Submersible Aircraft Carriers

Author

A.N. Other

Subjects

Ship design and development, History - WW2

Tags

Japanese operations, submarine design

RAN Ships

None noted.

Publication

March 2012 edition of the Naval Historical Review (all rights reserved)

By Driftwood

The Imperial Japanese Navy submarines I-yonhyaku-gata Sensuikan completed towards the end of WW11 were for many years the largest and potentially the most formidable submarines ever built. Displacing 6,670 tons with a length of 122m (400 ft) and 12 m (39 ft) beam they were nearly twice the displacement of the Collins class and, were of a size not surpassed until construction a second generation of nuclear submarines in the 1960s. These boats had a tremendous range and could be provisioned for four months enabling them to make one round voyage to any part of the globe without refuelling or re-provisioning. They were believed to be the brainchild of Admiral Yamamoto and were specifically designed to conduct attacks on the West Coast of the United States and the Panama Canal. The canal which formed a vital link with American east coast resources was surprisingly left untouched allowing the USN to concentrate its forces without the debilitating hindrance of convoy protection.

At the commencement of WW11 Japanese submarines in comparison to those of the Allied Forces were well prepared having practised live firing against obsolete warships, especially at night. Their optics and ranging finding equipment were first class and their ‘Long Lance’ torpedos were superior having greater range, running faster, with better detonation and more powerful explosive payloads than their American counterparts. Japanese boats were however were not technically advanced and lacked the strength of American and European designed boats making them vulnerable to blast damage from depth charges and bombs. In general they also lacked snorkels and radar and radar detection equipment. Because the Japanese sought greater range their boats were usually large, heavily armed, could reach high speeds but they were cumbersome and slow to dive and vulnerable to detection from superior Allied ASW technology.

To overcome some of these deficiencies the completed I-400 boats were fitted with surface radar and air search radar with a range to 80 km (43 nm); they also carried radar warning receivers. However by this stage Japanese technology had fallen well behind that available to the West. I-401 was fitted with a German supplied snorkel allowing the boat to run its diesels and recharge batteries while at periscope depth. In practice the large size of these boats worked against them as they were slow to dive and more easily detectable in the clear Pacific waters.

In addition to convention submarine armament of 8 x 533 mm torpedo tubes, they carried one 5.5 inch (140 mm) gun for surface engagements and shore bombardment and 4 x 25 mm anti-aircraft guns. Their pride possession was three Seiran torpedo-bombing aircraft, with a top speed of 295 mph they could carry a payload of 1,800 lbs to a range of 650 miles. When deployed the aircraft had a wing span of 39 ft and a length of 38 ft. These float planes were housed in a 115 ft long x 12 ft diameter hanger and launched from a 120 ft pneumatic compressed air catapult, they were recovered by landing alongside to be winched onboard by hydraulic crane. The planes had detachable floats and wings which could be assembled, armed and rigged for take-off in seven minutes, the detachable parts were coated with luminescent paint so that they could be assembled in the dark. All three planes could be launched in under 45 minutes, and much less in an emergency with floats not fitted.

While 18 of these craft were originally planned only six were laid down and four were launched but only three of these completed, these were designated as I-400, I-401 and I-402. The last boat to be completed I-402 did not carry aircraft as she had been converted to a tanker submarine to bring urgent oil supplies to blockaded Japan from the East Indies but with the war’s end she was never able to perform this mission. With deteriorating war conditions in Japan it was necessary to assign a special squadron of aircraft to protect construction of these boats in Kure and Sasebo building yards. Difficulties in obtaining supplies and finding suitable areas for work-up lead to delays with the three completed boats not available for offensive operations until July 1945. In June 1945 the air crews assigned to these submarines conducted practice strikes against full scale models of Panama Canal locks, however, this action was aborted in favour of a strike against US carrier forces expected to attack the Japanese mainland.

When on their first combat mission to launch their aircraft against American aircraft carriers news was received on 15 August 1945 of the Japanese surrender. Accordingly the boats returned to Japan and never firing a shot in anger were surrendered to the USN. All the crews survived excepting Captain Ariizumi of I-401 who took his own life rather than face the ignominy of surrender. Following inspections by American submarine experts and to prevent the technology falling into Soviet hands I-400 and I-401 were sunk as targets off Hawaii and I-402 was sunk as a target off the Japanese coast. The wreckage of I-401 was re-discovered by the Hawaii Undersea Research Laboratory in March 2005 at a depth of 820 meters.

A restored Seiran aircraft is displayed at the National Air and Space Museum at Washington, DC. This type of aircraft was unknown to Allied intelligence until after cessation of hostilities.