British 21 inch torpedo There have been several British 21 - inch ( 53 cm ) diameter torpedoes used by the Royal Navy since their first development just before the First World War . They were the largest size of torpedo in common use in the RN . They were used by surface ships and submarines rather than aircraft which used smaller 18 inch torpedoes . 21 inch Mark I . The first British 21 inch torpedo came in two lengths " Short " at 17 ft 10 . 5 in ( 5 . 45 m ) , and " Long " at 23 ft 1 . 25 in ( 7 . 04 m ) . The explosive charge was 200 lb of gun cotton increased later to 225 lb . 21 inch 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 agreement ) destroyers provided to the UK during the early part of the Second World War . The running speed was reduced from 45 knots ( 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 21 inch 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 . 21 inch Mark V . The Mark V was used by the A and B - class destroyers and , with modification , by the " Kent " - class heavy cruisers . 21 inch 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 . 21 inch 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 boat s . 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 was still in service with the Royal navy as late as 1983 , and with the Royal Norwegian Navy ( Coastal Artillery : Kaholmen torpedobattery at Oscarsborg fortress ) until 1993 . The Mark VIII * * was used in two particularly notable incidents : - On 9 February 1945 the Royal Navy submarine HMS " Venturer " sank the German submarine " U - 864 " with four Mark VIII * * torpedoes . This is the only intentional wartime sinking of one submarine by another while both were submerged . On 2 May 1982 the Royal Navy submarine sank the Argentine cruiser with three Mark VIII * * torpedoes during the Falklands War . This is the only sinking of a surface ship by a nuclear - powered submarine in wartime ( and only the second sinking of a surface ship by any submarine since the end of WWII ) . 21 inch Mark X . From 1939 , used by submarines , motor torpedo boats and destroyers . 21 inch Mark XI . Electric battery powered torpedo with a TNT warhead . Entering service during the Second World War it was used by destroyers . 21 inch Mark 12 . Codenamed first " Ferry " then " Fancy " , the Mark 12 never reached production . From 1952 , a warhead of Torpex . Powered by high test peroxide , giving it a speed of for . There were accidents during testing caused by the unstable nature of HTP . One such engine explosion , after loading on 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 . 21 inch 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 frigate s intended to use 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 . 21 inch 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 . 21 inch Mark 22 . A wireguided version of the Mark 20 produced by Vickers Shipbuilding and Engineering ( VSEL ) as a private venture 21 inch Mark 23 Grog . A wireguided version of the Mark 20 . Entered service in the 1971 although already obsolescent , serving only as an interim before Tigerfish entered service . The MK23 was fitted with a 10 000 M 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 .