The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth.[1] They displaced 1,133 tonnes (1,115 long tons) surfaced and 1,470 tonnes (1,447 long tons) submerged. The submarines were 80.5 meters (264 ft 1 in) long, had a beam of 7 meters (23 ft 0 in) and a draft of 4.07 meters (13 ft 4 in). They had a diving depth of 80 meters (260 ft).[2]
For surface running, the boats were powered by two 2,100-brake-horsepower (1,566 kW) diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600-horsepower (447 kW) electric motor.[3] They could reach 19.75 knots (36.58 km/h; 22.73 mph) on the surface and 8 knots (15 km/h; 9.2 mph) underwater. On the surface, the K6s had a range of 11,000 nautical miles (20,000 km; 13,000 mi) at 12 knots (22 km/h; 14 mph); submerged, they had a range of 45 nmi (83 km; 52 mi) at 5 knots (9.3 km/h; 5.8 mph).[1]
Submarine No. 645 was laid down on 2 December 1943 by Mitsui ZosenshoatTamano, Japan, with the provisional name Ro-75.[4] She had been renamed Ro-56 — the second submarine of that name — by the time she was launched on 5 July 1944.[4] She was completed and commissioned on 15 November 1944.[4]
Upon commissioning, Ro-56 was attached to the Maizuru Naval District and assigned to Submarine Squadron 11 for workups.[4] She was reassigned to Submarine Division 34 in the 6th Fleet on 10 February 1945.[4] She arrived at Saeki, Japan, on 16 March 1945.[4]
On 18 March 1945, Ro-56 departed Saeki to begin her first war patrol, ordered to operate southwest of Kyushu along with the submarines I-8, Ro-41, and Ro-49.[4] She reported on 22 March 1945 that she had reached her patrol area.[4] While she was at sea, the Battle of Okinawa began with the U.S. landings on Okinawa on 1 April 1945.[4]
The submarine Monssen and Mertz sank probably was Ro-56.[4] On 15 April 1945, the Imperial Japanese Navy declared her to be presumed lost off Okinawa with all 79 men on board.[4] The Japanese struck her from the Navy list on 25 May 1945.[4]
Some accounts claim that the U.S. submarine USS Sea Owl (SS-405) sank Ro-56 northeast of Wake Island on 18 April 1945,[4] but Ro-56 never operated near Wake. The submarine Sea Owl attacked was I-372 — and I-372 survived the attack.[4]
Bagnasco, Erminio (1977). Submarines of World War Two. Annapolis, Maryland: Naval Institute Press. ISBN0-87021-962-6.
Carpenter, Dorr B. & Polmar, Norman (1986). Submarines of the Imperial Japanese Navy 1904–1945. London: Conway Maritime Press. ISBN0-85177-396-6.
Chesneau, Roger, ed. (1980). Conway's All the World's Fighting Ships 1922–1946. Greenwich, UK: Conway Maritime Press. ISBN0-85177-146-7.
Hackett, Bob; Kingsepp, Sander (2017). "IJN Submarine RO-56: Tabular Record of Movement". SENSUIKAN! Stories and Battle Histories of the IJN's Submarines. Combinedfleet.com. Retrieved 26 September 2020.
Hashimoto, Mochitsura (1954). Sunk: The Story of the Japanese Submarine Fleet 1942 – 1945. Colegrave, E.H.M. (translator). London: Cassell and Company. ASIN B000QSM3L0.
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