Japanese submarine Ro-3

Ro-3, originally named Submarine No. 31, was an Imperial Japanese Navy Type F submarine of the F2 subclass. The Type F submarines were the first truly seagoing Japanese submarines, and the earliest to be classified as "second-class" or "medium" submarines of the Ro series. Ro-3 was commissioned in 1922 and operated in the waters of Chōsen and Japan. She was stricken in 1932.

Design and description
The Type F submarines were designed by the Italian firm Fiat-Laurenti and built under license by Kawasaki at Kobe, Japan. They were the Imperial Japanese Navy′s first true seagoing submarines, and when the Japanese adopted a three-tiered classification system of its submarines as first-class (I), second-class or medium (Ro), and third-class (Ha) on 1 November 1924, the Type F submarines were the earliest to receive the second-class classification, as reflected in their low numbers in the Ro series, and in fact they were the earliest Japanese submarine classified as anything higher than third-class. They had non-cylindrical hulls intended to provide extra internal space, but the Japanese considered the hulls weak despite the provision of additional scantlings during construction to reinforce them.

The submarines of the F2 subclass displaced 689 LT surfaced and 1,047 LT submerged. The submarines were 65.6 m long and had a beam of 6.1 m and a draft of 4.1 m. They had a diving depth of 40 m. For surface running, the submarines were powered by two 1,300 bhp Fiat diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a Savigliano 600 hp electric motor. They could reach 14 kn on the surface and 8 kn underwater. On the surface, they had a range of 3,500 nmi at 10 kn; submerged, they had a range of 80 nmi at 4 kn.

The submarines were armed with five 450 mm torpedo tubes, three in the bow and two in the stern, and carried a total of eight Type 44 torpedoes. As built, they were armed with a 7.7 mm machine gun. Soon after completion, however, a 76.2 mm deck gun was added.

In the F2 subclass, the Fiat diesel engines were unreliable and the F2 subclass′s top surface speed of 14 kn was well below the expected 17 kn. Because of their disappointing performance, the Type F submarines did not serve as the basis for any later Japanese submarine classes.

Construction and commissioning
Ordered in 1918, Ro-3 was laid down as Submarine No. 31 on 28 October 1918 by Kawasaki at Kobe, Japan. Launched on 10 March 1921, she was completed and commissioned on 15 July 1922.

Service history
Upon commissioning, Submarine No. 31 was attached to the Sasebo Naval District and assigned to Submarine Division 23. On 1 December 1922, Submarine Division 23 was assigned to the Chinkai Defense Division on the southern coast of Chōsen. Submarine No. 31 was renamed Ro-3 on 1 November 1924.

Submarine Division 23 was reassigned to the Sasebo Defense Division in the Sasebo Naval District on 1 December 1924. On 1 June 1925, the division was assigned to the 1st Fleet, and on 1 December 1925 it returned to the Sasebo Defense Division in the Sasebo Naval District, where it remained for the rest of Ro-3′s active service.

Ro-3 was stricken from the Navy list on 1 April 1932.