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	CHAPTER XIV
	GERMAN U-BOAT TORPEDOES AND MINES
	(A.)  TORPEDOES
	(i)  Introductory.
	1.  The information given below is not confined to those torpedoes of which we have actual specimens, but is all supported by strong evidence, and where necessary probability values are indicated in the text.
	It is emphasized that C.B.1815 remains the authority on torpedoes and that any subsequent amendments will appear in that book.
	2.  U-boats are using several types of 21-inch torpedoes; briefly:
	An acoustic homing torpedo, the "Gnat"; (see paragraphs 10-29 below).
	An electric torpedo, trackless, but short-ranged; (see paragraphs 40 to 61).
	An air-driven torpedo, long-ranged or fast; (see paragraphs 70 to 80).
	3.  The two latter types may be fired as straight torpedoes, or may have a to-and-fro "Curly" setting (paragraphs 90 to 114).
	4.  All three types may be used with magnetic non-contact or impact pistols.
	(ii)  "GNAT" Torpedo.
	10.  German name "T.5".
	THe following is a synthetic appreciation built on evidence.
	11.  The "Gnat" is a 21-inch electric-driven acoustic homing torpedo, and therefore trackless.
	12.  Although primarily an anti-escort weapon it has not been confined to these targets.
	13.  It can be fired from the bow or stern tubes of a U-Boat, on the surface or down to a depth of about 100 feet.
	14.  Normal gyro angling, probably up to 90°, can be applied.
	15.  Its speed is probably 24 knots, and range about 6,000 yards.  It may run on for some time at a tailing off speed before finally sinking.  The overall duration of its run cannot exceed 15 minutes.
	16.  If at any time during the "Gnat's" listening run it should come within receptive range of sound from its target, acoustic control will override its straight running or circling control, and it will close its target on a curve pursuit.
	17.  This receptive range will of course depend on the sound output of the target and on background noise, but it is of the order of 300 yards from an escort doing 15 knots.
	18.  The curve of pursuit course is such that the first crossing of the target's track must be close astern of the source of the sound.  The position of a subsequent hit depends on a large number of factors, but will tend to be on the after part of the vessel, the more so as her speed approaches the speed of the "Gnat" torpedo.
	19.  It probably does a straight safety range run, of about 700 yards, and then begins to "listen".
	20.  By an external tube setting spindle the "Gnat" may be pre-set so that, after the safety range is run off, it will behave in one of three ways, which are not fully known:
	(i)  suitable for an approaching target; possibly circling;
	(ii)  suitable for a retiring target;
	(iii)  to run as a normal straight torpedo, with acoustic control cut out.
	21.  The particulars of the "Gnat's" acoustic control are uncertain, but there are grounds for thinking that a noise decoy on each quarter affords much better protection than one alone.
	22.  Operative acoustic frequency is not known, but is probably between 1 and 50 k.c.
	23.  The Gnat" is probably unaffected by Asdic, and almost certainly cannot home on Asdic transmission.
	24.  The turning circle of the "Gnat" when in acoustic control (as opposed to when doing a pre-set "lurking" circle) will be as small as can be designed.  A turning circle diameter of about 150 yards is likely.
	25.  The pistol in the "Gnat" is believed to be in a top pocket in the warhead.  This is probably a sensitive impact pistol of the inertia type.  Almost certainly the "Gnat" has magnetic non-contact firing in addition.
	26.  The depth keeping gear probably has the range of settings down to 40 feet common to German torpedoes, but the depth setting used against escort vessels is likely to be about 15 feet.
	27.  The warhead charge in the "Gnat" is thought to be about 600 lbs.
	28.  It is uncertain whether the "Gnat" is fitted with a self-destructor or not.  If so it might either detonate the warhead, or less noisily wreck the mechanism.

 

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	29.  The explosion of a "Gnat" may therefore, it is thought, be caused in any of the following ways:
	Impact hit on its target.
	Non-contact hit.
	By wake effect close under a ship's stern.
	By perturbation in a rough sea, causing a premature of its magnetic pistol.
	By effects of a nearby depth charge, firing either its magnetic or inertia pistol.
	By perturbation when sinking unsteadily at the end of its run, which might be a deep explosion without plume.
	By self-destructor gear, possibly.
	(iii)  21-inch Electric Torpedo.
	40.  German name G.7.e", alias"T.2" and "T.3".
	41.  The standard 21-inch electric torpedo, "G.7.e", exists in at least two varieties (paragraphs 52 and 53).
	42.  Knowledge is based on specimens dating from 1941, but the general design is not thought to have changed.
	43.  It is driven by lead-acid secondary battery and electric motor.  Has an R.G.F. pattern tail, with two-bladed propellers.  Gyro and depth gear are operated by air from bottles, with exhaust into battery chamber.
	44.  It has one speed setting only, 30 knots to 5,400 yards.  To obtain this performance the battery must be pre-heated electrically, and if this is not done only 28 knots to 3,300 yards will be obtained.
	45. Warhead contains 660 lbs. of TNT/HND/AL.
	46.  Gyro Angling is up to 90° Right or Left in 1° steps.
	47.  Depth Setting is from 0 to 40 feet.
	48.  The electric torpedo is quite trackless.
	49.  It is cheaper and more quickly produced than an air torpedo, and has been mass produced.
	50.  It is the torpedo most used by U-boats, probably because of tracklessness.
	51.  The electric battery requires frequent attention, so electric torpedoes are not carried in the reserve upper deck stowages in U-boats.
	52.  The original electric torpedo "T.2" is still used with the impact pistol "G.7.H" alias "Pi 1", but is becoming obsolete.
	53.  A later model of the same torpedo "T.3", which probably only differs in having electric connection from the main battery to pistol pocket enabling it to employ the German magnetic pistol "Pi 2" (paragraph 122).
	54.  Recent electric torpedoes are fitted with "Curly" gear type 2 (paragraph 110).
	(iv)  Improved Electric Torpedo.
	60.  Information has hardened and points to a 21-inch electric torpedo with larger battery capacity being in existence.
	61.  Performance of 30 knots to 7,000 yards is estimated.
	(v)  21-inch Air Torpedo.
	70.  German name "G.7.a", alias "T.1".
	71.  Our specimens date back some years, but the general design continues.
	72.  Is still used by U-boats especially on long cruises.  Also appropriated to Cruisers, Destroyers, and E-boats.
	73.  The "G.7.a" is an air-driven torpedo of conventional design.  Wet heater cycle.  Fume tail, with six-bladed propellers; earlier models had four blades.
	74.  It has three speed-settings:
	44 knots to 6,500 yards.
	40 knots to 8,700 yards.
	30 knots to 15,300 yards.
	75.  Warhead contains 660 lbs. of TNT/HND/AL.
	76.  Gyro Angling is up to 90° Right or Left in 1° steps.
	77.  Depth Setting is from 0 to 40 feet.
	78.  Being an air torpedo it makes a visible, though not a pronounced track.  For this reason its use by U-boats is mainly by night, especially when fired with a "Curly" setting.
	79.  Its maintenance needs less frequent attention than does an electric torpedo.  Hence its use in reserve upper deck stowages in U-boats.
	80.  Hitherto the "G.7.a" has been used chiefly with an impact pistol "G.7.H", alias "Pi 1".  It is now believed to employ also a magnetic pistol "Pi 3" probably with a special warhead (paragraph 123).

 

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	(vi)  "Curly".
	90.  "Curly" is our name for torpedoes using mechanism causing them to describe some to-and-fro or circling course to obtain greater chance of a hit.
	91.  The Germans have several varieties of this gear, which are described below, and loosely refer to such torpedoes as "Geleitzugtorpedos" or Convoy Torpedoes.
	92.  The first "Curly" gear, German name "Fat", was introduced for U-boat torpedoes in 1942.  An improved "Curly" gear, German name "Lut", has come into operational use early in 1944, and is being fitted increasingly.
	93.  Both "Fat" and "Lut" require additional external tube setting gear, whereby the torpedo is set to run straight for a given range, and then begins its pre-set convolutions.  The "Lut" requires in addition an alteration of the torpedo control computer instrument.
	94.  Both "Fat" and "Lut" torpedoes are being used with magnetic and impact pistols.
	95.  "Fat 1" has recently been used in E-boat torpedoes, and it is reasonable to suppose that E-boats will later be supplied with "Lut".
	(vii)  "Fat 1".
	96.  "Fat 1" is the German name for a form of "Curly" gear fitted in 21-inch air torpedoes.
	97.  Probably used only by night, when its track will not be seen; and chiefly with the long-range 30 knot setting.
	98.  The capabilities of torpedoes with "Fat 1" are now known definitely.  The torpedo can be fired with normal gyro angling, and will then run straight to any set range.  On reaching this point the torpedo will begin to execute either Long or Short legs either to the Right or Left, as may have been pre-set.
	99.  The turns are 180° in every case and in alternate directions; that is to say the legs are parallel to the original straight run, and the mean line of advance of the "curly-ing" torpedo is at right angles to its straight run.  Figures 1 and 2 make this clear.
	100.  The dimensions of the "Fat 1" track are:
	Straight Run Any multiple of 110 yards, up to the full range of torpedo.   Turns The diameter of all 180° turns, and therefore the distance apart of the "legs", is 350 yards.   Long Legs The straight run on each leg when set to Long is 1,640 yards.   Short Legs The length of each leg when set to Short is 900 yards.   Number of Legs On the Long setting the torpedo will, if its range allows, describe 4 complete "there-and-backs", and on the Short setting 6.
	101.  On the Long leg setting the mean speed of advance of the "curly-ing" torpedo will be about 5 knots, and on the Short leg setting 7 knots, always at right angles to its original straight run.
	102.  The "Fat" torpedo is therefore best suited for shots from roughly the beam of a convoy, and not for shots from dead ahead.
	(viii)  "Fat 2".
	103.  "at 2" is the German name for a very similar "Curly" gear fitted in 21-inch electric torpedoes.
	104.  Its track is not known with certainty; but is believed to be the same as "Fat 1" except:
	(a)  Being a comparatively short-ranged torpedo only Short legs can be set, to Right or Left.
	(b)  The other two setting are utilised to execute plain Circles, either to the Right or Left.  See Figure 3.
	(ix)  "Lut".
	105.  "Lut" is the German name for a "Curly" gear which is believed to be the fully developed form of "Fat", having more general application.
	106.  Its improvements are believed to ne, briefly:
	(a)  The line of advance when curlying can be pre-set to any angle from its straight run.
	(b)  The mean speed of advance along its curlying direction can be pre-set at will from 5 to 19 knots.
	107.  This variation of mean speed of advance is believed to be obtained by varying the length of straight in each leg.  The longest leg setting is probably the same 1,640 yards as "Fat 1", which in a 30 knot torpedo, gives a mean speed of advance of 5 knots.  The shortest leg setting is probably nil, when successive semi-circles in alternate directions would give a 30 knot torpedo an advance of about 19 knots.
	Figures 4 and 5 show there probable "Lut" tracks.
	108.  "Lut" is believed to stand for "Lagenunabhangigertorpedo", which freely translated means "Torpedo which is independent of target's inclination".
	109.  There are reports of two varieties, "Lut 1" and "Lut 2".  The distinction is not known.  "Lut 1" is certainly fitted in some of the longer range electric torpedoes (paragraph 60).

 

 

 

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	110.  It will be seen that a torpedo with "Lut" will be valuable for browning a convoy from any direction, and will also be useful against independent ships.  Fired against single ships from right ahead or astern, and with curly advance set just in excess of target's speed, the chance of a hit should be great, unless the target zig-zags during the torpedo's approach.
	(x)  Pistols.
	120.  The pistols used in U-boat's torpedoes are of several kinds, viz:
	121.  An impact pistol, "G.7.H", "Pi 1".  This is described in C.B.(R) 3116, Section 15 (also in C.A.F.O.659/43), with advice for disarming it.  It can be used in 21-inch air or electric torpedoes; and by surface ships or U-boats, but 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.
	122.  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 probably embodied with the pistol itself in the nose pocket.  It is used probably only in the electric torpedo "T.3" (paragraph 53), as it requires a supply from the electric battery.  Its sensitivity is sufficient to fire 19 feet 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.
	123.  Another pistol "Pi 3" also impact and magnetic, and again the external parts are 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 conjunction with magnetic detonator gear of the Italian "S.I.C." kind (paragraph 320) situated at the after end of the warhead.  If so, degausing will have no appreciable affect against it.
	124.  Yet another type of pistol "Pi-4" is used in the "Gnat" torpedo (paragraph 25).  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.
	(xi)  General Information on U-boat Torpedoes.
	130.  The outfit of a typical 500-ton U-boat about December 1943 was 4 "Gnats", and 8 other electric torpedoes most of which could be "Curlies".  A few air torpedoes might be substituted, especially on distant stations.
	131.  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 especially an escort ship.  With air "Curlies" she might brown a convoy from ranges up to about 10,000 yards.
	132.  The following trends of U-boat torpedo development are apparent:
	Greater use of "Gnat" torpedoes.
	Greater use of magnetic pistols, not only in "Gnat" and "Curly", but also in straight torpedoes.
	Longer range in electric torpedoes.
	Discharge of torpedoes from greater depths.  The present limit of about 100 feet is imposed by the torpedo tubes, but both "Gnat" and "Curly" torpedoes would be effective weapons from deeper depths if discharge gear permitted.
	(xii)  Torpedo Tubes.
	Some U-boats have been fitted with steel torpedo tubes specially treated to overcome rust.  It is thought that this is only due to the copper shortage in Germany - and not for the purpose of firing deep.
	(B)  Mines (B2)
	(i)  Torpedo mines.
	There are 3 known types of mine laid by U-boats through their torpedo tubes.  These are carried in lieu of torpedoes:
	Type Charge Diameter Length No. per tube Details TMA 500 lbs. 21" 9' 2 Moored magnetic.  May have delay clock. TMB 1,220 lbs. 21" 7' 7-1/4" 3 Ground mine, magnetic or acoustic.  May have delay clock. TMC 1,850 lbs. 21" 11' 1" 2 Ground mine, magnetic, acoustic could be fitted.  May have delay clock.
	(ii)  Type "GO" mines (E.M.F.).
	Has been laid by minelaying U-boat types VII D, IX D and X B in up to 290 fathoms.
	Charge:  Approximately 750 lbs.
	Diameter:  46 ins.
	Moored/Magnetic (recovered).
	May have delay clock.
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