much would this take away from the resources needed to escort the carriers for engaging the Japanese surface fleet?
Not very. The Brits weren't even operating west of Ceylon for a solid two years. And the US was constantly running convoys in the Pacific.
 
And the biggest problem is that you need hind-site to make these changes.
Japan KNEW for a fact that a long war was a war Japan could NOT win.
The type of submarine campaign the OP is suggesting is only useful in a long war.
So the OP wants Japan to put into place plans and build Subs solely for a war that it is 100% sure it will lose.

And be very very clear, Japan built good subs and had the best torpedo at the start of the war. Yes they could have concentrated on more traditional subs and used them in a more traditional anti merchant ship role however doing so will have ZERO effect on the outcome of the war.
As for trying yo have an effect on shipping around India.. That is a long way to support a large number of subs so they will be less effective due to travel times and distances.
Yes it would be a bit worse for GB and the US but in truth it will basically have no effect on the war.

So we are looking at a change that requires Japan to plan for a long war and thus build a bunch of subs and creat a doctrine. to help the fight a war they know they will lose.
And then if they do we still have negligible effect on the outcome.
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.
 
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Driftless

Donor
One of my pet ideas for the Axis is for the Japanese to take Ceylon in the spring of 1942 when it was lightly garrisoned. With 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.
That would be an interesting TL. I have no clue about the plausibility of that campaign, but even a serious attempt has dimensions of the "unspeakable sea-mammal". All sorts of potential butterflies could take wing.
 
Unless they figure out some way to find and attack that merchant shipping, all of what you listed is just rearranging deck chairs on the Titanic. A survey of Japanese submarine operations leaves one with the inescapable conclusion that something was fundamentally wrong with Japanese submarine search tactics. Post-Pearl Harbor the Japanese posted multiple submarines around Oahu that accomplished basically nothing of note, and their submarine picket line at Midway was a comprehensive failure that is an underrated part of the disaster at Midway.

In fairness, a large part of this is simply that finding ships from a submarine is hard. Despite the advantages of excellent intelligence work, Wolfpack tactics, and predictable convoy routing, the Germans only even attacked one-third of inbound British convoys. Nonetheless, Parshall and Tully go into detail on the picket line at Midway and are very emphatic that it failed due to not being dense enough - the Americans just slipped right through the gaps. It’s noteworthy that the greatest Japanese submarine successes were when they knew where the fleet would be ahead of time (Yorktown at Midway, Juneau) or the Americans got careless with their patrol routes (the other Guadalcanal successes).
Parshall and Tully note that sub picket line was too late for the Battle of Midway, because the subs were sent too late. Subs were also expected to find the US ships sailing directly from Hawaii towards Midway, that was not the case.
(officer that was responsible for the subs being sent too late was very close to the Emperor, so he was not scrutinized after the battle)
 
Their sub drivers talked too much.

Forget the codebreaking - from early on the USN was able to predict future positions quite accurately based on DF, and was able to lay on a couple of successful ambushes.

This is simply a matter of proper security.
 
If you crunch the numbers the Japanese submarine fleet was too small to run a effective guerre de course or cargo ship campaign. The Germans managed a maximum effort surge for Operation Drumbeat on the US East Coast in January/February 1942. The did garner a lot of publicity, scare the hell out of a lot of mariners, damaged Admiral Kings reputation, and actually did sink some cargo ships. The also failed to sink even 1% of the cargo afloat off the US coast in those months. Halting the cargo ships for several weeks to organize convoys would have disrupted deliveries to US industry more than the cargo lost to operation Drumbeat.

For a campaign against merchant ships off the US west coast it would have been limited to the large fleet submarines, mostly the I class. After allowing for travel time from and to home base, breakdowns (All submarine fleets had a lot of aborted patrols for mechanical problems ) and any lost to enemy action. the Japanese would be hard pressed to keep a half dozen boats off the US coast for a sustained campaign. The rest would be in transit, or refitting/training. In 1944-45 the USN seldom had more than 20% of its huge submarine fleet on patrol stations any week.

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.

The German submarine losses were massive. By the last quarter of 1942 losses for all causes were approaching the replacement rate & in early 1943 the losses of crews exceeded the ability of the training establishment to replace them with skilled crews. The Japanese would be facing the same sort of aggressive defense, but with a far lower ability to build and deploy combat ready boats.

Sending the big I class boats off to the US west coast to hunt cargo ships probably adds another 5% to Allied losses in 1942. it also waives away the loss of the Wasp, at least the second hit on the Saratoga, and allows the salvage of the Yorktown. Thats probable gain of two US Fleet carriers for the naval battles of late 1942.
 
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That would be an interesting TL. I have no clue about the plausibility of that campaign, but even a serious attempt has dimensions of the "unspeakable sea-mammal". All sorts of potential butterflies could take wing.
Unlike the unspeakable sea-mammal in April 1942 the IJN would have naval & air superiority and the landing force would outnumber the defenders. The IJA had the advantage in the kind of terrain you find in places like Malaya, and Ceylon. I should add that most of the Allies remaining supply of natural rubber was on Ceylon, so losing the Island would leave them with only a limited supply of synthetic rubber. It took time to ramp up synthetic rubber production.
 
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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.

That would be an interesting TL. I have no clue about the plausibility of that campaign, but even a serious attempt has dimensions of the "unspeakable sea-mammal". All sorts of potential butterflies could take wing.

Unlike the unspeakable sea-mammal in April 1942 the IJN would have naval & air superiority and the landing force would outnumber the defenders. The IJA had the advantage in the kind of terrain you find in places like Malaya, and Ceylon. I should add that most of the Allies remaining supply of natural rubber was on Ceylon, so losing the Island would leave them with only a limited supply of synthetic rubber. It took time to ramp up synthetic rubber production.
I love this idea too, although the more historically plausible option is probably for the Japanese to invade India in early 1943 and then use Indian ports and airfields to blockade and bomb Ceylon into starvation/irrelevance.
 
In fairness, a large part of this is simply that finding ships from a submarine is hard.
How many optional routes does the U.S. have in the island hopping campaign?

Wait around those islands, and prioritize troop transports equally or more than supply ships.

The Pacific is big, but the Americans have to cross all of it to reach Japan.

You need the Japanese military to stick to the defensive "whittle them down" plan.
And an early POD that gets a combined defense ministry that doesn't fight with itself more than the enemy would really help.
 
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?
  • Focus on the Kaidai type (1400tons) rather than the Junsen type (2000tons)
  • Develop smaller fast subs earlier from the no71 Research sub (late 30s).
  • Build squadrons around fast seaplane carriers like Chitose rather than a light cruiser or Oyodo using coordinated seaplane and sub search.
  • Design a 21" mine.
  • Don't bother with aircraft from subs - more fast seaplane carriers/squadron flagships instead
  • Collaborate with the Italians who can teach you about divers from minisubs to infiltrate harbors not the miniature 2 man 2 torpedo subs
 
eI love this idea too, although the more historically plausible option is probably for the Japanese to invade India in early 1943 and then use Indian ports and airfields to blockade and bomb Ceylon into starvation/irrelevance.
Interesting. Could the Japanese supply an invasion in the dry season that year? If I remember correctly the British/Indian Army launched an offensive in Arakan which failed. Are you suggesting they launch a counter offensive after that? What kind of strategy are you suggesting. By that point I don't think the IJN could gain control of the Bay of Bengal or gain air superiority. In 1943 the RAF in India was flying Spitfire Mk V's and the U.S. 10th AF had P-40F & L models that were faster than the Zero or Oscar. By then they'd learned the right tactics to deal with their faster turning and climbing opponents.
 
Interesting. Could the Japanese supply an invasion in the dry season that year? If I remember correctly the British/Indian Army launched an offensive in Arakan which failed. Are you suggesting they launch a counter offensive after that? What kind of strategy are you suggesting. By that point I don't think the IJN could gain control of the Bay of Bengal or gain air superiority. In 1943 the RAF in India was flying Spitfire Mk V's and the U.S. 10th AF had P-40F & L models that were faster than the Zero or Oscar. By then they'd learned the right tactics to deal with their faster turning and climbing opponents.
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.
 
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.
 
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