Long before then, submarine commanders had been reporting the third major fault, that even when the torpedo struck fair and square it failed to explode. Again the BuOrd would not acknowledge this failing, until Vice Admiral Charles Lockwood authorised trial firing of live torpedoes against a cliff on Hawaii. Of three torpedoes fired, one failed to explode, and after a dangerous recovery operation the exploder was dismantled, and was found to have distorted on impact, jamming the firing pin. The same type of impact exploder that had functioned satisfactorily in the 33.5-knot Mark 13 aerial torpedo was not strong enough to withstand impacts at the 46.3 knots of the Mark 14. Again, realistic test launches, or even the simulation of live launches using live exploders, had simply not been carried out during the cash-strapped 1930s. Fixes were applied, and the Mark 14s began the systematic destruction of Japanese naval and merchant ships at an impressive rate.
It was suspected that identical failures had occurred in the destroyer Mark 15 torpedo, which was virtually identical apart from a larger warhead. Due to the nature of high-speed destroyer surface torpedo attacks, such as the night actions around Guadalcanal, more misses were to be expected than with the slow, calculated submarine approach. Nevertheless, the same modifications that had been implemented for the Mark 14 were also applied to the Mark 15. By the time they went into effect, the US destroyer force would have few remaining opportunities to use their torpedoes. But a major success came in the Surigao Strait in 1944, as related in Chapter 22.
Three important torpedo developments during the Second World War implemented by the US Navy were the electric torpedo, the homing torpedo and the hydrogen peroxide torpedo.
Following their abortive essay in the Great War, the electric Mark 18 torpedo was finally developed as a result of studies of captured German G7e torpedoes. The basic problem to be solved was producing an electric motor of high power, yet small enough to fit inside the shell of a torpedo. To do this, traditional construction requirements were relaxed, accepting temporary overheating and sparking commutators, not fatal in view of the intended short run to the target. Practice runs would involve refurbishment or replacement of the motor. By these means the motor weight was reduced from between 500lbs and 1000lbs (227–454kg) down to only 250lbs (113kg), the same weight as the electric motor in the German G7e used as a model. Although slow, it was cheap to produce and accurate, and left no wake to lead escorts back to the firing point. Introduced in 1943, some 9000 would be produced, and the Mark 18 would go on to sink around a million tons of Japanese shipping before the war ended.
A modification of the Mark 24 mine (see Chapter 8) for submarine use, with wooden strakes running along its sides to allow for launching the 19in torpedo from standard tubes, resulted in the homing torpedo Mark 27, popularly known as the ‘Cutie’. One basic change incorporated a ‘floor switch’ instead of a ‘ceiling switch’. The latter prevented the Mark 24 from attacking surface ships, the former prevented the Cutie from attacking its own launch submarine. Of the 106 fired against enemy escorts, thirty-three hit, and twenty-four of these sank the target, the other nine being damaged.
A full 21in diameter homing torpedo used before the end of the Second World War by the US Navy was the Mark 28, with a 585lb (265kg) warhead; fourteen were launched against escorts, and four hit.
Being at the receiving end of the Japanese Type 93 torpedo spurred the navy into resuscitating its pre-war plans for production of the Navol torpedo, and in addition to the planned destroyer Mark 17, a new Mark 16 was developed for submarines. As a result of the savings in weight of propulsion components, allowing for a larger warhead, and the higher efficiency of the fuel, the Navol Mark 16 Mod 1 achieved a range of 11,000yds (10,058m) at 46.2 knots compared with 4500yds (4115m) for the Mark 14. Several hundred of each mark were produced before the war ended but none were fired in anger.
One development shared by the British and the Americans was the introduction of a powerful new explosive for warheads. Torpex, introduced in 1943, was a mixture of 42 per cent RDX (cyclonite), 40 per cent TNT and 18 per cent powdered aluminium. Its use in torpedoes was desirable because the addition of aluminium increased the duration of the explosive pulse, enhancing the destructive underwater effect.
In 1939 the standard German torpedo used by U-boats, surface ships and S-boats was the 21in G7a, developed from the G7 of the Great War. It differed from the torpedoes of most other navies by burning decalin (decahydronaphthalene) instead of kerosene. It was fitted with the ‘Fiume’ tail. The early models had four-bladed propellers but later models had six-bladed. Starting in 1938, the initial production of the G7a or ‘Ato’ required about 3730 man-hours. By 1943 the process had been streamlined, and production took just 1707 man-hours. Similarly, between 1939 and 1943 the Germans managed to reduce the amount of non-ferrous metals used in the G7a as follows: copper reduced from 370kg to 169kg; tin from 61kg to 22kg, and nickel from 46kg to just 2kg.
The G7a formed the basis for the G7e electric torpedo. This was similar in shape except for an ‘RL’ form of tail. Compared with the 2300 G7a thermal torpedoes fired by the Kriegsmarine (including surface ships and S-boats) up to the end of January 1945, 7000 electric G7e models had been fired, mostly by U-boats, showing the importance of this wakeless model. It was, of course, much cheaper to produce, using materials and labour not normally required for torpedo production. It did, however, have one special requirement: while the boat was on patrol, the batteries needed constant heating to 30 °C, otherwise the range performance was seriously affected. Because of this, the G7e could not be carried in the upper deck storage compartments of U-boats. Both types could be preset to run a pattern course, but on the G7e the battery was changed from 93 amp/hours to 125 amp/hours. This gave it a range of 8200yds (7500m) at 30 knots.
The FAT (‘Federapparattorpedo’ or ‘spring-operated torpedo’) version was a pattern-runner introduced in 1942 to attack dense convoy formations; it ran a wandering course with regular 180-degree turns. It was best suited for beam firing rather than from dead ahead. The U-boat crew would input the chosen pattern using a control panel. From the end of 1942 onwards it was manufactured at the rate of roughly one hundred per month.