Numerous 53 cm (21-inch, 533 mm) torpedoes have been used by the Imperial Japanese Navy and its successor, the Japan Maritime Self-Defense Force, since their first development just before the First World War.
Torpedoes of 21-inch caliber are the primary size category used worldwide. In Japan, they are used by surface ships and submarines, and comprise the predominant majority of submarine torpedoes; historically, aircraft and midget submarines used smaller 45 cm torpedoes, and surface ships additionally used 61 cm torpedoes. Japan also employs 32 cm torpedoes which conform to the NATO 12.75-inch (323.8 mm) standard; these are dedicated ASW weapons, often delivered via aircraft.
Prior to 6 October 1917, imperial measurements were used. After this date, metric units were used. As such, the 21-inch torpedoes were designated as 53 cm torpedoes. Japanese torpedoes have usually conformed to the 45 cm (17.7-inch or 18-inch), the 53 cm (21-inch), and the 61 cm (24-inch) calibers.
The Japanese type designation scheme has mostly used three different approaches. Units designed prior to the end of the Second World War were designated by either the regnal era year or the imperial year. In 1873, the Gregorian calendar was introduced in Japan; during the latter half of the 20th century, Japan increasingly switched to using this system, and as such, more recent torpedoes have type designations denoting Gregorian years. As an example of all three systems, a torpedo designed or accepted for service in 1980 could potentially be called either a Type 55 (Showa Era year 55), a Type 40 (Imperial Year 2640), or a Type 80 (Gregorian year 1980).
During the Second World War, Japanese torpedoes were superb.[1] Rigorous live-fire testing in real-world conditions resulted in highly reliable designs. The Japanese were more willing to conduct dangerous experiments, an attitude that was also reflected in their highly realistic fleet exercises. Japan also possessed the fastest torpedoes in the world, having persevered in their pursuit of using pure oxygen as the oxidizer instead of air, whereas other navies abandoned the idea.
After the Second World War, Japan imported American torpedoes. These included the Mark 14, Mark 23, Mark 32, Mark 34, Mark 37, Mark 44, and Mark 46. This page presently only lists torpedoes which were indigenously produced in Japan.
After a brief period of postwar stagnation in the field, the Japanese experienced a revival of torpedo development. During the 1970s, indigenous designs became globally competitive.