The submarines of the Kaichu II sub-class were larger and had a greater range than the preceding Kaichu I subclass, but they had the same powerplant, so their greater size resulted in a loss of some speed.[1] They also had a modified conning tower, bow, and stern, and the stern was overhanging.[1] They displaced 7400NaN0 surfaced and 1003.10NaN0 submerged. The submarines were 70.1m (230feet) long and had a beam of 6.1m (20feet) and a draft of 3.68m (12.07feet). They had a diving depth of 30sp=usNaNsp=us.
For surface running, the submarines were powered by two 14500NaN0 Sulzer Mark II diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600hp electric motor. They could reach 16.5kn on the surface and underwater. On the surface, they had a range of 6000nmi at ; submerged, they had a range of 85nmi at .
The submarines were armed with six 450mm torpedo tubes, four internal tubes in the bow and two external tubes mounted on the upper deck, and carried a total of ten Type 44 torpedoes. They were also armed with a single 76.2abbr=onNaNabbr=on deck gun mounted aft of the conning tower.[1]
Ro-15 was laid down as Submarine No. 24 on 12 June 1920 by the Kure Naval Arsenal at Kure, Japan.[2] Launched on 14 October 1920,[2] she was completed and commissioned on 30 June 1921.[2]
Upon commissioning, Submarine No. 24 was attached to the Kure Naval District, to which she remained attached throughout her career.[2] She was assigned to Submarine Division 15 — in which she spent the rest of her career — and to the Kure Defense Division on 1 July 1921.[2] On 19 July 1921, a fire broke out in her galley due to faulty electrical wiring and spread to the adjacent torpedo room.[3] She was flooded to extinguish it, but not before much of her interior was burned out.[3] There were no casualties,[3] and she was repaired and returned to service.
Submarine Division 15 served in the Kure Defense Division until 1 December 1921 and again from 1 December 1922 to 1 December 1923.[2] While cruising off Moji, Japan, on 29 July 1924, Submarine No. 24 collided with a steamer, suffering no casualties.[4]
Submarine No. 24 was renamed Ro-15 on 1 November 1924.[2] On 1 December 1926, Submarine Division 15 began another assignment to the Kure Defense Division that lasted through the end of Ro-15′s active service.[2]
Ro-15 was stricken from the Navy list on 1 September 1933.[2] She remained moored at Kure as a hulk after that, and was renamed Training Hulk No. 3036 on 7 March 1934.[2] She served on training duties through the end of World War II in August 1945, and was scrapped in September 1948.[2]