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-boats 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 regualtor feeeding 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 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 initially fitted with a combined mechanical/magnetic exploder, which was inadequately tested (like the U.S. Navy's Mark XIV), having never been live fired. In addition, because the G7's performance had been 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."
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