The G7a(TI) torpedo was 533.4 mm (21 in) in diameter, 7163 mm (23 ft 6 in) in length (with a type Ka or Kb warhead and Pi1 or Pi2 pistol), the warhead holding a charge of approximately 280 kg (617 pd) of Hexanite. It was the standard issue torpedo for all German U-boats and surface torpedo-bearing vessels of the war.
The torpedo was of a straight-running unguided design, controlled by a gyroscope. The G7a(TI) 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 on torpedoes with reinforced engine.
The G7a(TI) was the last naval 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 propellers.
Though this system of propulsion gave the G7a(TI) 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. For U-boats, this relegated the G7a(TI) 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 G7a(TI) 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 G7a's performance had never been assessed, between deep running and premature explosions (both also familiar to the Mark 14), the G7a 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."
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 14 September 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 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 fuses.
Technical data for G7a(TI) torpedo
- Type: Standard, straightrunning torpedo with wet-heater propulsion and whitehead tail
- Design: Ca. 1930
- Operative service in Kriegsmarine: 1938–1945 1)
- Length (prepared for launch with warhead): 7163mm
- Diameter: 533.4mm (21")
- Weight (prepared for launch with warhead): 1538 kg
- Charge, warhead: Ca. 280 kg Hexite
- Boostercharge (Pi1 pistol): 300g Pentrite
- Minimum angle of impact to detonate (Pi1 pistol, mechanical mode): 16 degrees
- Speed: 30kn ("Weitschuß", WS) / 40kn ("Nahschuß", NS) / 44kn ("Schnellschuß", SS) 2)
- Depthsetting: 1-12m (TA-I), 1-15m (TA-II)
- Armingdistance (Pi1 pistol): 100m (black impeller) / 150m (red impeller) / 300m (blue impeller) 3)
- Propulsion: 4-cylinder steam engine (powered by a mixture of overheated steam and compressed air)
- Fuel: Decaline
- Power output, engine: 110Hp (30kn) / 255Hp (40kn) / 350Hp (44kn)
- Engine RPM: 1170 (30kn) / 1280 (40kn) / 1470 (44kn)
- Propeller: 2 x 6-blade (contra-rotating) 4)
- Max running distance: 12000m (30kn) / 7500m (40kn) / 5000m (44kn)
- Capacity, air-tank: 676l (200 kg/cm^2)
- Freshwatercompartment: 57l
- Fueltank: 14,5l
- Oiltank (engine): 6,5l
- Oiltank (low-pressure regulator): 0,3l
- Steering: Mechanical gyroscope powered by pressurized air (GA VIII) with anglesetting mechanism (+/- 90 degrees from initial course after launch)
- Depthcontrol: Hydrostatisc mechanism with pendulum (TA-I or TA-II)
1) Used in post-war service by several navies with different modifications and designations (Marina Espoñola: G-7a, 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
- G7a(TI) Fat I: TI with Fat I programsteering 1)
- G7a(TI) Lut I: TI with Lut I programsteering 1)
- G7a(TI) Lut II: TI with Lut II programsteering 1)
- G7a(TIü): TI dedicated for use with Schulboote at U-Flotillen. The torpedo was equipped with the older type depth-mechanism, the lead ballast was removed from the water chamber and a special "light" exercise head was fitted (it was empty, i.e. no water and blowing mechanism). This was done to ensure positive buoyancy at the end of the run. Minor adjustments were also done to the air regulators and combustion chamber to lower the temperature of the drive gas (i.e. mixture of steam and pressurized air). These measures lead to a decrease in range, but achieved the overall goal of significantly less damage/losses of torpedoes and a higher number of exercise-shots for the crews.
- G7a(TIü) Lut II: TIü with Lut II 2)
- G7a(TXIV): TI with changed buoyancy (max airpressure reduced from 200 to 100 kg/cm^2) for the Seehund midget submarine, with sinker-mechanism (never fielded).
- G7as: TI with acoustic seeker (never fielded)
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:
- Type 1210 or type 1215 (improved head with dual blow-mechanisms)
Warhead/pistol statutory combinations according to Kriegsmarine regulations:
- Ka, Ka1 or Ka2 with Pi1, Pi1a or Pi1c pistols
- Kc with Pi3 or Pi3c pistols
- Kc1 or Kc2 with Pi3, Pi3a or Pi3c pistols
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:
- Kb or Kb1 with Pi1, Pi1a, Pi1c, Pi2*, Pi2c* or Pi2c* pistols (* only heads with extended pocket)
- Bekker, Cajus. Hitler's Naval War (New York: Zebra, 1977), p.132.
- Bekker, pp.128, 133, and 137.
- Blair, Clay, Jr. Silent Victory (New York: Bantam, 1976), pp.160 & 413.
- Bekker, p.125.
- Bekker, p.125.
- Blair, pp.62 & 361.
- Bekker, quoted p.132.
- Bekker, Cajus. Hitler's Naval War. New York: Zebra Books/Kensington Publishing Corp., 1977. (Reprints 1974 MacDonald and Jane's 1971 translation of Gerhard Stalling Verlag edition)
- Blair, Clay, Jr. Silent Victory. New York: Bantam, 1976. (Reprints Lippincott 1975 edition)
- Eberhard Rössler. Die Torpedos der deutschen U-Boote (2.ed), 2005 (ISBN 3-8132-0842-7)
- Kriegsmarine (T.V.A.): Technische Dienstvorschrift Nr. 194 Torpedo-Kopf-Pistolen-Tabelle, 1944
- Bundesmarine: Deutsche Unterwasserwaffen - Torpedos, 1967 (German translation of U.S. Navy Ordnance Pamphlet 1673B)
- Bundesmarine: Torpedo Zünder der Deutschen Marine von 1914 bis 1968 mit historischen Rückblick Band 1, ca 1975
- Bundesmarine: TDv 1356_004-20 Teil 2 - Bedienungsanweisung einschlisslich Pflege und Sicherheitsmassnahmen für Torpedo DM 11 (G7a), 1965
- Royal Norwegian Navy. SEP-116 Technical manual for Torpedo T1 mod 1, ca 1990 (unofficial - intended as replacement for several regulations like E.12, E.19 etc)
- Royal Norwegian Navy. E.12 Manual for torpedo mod. T1, ca 1950