Oogata Unkato No. 1 (Large Type Cargo Transporting Tube) was a lacked propulsion and was designed to be towed
behind a submarine. The cargo tube had a cargo capacity of 377 tons (375 cubic meters). It was capable of being submerged to a depth of 120 meters (394')
On October 12, 1943, Oogata Unkato No.1 was tested by the Imperial Japanese Navy at Rabaul. Test found to very difficult to turn. An American air attack interrupted the tests and killed the chief engineer Lt Commander Hirano. Oogata Unkato No. 1 was never used in combat operations.
I-Boat Captain page 139
"The unpoto was already working successfully when the unkato was introduced. This was truly a remarkable experiment. The unkato was a cylinder 136 feet long with cone-shaped ends. It was 16 feet in diameter. Both the cones were ballast tanks and there was a third ballast tank in the unkato's center. These three compartments were separated from the long cargo space, which had a capacity of 377 tons of food, ammunition and medicines. With a strong hull structure that permitted submerging to depths of 400', the unkato was adjusted after loading so that it had slightly negative buoyancy, then it would be towed out in the middle of Simpson Harbor and secured by a long tow line to the stern of a submarine. Once the submarine cleared Rabaul, it would cruise along the northern coast of New Britain to the point where submerged run was to begin. When the submarine dived, the unkato would submerge with it. On arrival off New Guinea, the submarine would surface and release the unkato, which was then towed to the beach by men from the shore."
Oogata Unkato No. 1 was abandoned on a coral reef off Kokopo. Sometime postwar, salvaged by Pat Roberts and put into use carrying fresh water to vessels in Simpson Harbor. During the early 1980s sunk due to old age and neglect or possibly scrapped.
Rod Pearce adds:
"I remember this Japanese cargo tube very well even as a small boy in Rabaul. I remember it was found on a reef off Kokopo. Later, Pat Roberts salvaged it and fixed it up as a water carrier. It was part of Rabaul and Simpson Harbor for may years, lots of pictures around of it. As far as I recall, it was later sunk due to old age and neglect. As ships anchored out in the harbor, Roberts used this to carry water out to them. It held about 20-30 tons (not sure)."
I-Boat Captain page 139
Rust in Peace page 42
Thanks to Rod Pearce, Go Okumoto, and Yasufumi Kunimoto for additional information
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February 4, 2018