Towards the end of 1943, in response to unfavorable progress in the war, the Japanese command heard suggestions for various suicide craft. These suggestions were initially rejected but later deemed necessary. For the naval department this meant kamikaze planes, kaiten submarines, fukuryu suicide divers or human mines, and shinyo suicide boats.
These fast motorboats were driven by one man, to speeds of around 30 knots. They were typically equipped with a bow-mounted charge of up to 300 kg (660 lb) of explosives that could be detonated by either impact or from a manual switch in the driver's area. These attack boats also carried two anti-ship rockets mounted on launchers located on either side of the boat behind the driver.
The similar Maru-ni, which were used by the Imperial Japanese Army, were equipped with two depth charges, and were not actually suicide boats, as the idea was to drop the depth charges and then turn around before the explosion took place. Although the chances of boat and crew surviving the wave from the explosion might seem slim, a small number of crewmen successfully escaped. The depth charges used were known as the Experimental Manufacture Use 120 kg Depth Charge, and were armed by a delayed-action pull igniter.
Approximately 6,200 Shinyo were produced for the Imperial Japanese Navy and 3,000 Maru-ni for the Imperial Japanese Army. Around 400 boats were transported to Okinawa and Formosa, and the rest were stored on the coast of Japan for the ultimate defense against the expected invasion of the Home islands. The main operative use took place during the Philippines Campaign of 1944–45.