British 21-inch torpedo
There have been several British 21-inch (533 mm) torpedoes used by the Royal Navy since their first development just before the First World War. Torpedoes of 21 inch calibre were the largest torpedoes in common use in the RN. They were used by surface ships and submarines rather than aircraft, which used smaller 18-inch torpedoes. Mark I. The first British 21-inch torpedo came in two lengths, "Short" at , and "Long" at . The explosive charge was of gun cotton increased later to . Mark II. The Mark II, chiefly used by destroyers, entered service in  1914. Apart from some older British ships, it was used with the old US (destroyers-for-bases deal) Town-class destroyers provided to the UK during the early part of the Second World War. The running speed was reduced from (over 3,000 yards) for better reliability. The Mark II*, an improved Mark II was used by battleships and battlecruisers. A wet heater design, it could run for at . Mark IV. From 1912, used by destroyers and other surface ships and was an important weapon in the first World War. In the Second World War they were carried on . Mark V.  The Mark V was used by the A and B-class destroyers and, with modification, by the "Kent"-class heavy cruisers. Mark VII. The Mark VII was issued for use on the British heavy cruisers, i.e. cruisers with 8-inch guns. Designed in the mid-1920s the s were built at the same time in the post Washington Naval Treaty period. The power came from the use of oxygen-enriched air, though torpedo stocks were converted to run on normal air at the start of the Second World War. Mark VIII. Specifications: Mark VIII Early Mark VIII** Late Mark VIII** The Mark  VIII was designed around 1925 and was the first British burner cycle design torpedo. It was used from 1927 on submarines of the O class onwards and motor torpedo boats. The principal World War II version was the improved Mark VIII**, 3,732 being fired by September 1944 (56.4% of the total number). The torpedo is still in service with the Royal Navy albeit in a limited role, and was used by the Royal Norwegian Navy (Coastal Artillery: Kaholmen torpedo battery at Oscarsborg Fortress) until 1993. The Mark VIII** was used in two particularly notable incidents: Mark IX. First appeared in  1930 and was considerably improved by 1939. Used on "Leander" and later cruisers, "A" and later destroyer classes. Also replaced the old Mark VII in some cruisers during the war. Mark X. From 1939, used by submarines, motor torpedo boats and destroyers from other navies such as the Grom-class destroyer. Mark XI. Electric battery-powered torpedo with a TNT warhead. Entering service during the Second World War it was used by destroyers. Mark 12. At first codenamed Ferry, then Fancy, the Mark 12 never reached production. From 1952, it had a warhead of Torpex. Using high test peroxide fuel , it attained a top speed of for . There were accidents during testing caused by the unstable nature of high test peroxide. One such engine explosion, after loading aboard the submarine HMS "Sidon", caused enough damage to have the submarine taken permanently out of service. Mark 12 torpedoes were out of service in 1959 and the programme was cancelled. Mark 20 Bidder. Developed under the codename "Bidder", the Mark 20 was a passive-seeker battery-powered torpedo for use by surface ships (the Mark 20E – for "Escort") and submarines (Mark 20S). The E variant was not long in service due to  problems with its programming. This led to several of the frigates that were intended to have used them ("Rothesay" and "Whitby" classes) never being fitted with torpedo tubes or having them removed. It was replaced in the submarine service in the 1980s by Tigerfish. Mark 21 Pentane. A project for an autonomous active/passive sonar torpedo to be carried by the Short Sturgeon anti-submarine aircraft. It was cancelled after protracted work but the seeker development was used in Tigerfish. Mark 22. A wire-guided version of the Mark 20 produced by Vickers Shipbuilding and Engineering (VSEL) as a private  venture. Mark 23 Grog. A wire-guided version of the Mark 20. Entered service in 1966 although already obsolescent, and did not become fully operational until 1971, serving only as an interim before Tigerfish entered service. The MK23 was fitted with a outboard dispenser that contains a control wire to guide the weapon, During 1973, all of the RN torpedoes had to be taken out of service as the control system was failing at extreme range. After months of investigation, it was discovered that the fault lay in the Guidance Unit made by GEC. A germanium diode in the AGC  circuit had been replaced by a silicon diode, following an instruction by RN stores that all germanium diodes had to be replaced by silicon diodes. Unfortunately, the silicon diode's different characteristics caused the automatic gain control circuit to fail. Once the mistake was found, replacing the diode with the original type cured the problem. Mark 24 Tigerfish. The first Tigerfish (Mod 0) entered service in 1980. Tigerfish was removed from service in 2004. There were several models of Tigerfish due to the modifications made to tackle deficiencies.