I-400 Aircraft Carrying Subs

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Giant World War II aircraft-carrying submarine discovered off Oahu coast

A World War II-era Imperial Japanese Navy mega-submarine, the I-400, lost since 1946 when it was intentionally scuttled by U.S. forces after its capture, has been discovered in more than 2,300 feet of water off the southwest coast of O‘ahu. The discovery resolves a decades-old Cold War mystery of just where the lost submarine lay, and recalls a different era as one war ended and a new, undeclared conflict emerged.

Longer than a football field at 400 feet, the I-400 was known as a “Sen-Toku” class submarine—the largest submarine ever built until the introduction of nuclear-powered subs in the 1960s. With a range of 37,500 miles, the I-400 and its sister ship, the I-401, were able to travel one and a half times around the world without refueling, a capability that, to this day, has never been matched by any other diesel-electric submarine.

The new discovery of the I-400 was led by veteran undersea explorer Terry Kerby, Hawaiʻi Undersea Research Laboratory (HURL) operations director and chief submarine pilot. Since 1992, HURL has used its manned submersibles Pisces IV and Pisces V to hunt for submarines and other submerged cultural resources as part of the National Oceanic and Atmospheric Administration’s (NOAA) maritime heritage research effort.

Heritage properties like historic wreck sites are non-renewable resources possessing unique information about the past. This discovery was part of a series of dives funded by a grant from NOAA’s Office of Exploration and Research and the University of Hawai‘i at Mānoa’s School of Ocean and Earth Science and Technology (SOEST). Working with Steven Price of HURL, Kerby has researched the subject of lost submarines off O‘ahu for decades. On these recent dives, Kerby was joined by two NOAA archaeologists with experience in documenting World War II vessels and submarines, Drs. James Delgado and Hans Van Tilburg.

“The I-400 has been on our ‘to-find’ list for some time. It was the first of its kind of only three built, so it is a unique and very historic submarine,” said Kerby. “Finding it where we did was totally unexpected. All our research pointed to it being further out to sea. The multi-beam anomalies that appear on a bottom survey chart can be anything from wrecks to rocks—you don’t know until you go there. Jim and Hans and I knew we were approaching what looked like a large wreck on our sonar. It was a thrill when the view of a giant submarine appeared out of the darkness.”

The I-400 and the I-401 aircraft-carrying submarines held up to three folding-wing float-plane bombers that could be launched by catapult just minutes after the submarines surfaced. Each aircraft could carry a powerful 1,800-pound bomb to attack the U.S. mainland. But neither was ever used for its designed purpose, their missions curtailed by the end of armed conflict in the Pacific.

“The innovation of air strike capability from long-range submarines represented a tactical change in submarine doctrine,” said Delgado, director of NOAA’s Maritime Heritage Program, within the Office of National Marine Sanctuaries, Washington, D.C. “The large I-400, with its extended range and ability to launch three M6A1 Seiran strike aircraft, was clearly an important step in the evolution of submarine design.”

Up until the Sen-Toku’s day, submarines had been almost exclusively dedicated to sinking surface ships (and other submarines) by stealth attack from under water.

“The I-400 is technologically significant due to the design features associated with its large watertight hangar,” Delgado said. “Following World War II, submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for U.S. submarines at the advent of the nuclear era.”

At the end of WWII, the U.S. Navy captured five Japanese subs, including the I-400, and brought them to Pearl Harbor for inspection. When the Soviet Union demanded access to the submarines in 1946 under the terms of the treaty that ended the war, the U.S. Navy sank the subs off the coast of Oʻahu and claimed to have no information on their precise location. The goal was to keep their advanced technology out of Soviet hands during the opening chapters of the Cold War. HURL has now successfully located four of these five lost submarines.

The HURL crew identified the wreck site by carefully combing through side-scan sonar and multi-beam sonar data to identify anomalies on a deep sea floor littered with rocky outcrops and other debris. The wreck was positively identified as the I-400 based on features including its aircraft launch ramp, deck crane, torpedo tube configuration, and stern running lights. The remains of the submarine’s aircraft hangar and conning tower appear to have been separated from the wreck, perhaps in the blunt trauma of the three U.S. Navy torpedo blasts that sunk the ship in 1946.

The I-400 was discovered in August 2013 and is being announced today after NOAA has reviewed its findings with the U.S. state department and Japanese government officials.

“These historic properties in the Hawaiian Islands recall the critical events and sacrifices of World War II in the Pacific, a period which greatly affected both Japan and the United States and shaped the Pacific region as we now know it,” said Van Tilburg, maritime heritage coordinator for NOAA in the Pacific Islands region. “Our ability to interpret these unique weapons of the past and jointly understand our shared history is a mark of our progress from animosity to reconciliation. That is the most important lesson that the site of the I-400 can provide today.”
 
Re: I-400 Aircraft Carrying Submarine found...

Looks like the I-402 has been found, off Nagasaki where it was sunk at the end of the war as part of the USN's Operation Road's End: http://the-japan-news.com/news/article/0002342386

23 other submarines were also apparently discovered along with it (looks like the long lost Point Deep Six submarine 'graveyard'): http://ajw.asahi.com/article/behind_news/social_affairs/AJ201508080037
 
I-400 Subs

;D
 

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Following Soviet demands to take over Japanese submarines, the Americans sank the submarines as targets in explosive tests off Pearl Harbor on 4 June 1946.
 
Are these the mega-subs whose planes were intended to take out the Panama locks ??

IIRC, auxiliary gates had already been fitted that would have prevented loss of the essential 'header' lake-water...
 
Are these the mega-subs whose planes were intended to take out the Panama locks ??
To be exact, they were designed to the generic idea of "attacking America homeland" in mind; Yamamoto wanted some "strategic weapon" that could put America into defense (presumably hoping that it would at least drag some resources from the frontlines). He ordered eighteen such submarines, mainly specifying that their range should be enough to circumnavigate the globe without refueling.

Panama strike idea was finalized later, in 1943, as an idea to implement I-400 submarines in a surgical strike with maximum outcome, rather than generic "attacks against American coastlines". The air crews were trained initially to usual bomb runs, then to kamikaze attacks on gates.

Before such operation could be implemented, the deteriorating situation caused to re-evaluate the whole plan. It was decided, that Panama attack would not have much of immediate effect, and Japan desperately needed some relief now. So the plan was made instead to make a surprise attack against American carriers gathered at Ulithi atoll. The two-stage plan was made:

* To move diassembled C6N Saiun recon planes by submarines (old aircraft-capable I-13 and I-14) to Truk, to make high-altitude recon flight over atoll.
* To attack the atoll with six Seirans from I-400 and I-401. The bombers were supposed to make a low-altitude approach less than 50 meters over the water, to avoid detection by radars and patrolling US fighters. To maximize chances, Japanese even decided to camouflage the attacking planes into American marking (!)

After that attack, all four submarines were supposed to relocate to Singapore - where fuel was more available - and replenish their air groups for further attacks.

The operation was put in action, but firstly I-13 was sunk by US destroyer, and before I-14 was able to deliver recon planes to Truk, Japan surrender3ed.
 
I suppose an option would have been air-delivery of CBW per that unspeakable 'research' site in China...

Given the Allied 'nasties' that we know US & RN deep-dumped post-war, IJN must have accepted that depopulating several US West Coast ports & bases would have been a very, very bad mistake.

Tangential, we know Japan's nuke program was more focussed, but far behind Germany's plurality shambles. Were they aware US was busy thus ??
 
I suppose an option would have been air-delivery of CBW per that unspeakable 'research' site in China...
Probably, but... Japanese military was actually reluctant to launch bioweapon campaign due to simple reason; by 1945, Japanese population was nearly starving. No matter how bioweapon attack may hurt USA, the retaliation bioweapon attack would cause orders of magnitude worse effect on Japanese population. Even Japanese generals and admirals - not exactly known for rational thinking - understood that Japanese suffering from malnutrition are much more vulnerable than healthy Americans.

Tangential, we know Japan's nuke program was more focussed, but far behind Germany's plurality shambles. Were they aware US was busy thus ??
Most likely no. They assumed that US have atomic project of their own, of course, but they have very little clues how advanced American research are. Japanese intelligence efforts in USA were almost nonexistent; basically their only way to gather information were embassies in the neutral countries.
 
I agree with Dilandu, the Germans and the Japanese had much more to lose thru the
use of bioweapons than they had to gain. They also didn't know what our ability of
these type of weapons where.
 

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