The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth. They displaced 11150NaN0 surfaced and 14470NaN0 submerged. The submarines were 80.5m (264.1feet) long, had a beam of 7m (23feet) and a draft of 4.07m (13.35feet). They had a diving depth of 80m (260feet).[1]
For surface running, the boats were powered by two 21000NaN0 diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600hp electric motor.[2] They could reach 19.75kn on the surface and underwater. On the surface, the K6s had a range of 11000nmi at ; submerged, they had a range of 45nmi at .
The boats were armed with four internal bow 53.3cm (21inches) torpedo tubes and carried a total of ten torpedoes. They were also armed with a single 76.2mm L/40 anti-aircraft gun and two single 251NaN1 AA guns.[3]
Ro-55 was laid down as Submarine No. 396 on 5 August 1943 by Mitsui Zosensho at Tamano, Japan.[4] She was launched on 23 April 1944 and was renamed Ro-55 that day, the second Japanese submarine of that name.[4] She was completed and commissioned on 30 September 1944.[4]
Upon commissioning, Ro-55 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 4 January 1945.[4]
On 27 January 1945, Ro-55 departed Kure, Japan, to begin her first war patrol, assigned a patrol area in the South China Sea west of Mindoro in the Philippine Islands.[4] While in the Philippine Sea east of Luzon on 2 February 1945, she reported that Allied aircraft had attacked her and that she would reach her patrol area five days late.[4] The Japanese never heard from her again.[4]
After dark on 7 February 1945, the United States Navy destroyer escort detected a surfaced submarine on radar while escorting a Leyte Gulf-bound Allied convoy off Iba, Luzon. As she closed the range, the submarine submerged, and at 23:30 Thomason began attacks against it, firing 24-projectile Hedgehog barrages that sank the submarine at 15.45°N 144°W.[4]
The submarine Thomason sank probably was Ro-55.[4] On 1 March 1945, the Imperial Japanese Navy declared her to be presumed lost off the Philippine Islands with all 80 men on board.[4] The Japanese struck her from the Navy list on 10 May 1945.[4]