Ships
- IJN Yubari Captain from 15 August 1942
Yasuji Hirai was a career officer of the Imperial Japanese Navy. He was born on 2 May 1893 and was a member of the 43rd class of the naval academy. After the academy he passed through the torpedo school and the gunnery school, and the appointments that followed were those of a torpedo specialist who alternated between destroyer command and the schools that trained other torpedo officers.
On 1 December 1930 he became commanding officer of the destroyer Nagatsuki. From 16 July 1931 he commanded Fumizuki at the same time, and on 2 November 1931 he left both ships for the naval academy, where he served as an instructor and supervisor. He returned to sea on 15 November 1933 as commanding officer of the destroyer Fubuki and stayed with her until 15 October 1935. He then taught again at the torpedo school. From 15 December 1936 that teaching was combined with duty in the torpedo experimental section of the Kure Navy Yard.
On 20 October 1937 Hirai was appointed executive officer of the heavy cruiser Ashigara. He left her on 20 April 1939 for the port affairs staff at Sasebo. In July 1941 he was attached to the Second Fleet, and on 25 July he took command of Destroyer Division 24. He led that division through the first year of the Pacific War. On 30 July 1942 he was ordered to the Yokosuka Naval District, and on 10 August he was attached to the staff of the Fourth Fleet.
He was appointed captain of the light cruiser Yubari on 15 August 1942, relieving Masami Ban, who had commanded her in the night battle off Savo Island on 9 August. The cruiser was at Rabaul. Blade trouble in a turbine already limited her to about 26 knots on two shafts. Yubari, completed in 1923, was the only ship of her design, a small hull carrying an unusually heavy gun and torpedo armament, and she had entered the war as flagship of a destroyer squadron in the central Pacific.
Hirai took her out of Rabaul on 18 August 1942. Late that month Yubari covered the landings that seized Nauru and Ocean Island. Through the autumn she remained on escort and patrol duty among the island bases of the south and central Pacific. She then went back to Japan for a refit that kept her home through the winter. In April 1943 she returned to the south Pacific, with Hirai still in command.
On 3 May 1943 he turned Yubari over to Morie Funaki and was attached again to the Yokosuka Naval District. On 25 June he became chief of port affairs at Maizuru. From 20 July 1943 until 18 March 1944 he also held the appointment of captain of the light cruiser Natori, while his principal duty stayed at Maizuru, and after that date he continued as chief of port affairs alone. Later captains of Yubari included Ranji Oe and Takeo Nara. On 5 May 1944 Hirai was appointed chief of the 30th Port Affairs Department. He was killed in action on 20 June 1944, and the navy promoted him to rear admiral the same day.