During the Second World War, submarines saw more combat than ever before. Controlling the seas and oceans has turned into an arms race with each side trying to field improved submarines and counter-submarine measures that would trump the opponents, and give that side a winning advantage. Near the end of the war, the biggest submarines of the war have been developed by the Japanese empire, but fortunately for the allies, never saw actual use. Japan surrendered before the 3 newly built behemoths could be used.
The Admiral’s Brainchild
The I-400 submarines, known as a Sentoku type in Japan, were a brainchild of Admiral Isoroku Yamamoto, and were more than twice the size of any previously built submarine made by allies. These 120-meter long submarines were powered by four diesel 1,680 kW (2,250 hp) engines and carried enough fuel to go around the world 1.5 times, which was more than enough to allow them to approach the US coast from either east
or west. Additionally, the subs were covered with a rubberized coating which reduced their sonar signature. One of its most distinct features was a figure eight double hull cross-section, designed in such way to provide necessary reinforcement, support and stability to handle the weight of the integrated aircraft hangar. Since the stowed aircraft were located along the submarine’s centerline, the conning tower had to be offset to port, and the slightly eccentric design of the submarine affected its handling and maneuverability.
The initial plan, conceived and agreed upon in 1942, accounted for the construction of eighteen of these super-submarines. Unfortunately for the Japanese submarine program, on the April 18, 1943, US Air Forces managed to kill Admiral Yamamoto, after shooting his plane down. Since the main proponent of the program was dead, the Japanese government decided to reduce the number of the submarines that was to be built first to 11, and subsequently, as the war tides were changing, to 5. In the end, only three of these submarines were built in the 1945, five weeks before the end of the war.
Everything has its Flaws
While the design was sound, and plans for the I-400 ambitious, had numerous flaws. The Sentoku had relatively small rudders, which made them sluggish and difficult to maneuver while surfaced. Because of their size and prominent features, such as their large hangar and conning tower, they would veer off course under strong winds, and they had a very large radar and visual signature that could easily be spotted by enemy recon aircraft and vessels. Besides this, these subs could dive to the depth of only around 100 meters, with a very shallow dive angle, which posed problems in cases of emergency. Their slow dive time of 56 seconds, made these boats extremely vulnerable to enemy air attacks.
Due to the asymmetric cross-section, caused by the offset conning tower, these submarines also had issues when traveling underwater. When traveling at lower speeds, they tended to veer to their port side, which meant that the helmsman had to input a 7 degree course-correction to the starboard. When conducting a torpedo attack, or other combat maneuvers, the command crew had to take into account the difference between the turning circles, since the starboard turning circle was larger by a significant margin.
The living conditions in these submarines were subpar, like in all the other Japanese submarines of that time. These submarines had no air conditioning, which posed issues when traveling in tropic waters, no toilets with running water, no cold storage and insufficient number of sleeping quarters. This meant that the health of the crewmen was compromised, due both the hygiene issues and a very limited diet caused by the lack of refrigeration. When fully crewed, the I-400 carried 157 officers, engineers, electricians and pilots, and some of the men had to sleep on the decks and in passageways.
Death from Above
Originally, these subs were designed to hold two fighter planes, but this has been improved to three in 1943, on request of the Submarine Staff Officer of the Naval General Staff, Yasuo Fujimori. After redesign and remodeling, the Sentoku could hold three fully operational aircraft and parts for a fourth. In addition to these specially designed Aichi M6A Seiran floatplanes, the firepower of these submarines consisted of three triple 25mm anti-aircraft guns, a 25mm bridge anti-aircraft gun, a large 14 cm 50 cal deck gun, and eight torpedo tubes that fired the largest, most advanced torpedo of the time, the Long Lance.
The floatplane Seiran was designed specifically for use on these submarines, and was a closely guarded secret, unknown to allied forces. It had a 
range of 1,000 km (620 mi), top speed of 475 km/h (295 mph), and it could carry an 800 kg (1,800 lb) bomb. It had a length of 11.6 m (38 ft), and a wingspan of 12 m (39 ft). Due to the narrow storage space on the submarine, special modifications had to be made to the planes, to make their profile as narrow as possible. They had a specialized hydraulic system that could rotate wings 90 degrees and fold them backwards against the fuselage, and that could also be used to fold the horizontal and vertical stabilizes in such a way that the forward profile of the aircraft was defined by the propeller. It took 45 minutes to assemble, prepare, and launch all three aircraft, by a trained four-man crew. In case of a kamikaze attacks, which were also considered, it would take only 5 minutes per plane, since the landing pontoons would be left out.
Since Serian was designed for hit-and-run terror missions, it meant it was mainly to be used in night sorties and assembled in the dark, so they were coated in special luminescent paint to make the assembly process easier. The aircraft were to be launched by a 26 m (85 ft) long, Type 4, compressed-air catapult, fly their missions, and navigate back to their home by dead reckoning. This meant that the submarine would have to stay submerged in one place, so that the pilot can find his way back, land on the water, and be hoisted up by submarine’s crane. While the process was similar to that used by Japanese navy cruisers, the biggest issue for the pilot is finding the hidden vessel on his return.
The Unfortunate End
After the war had ended, the U.S. navy seized the two submarines that remained operational, while they were on their way to Japan. These submarines were then sent to Hawaii where they were studied extensively, until the Soviet Union requested access to them, as per the terms of the treaty that was signed between the warring nations. Rather than comply, the U.S. felt the looming threat of the coming cold war, and decided to scuttle the submarines instead. The one recovered Sieran bomber is displayed in the Smithsonian museum, near Dulles international airport.
While these submarines, thankfully for the allied forces, never saw action, they still remain an amazing piece of engineering that showed the genius of Japanese engineers, and they inspired numerous future submarine designs.
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