Photo story (Clockwise from top left): (1) WWII I-400 class Japanese mega submarine I-400 with its forward catapult and long Plane Hanger during WWII. (2) Aichi M6A1 Seiran during WWII (3) Pearl Harbor attack was successfully carried out under the auspices of Admiral Isoroku Yamamoto. (4) After being captured by U.S. Navy, officers of I-400 photographed in 1945. (5) The 14cm /40 11th Year Type naval gun aboard I-400 being examined by U.S. Navy Personnel (6) HURL photo of I-400 submerged off the coast of Oahu.
The WWII I-400 class Japanese mega submarines were the largest subs of WWII and remained largest ever built till the 1960s. Three Aichi M6A1 Seiran floatplanes, which had foldable wings & stabilizers, were carried by the mega subs to their desired destinations, undetected under water. Each Aichi M6A1 Seiran aircrafts were armed with an 800 kg or 1,800 lb bomb or a torpedo. 8 torpedo tubes were also mounted in the bow of the I-400, four above & four below. A ‘14cm /40 11th Year Type’ naval gun & four more anti aircraft machine guns gave the mega sub further aerial protection.
I-400s were designed to surface, launch the Aichi M6A1 Seiran aircrafts aboard and then quickly dive under water before the enemy could discover them. The mega sub had the displacement of 6,670 tons. The weight of water that a naval vessel displaces when it’s floating is called its displacement. It’s the same as the weight of the loaded naval vessel according to principle of Archimedes. The sub was 400 ft long, with a beam or width of 39.4 ft. Its draft or the distance from the keel to the waterline was 23 ft. Four 1,680 kW or 2,250 hp diesel engines propelled it while surfaced and two 1,600 kW or 2,100 hp electric motors propelled the sub while submerged. The mega sub carried 144 naval officers and men.
The WWII I-400 class Japanese mega submarine was the brainchild of Commander in Chief of Japanese fleet, Admiral Isoroku Yamamoto. On 7th December 1941, the surprise military attack against U.S. naval base at Pearl Harbor, Hawaii, was successfully carried out under the auspices of Yamamoto and Chuichi Nagumo, Commander in Chief of the Japanese Navy’s main aircraft carrier force, the First Air Fleet. The United States had 8 battleships, 30 destroyers, 8 cruisers, 1 coast guard Cutter, 4 submarines, 49 other ships and around 390 aircrafts at Pearl Harbor. The Japanese used 2 battleships, 6 aircraft carriers, 1 light cruiser, 2 heavy cruisers, 8 tankers, 9 destroyers, 5 midget submarines, 23 fleet submarines and 414 aircrafts in the attack.
In the assault, United States lost 2,402 troopers with 1,247 more wounded. The U.S. losses include- 4 battleships & 2 other ships were sunk, 1 battleship was grounded, 3 battleships, 3 cruisers, 3 destroyers, 3 cruisers, 3 other ships & 159 aircrafts were damaged and 188 aircrafts were completely destroyed. The Japanese forces lost 64 men, 1 got captured. Japanese losses include- 1 midget submarine was grounded, 4 midget submarines were sunk and 29 aircrafts were destroyed.
At Pearl Harbor, the Japanese achieved a major tactical victory. After it, the United States joined the Allies against the Axis forces and declared war on the Japanese Empire. Germany & Italy declared war against the U.S. subsequently.
Shortly after the Pearl Harbor onslaught, Yamamoto planned aerial strikes to the U.S. cities along the eastern and western coasts. The plan was to prepare a fleet of 18 mega submarines capable of making 3 round trips to the U.S. west coast carrying aircrafts armed with torpedo or bomb. Construction of I-400 began on 18th January 1943. I-401, I-402, I-403 & I-404 were also scheduled to follow. But as part of Operation Vengeance, the U.S. operation to Kill Yamamoto, the Japanese admiral was killed on Bougainville Island in the Solomon Islands area when U.S. army aircraft P-38s operating from Kukum Field on Guadalcanal shot down Yamamoto’s Mitsubishi G4M Betty bomber on 18th April 1943.
The number of WWII I-400 class Japanese mega submarines to be built was dropped from 18 to 9, then 5 and finally 3, following the death of Yamamoto. I-400 and I-401 were the only two mega subs to enter service. I-402 was completed on 24th July 1945, just 5 weeks before the end of the WWII, therefore, never made it to the sea.
I-400 was completed on 30th December 1944. In April 1945, I-400, I-401 super subs along with I-13, I-14 were prepared for the Panama Canal Strike, a Japanese attack plan to destroy the locks of Panama Canal to cut down the U.S. supply lines to the Pacific Ocean. But after Okinawa fell, the plan was cancelled and the fleet planned to attack 15 U.S. aircraft carriers assembled at Ulithi atoll. While refueling at Ominato, I-13 was sunk by an American destroyer in July 1945. Before the Ulithi attack was launched, Japan surrendered on 15th August 1945, following atomic bombing of Hiroshima on 6th August 1945 and of Nagasaki on 9th August 1945.
Crew of the U.S. destroyer, to which I-400 surrendered, was astounded at the sheer size of the sub. 24 submarines were recovered and boarded by the U.S. navy including the three I-400s. U.S. Navy was taking the subs to Sasebo bay to study them. But as they received a message that an inspection team was being sent by the Soviets to examine the submarines, Operation Road’s End was initiated by United States to destroy the captured subs. Most of these were destroyed with explosive charges near Nagasaki.
I-400, I-401, I-201 & I-203 were taken to Hawaii by U.S. Navy for further inspection. After examining, U.S. submarine USS Trumpetfish scuttled the Japanese subs in the waters near Oahu in Hawaii by torpedoes on 4th June 1946. Website of news unit shared by three TV stations- KGMB, KFVE & KHNL in Honolulu, ‘Hawaii News Now’ reported that Hawaii Undersea Research Lab (HURL) at the University of Hawaii discovered I-400 in 2,300 ft water off Oahu’s southwest coast. The announcement came in the same week when the preparations are underway to commemorate the anniversary of the attack on Pearl Harbor. I-400 had been buried at sea off Oahu for over 60 years.
HURL detected an anomaly 5 miles south of Barbers Point in August 2013. Four of the five Japanese submarines underwater off Oahu have been located by them now. HURL’s operational director Terry Kerby called the massive submarine ‘a unique piece of history’. ‘The ultimate needle in the haystack is the I-23 and we believe it’s located south of Oahu’ he added.
Video story: University of Hawaii scientists found a Lost WWII I-400 class Japanese mega submarine off Barbers Point
Video story: Documentary on WWII I-400 class Japanese mega submarines, the largest submarines till the 1960s.