The G7a or G7a/T1 was the standard issue Kriegsmarine torpedo during the early years of World War II.
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The torpedo was 53.3cm (21in) in diameter, 7.163 m (23 ft 6 in) in length, and with a warhead of 280 kg (617 pd) of Hexanite, and was standard issue for all U-boats of the war.
The torpedo was of a straight-running unguided design, stabilized by a gyroscope. The G7a was of variable speed, running a distance of 6,000 m at 81 km/h (6,600 yd at 44kt), 8,000 m at 74 km/h (8,800 yd at 40 kt), and 14,000 m at 55.6km/h (15,300 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 fueled by a mixture of compressed gas, decalin and water. The compressed gas was forced into a combustion chamber along with petrol and water, where the mixture was ignited; after which the resulting superheated steam powered a four cylinder reciprocating engine, in turn powering a pair of contra-rotating propellers.
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 fitted with a magnetic exploder, which was inadequately tested (like the U.S. Navy's Mark XIV), having never been live fired.[1] In addition, because the G7's performance had been never been assessed, between deep running and premature explosions[2] (both also familiar to the Mark 14),[3] the G7 suffered a thirty percent failure rate early in World War II.[4] The response of the high command, ignoring complaints and blaming the operators,[5] was also common to the U.S. Pacific Fleet's Submarine Force.[6] 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."[7]
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 bombers 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 was easily spotted by surface ships, it remained the torpedo of choice for some U-Boat captains until the release of the G7e/T3 electric torpedo in 1942, largely due to the inferior performance and tendency of the G7e/T2 (the wakeless electric torpedo available to U-boats from 1939-1942) to fail to detonate, both on proximity and contact fuses.
The G7a could be fitted with both the FaT (ladder search pattern) and LUT pattern running guidance systems for attacking convoys.