The Imperial Japanese Navy’s I-400-class submarine carried three special-attack Seiran aircraft and theoretically possessed sufficient range to travel one and a half times around the globe.

2020-08-05
This article records the concept, capabilities, wartime history, rediscovery, and technological legacy of an unprecedented “submarine aircraft carrier” that was among the largest submarines commissioned during the Second World War and is said to have influenced the later development of missile-launching submarines.
About This Article
The I-400 class was a group of Imperial Japanese Navy submarines constructed during the Greater East Asia War and designed to carry three Seiran special-attack aircraft.
Its theoretical cruising range was sufficient to travel one and a half times around the Earth.
It was therefore conceived as a submarine capable of departing from the Japanese home islands, reaching almost any target on the globe, carrying out an attack, and returning to Japan.
Although three submarines of the class entered service, the war ended before they could achieve major combat results.
The Allies remained unaware of their existence until Japan surrendered.
It is said that American naval officers were astonished by their enormous size and aircraft-carrying capability when the submarines were taken into custody after the war.
The I-400 concept treated the submarine not merely as an anti-ship weapon, but as a long-range strategic platform capable of carrying aircraft and other offensive weapons.
It has therefore been described as a forerunner of later cruise-missile and ballistic-missile submarines.
Source
The free encyclopedia Wikipedia, “I-400-class submarine”
Quotation
“Theoretically, it possessed the extraordinary cruising range required to sail one and a half times around the globe, allowing it to attack any chosen point on Earth from the Japanese home islands and then return to Japan.”
The I-400-Class Submarine
The I-400-class submarine was a class of submarine operated by the Imperial Japanese Navy during the Greater East Asia War.
It was a “submarine aircraft carrier” capable of carrying three Seiran special-attack aircraft.
The class was also known as the Sen-Toku type.
To compensate for the reduction of the original I-400 construction program, the I-13 class was developed by modifying the Type A cruiser-submarine design.
The two classes were similar in outward appearance.
The I-400 class was capable of carrying three Seiran special-attack aircraft.
It was therefore a genuine submarine aircraft carrier.
Among the submarines commissioned during the Second World War, it was the largest, exceeding the length of the United States Navy’s Gato class by 27 metres.
It remained the world’s largest conventionally powered submarine until it was surpassed by the Chinese People’s Liberation Army Navy’s Type 032 submarine, completed in 2012, with a surface displacement of 3,797 tonnes and a submerged displacement of 6,628 tonnes.
Theoretically, it possessed the extraordinary cruising range required to sail one and a half times around the globe.
It could therefore depart from the Japanese home islands, carry out an attack at virtually any chosen point on Earth, and return to Japan.
Although it had an enormous hull with a displacement of 3,350 tonnes, larger even than the light cruiser Yūbari, its underwater performance was good.
It required one minute to complete an emergency dive.
Three submarines of the class entered service.
However, the war ended before any of them could achieve major combat success, and the Allies remained unaware of their existence until Japan surrendered.
An account survives stating that American naval officers were astonished by their size when the vessels were taken into United States custody immediately after the war.
Following examination by the United States military, the possible use of I-400 and I-401 by the United States Navy was considered.
However, shortly after representatives of the Soviet government requested permission to inspect them, the United States decided to prevent the possible transfer of information to the Soviet Union.
The two submarines were therefore sunk by torpedoes off Hawaii.
Their precise locations were not recorded after the disposal.
Following many years of underwater investigation by American researchers, I-401 was discovered in March 2005 and I-400 in August 2013.
I-402 was subsequently discovered on the seabed in August 2015.
According to specialists, the I-400 class transformed the perceived role of the submarine, which had previously been regarded primarily as an anti-ship weapon.
It significantly influenced the design and operational philosophy of submarines after the Second World War.
This development is said ultimately to have led to submarines capable of launching ballistic missiles in the nuclear age.
Indeed, the submarine used by the United States military in postwar experiments involving the launch of a pulse-jet missile from a surfaced submarine bore a striking resemblance to the I-400 class.
Submarines Capable of Carrying Aircraft
After the First World War, the Imperial Japanese Navy developed the Yokosho Type 1 Reconnaissance Seaplane, a submarine-borne reconnaissance aircraft based on a small German-built seaplane.
By the early Shōwa period, the Japanese Navy had broadly established the practice of using submarines for monitoring, shadowing, and maintaining contact with enemy fleets.
The installation of small reconnaissance seaplanes aboard submarines strengthened their reconnaissance and surveillance capabilities.
Aircraft carried by these earlier aircraft-capable submarines, including I-5 and I-12, were intended for reconnaissance during conventional submarine operations.
They included aircraft such as the Type 96 Small Reconnaissance Seaplane and the Type Zero Small Reconnaissance Seaplane.
The special-type submarine, however, was initially intended to carry the Suisei carrier-based dive bomber.
When this was judged impractical, the Seiran special-attack aircraft was developed specifically for the mission.
In the original design, the Seiran was to be operated without floats as a conventional aircraft.
Under this arrangement, recovery of the aircraft would have been impossible, and each aircraft would effectively have been expendable.
The principal purpose of the project was the strategic employment of aircraft intended purely for bombing and attack missions.
In this respect, the I-400 class was fundamentally different from earlier submarines that carried small dedicated seaplanes solely for reconnaissance.
To be continued.