Japanese I-400 aircraft carrier submarine and the planned attack on Panama Canal locks


artist formerly known as quad destroyer and...
Dec 13, 2001
Austin, TX

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Pictures randomly inserted for your viewing and reading pleasure.



The I-400 class [submarine] was the brainchild of Admiral Isoroku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet.
Shortly after the attack on Pearl Harbor, he conceived the idea of taking the war to the United States mainland by making aerial attacks against cities along the U.S. western and eastern seaboards using submarine-launched naval aircraft.

Yamamoto submitted the resulting proposal to Fleet Headquarters on 13 January 1942.
It called for 18 large submarines capable of making three round-trips to the west coast of the United States without refueling or one round-trip to any point on the globe.
They also had to be able to store and launch at least two attack aircraft armed with one torpedo or 800 kg (1,800 lb) bomb.


Each submarine had four 1,680 kW (2,250 hp) engines and carried enough fuel to go around the world one-and-a-half times—more than enough to reach the United States travelling east or west. Measuring more than 120 m (390 ft) long overall, they displaced 5,900 t (6,500 short tons), more than double their typical American contemporaries.

Located approximately amidships on the top deck was a cylindrical watertight aircraft hangar, 31 m (102 ft) long and 3.5 m (11 ft) in diameter. The outer access door could be opened hydraulically from within or manually from the outside by turning a large hand-wheel connected to a rack and spur gear.

Eight torpedo tubes were mounted in the bow, four above and four below. There were no aft tubes.

The I-400-class subs were unwieldy and relatively difficult to maneuver while surfaced owing to their small rudders. The large superstructure also caused the sub to veer off course during any strong wind. The maximum safe diving depth of the I-400-class submarine was only 82% of its overall length, which presented problems if the submarine dived at too steep an angle in an emergency. Because of their large aircraft hangars and conning tower, all I-400-class boats had significant visual and radar signatures on the surface, and could be detected by aircraft relatively easily. Dive time was 56 seconds, nearly double that of U.S. fleet subs, which made the boats easier to destroy from the air when caught on the surface.


When submerged and traveling at a slow speed of two knots, the offset superstructure forced the helmsman to steer seven degrees starboard in order to steer a straight course. When conducting a torpedo attack the captain had to take into account his larger turning circle to starboard than to port, again because of the offset design. Like other Japanese submarines, crew members in Japanese subs had no air conditioning to control temperatures in tropic waters and no flush toilets. Lack of cold storage greatly limited the crew's diet, while inadequate sleeping quarters forced some of the crew to sleep on the decks or in passageways.

I-400s could stow up to three Aichi M6A Seiran floatplanes.

The Seiran was specifically designed for use aboard the submarines and could carry an 800 kg (1,800 lb) bomb 1,000 km (620 mi) at 475 km/h (295 mph). To fit inside the narrow confines of the hangar, the floats were removed and stowed, the wings rotated 90 degrees and folded backward hydraulically against the fuselage, the horizontal stabilizers folded down and the top of the vertical stabilizer folded over so the overall forward profile of the aircraft was within the diameter of its propeller.


Panama Canal strike


Following an inspection of Rabaul in August 1943, Captain Chikao Yamamoto and Commander Yasuo Fujimori conceived the idea of using the sen toku (secret submarine attack) to destroy the locks of the Panama Canal in an attempt to cut American supply lines to the Pacific Ocean and hamper the transfer of U.S. ships. Intelligence gathering on the proposed target began later that year.

The Japanese were well aware that American fortifications existed on both sides of the Canal. On the Atlantic, the large coastal artillery batteries of Fort Sherman had a range of 30,000 yards (17 miles (27 km)), preventing enemy ships from getting near enough to shell the locks. In the months following the attack on Pearl Harbor, air and sea patrols had been strengthened around both entrances, and barrage balloons and anti-submarine nets erected. In August 1942, the 88th Coast (Anti-Aircraft) Artillery unit was added to help defend against aerial attacks.

As the war continued and Japan's fortunes declined, however, security around the Canal grew increasingly lax. In January 1944 Commander Fujimori personally interviewed an American prisoner-of-war who had done guard duty there. He told Fujimori that defensive air patrols had virtually ceased, since it was considered increasingly unlikely the Axis powers would ever attack the locks. This further convinced Fujimori of his plan's feasibility.

A Japanese engineer who had worked on the Canal during its construction handed over hundreds of documents to the Naval General Staff, including blueprints of the Canal structures and construction methods. A team of three shipping engineers studied the documents and concluded that the locks at Miraflores on the Pacific side were the most vulnerable to aerial bombing, but the Gatun locks on the Atlantic side offered a chance of causing greater damage, since it would be harder to halt any outflow of water. They estimated the Canal would be unusable for at least six months following a successful attack on the locks.


To increase the size of the airborne attack force, Commander Fujimori requested that two additional fleet submarines still under construction at Kobe, I-13 and I-14, be modified to house two Seirans each, bringing the total number of planes available to ten. It was originally planned that two of the Seirans would carry torpedoes and the other eight would carry 800 kg (1,800 lb) bombs. They were to make a combined torpedo and glide-bombing attack against the Gatun Locks. Eventually though, torpedo-bombing was dispensed with, because only one Seiran pilot had mastered the technique.


The Panama Canal strike plan called for four aircraft-carrying submarines (I-400, I-401, I-13 and I-14) to sail eastward across the Pacific to the Gulf of Panama, a journey expected to take two months. At a point 185 km (100 nmi) off the coast of Ecuador, the submarines would launch their Seiran aircraft at 0300hrs on a moonlit night. The Seirans, without floats, would fly at an altitude of 4,000 m (13,000 ft) across the northern coast of Colombia to the vicinity of Colón. Now on the Caribbean side of the isthmus, they would turn westward on a heading of 270 degrees, then angle south-west and make their final approach to the Canal locks at dawn. After completing their bombing runs, the Seirans were to return to a designated rendezvous point and ditch alongside the waiting submarines where the aircrews would be picked up.

Around April 1945, Captain Ariizumi, the man appointed to carry out the attack, decided the Seiran pilots would make kamikaze ramming attacks against the gates, rather than conventional bombing runs, a tactic becoming increasingly common as the war went against the Japanese. The Seiran squadron leader had already suggested as much to Ariizumi earlier that month, though for a time this was kept secret from the other pilots. At the end of May, however, one pilot happened to observe a Seiran having its bomb-release mechanism removed and replaced with a fixed mount. Realizing the implications of this change, he angrily confronted the executive officer of the squadron, who explained that the decision to withhold this intention from the other men was made to "avoid mental pressures on the aircrews."

By 5 June 1945, all four aircraft-carrying submarines had arrived at Nanao Wan where a full-scale wooden model of the Gatun Locks gate had been built by the Maizuru Naval Arsenal, placed on a raft and towed into the bay. The following night, formal training commenced with the Seiran flight crews practising rapid assembly, catapult launch and recovery of their aircraft. There was also rudimentary formation flying. From 15 June the Seiran pilots made practice daylight bombing runs against the wooden gate mock-up. By 20 June, all training ended and the operation was set to proceed

Before the attack could commence, Okinawa fell, and word reached Japan that the Allies were preparing an assault on the Japanese home islands. The Japanese Naval General Staff concluded the Panama Canal attack would have little impact on the war's outcome, and more direct and immediate action was necessary to stem the American advance.

Fifteen American aircraft carriers had assembled at the Ulithi atoll, preparatory to making a series of raids against the home islands. The Japanese mission was changed to an attack on the Ulithi base.

The attack on Ulithi Atoll was to take place in two phases. The first, codenamed Hikari (light), involved transporting four C6N Saiun (Myrt) single-engined high-speed reconnaissance planes to Truk Island. They were to be disassembled, crated and loaded into the water-tight hangars of submarines I-13 and I-14. Upon reaching Truk, the Saiuns would be unloaded, reassembled and then flown over Ulithi to confirm the presence of American carriers anchored there. Following the delivery, I-13 and I-14 were to sail for Hong Kong, where they would embark four Seiran attack planes. They would then head to Singapore and join I-400 and I-401 for further operations.


The second phase of the Ulithi attack was codenamed Arashi (storm). I-400 and I-401 were to rendezvous at a predetermined point on the night of 14/15 August. On 17 August they would launch their six Seirans before daybreak on a kamikaze mission against the American carriers. The Seirans, each with an 800 kg (1,800 lb) bomb bolted to its fuselage, were to fly less than 50 m (160 ft) above the water to avoid radar detection and the American fighters expected to be patrolling 4,000 m (13,000 ft) above.

Just before departing Maizuru Naval Station, the Seirans were completely over-painted in silver with American stars and bars insignias covering the red Hinomarus, a direct violation of the rules of war. This was an attempt to further confuse recognition if the aircraft were prematurely spotted, but it was not well received by the pilots. Some felt it was both unnecessary and a personal insult to fly under American markings, as well as dishonorable to the Imperial Navy.

Following the attack on Ulithi, I-400 and I-401 would sail for Hong Kong. There they would take on six more Seirans and sail for Singapore, where fuel oil was more readily available. They would then join I-13 and I-14 and stage further attacks with a combined force of ten Seiran aircraft.

On 22 June, I-13 and I-14 arrived at Maizuru Harbor to take on fuel. They reached Ominato on 4 July to pick up their Saiun reconnaissance aircraft. I-13 departed for Truk on 11 July but never reached her destination. She was detected running on the surface, attacked, and damaged by radar-equipped TBM Avengers on 16 July. An American destroyer escort later arrived and sank her with depth charges.


Japan surrendered before the Ulithi attack was launched, and on 22 August 1945, the crews of the submarines were ordered to destroy all their weapons. The torpedoes were fired without arming and the aircraft were launched without unfolding the wings and stabilizers. When I-400 surrendered to the American destroyer, Blue, the U.S. crew was astounded at her size, nearly 24 ft (7.3 m) longer than the USS Blue and just as wide – considerably longer and wider than the largest American battle submarine of the day.

American inspections after Japanese surrender:


The U.S. Navy boarded and recovered 24 submarines, including the three I-400 submarines, taking them to Sasebo Bay to study them. While there, they received a message that the Soviets were sending an inspection team to examine the submarines. To prevent this Operation Road's End was instituted. Most of the submarines were taken to a position designated as Point Deep Six, about 35 km (19 nmi) southeast of Fukue Island, packed with charges of C-3 explosive and destroyed; they sank to a depth of 200 m (660 ft).

Four remaining submarines, I-400, I-401, I-201 and I-203, were sailed to Hawaii by U.S. Navy technicians for further inspection. Upon completion of the inspections, the submarines were scuttled in the waters off Kalaeloa near Oahu in Hawaii by torpedoes from US submarine USS Trumpetfish on June 4, 1946, to prevent the technology from being made available to the Soviets who were demanding access to them.

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artist formerly known as quad destroyer and...
Dec 13, 2001
Austin, TX
yw, I enjoyed reading it all and finding the pics.
I'd always known the Japanese had planned an attack on the canal during WWII but never knew any of the details until today, when I read all this.

Found out that one of the older zonian's Grandfather was one of the pilots during the raid that killed Yamamoto.
Not *the* pilot, but was in the air with him on that raid.



To boost morale following the defeat at Guadalcanal, Yamamoto decided to make an inspection tour throughout the South Pacific (spoiler: this is a big "doh" decision).

On April 14, 1943, the US naval intelligence effort, code-named "Magic", intercepted and decrypted a message containing specifics of Yamamoto's tour, including arrival and departure times and locations, as well as the number and types of aircraft that would transport and accompany him on the journey.

US President Franklin D. Roosevelt may have authorized Secretary of the Navy Frank Knox to "get Yamamoto," however no official record of such an order exists and sources disagree whether he did so. Knox essentially let Admiral Chester W. Nimitz make the decision.[29] Nimitz first consulted Admiral William Halsey Jr., Commander, South Pacific, and then authorized the mission on April 17 to intercept Yamamoto's flight en route and shoot it down. A squadron of USAAF Lockheed P-38 Lightning aircraft were assigned the task as only they possessed sufficient range to intercept and engage. Select pilots from three units were informed that they were intercepting an "important high officer" with no specific name given.

Sleeve insignia of Kaigun Taishō (Admiral); the rank Yamamoto held at the time of his death.
On the morning of April 18, despite urging by local commanders to cancel the trip for fear of ambush, Yamamoto's two Mitsubishi G4M bombers, used as fast transport aircraft without bombs, left Rabaul as scheduled for the 315 mi (507 km) trip. Sixteen P-38s intercepted the flight over Bougainville and a dogfight ensued between them and the six escorting Mitsubishi A6M Zeroes. First Lieutenant Rex T. Barber engaged the first of the two Japanese transports, which turned out to be T1-323 (Yamamoto's aircraft). He fired on the aircraft until it began to spew smoke from its left engine. Barber turned away to attack the other transport as Yamamoto's plane crashed into the jungle.

Yamamoto's body, along with the crash site, was found the next day in the jungle of the island of Bougainville by a Japanese search and rescue party, led by army engineer Lieutenant Tsuyoshi Hamasuna. According to Hamasuna, Yamamoto had been thrown clear of the plane's wreckage, his white-gloved hand grasping the hilt of his katana, still upright in his seat under a tree. Hamasuna said Yamamoto was instantly recognizable, head dipped down as if deep in thought. A post-mortem disclosed that Yamamoto had received two 0.50-caliber bullet wounds, one to the back of his left shoulder and another to the left side of his lower jaw that exited above his right eye. The Japanese navy doctor examining the body determined that the head wound killed Yamamoto. The more violent details of Yamamoto's death were hidden from the Japanese public. The medical report was whitewashed, changed "on orders from above", according to biographer Hiroyuki Agawa.

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Dec 25, 2006
Long Beach California
"Yamamoto had been thrown clear of the plane's wreckage, his white-gloved hand grasping the hilt of his katana, still upright in his seat under a tree".
Bet that was a little creepy to walk up on!


Nov 17, 2002
"Yamamoto had been thrown clear of the plane's wreckage, his white-gloved hand grasping the hilt of his katana, still upright in his seat under a tree".
Bet that was a little creepy to walk up on!
The through-and-through .50 round to the face probably didn't help. :eek:


Oct 4, 2005
On the road
Interesting read. pood design tho.

What the story about Yamamoto doesnt say, is that the US had broken the ciphers nd had intelligence about Yamamoto trip.

That was Mac Arthur at the helm, the man who became the emperor of Japan after WW2 and rewrote their constitution, legal system and redesigned thier economy from the ground up. When he travelled around in his car, all traffic stopped. bicycles to trams to trains. They all bowed.

Japan is Mac Arthurs creation
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