Shipbucket

Not really. They were really, really big, and were virtuous in that Japanese submariners were willing and able to put up with much more spartan conditions than were their adversaries. They were perfectly decent, but they did not exhibit any particular stupendous technological virtue like, say, a Type XXI could be said to.

And that's was the case, size. How the build so enormous submarine, in that time there was a belief in navy that there is impossible to build something bigger then Surcouf. And Japanese done that, they build gigantic sub, with hangar, enough supplies to swim around the word, quiet and pack with very high tech for that time. Crew condition for a Japanese sub were very good, in western ways only moderate of course. They build proto-boomer, first real strategic submarine. Guess what would be look like the advance I-400 when you join Germany tech from XXI and Japanese skill to build such enormous subs ?

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Something that's essentially useless until the advent of sub-launched strategic missiles.

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“Close” only counts with horseshoes, hand grenades, and tactical nuclear weapons.
That which does not kill me has made a grave tactical error

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Source Materiel is always welcome.

Useless indeed, but why nobody tell that to poor yanks...

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Just curious but have you ever written a post that was more than just a one-line snark?

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USN components, camouflage colors, & reference links (World War II only)

Like I said, not much use until Strategic missiles become a thing
USS Tunny was a modified Gato class submarine so I don't see how that's going to be relevant.
USS Grayback didn't enter service until 1958, by which time anything they could have learned from the I-400 would have been outdated anyway.
Every now and then when inspiration strikes.

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“Close” only counts with horseshoes, hand grenades, and tactical nuclear weapons.
That which does not kill me has made a grave tactical error

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Source Materiel is always welcome.

I'm afraid I can't make heads or tails of that.

If you're suggesting that being able to build a huge noisy diesel boat was some kind of amazing technical achievement and/or war-changing capability, one wonders why nobody made any attempt to build such creatures postwar (even after their size and capabilities became known), why the I-400s made no truly meaningful contribution to the Japanese war effort that could not be made by other classes, and indeed why similarly huge boats (once they eventually appeared) bore no meaningful resemblance whatsoever to I-400.

Regulus I missile hangar on Tunny was a direct copy of Japanese tech, in that time US Navy posses 0% knowledge of build that kind of equipment on subs. But they had a specification and plans of I-400 plane hangar and they use it to built missile hangars for Tunny and Barbero. Grayback also used that tech for his hangars. And outdated ? good to know that nobody use now that old and outdated tech like snorkel.

And hard to say that Regulus I was a strategic missile with range under 1k.
Ability to build huge, not that noisy ( I-400 even if was a big sub it was a pretty quiet ) is the amazing technical achievement. The war changing ? ... who knows ? What would happen if at the start of war on Pacific I-400 planes destroy Panama gates. That would be a war changing event ? I know that that sub enter service in 44 when war was already lost but idea of that class of sub carrier/cruiser is a war changing ability.

Why nobody build something like that after the war ? For many reasons, money, end of War, lot's of other equipment around, ect. Sub carrier don't have a role anymore, why build for US Navy that ship when you have a horde of carriers at your disposal and the biggest war fleet on whole world. The idea of I-400 come back when yanks need a sub carrier for Regulus missiles. Why they have not build a copy or something on pair with Japanese sub ? For what, the more cheaper was to mod existing subs and they need to do this fast. Next incarnation of I-400 idea was the first full boomer George Washington class.

We know that concept of sub carrier are alive and come back some way or another in every year. In the last one and very probably the one that will be realize is a boomer armed with unmanned reusable UAV that was already tested. Who know what future brings.

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Nobody expects the Imperial Inquisition!