This is a post for all of you who didn’t know about this monster before. So let me present a none ordinary Japanese Navy built Submarine. Because these were the largest submarines of WWII, the I-400 Class. Over 400′ long, weighing 5,700 tons, carrying a crew of 200 and possessing a range of over 50,000 miles. Nearly 70 years ago, the Imperial Japanese Navy’s built the I-400-class submarine aircraft carriers until they were captured and scuttled off the coast of Hawaii. These subs were HUGE let me tell you that. And during World War II, Imperial Japan’s massive I-400 submarine was shrouded in the highest levels of secrecy.

The Japanese built five (three were launched) giant, seaplane carrying submarines. One made a trip around the Horn of Africa to Germany. Another was converted as a tanker. A submarine fleet was formed intending its aircraft to torpedo the Gatun Locks, Panama Canal, but was diverted to attack the US anchorage at Ulithi just as the war ended. The I-400 series submarine had an energy absorbing skin, snorkel, radar, degaussing, and a range of 1-1/2 times around the world and four months duration. Displacement 4760 tons, length 355 feet.

For the next 68 years, 2,300 feet of ocean darkness concealed the world’s first sub capable of launching an offensive strike on mainland targets. But earlier this year, the twisted 400-foot colossus was discovered resting peacefully on the ocean floor off the southwest coast of Oahu.

The groundbreaking submarine scuttled by the U.S. Navy after the war to keep the technology out of the Soviets’ hands was discovered by veteran Hawaii Undersea Research Laboratory explorer Terry Kerby and colleagues from the National Oceanic and Atmospheric Administration and the University of Hawaii at Manoa’s School of Ocean and Earth Science and Technology.

The find not only solved a seven-decade mystery, but opens a window into the potency of the Imperial Japanese war machine and the American foresight of the looming Cold War.

i-400 i-400c

“The I-400 is the big prize,” Kerby told Stars and Stripes earlier this month.

These submarines included a unique weapon system: a watertight deck hanger capable of transporting three Aichi M6A1 Seiran float-planes for offensive operations. These folding float-planes could be readied for flight in only minutes and launched by catapult. Submarine I-400 was to be used to attack the Panama Canal, or even plant to attack American cities. No book to date has tackled this incredible subject matter, and the authors debunk myths and facts of the I-400’s wartime service and mission.

Even thou Yamamoto Isoroku had reservations about taking on the sleeping giant that was America. In order to get the Americans to the peace talk table quickly, the Japanese would have to act swiftly and decisively. The first part of his plan was the attack on Pearl Harbor on Dec. 7, 1941. The second part was a reign of terror to frighten the Americans into submission, with attacks on both the east and west coasts, possibly even Washington, D.C.

To accomplish this, plans were devised in January 1942 for a hybrid weapon: part aircraft carrier to launch planes that could attack mainland targets, and part submarine for stealth and the element of surprise. The I-400 was born. Development of the sub took years, but when the I-400 was completed in 1944, it was the largest submarine ever built at 400-feet long and a surface displacement of 3,530 tons. The I-400 carried 157 officers, engineers, electricians and pilots, Ando said. It had a twin-hulled design to support the weight of aircraft on its top deck.

“It is the only submarine that carried fighters,” Ando said. “There is no other example.”

I-400-Class-Japanese-Submarine-1-with aircraft

The I-400 featured a 115-foot long, 12-foot diameter, water-tight hangar housing three M6A1 Seiran (Storm from a Clear Sky) torpedo-bombers above its main deck, according to the writings of U.S. Navy Lt. T.O. Paine, who sailed the vessel to Hawaii as executive officer of the prize crew at the war’s end. The fighters, with their wings folded in, were rolled out through a massive hydraulic door onto an 85-foot pneumatic catapult that launched them into the sky within minutes of the order being given. After completing their mission, they would land in the sea and be picked up and loaded back onto the sub using an onboard hydraulic crane.

“She was armed with eight torpedo tubes, a 5.5-inch, 50-caliber deck gun, a bridge 25mm anti-aircraft gun, and three triple 25mm A/A mounts atop her hangar,” Paine wrote. “Meals for her oversize crew were prepared in a galley in the starboard hull, where large steam kettles turned out great quantities of rice.”

In addition to its armament, the Japanese used a groundbreaking rubberized coating that would help the submarine remain undetected by allied sonar as it traversed the globe. A technology adapted from sharpening swords was used to keep the vessel sealed water-tight, Ando said: Too little, too late. When all was said and done, the Japanese had a super weapon on their hands.

The submarine had a range of 37,500 miles and was able to travel around the world 1 1/2 times before it needed refueling, something that remains unmatched to this day by any other diesel-electric submarine, according to a University of Hawaii at Manoa statement. And plans were to develop 18 of the subs to bring the war to America’s doorstep. However, the I-400 program suffered a setback when Yamamoto was killed by U.S. Army Air Force pilots on April 18, 1943, after the allies broke Japanese codes and shot down his plane while he toured the Northern Solomon Islands.

In July of 1943, the number of subs to be built was reduced to 11, then eventually to five in December, Ando said.

The I-400 was built in Kure and completed on December 30, 1944. The I-401 was completed in Sasebo on January 8, 1945 and the I-402 on July 24, 1945.

“We finally managed to complete underwater carrier even though the process did not go as planned since the submarine was tremendously big and there were lack of materials since the war was unfavorable [to Japan] and damages to the factories were increasing and the plan had to be changed many times,” wrote Tsugio Yata, the gunnery officer on I-401, in a newsletter for his veterans group, Naniwa Kai. Can you imagine the capability of these subs if completed in full-scale production?

I-400-Class-Japanese-Submarines-1-with aircrafts

Both submarines were however seized by the U.S. Navy on Aug. 28, 1945, as they made their way back to Japan. Their bombers had earlier been pushed overboard to avoid capture, but U.S. troops in Japan later found one surviving example and shipped it back for testing. That aircraft is now displayed by the Smithsonian Institution at its museum near Dulles International Airport.

As the occupation of Japan began, 24 operational submarines with Cherry Blossoms attached to their periscopes as a farewell from their crews were sunk off Goto Island near Sasebo in Operation Roads End, including the I-58, which had sunk the USS Indianapolis. However, five of the more unusual vessels, the I-400 and I-401, the 400-foot I-14c, and the two fast attack subs I-201 and I-203, were to be taken to Hawaii for closer inspection.

“The high-ranking [American] officers visiting Japan all said, ‘wonderful’ and ‘big one,’” Yata wrote. “These were the words that anyone who saw that submarine says first.”

The I-400 departed from the bombed out city of Sasebo for Pearl Harbor on Dec. 11, 1945, escorted by the submarine rescue vessel USS Greenlet, The I-400 was sunk on May 31, 1946 and the I-401 on June 4, 1946.


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