Thought this might be of interest, its a document published by British Naval Intelligence, April 1944 TORPEDOES USED IN GERMAN U-BOATS German U-Boats are using at least three kinds of torpedoes, an acoustic homing torpedo—the " Gnat," an electric torpedo, and an air-driven torpedo. The two latter may be fired as straight torpedoes or may have " Curly " settings. All three types may be used with magnetic non-contact or impact pistols. The " Gnat " From the evidence at our disposal, the " Gnat " appears to be a twenty-one inch electric torpedo, which can be fired from bow or stern tubes on the surface or down to a depth of one hundred feet. Normal gyro angling, probably up to 90°, can be applied. The speed of the torpedo is probably twenty-four knots and the range about 6,000 yards. The overall duration of its run cannot exceed fifteen minutes. The torpedo probably does a straight safety range run of about seven hundred yards and then begins to " listen." On coming within receptive range of sound from its target—about three hundred yards in the case of an escort vessel making fifteen knots—the " Gnat's " acoustic control will override its straight running or circling control and it will close its target on a curve of pursuit. This curve of pursuit is such that the first crossing of the target's track must be close astern of the source of sound. The position of the subsequent hit depends on a large number of factors, but it will tend to be on the after part of the vessel, according to the approximation of the two speeds. The nature of the " Gnat's " acoustic control is not known for certain, but there are grounds for thinking that two noise decoys, one on each quarter, give much better protection than one alone. The " Gnat " is probably unaffected by Asdic and almost certainly cannot home on Asdic transmissions. Its turning circle when in acoustic control is likely to be about one hundred and fifty yards in diameter. The pistol in this type of torpedo is believed to be in a top pocket in the warhead and to be a sensitive impact pistol of the inertia type. It almost certainly has magnetic non-contact firing in addition. The Electric Torpedo The twenty-one inch electric torpedo has one speed setting only, 30 knots to 5,400 yards but this is subject to the battery being pre-heated electrically ; unless this is done, the speed and range will be only 28 knots to 3,300 yards. It is entirely trackless and it is probably for this reason that it is the one most used in U-Boats. It is possible that the enemy has made some improvements in this design, giving it an estimated performance of 30 knots to 7,000 yards. The Air Torpedo As compared with the electric torpedo the 21-in. air torpedo requires much less attention and is, therefore, generally used by U-Boats carrying out long patrols. It has three speed-settings, 44 knots to 6,500 yards, 40 knots to 8,700 yards and 30 knots to 15,300 yards. Being an air torpedo it makes a visible, though not a pronounced track, and is therefore mostly employed in night attacks. It generally carries an impact pistol. " Curly " Torpedoes " Curly " is our name for torpedoes using mechanism which causes them to describe some to-and-fro course. They may be either electric or air torpedoes. " Curly " Type 1 is fitted in the latter and is chiefly used with the long range 30-knot setting. After a straight run, the torpedo begins to make turns of 180° describing a succession of long or short legs of about 1,000 yards and 700 yards respectively. The mean speed of zig-zag advance is about 5 knots when doing long and 7 knots when doing short legs. The configuration of the run is, however, uncertain. The legs are probably in line with the straight run and so suitable for an attack from the beam, but they might well be at right angles to the straight run and so give good prospects for attack ahead of a convoy. " Curly " Type II is used in electric torpedoes. Owing to their comparatively short range, there are no long leg settings. The torpedo describes either short legs or circles. Pistols There are four types, first an impact pistol, known as " G.7.H ", or " Pi 1." This can be fitted in 21-in. air or electric torpedoes, but it is now little used by U-Boats. It is fired by a blow end on or on any of its four lever whiskers. It should be safe whilst in the tube, and should not fire hydrostatically, but its lever whiskers are likely to detonate the torpedo when it strikes the sea bottom. Secondly, there is a pistol, " Pi 2," introduced in 1942, which is impact and magnetic at option. Externally it is believed to resemble " Pi 1 " with the same impact whiskers. Its magnetic portion is of German design and is probably embodied with the pistol itself in the nose pocket. It is used probably only in the later model electric torpedo " T.3," as it requires a supply from the electric battery. Its sensitivity is sufficient to fire 10 ft. below the keel of a heavy ship. It is likely to be less liable to premature than was the spinning armature magnetic pistol in use before 1941. " Pi 3, ";the third type, which is also impact and magnetic, is believed to resemble " Pi 1." It is probable that " Pi 3 " is used only in air torpedoes, where no electric supply is available, and that it acts in conjimetion with magnetic detector gear of the Italian " S.I.C." kind situated at the after end of the warhead. If so, degaussing will have no appreciable effect against it. Pistol " Pi 4 " is used in the " Gnat " torpedo. This is thought to be a top insertion pistol, probably of the inertia impact type, allied with magnetic detector gear possibly of the " S.I:C." type. It is likely that several models of this pistol exist. General Information on U-Boat Torpedoes The outfit of a typical 500-ton U-Boat about December, 1943, was four " Gnats," and eight other electric torpedoes, most of which could be " Curlies." A few air torpedoes might be substituted, especially on patrols to distant areas. This armament would enable her to fire a salvo of three electric " Curlies " at ships in convoy, or to fire a " Gnat " at any ship. With " air Curlies," she might brown a convoy from ranges up to about 10,000 yards. The following trends of U-Boat torpedo development are apparent. There is a greater use of " Gnats " and of magnetic pistols ; electric torpedoes now have a longer range and torpedoes are fired from greater depths. The present limit is about 100 ft. but both " Gnat " and " Curly " torpedoes would be effective weapons from greater depths if discharge gear permitted.