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Japan's I-400 Super Sub with Seiran Aircraft & Their Last Missions in 1945

Updated on November 16, 2009
The I-14 sub- 400 ft long
The I-14 sub- 400 ft long
The submarine aircraft hangar
The submarine aircraft hangar
The hangar closed and catapault
The hangar closed and catapault

Biggest Submarine until 1962

Like Germany, Japan also had its few secret weapons. The most famous were the four super subs unlike the world had seen up to that time; the I-401, I-400, I-14, I-13 and I-174. Until 1962, these subs would remain the largest every made at 400 ft long and the only aircraft carrier submarines, each carrying three Aichi M6A1 Seiran torpedo bombers. Each of the aircraft could carry 1764 lbs. of bombs or one 17 inch torpedo some 654 miles. The I-14 width was 38 ft wide, a draft of 19 ft., crew of 114 and weighed 2800 tons. The subs were also the fastest traveling at 23 knots on the surface or 8 knots submerged. It could submerge to 340 ft and contained two 2400 hp engines. The I-I4 was equipped with a snorkel, radar, radar detectors, and huge fuel tanks that gave her a range of 37,500 miles: One and a half times around the world. She was armed with eight torpedo tubes, a 5.5-in 50-cal deck gun, a bridge 25mm antiaircraft gun, and three triple 25 mm A/A mounts atop her hangar. Also the I-14 had great cruising range which enabled them to launch her three bombers within striking distance of targets as far from Japan as San Francisco, the Panama Canal, Washington, or New York. All of these missions were considered by the Tokyo Naval Strategists. The I-201 could travel submerged at a speed of 20 knots! The subs also contained a rubber coating to absorb internal noises and radar from enemy ships. All these features made it a dangerous foe to the USN.

Several plans were formulated to attack the US mainland during the Summer of 1945 (after the first mass bombing of Japan on March 10 by 280 B-29s, which killed 83,000) , the Japanese planned to use the I-400 to bomb San Francisco, CA in retaliation. This was rejected but the attack on the Panama Canal gates was accepted.

The combined force consisted of the submarine carriers I-400 and I-401, two AM class submarines, the I-13 and I-14, and 10 Seiran bombers when IJN selected the Gatun Locks of the Panama Canal (on the Caribbean side of Gatun Lake) using the 631st Air Corps embarked aboard the 1st Submarine Flotilla. Ten Seirans were to strike the locks with six torpedoes and four bombs. Once destroyed, US ships would have to travel around the south tip of South America to reach the Pacific.

The pilots studied a large-scale model of the lock system and memorized important features of the canal, just as their predecessors did before attacking Pearl Harbor. During these preparations, the Japanese decided to strike first at the U. S. Navy fleet anchored at Ulithi Atoll in the Carolines. This a was major USN base containing over 600 ships. It was also a key staging area for the US. It was June 12th.

Operation Arashi (Mountain Storm)

This plan required six Seirans and four Nakajima C6N1 reconnaissance aircraft. The I-13 and I-14 were planned to carry two each and offload them at Truk Island. The pilots would then take off and scout the American fleet at Ulithi, relaying target information to the Seiran crews. The six Seirans would carry out kamikaze attacks on the most important targets-American aircraft carriers and troop transports. The date of the attack was to be on August 17th.

The attack was to be made under a full moon. Prior to the start of the attack, the pilots were to receive a special hormone injection to enhance their night vision. Each Seiran were tasked to each carry an 800-kg bomb and land near their submarines. After the attack, all four of SubDiv 1's boats were to proceed to Singapore, refuel and embark new planes for the new attack. Ten Seiran were to be stationed there prior to the attack on Ulithi.

On July 3rd, both the I-401 and 400 departed from Ominato on separate paths to meet on August 16th off Ponape Island, Carolines. On August 14th, the I-401 makes a detour due to American acitivity near the Marshall Islands and sends a message to I-400 revealing a new rendevous point. The I-400 never received it. In any case, the subs move to the Ulithi area and rendevous for the planned strike on August 17th. Their superiors cancel the attack until the 25th. The next day, Japan surrenders but the crews of the subs ignore it and do not believe it. Both subs are in position for the attack the morning of August 16th. The attack of the 17th fails to occur. On the 18th, the captain of the I-401 receives direct orders from the Vice Admiral of the Navy to cancel the plan and return to Japan. The I-401 hesitates and loiters in the area. On the 26th, the captain is ordered to hoist the black flag of surrender and to destroy its aircraft, logs, torpedoes and secret documents. On the 30th, the captain blows his brains out with a gun, his body dumped into the sea.

The Americans capture four of the super submarines and quickly sail them to Hawaii to study before the Soviets can. Once completed, they are sailed of Oahu and sunk not to be found until 2005 or so.


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      M. Naim Shaikh 8 years ago from Greater Toronto Area, Ontario

      Now this is a nice history of the Japanese hybrid force which consisted of undersea-cum-aereal capabilities. These subs should have been studied further for their technology is fascinating for any age. Thanks for sharing with us.

      M. Naim Shaikh