Different IJN submarine types during WW 2

I would think that the major design concerns would be range, being able to carry a decent load of torpedos and a design that can be built in large numbers. Would the Type IX cut it? I suspect not as the Japanese subs would surely be using Long Lance torpedos which were 9 m long as against the 7.163 m for German torps. For a sub when you include reloads thats a fair bit of difference in lenght

The Type 95 was issued, and widely used. Ask the Indianapolis.

Some say they used Kaiten suicide torps for that.

Hashimoto is on the record saying 95s were used on the Indianapolis, but he also logged three ships sunk with his four Kaitens after sinking the Indianapolis, losses that don't match of with anything on USN records.
 
Another possibility: a closer post war - or pre-1922, anyway - link between the IJN and the RN. The 'R' class submarines were the first boats optimised for underwater rather than surface performance - 14 knots underwater, not bad for a 1917 design. IRL they were mainly scrapped postwar (no operational requirement, they couldn't fulfill peacetime patrol boat duties), bar two used for ASW training.

POD: they're sold (or the designs are) to the IJN, post war, rather than broken up. A fast submarine, if it could be developed - and the IJN was capable of putting a lot of development work into something it liked - might fit into existing Decisive Battle doctrine, and over time could be used as an ASW platform. Wikipedia basic, but seems accurate enough - here.

I'd forgotten about the 'R' class submarines, and that could evolve into a mature weapons platform with twenty years of development. Indeed I had the redoubtable Max Horton, indirectly act as a catalyst for the revitalisation of the submarine arm, courtesy of a speech given at the Naval General Staff.
 
A lot, particularly given the vast distances of the Pacific Ocean. I guess the best place to employ them would have been on the convoy routes between Hawaii and Australia and at points in between. However, those sea lanes are about 2000 miles from forward bases in the Marshalls and over 3500 miles from more developed bases in the Mariannas and Carolines.

They could use merchant cruisers to resupply them like they did off East Africa in the early summer of 1942.
 
Hmm... How many submarines did the Germans & Italians build, and fail to win their interdiction campaign?

The IJN submarine arm will be more effective than their counterparts in OTL, but in the end the result will be the same i.e. occupation by the Allies.

Any interdiction campaign would concentrate I think on the SLOCs supplying Australia, New Zealand and other staging areas, while seeking to interrupt Australian coastal shipping. Although this will slow down the Allied island hopping campaign, it will not stop it. When I get around to writing the TL it will be a tragedy, about a man fighting well for a forlorn cause as well as an evil regime.
 
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The IJN submarine arm will be more effective than their counterparts in OTL, but in the end the result will be the same i.e. occupation by the Allies.

Any interdiction campaign would concentrate I think on the SLOCs supplying Australia, New Zealand and other staging areas, while seeking to interrupt Australian coastal shipping. Although this will slow down the Allied island hopping campaign, it will not stop it. When I get around to writing the TL it will be a tragedy, about a man fighting well for a forlorn cause as well as an evil regime.
Sounds like a cool TL, send me a link when you finally get around to posting it.
 
For the USA and Japan, the size of the Pacific meant that their subs had to have much longer range than those of the European powers. Habitability is also an issue for long cruises (and tropical temperatures). You can tell the crews be tough, but no matter what efficiency suffers if you ignore these issues. As noted, the major problem is to change Japanese submarine doctrine, absent that technical changes are almost irrelevant. The other major problem is that while Japan is highly dependent on imports that have to pass through restricted and well defined waters. On the other hand, the US has a lot of coastwise West Coast traffic, but during WWII not much in the way of incoming traffic from the Pacific to the West Coast. Once you get past Hawaii, the routes to various places like Australia/New Zealand, various islands are quite long, can be shifted a little, and cover a huge area for a submarine to search.

If the Japanese do adopt interdiction strategy, and they develop "milch cows" to help maintain subs at a distance, and improve habitability, etc of subs, it still won't really change things. To be effective the Japanese would need many, many more submarines than they had to cover such a vast area. They simply cannot build that number, even if they start well before the war - what ships WON'T they build to build more subs? Frankly any ship type, including merchants, they short to make more subs will make things worse for the IJN. A better doctrine, and a few changes/new subs would help, but not that much. The USA can shift a little construction for escorts for the Pacific if need be.
 
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