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I-400 Class Submarines

We have the Japanese to thank for many ingenious creations over the years. The Bullet Train, the QR code, Sushi, car navigation systems, emojis and who can forget our favourite adventurous plumbing duo, the Mario Bros. On a more serious note, the Japanese are also to blame for the selfie stick, which, surprisingly to me at least, was first created all the way back in 1983 – but I think we can let them off from that one. 

Japanese ingenuity is nothing new and goes back decades as a shattered country valiantly picked up the pieces after World War II and came roaring back and at one point became the third-largest economy in the world. But this inventiveness was also clearly evident during the second major war of the 20th Century as Imperial Japan looked to shift its borders a few thousand miles or so. 

The Second World War saw some extraordinary Japanese military hardware appear, including the two Yamato-class battleships which were at the time the heaviest ever to set sail, and which we have already covered here on Megaprojects, and the advanced fighter jet the Mitsubishi A6M “Zero” series. 

Battleship Yamato during sea trials October 30, 1941
Battleship Yamato during sea trials October 30, 1941

But the focus of our video today is on the vast submarines that began appearing in 1944. The I-400 Class Submarines were, until the introduction of nuclear ballistic missile submarines in the 1960s, the largest submarines ever built – which in itself is quite extraordinary, but is still not the most intriguing aspect of these submarines. You see, what the Japanese Navy called the Sentoku type submarines, were technically also aircraft carriers.   

Submarine Aircraft Carriers

Now, I know what you’re thinking, a submarine/aircraft carrier – how do I not know about this already? Well, primarily because only three were ever finished towards the end of the war, with one even converted into a tanker at the last minute, and as much as our imagination wishes it so, these were not exactly the colossal USS Nimitz but underwater – though that would be very cool indeed. 

Capable of only carrying three planes on board, these were much more submarines than aircraft carriers, but nevertheless, they provided a unique threat that has never been repeated since. But that’s not to say it wasn’t trialled before World War II. 

The Germans gave it a damn good go during World War I when they tested an FF-29 taking off from the deck of a U boat, while between the wars, France, the UK, the U.S and Italy also experimented with having a plane onboard, mainly for observation purposes, and often without much success. During World War II, the Germans attempted to resurrect the idea but it never really got past the experimental stage and it was their Asian allies who took things to a different level. 

Type B1   

Also known as the I-15-class submarines, the Type B1 class first appeared in 1939 and went on to become the most numerous submarines within the Japanese fleet. Again, to call them aircraft carriers might be a bit of s stretch, but they did come with a single Yokosuka E14Y seaplane, located in a hangar in front of the conning tower, which could be launched with the help of a catapult. 

Twenty of these submarines were built through the late 1930s up until the end of the war and one of them, I-25, was responsible for the only bombing raid on the continental U.S during the conflict.

On 15th August 1942, I-25 steamed clear of Yokosuka carrying six 76-kilogram (168 lb) incendiary bombs with the sole intention of starting a large-scale forest fire. The Japanese had done their research and the two fires in 1933 and 1339, together known as part of the Tillamook Burn, which would eventually destroy 350,000 acres across four major fires between 1933 and 1951, had shown just how destructive these fires could be. 

As luck would have it, for the Americans at least, the bombs dropped close to Brookings in Oregon failed to start a fire, mainly down to the light winds, wet weather and due diligence of the fire lookouts that night. 

On a loosely related note, this was not the only time the Japanese attempted to start forest fires in the continental U.S. The Fu-Go balloon bombs were hydrogen balloons with either incendiary bombs or anti-personnel devices attached to them that the Japanese launched hoping that the jet stream would carry them the thousands of miles to their intended targets on the west coast of the U.S. The effort was largely unsuccessful but one bomb did kill a pregnant woman and five children near Bly in Oregon. As bizarre as these bombs sound, they did represent the first intercontinental weapon, and until the British attacks on the Falkland Islands in 1982, known as Operation Black Buck, which we have already covered on Megaprojects, the Fu-Go balloon bombs were the longest ranged attacks ever carried out. 

Anyway, let’s get back to the submarines.  

Mainland Strikes  

Shortly after the attacks on Pearl Harbor, the Japanese sought to hammer home their advantage, but it didn’t take long for the Americans to find their feet and begin striking back. Admiral Isoroku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet, wanted to begin hitting the U.S mainland with aerial attacks – the only problem was it was way too far for any aircraft to reach and getting aircraft carriers within range would almost certainly be met with fierce resistance. 

Therefore, he planned to build a series of submarines that would be able to carry at least two aircraft and that could make three round-trips to the west coast of the United States without refuelling. His ambitious proposal was met with equally ambitious approval on 17th March 1942 and why not, the Americans had been given a good hiding and the Japanese were riding high, now was the time to strike the enemy where it really hurt. 

The hull for the first I-400 submarine, rather boringly also called I-400, was laid down on 18th January 1943, with another four planned close behind, and a total of 18 to be built in the coming years. But by this point, things had already begun to change. The Japanese Navy had met their match during the Battle of Midway in June 1942 and by March 1943, when Admiral Yamamoto died, the fleet had already lost six aircraft carriers. 

With the death of the Admiral, the number of proposed I-400 submarines fell from eighteen to nine, then five and eventually just three. 

The I-400 Class Submarines 

At 120 metres (400ft) in length, the I-400s were slightly longer than the Los Angeles Class submarines, but a good 50 metres (164ft) shorter than the Ohio Class. They came with a beam of 12 metres (39.4 ft) and a draft of 7 metres (23 ft), while their displacement of 6,670 tons was roughly twice that of American submarines at the time.  

Each of the submarines had four 2,250 hp engines and carried enough fuel to go around the world one-and-a-half times – easily enough for a few secretive jaunts to the U.S West Coast. They had a top speed on the surface of 18.7 knots (34.6 km/h; 21.5 mph) and a submerged top speed of 6.5 knots (12.0 km/h; 7.5 mph), along with a recorded test depth of 100 metres (330ft) – and if you’re paying attention you’ll have noticed that its dive depth was, in fact, less than its total length, meaning it had to be careful during steep dives.   

On the top deck was a cylindrical watertight aircraft hangar, 31 metres (102 ft) long and 3.5 metres (11 ft) in diameter with a 51-millimetre-thick (2.0 in) rubber gasket to ensure no water could get in. The submarine also came with its own mini-crane, because, well, the planes couldn’t land on the deck of course, so these seaplanes would make a splashdown before sidling up to the mothership who then pluck them up with the help of the crane to then be stowed away. It came with an electrically operated hoist and could lift a maximum of 4.5 tons.    

Directly above the aircraft hangar were three waterproofed Type 96 triple-mount 25 mm (1 in) autocannons, with an additional 25 mm (1 in) autocannon just aft of the bridge. It also came with a single long-range Type 11, 140 mm (5.5 in) deck gun capable of hitting targets 15 km (9.3 mi) away and eight torpedo tubes all found in the bow of the submarine.   

Like many Japanese ships at the time, the I-400s came with two parallel sets of demagnetization cables, running from the stern to the bow plane. These were designed to nullify the magnetic field caused by the submarine which could trigger magnetic mines in the area. They also carried a special trim system that would allow them to loiter underwater while waiting for the planes to return. No doubt a great idea in theory, but in practice it was said to be incredibly noisy and it’s not clear how often it was actually used.

Another ingenious addition, of German origin it must be said, was the anechoic coating that was applied from the waterline to the keel. The mixture of gum, asbestos, and adhesives was said to absorb or diffuse enemy sonar pulses and dampen reverberations from the boat’s internal machinery. This meant that the submarines should have been much harder to detect while submerged, but again, it’s not entirely clear just how useful it turned out to be, mainly because they were never fully operational in that way.  

The Japanese also trusted the Germans to make their periscopes and each I-400 came with two 12.2 m (40 ft) long periscopes, one for daylight and the other at night. During the final days of the war, one of the I-400s was fitted with a German-supplied snorkel, and with this hydraulically raised air intake system, the submarine could effectively run its engines and recharge its batteries while remaining at periscope depth.   

Aichi M6A Seiran

Aichi M6A1 Seiran
Aichi M6A1 Seiran. By HawkeyeUK, is licensed under CC-BY-SA

The aircraft launched from the I-400s was the Aichi M6A Seiran, a plane designed specifically with its rather unique mission in mind that remained a secret from the allies until the end of the war. This was a two-seat, low-winged monoplane with a 1,410 hp Aichi AE1P Atsuta 30 engine, which was a licence-built copy of the Daimler-Benz DB 601. As we’ve seen, the Axis powers were certainly quite open about sharing technology with fellow would-be rulers of the world. It could be folded up to fit nicely into the hangar on the submarine and was launched via a 26 metres (85 ft) compressed-air catapult mounted on the forward deck.  

The launch process was painstakingly practised by a team of four that could get an aircraft into the air with just seven minutes’ notice. But this aircraft did need some special attention because their engines couldn’t be warmed up on deck as you would on a normal aircraft carrier. To save time, and using another helpful hint from their German buddies, the crews heated the engine oil to roughly 60 °C (140 °F) in a separate chamber and pumped it, along with hot water, through the engine just before launch. This meant that the aircraft could be ready to go from a cold start and drastically reduced preparation time.  

In terms of weaponry, the Seiran carried a single 850 kg (1,870 lb) torpedo or an equivalent weight in bombs and a lone 13 mm (0.51 in) Type 2 machine gun to be used by the observer behind the pilot. 


Only three I-400 submarines were completed and far too late in the day to make any meaningful impact on the war. The first, I-400, was completed on 30th December 1944, the second, I-401 on 8th Jan 1945 and the third, I-402, on 24th July 1945. The final submarine was converted into a tanker shortly before its completion, so technically the Japanese only had two I-400s with aircraft capabilities.

If you know your World War II history, and if you’ve come almost all the way through a video about an obscure Japanese submarine class during the Second World War I’m guessing you might have some sort of idea, you’ll know that by this point the war was spiralling out of control for the Japanese as they were pushed further and further back. 

In fact, such was the collapse of the Japanese forces, neither of the operational submarines ever even took part in combat. One plan to attack the locks in the Panama Canal got to the stage of full-scale mock-ups being created and the pilots practising dropping torpedoes, but shortly before the attack was scheduled, the Japanese island of Okinawa fell to the Americans, and the tattered remnants of the Japanese Navy were ordered back to defend the home islands. 

A second attack was conceived that would target the Ulithi U.S base on the Caroline Islands in the Western Pacific, where a group of 15 U.S aircraft carriers had rendezvoused to commence further airstrikes. Shortly before leaving the Maizuru Naval Station, each aircraft had two stars and stripes painted over the Japanese rising suns on each wing, which is a clear violation of the rules of war and went down very badly with the aircrew who saw it as dishonourable to fly under the American flag.

But it didn’t matter. On 6th December 1945, the city of Hiroshima was left a smouldering wreck by the world’s first nuclear attack and three days later, the Americans followed it up with a second strike on Nagasaki. Six days later, the Japanese Emperor formally announced the nation’s surrender and all planned military operations were of course cancelled, including the attack on the Ulithi base. 

There was apparently another mission that the Japanese had in the pipeline that would have needed the I-400 submarines. Operation Cherry Blossom at Night actually has quite a pleasant ring to it, but the proposed biological attack on San Diego would have been anything but. The plan to drop infected flea bombs on the Californian city had been scheduled for 22nd September 1945, but by that point, U.S ships were sitting comfortably in Tokyo Bay. 

The Dark Depths   

When the U.S arrived in Japan, they took possession of numerous submarines and other ships. Most of these were scuttled off the coast of Japan after the Soviets demanded the right to have a snoop around this new technology and no doubt take a few notes. 

The three finished I-400s were sailed back to Hawaii where they underwent further inspection before being scuttled when once again their communist ‘allies’ requested access to the submarines under the terms of the treaty that ended the war. The I-400s were sailed off the coast of Oʻahu where they were torpedoed and sent down below the waves and into the dark depths of the Pacific Ocean. 

In 2005, I-401 was discovered by the Pisces deep-sea submarines operated by the Hawaii Undersea Research Laboratory at a depth of 820 metres (2,690 ft) and in 2013, I-400 was found by the same team at a depth of 700 metres (2,300 ft), its skeletal form still largely intact, its guns still mounted and gradually decaying. 

It wasn’t until the introduction of the nuclear ballistic missile submarines in the 1960s that the crown of the largest submarine in the world was finally passed. The I-400s weren’t exactly great submarines, and the Japanese certainly had much better, but they did provide something entirely unique. And to this day, they remain the only submarines ever created that could hold more than 2 aircraft.  

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