G7a torpedo The G7a ( T1 ) was the standard issue " Kriegsmarine " torpedo during the early years of World War II . Design . The torpedo was 533 . 4 mm ( 21 in ) in diameter , 7163 mm ( 23 ft 6 in ) in length , and with a warhead of 280 kg ( 617 pd ) of Hexanite , and was standard issue for all U - boat s of the war . The torpedo was of a straight - running unguided design , controlled by a gyroscope . The G7a was of variable speed , running a distance of 5 , 000 m at 81 km / h ( 5 , 500 yd at 44kt ) , 7 , 500 m at 74 km / h ( 8 , 250 yd at 40 kt ) , and 12 , 000 m at 55 . 6 km / h ( 13 , 200 yd at 30 kt ) . The 44 kt setting was used only by torpedo boats like the " Schnellboote " . The G7a was the last torpedo of German design to use a wet heat method of propulsion . The torpedo was powered by a steam - engine fed by a mixture of compressed air and steam . Fuel decalin was burning in a combustion - chamber , creating steam from fresh water . The torpedo ' s speed was determined by the level of pressure ( three settings for 30 / 40 / 44 kn ) from the low - pressure regulator feeding air to the bottom of the combustion chamber . The resulting superheated steam powered a four cylinder reciprocating engine , in turn powering a pair of contra - rotating propeller s . Though this system of propulsion gave the G7a great speed and endurance - the greatest of any production model German torpedo of World War II - it had the distinct disadvantage of being very noisy and leaving a long wake of bubbles , common to most torpedoes of the period , with the exception of the Japanese Type 93 and submarine Type 95 , which were fuelled by enriched oxygen . This relegated the G7a for use mainly at night , when its wake was least noticeable , so as to not give away the element of surprise and the location of the submarine that fired it . The G7 was initially fitted with a combined mechanical / magnetic exploder , which was inadequately tested ( like the U . S . Navy ' s Mark 14 ) , having never been live fired . In addition , because the G7 ' s performance had never been assessed , between deep running and premature explosions ( both also familiar to the Mark 14 ) , the G7 suffered a thirty percent failure rate early in World War II . The response of the high command , ignoring complaints and blaming the operators , was also common to the U . S . Pacific Fleet ' s Submarine Force . The problems were so serious , Admiral Dönitz said , " . . . never before in military history has a force been sent into battle with such a useless weapon . " Use . There is at least one recorded case of a U - boat being bombed based upon her position being given away by a G7a ' s wake . On September 14 , 1939 , " U - 30 " was attacked by loitering United Kingdom Fairey Swordfish naval bomber s when she fired a G7a from her stern torpedo tube at the SS " Fanad Head " . " U - 30 " was undamaged in the attack and served until she was scuttled at the end of the war . Though the G7a could easily be spotted by surface ships , it remained the torpedo of choice for some U - Boat captains until the release of the G7e ( TIII ) electric torpedo in 1942 , largely due to the inferior performance and tendency of the G7e ( TII ) ( the wakeless electric torpedo available to U - boats from 1939 – 1942 ) to fail to detonate , both on proximity and contact fuse s . The G7a were also issued in versions with programsteering gyroscopes , using the Fat I ladder search pattern and the Lut I or Lut II pattern running for attacking convoys . Technical data for G7a TI . 1 ) Used in post - war service by several navies with different modifications and designations ( Bundesmarine : DM11 , Royal Danish Navy : T1T , Royal Norwegian Navy : T1 mod 1 ) . Last known operator was RNoN with the T1 mod 1 ( G7a with wire - guidance : Although 4 modifications to the torpedo where carried out over the years - the last with computerized control in 1992 - the same designation " mod 1 " was applied throughout its service ) . T1 mod 1 was used from FAC ' s ( until 1992 ) , submarines ( the 80 ' s ) and Coastal artillery shore - batteries ( 1999 ) . 2 ) 44kn was to be used by S - Boote ( problems with initial design lead to breakdown on motors running 44kn , so this setting was banned until the new engine was available ) 3 ) Black used only from shore torpedobatteries , red from surfacevessels and blue from submarines 4 ) Early versions had 4 - blade propellers Other versions : 1 ) For launch with program setting , speed could only be set to 30kn 2 ) For launch with program setting , speed could be set to 30 or 40kn Exercise - and Warheads / pistols used on the TI : Exersiceheads : Warhead / pistol statutory combinations according to Kriegsmarine regulations : Warhead / pistol possible ( technically ) combinations - only on special demand , approved by torpedo - arsenal and with special consideration to depth - settings in order to assure proper function of the pistol :