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The I-400, The Underwater Aircraft Carrier of World War II

Updated on May 27, 2019
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Mamerto Adan is a feature writer back in college for a school paper. Science is one of his many interests, and his favorite topic.

When I was young, I saw the Second World War as primitive fights of the early 1900s. I was in grade-school back then so please, excuse my childish ignorance. I mean I came to that thinking after watching movies with my dad. Most of the WWII films are either in black and white, and with lead characters played by actors that my grandma could have adored. Propeller planes never turned me on, as well as retro battleships. Again, childish ignorance got the best of me.

Yet as I got older and got a degree in engineering, I slowly became fascinated with the hardware used in the Second World War. And as it turns out, they are not that primitive and clunky piece of antiques.

For one thing, modern day technologies both in everyday life and military weapons owed their existence from World War-2 technologies. Those years saw the birth of radars, jet powered aircrafts, modern day assault rifles and many more. Hitler’s delusional obsessions with wonder weapons that will soon bring him down did contributed to the advancement of modern-day technology. The same can be said to the Allies’ research and development.

I’m not really glorifying war. World War II did bring untold casualties and destruction. It’s just that people are good at coming up with clever ways to cut each other’s throats. And one of which is launching an aircraft to bomb enemy territories, from a submarine.

Yes, Underwater Carriers Exist

A British submarine carrier launching a float plane.
A British submarine carrier launching a float plane.

Launching armed aircrafts from underwater is the stuff of science fictions and spy films. I bet that one will say that you read too much comic books if you mentioned this to someone. But historically, these exotic weapons exist.

Technically they are known as submarine aircraft carriers. Their history goes way back to the first World War, when Germany began to launch modified sea planes from U-boats. The honor of becoming the first submarine carrier went to SM U-12. The idea here was extending the range of sea planes, so they could initiate raid at distant targets. For convenience a sea plane was used so no more messing around with vast landing deck. More of these submarine carriers followed and the British also experimented with their own. Their answer to this early German wonder weapon is the modified HMS E22. If successful, it will be used to intercept German airships. It operated the same way as its German counterpart, from how it launched and how it was retrieved.

But the problem here was that these first submarine carriers cannot submerge with their aircraft on deck. There were no internal hangars where the planes can be stored, hence the subs never sailed submerged, with their aircraft perched on their decks unprotected. But that doesn’t stop the nations from exploring the idea of a full submarine aircraft carriers.

Advantages of an Underwater Aircraft Carrier

In between wars, many nations did consider making their own submarine carriers. France, Italy, Japan, United Kingdom and the United States all invested time and money exploring the concept of an underwater aircraft carrier. One might wonder why even bother, as a fully functional aircraft carrier was already available at that time. A surface aircraft carrier has many advantages over a submarine aircraft carrier. But stealth attacks from underwater with armed aircraft has its allure even today. Submarine aircraft carriers have an advantage of going undetected, unlike their surface counterpart despite of carrying only one or two aircraft. They could steam underwater until they are close enough for their aircraft to fly into enemy territory to bomb them, and dive again before they are discovered.

And the Japanese Did It

Admiral Isoroku Yamamoto, the brain behind the submarine carrier.
Admiral Isoroku Yamamoto, the brain behind the submarine carrier.

And during World War II, the Imperial Japanese Navy unveiled the massive 1-400 class submarine, or the Sentoku type. Like any normal submarine at that time, it carried torpedoes, but it also housed special kinds of weapons. Three Aichi M64 Seiran. Float planes armed with guns, bombs and torpedoes. What was great it was that it could carry these planes while fully submerged, unlike its German and British counterparts back in World War I.

It was the brainchild of Japanese Combined Fleet Commander-in-Chief Admiral Isoroku Yamamoto, with Captain Kameto Kuroshima doing the feasibility studies. The idea here is to attack on American soil undetected, with warplanes launched from underwater carriers. The proposal was submitted to the Fleet Headquarters on January 12, 1942 and the design was finalized by the 17th of March. By the next year (January 18, 1943), construction of the I-400 began and four more followed.

From the planned 18 submarine aircraft carriers, only two made it into service; the 1-400 and the 1-401. The third submarine 1-402 came too late, as it was completed before the end of the war.

The Submarine Itself

The I-400 sailing.
The I-400 sailing.

On the outside, this underwater carrier was an imposing sight to behold. Before the introduction of nuclear subs, it is the largest submarine in the world. It measured 390 ft long and with displacements of 5900 tons. The most distinct feature was the cross-sectional shape of the hull, which resembled the number eight. Basically, the submarine used two hulls fused into one, giving it the necessary structural strengths to handle a hangar. Yes, it had a hangar on the deck where the aircraft were stored when the sub went underwater. This cylindrical structure is 102 feet long and house 3 aircraft and located on the top deck. And since it was needed to stow the aircraft in the center water line, the conning tower was offset to the port side.

For armaments it carried autocannon, deck guns and eight torpedo tubes.

The sub had its downsides. The large superstructures and the small rudder meant that this monster could be unwieldy. It cannot dive too deep (82% of its length) and makes for a fat target when surfaced due to the large size.

How It Launched Its Planes

Aichi M6A Seiran attack plane.
Aichi M6A Seiran attack plane.

The aircraft it carried were three Aichi M6A Seiran. Again, they were float planes, with a pair of floats so they can land on sea. These aircraft could fold their wings and their floats removed, so they could fit into the submarine’s on-deck hangars. To launch the planes, the sub will surface, and the hangars opened. The plane will be pulled out, unfolded and floats attached (which was stored in waterproof compartments on either sides of the catapult rack). The planes were launched by catapults. As the aircraft fly into their missions, the subs submerged in one place, so the planes won’t have troubles locating them. Upon return, the planes will land on water, and will be retrieved by the subs’ cranes.

In case one might wonder, the Aichi M6A was a fully capable attack plane and not just a spotter aircraft. It could drop bombs and torpedoes and carried machine guns.

But They Came Too Late

With the tides turning against the Japanese during WWII, the need arised to take the fight to the American soil. Firstly, an attack on the Panama Canal was planned with the use of submarine carriers. This will hamper the transfer of U.S. ships and cut the supply lines in the Pacific. Training underwent, from rapid deployment of aircraft, to bombing runs and even Kamikaze attacks. But Okinawa fell and the attack was cancelled, so the Japanese could focus on a more direct attack on the Americans. This time the Japanese set their sights on the Ulithi atoll where the American aircraft carriers gathered. But again, the attack was cancelled after Japan surrendered. Japan’s surrender also stopped them from carrying a biological attack at the civilian population in San Diego, California. Eventually after destroying all their weapons and planes, the submarine 1-400 surrendered to the American destroyer USS Blue.

Underwater Carriers Were Later Abandoned

Submarine Launched Cruise Missile.
Submarine Launched Cruise Missile.

At present, surface aircraft carriers are fully preferred over their more exotic underwater counterpart. Aside from the fact that the number of aircraft was limited by the cramped size of the submarine, making the whole sub bigger will make it prone to detection. Nevertheless, launching something destructive from an undetectable position underwater was never abandoned. Today, submarines launching cruise missiles while submerged became the successors of the I-400. And modern subs like the German type 21 could launch drones.


1. "New Fold Plane For Submarines", February 1931, Popular Science

2. Francillon, R.J. (1970). "Japanese Aircraft of the Pacific War." London:Putnam

3. Geoghegan, John J. (2013). "Operation Storm: Japan's Top Secret Submarines and Its Plan to Change the Course of World War II." Crown Publishers.

4. Layman, R.D. and Stephen McLaughlin. The Hybrid Warship. London:Conway Maritime Press,

5. The Maru Special, Japanese Naval Vessels No.13, Japanese submarine I-13 class and I-400 class, Ushio Shobō, July 1977


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