Here at LSOZI, we are going to take out every Wednesday for a look at the old steam/diesel navies of the 1859-1946 time period and will profile a different ship each week.
– Christopher Eger
Warship Wednesday, June 19th
Here we see the Sen Toku I-400-class (I-yonhyaku-gata Sensuikan) giant submarine aircraft carrier I-401 at sunset. It’s an appropriate picture as the submersible was at the time one of the last remaining units of the WWII Imperial Japanese Navy left afloat in the world. The IJN’s battle flag was the now-infamous Rising Sun, and this beautiful picture was taken of the I-401 at sunset, as a captured prize ship of the US Navy, sitting in Pearl Harbor in 1946.
In 1942, the war in the Pacific was still winnable for Japan, and Admiral Isoroku Yamamoto conceived of a class of huge submersible warships, 18 overall, that could carry an armada of 54 submarine-launched attack floatplanes to attack far off strategic US targets such as the Panama Canal, or fuel manufacture/storage facilities on the West Coast, or logistical hubs like American Samoa. Furthermore, the ships would be capable of circumnavigating the earth 1.5 times (37,000 miles!) on one full load of fuel, which would enable even targets on the US East Coast within the reach of the Japanese Navy.
To make such a capable submarine in 1942 under wartime conditions was a challenge. Nevertheless, you have to admire the audacious plan. Each of these I-400 boats had to be some 400-feet long with a very wide beam to be able to carry and launch up to three combat airplanes. This gave them a displacement of some 6700 tons and an immense crew of over 140, including air wing. When you compare this to the subs of the time, they are super-sized. Even looking at today’s HY-80 steel nuclear propelled boats, the I-400s are larger than many of the modern hunter-killer of the sea. For example, the backbone of the US Navy since 1976, the “688 Boats” of the Los Angeles class SSNs have a length of 362 feet and a surfaced displacement of 6.082-tons.
The Germans helped a lot with the design, giving the Japanese the plans for the aircraft catapult as well as supplying them with snorkels and periscopes. Unlike many subs of the day, the I-400s had both air and surface search radars as well as a primitive radar warning receiver and sonar absorbing anechoic tile.
The I-400s had a huge armament punch. Not only could they carry a trio of M6A1 Seiran (Mountain Haze) attack planes, each of which could carry a 1800-pound bomb or torpedo load out to 300-miles from the submarine and return, but the ship itself carried 8 21-inch torpedo tubes, with 24 Type 95 torpedos, a 140mm deck gun and a number of 25mm cannons for small surface ships and aircraft defense. The Type 95 is considered by many to be the best torpedo of WWII, being an advanced design of the famous Long Lance, it had a 51-knot speed and a 1200-pound warhead, a performance envelope that is still formidable today.
The Seirans were to be launched via a 85-foot long compressed-air catapult mounted on the forward deck. A well-trained crew of four men could roll a Seiran out of its hangar on a collapsible catapult carriage, attach the plane’s pontoons and have it readied for flight in approximately 7 minutes. Although to get all three airplanes off the boat took up to 30-minutes.
The Gatun Locks at the Panama canal were supposed to be the I401s first target
Well, all did not go as planned for the I-400s. After Yammoto was killed in 1943, the Japanese Navy saw little use for the program and started slowly canceling the ships. Just three I-400s were finished and only two, I-400 and I-401, ever went to sea. Their primary reason for being, the Seiran float-plane, had only 28 examples made.
Commissioned 8 January 1945, I-401 was a late comer to the war. Already the US Navy had recaptured the Philipines and was breathing hard on the Japanese home islands. By June the two boats and a crew of float plane pilots were practising on wooden mock-ups of the Panama canal locks in preparation for their first attack. At the last-minute, the plan was halted and the two I-400s were sent to attack Ulithu Atoll, the forward base of the US Navy’s fast carriers. At any given time the US Navy had up to a dozen carriers there on “Murders Row”, taking a break from the war. To give the six Seirans a fighting chance against up to 2000 US aircraft and thousands of anti-aircraft guns in the atoll, they were painted in US markings and refitted as kamikaze aircraft.
Murderers Row at Ulithi atoll was the target of two submarines and six Seiran floatplanes.
While at sea on the way to the atoll, the war ended and the I-400 and 401 surrendered to US forces. Both ships shot away their torpedoes, threw their artillery shells overboard, and shot their unmanned floatplanes off the deck into the deep ocean. I401 surrendered to the USS Segundo (SS-398), a Balao-class submarine less than half her size.
The floatplanes on the I400 and 401 were given US markings and looked almost like a P-51 with a set of floats.
Both the I400 and I401 were taken to Pearl Harbor by prize crews where they were inspected at length by the US Navy. Odds were they would have been kept for years, and one of them may have even still been around as a trophy ship had the Soviets not wanted to inspect them. To prevent the Russkis from getting to the amazing Japanese-German hybrid tech of the I400s, the Navy sunk them as targets off Hawaii in 1946.
The US navy had these ships for almost nine months, and they would probably be gracing a museum somewhere today, had it not been for the Russians.
The I-401 was rediscovered in 2005 about a mile off Barber’s Point in 2600-feet of water. A few of her parts were saved prior to sinking, including the 140mm gun sight which is currently displayed at the Yokohama WWII Japanese Military Radio Museum.
The only remaining Seiran floatplane, captured intact at the Aichi Aircraft Factory following the end of the war in August 1945, is at the Smithsonian Air and Space Museum on current display.
True to Yammaoto’s vision, at least one Seiran made it all the way to Washington DC, just not how he thought it would.
In a twist of fate, the USS Segundo (SS-398), captor of the I-401, was herself sunk as a target by the USS Salmon (SSR/SS/AGSS-573), a Sailfish-class submarine, in 1970, her usefulness past. It should go without saying that the Salmon likewise was sent to the bottom 5 June 1993, as a target by the US Navy. History is funny like that.
Displacement: 5,223 long tons (5,307 t) surfaced
6,560 long tons (6,665 t) submerged
Length: 122 m (400 ft)
Beam: 12 m (39 ft)
Draft: 7 m (23 ft)
4 diesel engines, 7,700 hp (5,700 kW)
Electric motors, 2,400 hp (1,800 kW)
Speed: 18.75 knots (21.58 mph; 34.73 km/h) surfaced
6.5 kn (7.5 mph; 12.0 km/h) submerged
Range: 37,500 nmi (69,500 km) at 14 kn (16 mph; 26 km/h)
Test depth: 100 m (330 ft)
Armament: • 8 × 533 mm (21 in) forward torpedo tubes
• 20 × Type 95 torpedoes
• 1 × 14 cm/40 11th Year Type naval gun
• 3 × 25 mm (0.98 in) 3-barrel machine gun
• 1 × 25 mm machine gun
Aircraft carried: 3 × Aichi M6A1 Seiran sea-planes
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