The second Ro-56 was an Imperial Japanese Navy Kaichū type submarine of the K6 sub-class. Completed and commissioned in November 1944, she served in World War II and was sunk during her first war patrol in April 1945.
Design and description The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth. They displaced surfaced and submerged. The submarines were long, had a beam of and a draft of . They had a diving depth of .Bagnasco, p. 187
For surface running, the boats were powered by two diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a electric motor.Chesneau, p. 203 They could reach on the surface and underwater. On the surface, the K6s had a range of at ; submerged, they had a range of at .
The boats were armed with four internal bow torpedo tubes and carried a total of ten torpedoes. They were also armed with a single L/40 anti-aircraft gun and two single AA guns.Carpenter & Dorr, p. 124
Construction and commissioning
Submarine No. 645 was laid down on 2 December 1943 by Mitsui Zosensho at Tamano, Japan, with the provisional name Ro-75. She had been renamed Ro-56 — the second submarine of that name — by the time she was launched on 5 July 1944. She was completed and commissioned on 15 November 1944.
Service history Upon commissioning, Ro-56 was attached to the Maizuru Naval District and assigned to Submarine Squadron 11 for workups. She was reassigned to Submarine Division 34 in the 6th Fleet on 10 February 1945. She arrived at Saeki, Japan, on 16 March 1945.
First war patrol
On 18 March 1945, Ro-56 departed Saeki to begin her first war patrol, ordered to operate southwest of Kyushu along with the submarines , , and . She reported on 22 March 1945 that she had reached her patrol area. While she was at sea, the Battle of Okinawa began with the U.S. landings on Okinawa on 1 April 1945.
Loss On 9 April 1945, the United States Navy destroyers and were escorting the aircraft carriers of Task Force 58 west of Okinawa when Monssen detected a submerged submarine on sonar at a range of . Monssen dropped three patterns of depth charges, after which Mertz joined the attack and dropped another three patterns. Monssen then made two more depth-charge attacks, sinking the submarine at .
The submarine Monssen and Mertz sank probably was Ro-56. On 15 April 1945, the Imperial Japanese Navy declared her to be presumed lost off Okinawa with all 79 men on board. The Japanese struck her from the Navy list on 25 May 1945.
Some accounts claim that the U.S. submarine sank Ro-56 northeast of Wake Island on 18 April 1945, but Ro-56 never operated near Wake. The submarine Sea Owl attacked was — and I-372 survived the attack.
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