From 1883 until the end of the Meiji era, the Imperial Navy purchased torpedoes from European manufacturers, mainly from Schwarzkopff and Whitehead. The earliest ones adopted for service use were purchased from Schwarzkopff in 1884 and were designated as SHU 84 Shiki Nen Shiki gyorai, where SHU = Schwarzkopff, gyorai = "torpedo" (literally, gyo = fish and rai = thunder). Whitehead torpedoes were purchased in 1893 and were designated as HO 26 Shiki with HO = Whitehead and 26 Shiki = Type 26 (26 year of the Meiji era or 1893).
With the exception of the very earliest torpedoes, Japanese torpedoes built before 1945 were usually designated per their size and, similar to Naval Guns, the year in either the Meiji era or the old Imperial Calendar that design work started on them. However, some seem to have been given model years simply to fill holes in the numbering sequence. Major changes are denoted by model numbers, lesser changes are denoted by modification numbers. Post-World War II torpedoes use a similar designation system, but now use the Gregorian calendar for the model year.
For example, 61 cm Type 93 Model 1 Mod 3 means that the torpedo is 61 cm (24 in) in diameter, development started in Imperial Calendar year 2593 (1933) and it was the first version with the third minor change. Prior to 6 October 1917, the Imperial Navy used English units of measurements. After that date, metric units were used. Older torpedoes were re-designated in metric units. For example, the 21" Type 6 (1917) was redesignated as the 53 cm Type 6.
Regardless of designation, all Japanese 18" torpedoes were actually 17.7" (45 cm), all 53 cm torpedoes were actually 53.3 cm (21"). 61 cm torpedoes were 61.0 cm (24").
Production of oxygen torpedoes was at Kure Arsenal.
The excellence of Japanese torpedoes in World War II was due to the use of pure oxygen instead of air and, unlike the USA, British and German torpedoes, to the reliability of the whole torpedo including the explosive charge system. This can be attributed to the sinking of many old target hulks in numerous live-shoot trials prior to the war. However, the use of oxygen did make the torpedoes more prone to explosion and many Japanese cruisers were lost to torpedo explosions following damage from bombs or shell hits.
Experimental work on oxygen fuel began in 1917 but was abandoned in 1918 as a result of continual explosions in the generators. No more work was performed until a decade later when Lt/T Ōyagi Shizui (later Rear Admiral) during a trip to Britain in 1926 overheard that the torpedoes being developed for the British battleships Nelson and Rodney were oxygen fueled. While not strictly true - they were only oxygen enriched - this report did cause work on oxygen torpedoes to be restarted at the Kure Torpedo Factory. A successful all-oxygen torpedo was built in 1933 and was the forerunner of the "Long Lance," the most famous torpedo of World War II.
It should be noted that not all Japanese torpedoes of World War II were oxygen powered, many of the earlier ones still in service were conventional wet-heater air type.
The standard explosive charge was 60% TNT and 40% hexanitrodiphenylamine in blocks. This had first been developed by the Germans in 1907 and was very resistant to shock. This explosive was classified as Type 97 by the Japanese and was about 7% more powerful than 100% TNT.
Unlike most other navies before World War II, the Japanese spent much time improving their warhead pistols by conducting live-fire exercises. This led to a very reliable inertia pistol, although the USN reported that it was sensitive to disturbances in the water, such as when it passed through a ship's wake. Magnetic pistols were developed and accepted into service in July 1944, but the results, if any, are unknown.
"Naval Weapons of World War Two" by John Campbell
"Japanese Cruisers of the Pacific War" by Eric Lacroix and Linton Wells II
Biographical Information on Ōyagi Shizu from Imperial Japanese Navy (accessed 10 July 2007).
13 March 2008 - Benchmark
10 October 2020 - Converted to HTML 5 format 05 December 2020 - Updated spelling of Ōyagi Shizu