Japan’s Underwater Aircraft Carrier / part-one

I-400 Series Super-submarine

Lieutenant Commander Stephen L. Johnson had a problem on his hands; a very large problem. His Balao-class submarine, the Segundo, had just picked up a large radar contact on the surface about 100 miles off Honshu, one of Japan’s home islands, heading south toward Tokyo.  World War II in the Pacific had just ended, and the ensuing cease fire was in its 14th day. The official peace documents would not be signed for several more days.

As Johnson closed on the other vessel, he realized it was a gigantic submarine, so large in fact that it first looked like a surface ship in the darkness. The Americans had nothing that size, so he realized that it had to be a Japanese submarine.

This was the first command for the lanky 29-year-old commander. He and his crew faced the largest and perhaps the most advanced submarine in the world. The Japanese I-401 was longer than a football field and had a surface displacement of 5,233 tons, more than three times the Segundo’s displacement. More troubling though was the sub’s bristling weaponry that included a 5.5-inch gun on her aft deck, three triple-barreled 25mm antiaircraft guns, a single 25mm gun mounted on the bridge, and eight large torpedo tubes in her bow.

During a brief ceremony aboard one of the aircraft carrier submarines, the Japanese naval ensign is lowered and replaced by the Stars and Stripes as the vessel is turned over to the control of the U.S. Navy after Japan’s surrender

The large sub displayed the mandatory black surrender flag, but when the Segundo edged forward, the Japanese vessel moved rapidly into the night. The movement and the continuing display of the Rising Sun flag caused concern.  Johnson’s vessel pursued the craft that eventually slowed down as dawn approached. He brought his bow torpedo tubes to bear on the craft as the two vessels settled into a Mexican standoff.

Johnson and his crew had received permission by now to sink the reluctant Japanese vessel if necessary, but he realized he had a career-boosting and perhaps a technologically promising prize in his sights. Much depended on this untried American submarine captain and his wily opponent in the seas off Japan.

Supersub I-400 series

Little did Johnson know that the Japanese submarine was a part of the I-400 squadron, basically underwater aircraft carriers, and that the I-401 carried Commander Tatsunosuke Ariizumi, developer of the top-secret subs initially designed to strike the U.S. homeland in a series of surprise attacks. Ariizumi was considered the “father of the I-400 series” and a loyal follower of the emperor with years of experience in the Japanese Navy, so surrender was a disgrace he could not endure.

Johnson also had to contend with Lt. Cmdr. Nobukiyo Nambu, skipper of the I-401, who traced his combat experience back to Pearl Harbor. He now commanded the world’s largest submarine designed to carry three state-of-the-art attack planes in a specially built hanger located atop the vessel. These secret Aichi M6A1 planes were initially designed for “a second Pearl Harbor” or another surprise attack, possibly even against New York City or Washington, D.C. The I-400 series submarines were themselves full of technological surprises.  They were capable of traveling around the world one and a half times without refueling, had a top surface speed of 19 knots (or nearly 22 miles per hour), and could remain on patrol for four months, twice as long as the Segundo.

Neither Nambu nor Commander Ariizumi readily accepted the emperor’s surrender statement when it was broadcast on August 15. The subsequent communiqués from Tokyo were exceptionally confusing, especially Order 114, which confirmed that peace had been declared – but that all submarines were to “execute predetermined missions and attack the enemy if discovered.”

Admiral Isoroku Yamamoto

It was Admiral Isoroku Yamamoto, commander of Japan’s Combined Fleet and developer of the Pearl Harbor attack, who called for the construction of the I-400 series some three weeks after Pearl Harbor.  Once Japan was committed to war, he believed that submarine aircraft carriers dropping bombs “like rain” over major U.S. cities would surely cause the American people to “lose their will to fight.” A second surprise attack with even more to come would prove psychologically devastating to the Americans.

To be continued…

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Military Humor-

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Farewell Salutes – 

William K. Beers – Lewistown, PA; US Army, Vietnam, 101st Airborne & 17th Artillery

George E. Bernard (100) – Burlington, VT; US Navy, WWII, ETO, Seaman 2nd Class, LCT Rocket # 373

Charlotte Clark – Laconia, NH; Civilian, WWII, Scott & Williams Aircraft parts

Milton Cronk (100) – Iowa City, IA; USMC, WWII, PTO, Marines Raiders, Purple Heart

Frank Daniels – Watertown, MA; US Navy, WWII, PTO, gunner, Seaman 1st Class

R. James Giguere – St. Paul, MN; US Army Air Corps, WWII, ETO, B-17 “Miss Lace” bull turret gunner

William C. Jones (100) – Fort Smith, AR; US Navy, WWII

Ruth (Lias) Lowery (100) – Akron, OH; Civilian, WWII, B-17 production

Johnnie Mullenix – Unionville, MO; US Navy, WWII, PTO

Richard Shaug – Cambria, CA; US Army, Korea, HQ Co/187th RCT

Twila Wellsfry – Chico, CA; Civilian, WWII, bookkeeper, Mares Island shipyard

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About GP

Everett Smith served with the Headquarters Company, 187th Regiment, 11th A/B Division during WWII. This site is in tribute to my father, "Smitty." GP is a member of the 11th Airborne Association. Member # 4511 and extremely proud of that fact!

Posted on November 14, 2022, in WWII and tagged , , , , , , , , , , . Bookmark the permalink. 135 Comments.

  1. Sorry, GP! I had missing a lot of your last postings. I hope you had a wonderful Thanksgiving! Best wishes, Michael

    Liked by 1 person

  2. I had never read about this vessel before — what a marvel of engineering.

    Liked by 1 person

  3. This is an intriguing story, GP, and I am looking forward to the conclusion.

    I enjoyed the cartoons, too. 🙂

    Liked by 1 person

  4. Reblogged this on History and Hobbies and commented:
    Part One of an interesting true story.

    Liked by 1 person

  5. You commanded my attention with this post, GP.

    Liked by 2 people

  6. This was news to me, as well. I had no idea that such a warship even existed!

    Liked by 2 people

  7. It’s so fascination and neverheard of it.Thanks for the info.

    Liked by 2 people

  8. Just finished an article about a local aviation artist. He has hanging in his studio the propeller from one of those Japanese planes that were on the subs. I’ll be posting the article in the future although that is just a small part of the article.

    Liked by 2 people

  9. This reads like a fiction story, and it’s chilling to realize that’s it is true, and Americans were really faced with this monster.
    Looking forward to the next part, GP.

    Liked by 2 people

  10. What a cliffhanger! And thank you for the debrief.

    Liked by 2 people

  11. I never knew about this!!!

    Liked by 1 person

  12. A wonderful article. Thanks for providing such works for younger people to learn from.

    Liked by 2 people

  13. Stuff from Jules Verne science fiction.

    Liked by 2 people

  14. I’d heard of these submarines but don’t recall this encounter. I’m tuned in for the next chapter.😊

    Liked by 2 people

  15. Wow, and to think that we had the most advanced technological warfare at that time.

    Liked by 2 people

  16. This was a fascinating post! I never would have imagined an underwater aircraft carrier. I look forward to the next installment to learn more.

    Liked by 2 people

  17. The Hunt for Red October has nothing on this storyline. Your ability to dig out these fascinating details is amazing, GP. I must say, both this weapon and the complexities of the post-surrender announcements are fascinating. I’m eager to see what comes next.

    Liked by 2 people

  18. So hard to envision them coming up against a submarine like an underwater aircraft carrier

    Liked by 2 people

  19. I did not know about these submarines, GP. Can’t wait to read more.

    Liked by 2 people

  20. I watched the YouTube videos on this sub

    Liked by 2 people

  21. The things we never knew. I can’t wait for the follow up.

    Liked by 2 people

  22. An amazing thing….very imaginative…..chuq

    Liked by 2 people

  23. GP, this is fascinating stuff and info I never heard before. A reminder that when someone announces a war is over, stay alert and vigilant.

    Liked by 2 people

  24. When I first saw the headline, it took me straight back to a quote from the film “Butch Cassiday and the Sundance Kid”. The latter could well have said, “An Underwater Aircraft Carrier?? Just keep thinking, Butch, that’s what you’re good at!”.
    Seriously, an extremely interesting post and I look forward to hearing how this situation settles itself amicably.

    Liked by 2 people

  25. No doubt the huge length was required to allow the planes to take off. But as they were never tried in combat, it will be interesting to see if they were actually practical. Presumably the pilots would know they could almost certainly not return and land back on the sub? But given the Japanese way of thinking, they might well have been an early form of Kamikaze.
    Best wishes, Pete.

    Liked by 2 people

  26. Lots of wows, GP! It’s a wowful machine!

    Liked by 2 people

  27. These subs were the “ultimate weapons of destruction” of their time. Perhaps even more than A-bombs because we had only two with a mere one time use for each while the subs’ attack life was comparatively endless.

    Liked by 3 people

  28. Triple WOW from Canada! It almost sounds like a tall tale.

    Liked by 3 people

  29. I’ll have to join the crowd, as my reaction was also “WOW!” Can’t believe I’ve never heard of this boat before, nor the post surrender drama of the Japanese subs orders…

    Liked by 3 people

  30. I know North Korea shot planes out of mountains, but didn’t know about this, GP. My daughter will love the engineering funny. She was ACE on a cruiser in her early Navy days.

    Liked by 3 people

  31. Something I had never heard of

    Liked by 3 people

  32. What an enormous expense to launch THREE planes! No doubt that three planes could do serious damage, but in the scope of things, the results would be trivial. More than anything, it speaks to the Japanese navy’s almost total loss of airpower and its futile attempt to recover face.

    Liked by 3 people

  33. I can hardly wait for the rest of the story!

    Liked by 3 people

  34. This is fascinating, GP. I look forward to the next installment. It’s a very interesting concept.

    Liked by 3 people

  35. Double WoW! This is a part of WWII history that I have never heard of. I love your Navy ‘toon. On my husband’s first ship, the XO (who was more cerebral than functional) carried a phone out to the quarterdeck shortly after the ship docked. He asked “Why is the phone going (no noise) instead going (hum)?” “Well, Sir” one of the sailors replied, “it’s not yet connected to service on the pier.”

    Liked by 3 people

  36. What a beast! And what a dilemma for the Segundo’s skipper.
    Still laughing about today’s bad decisions not making themselves! Should be the watchword of our current politicians!

    Liked by 3 people

  37. At first glance, a submersible aircraft carrier seems like a good idea but unachievable with the technology of the day. Such thinking didn’t stop naval planners from going ahead with such a revolutionary idea. That ship was masterclass thinking.

    Liked by 3 people

  38. I’d echo Beth’s comment! WOW.

    Liked by 4 people

  39. This is a fascinating episode that was previously unknown to me! Thank you for sharing and I look forward to the second part!

    Liked by 4 people

  40. Thank you, Nesapy.

    Like

  41. Thank you once again.

    Like

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