Numerous 45 cm (17.7-inch or 18-inch) torpedoes have been used by the Imperial Japanese Navy and Imperial Japanese Navy Air Service. The size category has not been used by Japan since the end of the Second World War.

Torpedoes of 18-inch caliber were much more common during the early 20th century. Historically, this size category was often used by aircraft. Submarines and surface ships generally used 53 cm torpedoes, and surface ships additionally used 61 cm torpedoes. Japan also employs 32 cm torpedoes which conform to the NATO 12.75-inch (323.8mm) standard originally defined by the dimensions of the Mark 46 torpedo; these are dedicated ASW weapons, often delivered via aircraft. Due to their historical importance, this page also includes the smaller 14-inch (35.6cm) predecessors to the 45 cm torpedoes, and some of the miscellaneous developments during the Second World War.

Up to the beginning of the First World War, Japan had used more torpedoes against live targets than all the other navies of the world combined. During that time period, the torpedoes listed in this article were the primary ones employed.

Prior to 6 October 1917, imperial measurements were used. After this date, metric units were used. As such, the 18-inch torpedoes were designated as 45 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 relied on the traditional Japanese calendar and 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).

Predecessors

Following the proclamation of the Meiji Restoration, the subsequent Boshin War, and the ramifications of its resolution, Japan rapidly modernized. This included the adoption of new technologies for its navy. Prior to Japan developing its own torpedoes, specimens were purchased from Schwartzkopff and Whitehead during the 1880s and 1890s. These would see use in the Sino-Japanese War and the Russo-Japanese War. The use of torpedoes during these two wars proved highly successful, and had a long-lasting effect on Japanese naval doctrine.

These early torpedoes, all with a 14-inch (35.6 cm) diameter, were sold on the global market with very little restriction. International clients would often purchase knock-down kits and complete assembly of the weapons in their respective countries, both to save money and avoid the difficulty of having torpedo warheads shipped to them. Schwartzkopff's torpedoes were more expensive, but more desirable for smaller navies due to their bronze construction, because steel required constant careful maintenance to stave off corrosion. Whitehead would become especially prominent, with the factory in Fiume reaching an annual output of 800 torpedoes after 1875. Early models from both firms varied in size as they were incrementally developed. Upon adoption for Japanese service, the torpedoes received type designations, as well as the label of either "Shu" (シュ) or "Ho" (ホ), which corresponded to the first syllable of the developer's name ("Schwartzkopff" was spelled as シュワルツコフ, and "Whitehead" was spelled as ホワイトヘッド). After 1892, Whitehead torpedoes outclassed all rival designs of the 19th century. Japan would switch all of its future orders for torpedoes to Whitehead, beginning in 1895. The early 14-inch torpedoes would see active use on Japanese ships, such as Matsushima and Yoshino. Although purchases of Schwartzkopff torpedoes ceased in 1895, and numerous torpedoes were expended in warfare, British intelligence reported in 1908 that approximately 800 Schwartzkopff torpedoes in four variations remained in active service stockpiles, obtained from sources which were unaccounted for. According to the report, aside from the Shu Types 84 and 88, there was a faster version of the Shu Type 84 (24 knots) and a heavier version of the Shu Type 88 (90 kg warhead).

Specifications:

Shu Type 84 Entered service: 1884 Propulsion: Compressed air Weight: Length: Explosive charge: Guncotton Range and speed: at Notes: Purchased from Schwartzkopff, model of 1883. Bronze body. A total of 200 units were purchased.

Shu Type 88 Entered service: 1888 Propulsion: Compressed air Weight: Length: Explosive charge: Guncotton Range and speed: at , at Notes: Purchased from Schwartzkopff, model of 1885. Bronze body. A total of 307 units were purchased.

During the First Sino-Japanese War, Schwartzkopff torpedoes of various models were used by both of the belligerent sides. The novel weapon proved to be sensitive to deficient maintenance practices. At the Battle of the Yalu River, the Chinese launched multiple torpedo salvos at the ironclad Hiei as it executed a flanking maneuver. Torpedoes could be seen breaching the waves, indicative of a failure of the depth-keeping mechanism due to poor maintenance. This greatly reduced both the accuracy and propulsive endurance of the weapons, and none managed to hit the vessel. A second instance during the same battle was the Chinese torpedo attack against the Saikyo Maru, with the torpedo's depth-keeping mechanism likewise failing, and the weapon witnessed passing harmlessly below the vessel's keel. Later in the war, during the Battle of Weihaiwei, the Japanese attacked with Schwartzkopff torpedoes of their own, launched from torpedo boats, targeting Chinese vessels harbored in the naval base at the Shangdong peninsula. The torpedoes functioned reliably and proved to be decisive, sinking multiple vessels and sending the rest of the defending fleet into disarray.

Ho Type 26 Entered service: 1893 Propulsion: Compressed air Weight: Length: Explosive charge: Guncotton or Lyddite equivalent Range and speed: at , at Notes: Purchased from Whitehead, Fiume Mark IV model of 1885, with incremental improvements. Steel body, phosphor bronze internals. A total of 100 units were purchased. The best torpedo of the Sino-Japanese War.

Ho Type 30 Entered service: 1897 Propulsion: Compressed air Weight: Length: Explosive charge: Shimose Range and speed: at , at , at Notes: Purchased from Whitehead, Fiume Mark V model of 1893, upgraded after 1895. Featured gyroscopic guidance using the Obry gyroscope after the 1895 retrofit. A total of 50 units were purchased. Domestic production was set up at the Kure Naval Arsenal. A 45 cm version of this torpedo was also procured.

Ho Type 32 Entered service: 1899 Propulsion: Compressed air Weight: Length: Explosive charge: Shimose Range and speed: at , at , at Notes: Purchased from Whitehead, Fiume Mark V model of 1895. Featured gyroscopic guidance using the Obry gyroscope. A total of 76 units were purchased. Domestic production was set up at the Kure Naval Arsenal. A 45 cm version of this torpedo was also procured.

Yokosuka Naval Arsenal Aerial Torpedo Prototype

The first ever Japanese attempt at an aerial torpedo, constructed as a prototype in 1916. A 14-inch Ho Type 32, which had been shortened and reinforced by the Yokosuka Naval Arsenal. Designed as part of a project for an experimental float plane torpedo bomber, the Yokosho Twin-Engine, created in 1916 by Chikuhei Nakajima. The design did not result in a torpedo bomber, though it did contribute to the Yokosho Ro-go Ko-gata reconnaissance float plane. Japan would soon thereafter, in 1922, develop the unsuccessful 1MT1N torpedo bomber, followed by the successful B1M in 1923. Both of these would be designed to carry dedicated 45 cm aerial torpedoes.

Prototype Date: 1916 Propulsion: Compressed air Weight: Length: Unknown, approximately Range and speed: at Max airdrop speed: Approximately Notes: Warhead was likely removed; the range and speed suggest that the propulsion system and air flask were untouched.

In 1921, at the newly-constructed Kasumigaura Air Field near Tokyo, an intensive training program took place with the ultimate goal of developing a functional aerial torpedo, involving the assistance of foreign instructors. Torpedo bombing tests were carried out under the supervision of 29 British instructors, using the Sopwith Cuckoo and Blackburn Swift aircraft, as well as the Supermarine Channel and Felixstowe F5 flying boats. Japan would subsequently purchase a group of 7 Blackburn Swift aircraft, designated the Swift Mark II. Also purchased were a group of 3 Supermarine Channels. The Felixstowe F5 would later be license-built in Japan. During this time period, Britain utilized the 18-inch Mark VII and Mark VIII torpedoes for aerial launch, though it is unknown if live examples were furnished for the training program. Dummy torpedoes may have been used, based on the Type 38 - the last of the cold-runner compressed air designs.

Torpedo work at the Kasumigaura air field continued until January 1923, which is when a final torpedo design without an explosive payload was tested on the new Mitsubishi B1M bomber, and finally implemented with a warhead the same year. The torpedo thus used was a modification of the Type 44, which lasted until 1931, whereupon it was gradually replaced with prototypes of the Type 91 Mod 1, and retired from the aerial torpedo role after 1933.

Type 30 Main article: 18-inch Fiume Mark I Long

18-inch Fiume Mark I Long torpedo of 1892, purchased and slightly modified by the Japanese. The original wet guncotton filler was replaced with stabilized picric acid, transitioning to the Shimose system in later units. Procured in 1896, and in service in 1897, immediately after the First Sino-Japanese War. The Whitehead factory designation was W100/450 x 5, with deliveries spanning from 1896 to 1899. This was the first 45 cm torpedo in Japanese service. Used on surface ships.

Specifications:

Entered service: 1897 Propulsion: Compressed air Weight: Length: Explosive charge: Lyddite equivalent or Shimose Range and speed: at , at

Type 32 Main article: 18-inch Fiume Mark II

18-inch Fiume Mark II torpedo of 1893, fitted with the Obry gyrocompass of the contemporary 14-inch Fiume Mark III, purchased and slightly modified by the Japanese. The original wet guncotton filler was replaced with stabilized picric acid, transitioning to the Shimose system in later units. Procured in 1898, in service in 1899. The first Japanese torpedo to feature gyroscope guidance. Used on surface ships.