The Unpoto was a container that could store an artillery piece with ammunition or 15 tons of cargo. Also known as a "Cannon Carrier" or "Gun Sled". The Unpoto consisted of two cylinders attached to a platform and used two torpedoes engines for propulsion at six knots over several miles. Developed by the Japanese Navy to deliver supplies to isolated Japanese garrisons where using transports or barges was impossible due to Allied warships or aircraft.
After loading, the Unpoto was secured to the forward or aft deck of a submarine and transported. At the destination, the submarine would surface, release the unpoto and the submarine would submerge slightly so it would float free. An operator aboard would start the engines and guide the unpoto, beaching it ashore. The bow ramp lowered allowing Japanese forces to unload the cargo. Later, a larger submerged cargo pod, known as the Unkato was devised.
Used by the Japanese Navy based at Rabaul to land guns and supplies to Lae and Sarmi.
Use by I-38 Japanese Submarine
On May 8, 1943 at Kure Japanese submarine I-38 was the first to transport an unpoto mounted on the aftdeck traveling first to Osaka, Saeki then to Truk Lagoon arriving on May 14. The next day, the device was inspected by Vice Admiral Komatsu Teruhisa and repairs made. On May 16, I-38 departed for Rabaul arriving two days later where the unpoto was unloaded.
Later, Taisei Maru transported a load of unpoto to Rabaul for use on resupply missions.
On June 6, 1943 at Rabaul I-38 was loaded with an unpoto loaded with an artillery piece from Taisei Maru. Three days later the I-38 departed for Lae arriving on June 12 and successfully delivered the unpoto then returned to Rabaul.
On June 17, 1943 at Rabaul loaded with another unpoto from Taisei Maru departing two days later carrying 48.5 tons of provisions and makes another successfully delivery to Lae.
On June 24, 1943 at Rabaul loaded with another unpoto from Taisei Maru. Departs two days later carrying 48.5 tons of
provisions and makes another successfully delivery to Lae.
On July 23, 1943 at Rabaul loaded with another unpoto from Taisei Maru. Departs the next day and makes another successfully delivery to Lae during the night of July 26.
On August 26, 1943 at Rabaul loaded with another unpoto from Taisei Maru. Two days later, departs and makes another successfully delivery to Lae on August 30.
On September 2, 1943 at Rabaul loaded with another unpoto from Taisei Maru. Five days later departs for Kolombangara. Attacked by an enemy aircraft, the sub arrives at Shortland on September 12 unloads her cargo, is attacked again, then departs for Rabaul the next day.
On December 7, 1943 departs Rabaul towing an unpoto bound for Sarmi. Arrives two days later where a local Daihatsu barge tows the unpoto ashore.
Unpoto were also used by I-54, I-55 and I-26.
I-Boat Captain page 138-139, 144 (diagram)
"In replenishing New Guinea, Japanese submarines emplyoed two ingenious devices. They were called "unkato" (store carrier tub) and "unpoto" (cannon carrier). The unpoto was introduced first... The unpoto was already working successfully when the unkato was introduced."
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May 22, 2017