Aichi M6A

This sleek looking float-plane just missed combat in World War II and was part of a unique operation.

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After the jump, a look at a particularly intriguing “what if”.

Japan was the only combatant to seriously consider a submarine aircraft carrier during the War.  Germany and Japan both had some submarines with scout aircraft, but after the Pearl Harbor attack Admiral Yamamoto proposed a fleet of submarine aircraft carriers for attacks on the continental US.  Design work was done through 1942 and construction started in January of 1943.

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These were the largest submarines in the world until the nuclear age.  At almost 7000 tons they were bigger than destroyers of the time.  Their main feature was a waterproof tube on the top of the pressure hull for storing the aircraft.  The initial idea was to carry two float plane bombers on each submarine, this was increased to three before the boats were complete.  Other armament was normal for a submarine of the era (8 torpedo tubes, deck gun, 10 x 25 mm anti-aircraft guns in various mounts).  They would be known as the I-400 Class, or “Special Type Submarine”.  As I was writing this post, a very fitting I-400 model by Eric Morningstar was posted over at Aircraft Resource Center.  I would encourage taking the quick link over to that site for a good look at this large submarine.

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Original plans called for 18 such submarines to be built, but this was scaled back several times.  In the end, only I-400 and I-401 were completed as planned (I-402 was also finished, but as a fuel tanker).  When it became obvious the boats would be completed late two other large fleet submarines (I-13 and I-14) were converted to carry aircraft but these would only carry two planes.

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This special submarine attack fleet, with ten airplanes, started preparing for an attack on the Panama Canal in hopes it would reduce the flow of supplies to the war zone.  In Spring of 1945 the plans were redirected towards Ulithi Atoll which was a major US Fleet anchorage.  They were underway, and only a few days from launching their attack when word came of the surrender. The airplanes were dumped into the sea prior to surrender, at least in part because they had been stripped of paint and given US national markings for their coming operation in violation of international law. The boats were studied by the US Navy, until word came that the Soviets would be sending a technical team to take a look; they were all quickly scuttled in deep water.

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The I-401 after surrender to the US Navy (from en.wikipedia.org)

Manipulator arm of HURL’s submersible places the I-400 bell into a collection basket. (HURL/UH)

Ships bell from the I-400 being recovered in March 2016. (from Alert5.com)

The Aichi M6A Seiran (“Clear Sky Storm”) was a type unknown to western intelligence during the war and thus received no Allied Code Name.  It was designed for the very specific purpose of being used on a submersible aircraft carrier.  Its engine is a Japanese derivative of the German DB601 engine (but NOT the Ha-40 that was used in the Ki-61). It could carry about 1800 lbs of ordnance (bombs or torpedo).  Most significantly, it folded down tight for submarine storage.  The wings rotated 90 deg and folded back flat along the fuselage.  The horizontal tail surfaces folded back, the vertical tail folded down, and the floats detached.  The floats are an interesting topic on their own; they could be left off for launch which sped up the process and allowed for a much higher speed for the aircraft.  Obviously though that changed the recovery process; pilots could still be picked up by their boat but the plane was lost without floats. An amusing error in much written history is that many books and articles have suggested the floats could be jettisoned in flight, but it was determined in the late 1990s, during the restoration of the only M6A by the Smithsonian, that there is no control for the crew to do this.  Floats/no floats is a decision that has to made prior to launch.

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A single 850 kg bomb is the hitting power.  Apparently prior to the Ulithi raid aircrew discovered the bomb release mechanism had been replaced by a fixed bracket.  This caused a near mutiny over the aircrew being forced into a suicide mission without their agreement.

This is the Tamiya kit of the aircraft and beaching dolly.  The markings are pretty generic for an operational M6A prior to the Ulithi raid summer of 1945.  This was a completely painless build.

The surviving M6A on its dolly at the Smithsonian restoration facility. Here the defensive heavy machine gun is raised from the rear crewman’s position. (from jn.passieux.free.fr)

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The original plan to attack the Panama Canal was calculated, in part, that fighter defense was minimal.  But Ulithi was a major base, I suspect a small force of float planes would have faced tough opposition.

The best resource for more information on the I-400 and its mission is “I-400” by Henry Sakaida and Gary Nila.

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About atcDave

I'm 53 years old and live in Ypsilanti, Michigan. I'm happily married to Jodie. I've been an air traffic controller for 30 years; grew up in the Chicago area, and am still a fanatic for pizza and the Chicago Bears. My main interest is military history, and my related hobbies include scale model building and strategy games.
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13 Responses to Aichi M6A

  1. jfwknifton says:

    A wonderfully sharply made kit of an aircraft I had actually never heard of! The Japanese have always been very inventive, but I don’t see this system working very well in combat conditions, somehow!

  2. Very nice job on the aircraft, and interesting story about an unusual subject. This is one I had never heard of.

  3. Theresa says:

    The Japanese were working on several innovative aircraft at the war’s end.

  4. Another superb build and write-up Dave. I built a Seiran a few years ago, it’s a beautiful looking machine. I think the idea of disrupting shipping along the Panama canal I’d a sound idea, but in practice would have proved a wasted effort. Even a successful attack with so few aircraft would have caused minimal damage, which no doubt would have been quickly repaired, and the psychological impact of such a raid so late in the war would have been equally minimal. I suspect the diversion to attack the US fleet at Ulithi Atoll would have been equally costly for the M6A’s. Still, an interesting idea and a beautiful build. Thanks for the post Dave.

    • atcDave says:

      I agree exactly about the potential. They were looking for choke points, where a small strike might have maximum impact; but ten planes, even if they were completely unopposed, seems pretty weak.

  5. A gorgeous model you have there Dave. Interesting to see the Japanese tackling such an idea which I know was also considered elsewhere – that reaching a similar end. I think to have any significant effect there would have to have been been a very large number of vessels built.

    • atcDave says:

      Yeah and I think they’d need bigger capacity too. Even if they’d built all 18 subs as intended, that’s only a 54 plane strike, and all light bombers. That’s like a single fleet carrier with no fighters.

      The potential as a nuisance may be high, but its hard to imagine it as a strategic weapon.

  6. Pierre Lagacé says:

  7. Pierre Lagacé says:

    I watched it last year.

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