Hiraga was born in
Tokyo and grew up in
Yokosuka, Kanagawa although his family was from
Hiroshima, where his official family registration was located. He graduated from what is now
Hibiya High School, and entered the engineering department of Tokyo Imperial University in 1898, specializing in
marine engineering. He was drafted into the Imperial Japanese Navy in 1899, but allowed to continue his studies, and graduated in 1901 as a sub-lieutenant. He immediately went to work for the
Yokosuka Naval Arsenal as a design engineer for new warships. Promoted to lieutenant on 28 September 1903, he was transferred to the
Kure Naval Arsenal in 1905. From 1905, at the height of the
Russo-Japanese War, Hiraga was dispatched to the
United Kingdom for further studies. He left
Yokohama in January, and travelling across the
Pacific Ocean, the
United States and the
Atlantic Ocean, he arrived in
London in April. From October, he was enrolled in the
Royal Naval College, Greenwich, where he studied the latest techniques in warship design. He graduated in June 1908, and spent the next six months touring various shipyards in
France and
Italy before returning to Japan in early 1909. In September of the same year, he became a professor of engineering at Tokyo Imperial University, Hiraga was appointed a technical advisor to the Japanese delegation at the
Washington Naval Conference, and was in the United States from November 1923 to August 1924, becoming head of the
Imperial Japanese Navy Technical Department on his return. He was promoted to
vice admiral in 1926. Hiraga assembled a team of engineers to rebuild the Japanese navy in the aftermath of the terms imposed by the
Washington Naval Treaty, which severely restricted designs in terms of displacement and numbers of large capital warships. The innovative designs for cruisers and destroyers formulated by Hiraga, which were extraordinarily powerful for their size, were among the most advanced in the world. “They must be building their ships out of cardboard or lying” said the Royal Navy’s Director of Naval Construction (DNC) in 1935 when briefed by Naval Intelligence about the public displacement figure announced by the Japanese. In 1929, after Hiraga's design for the was shelved, he went into semi-retirement, and retired from active military service in 1930, becoming an advisor to
Mitsubishi shipyards. In April 1934, Hiraga faced a board of inquiry after the
Tomozuru Incident, a marine accident involving the
torpedo boat , which overturned and capsized during trials at the
Sasebo Naval Arsenal. Resulting investigation revealed what a number of Western engineers had long suspected: Hiraga's designs were top-heavy and tended towards instability. The
Tomozuru Incident sent shock waves through the Japanese military, as it called into question the safety and basic design concepts of the most modern warships in the Japanese inventory. Hiraga's reputation suffered a further setback due to the
Fourth Fleet Incident, in which many destroyers of the Hiraga-designed s were so damaged in a
typhoon that the whole class had to be reconstructed. In December 1938, Hiraga became the President of Tokyo Imperial University. In 1939, he conducted what journalists later termed the "Hiraga Purge", by expelling most of the faculty of the university’s School of Economics, for publicly supporting liberal political doctrines. On February 17, 1943, Hiraga died at Tokyo University Hospital of complications arising from
pneumonia. He was posthumously awarded with the Grand Cordon of the
Order of the Rising Sun and also with the
kazoku peerage title of
baron. His brain was removed on his death, and is preserved at the
Tokyo University Hospital. His grave is at the
Tama Reien in
Fuchū, Tokyo. ==Notes==