In 1937 the Japanese designed their third oxygen-fuelled torpedo, this time an even smaller 18in model, the Type 97, designed for midget submarines. It carried a 350kg (770lbs) charge at 45 knots over 5500m (6000yds). The original Type 97 would see action only once, during the attack on Pearl Harbor, as its leaky first air bottles were impossible to check and recharge, since the torpedoes were muzzle-loaded into the tubes before the start of a mission, and the crew had no access to the torpedoes when under way.
The Japanese pre-war torpedo arsenal was completed by the excellent Type 91 aircraft torpedo, and this is fully described in Chapter 8. They experimented with highspeed torpedoes for destroyers, the two built reaching 56 knots, and also with turbine torpedo engines, but neither development was pursued. Up until 1940 they used round-nosed torpedo heads, but in that year the Italian streamlined torpedo head was introduced and used on all types, with a claimed increase in speed of around 2 knots, with no increase in engine power.
The US Navy produced three new torpedo designs in the 1930s, which would continue to serve throughout the Second World War. The Mark 14 submarine torpedo was designed in 1931, and was a development of the previous Bliss-Leavitt designs. The destroyer version was the Mark 15, which was longer with a larger warhead, but otherwise differed in minor details only. The wartime experiences with these two types will be described in Part IV. The Mark 13 aerial torpedo is described in Chapter 8.
The US Navy had begun experiments with alternative fuels as early as 1915, and in 1929 had started a research programme at the Naval Research Laboratory. By 1934 they had produced ‘navol’, a concentrated solution of hydrogen peroxide in water to provide an oxygen source, burning alcohol as fuel. The projected Mark 17 Navol torpedo for destroyers was interrupted by the attack on Pearl Harbor and the urgent need to produce torpedoes of the existing types.
THE SECOND WORLD WAR
As in the early days of the Great War, German submariners quickly began to score successes against their Royal Navy adversaries. Divers sent down to examine the wreck of HMS Royal Oak, sunk in Scapa Flow by Prien, discovered parts of a G7e electric torpedo. A complete example, which had luckily failed to explode, was recovered embedded in the hull of the child refugee ship SS Volendam, and more G7e torpedoes were found on board U 570 when she surrendered to British forces in 1941. But the British had no tactical requirement for a slow torpedo, and high speed was their priority in view of the paucity of German warship targets.
When, tardily, it was appreciated that Royal Navy submarines in the Mediterranean could well profit from the use of wakeless torpedoes, development of a British electric torpedo was begun, but with a low priority. The weapons were eventually produced too late for use in the Mediterranean, and when they were despatched to the Pacific theatre they were never fired in anger before the war ended. They would, however, be used as the starting point for the successful post-war British electric torpedoes described in the following chapters.
Meanwhile, the German U-boat crews began to suffer the first of a series of frustrating failures. In the battle for Norway, U-boat torpedoes set to run under their target failed to explode, or detonated prematurely some distance from the ship. The deep running was eventually traced to faulty design of the depth mechanisms, which were subject to derangement if the air pressure inside the U-boat was higher than normal, for example after venting air into the interior. The designers of the magnetic influence exploders had failed to take into account the different properties of the Earth’s magnetic field in the northerly waters off Norway.
These combined failures saved many British warships from destruction, and in at least one case, led directly to the loss of the U-boat: on 14 September 1939 Gerhard Glattes in U 39 fired three torpedoes at the carrier HMS Ark Royal, only to have them explode prematurely. Alerted by the prematures, Ark Royal’s escorting destroyers hunted down and sank U 39, the first boat to be lost during the Second World War. This and other catastrophic failures off Norway had a devastating effect on the morale of the German crews. Dönitz, the commander of the U-boats, designated Rear Admiral Kummetz to investigate, and several officers responsible for the faulty designs were court-martialled.
British duplex exploders, incorporating contact detonators as well as magnetic influence devices, were prone to the same problems. This was fortunate for the crew of the cruiser HMS Sheffield, engaged in shadowing the German battleship Bismarck in May 1941. When she was attacked in error by Swordfish torpedo bombers whose crews had mistaken her for the Bismarck, she was saved when several magnetic influence exploders went off prematurely.
In the Pacific the IJN made no such mistakes with their torpedoes, beginning with the excellent Type 91 aircraft torpedoes described in detail in Chapter 7. By contrast the Allies long remained completely unaware of the oxygen-fuelled Type 93, 24in torpedoes fired from cruisers and destroyers. It left no wake, and in early surface battles such as in the Java Sea was often fired in salvoes at very long range. When their ships were sunk or seriously damaged by devastating underwater explosions, the Allied crews put the damage down to mines, or nearby Japanese submarines working in conjunction with their surface forces — as the British had tried to do in the Great War.
In later actions in narrow waters such as the seas around Guadalcanal, the crews of the Japanese cruisers and destroyers profited from the high speed of the wakeless Type 93s, typically 48 knots out to 20,000m. And when a Type 93 hit its unsuspecting victim, the explosive effect of its 490kg (1078lbs) warhead was devastating.
The smallest of the oxygen-fuelled torpedoes, the 18in Type 97, designed for use by midget submarines, still carried an effective warhead. One of these Type 97s may even have struck USS Oklahoma at Pearl Harbor, hastening her sinking and ensuring she capsized instead of ending up on an even keel like the other torpedoed battleships at Pearl (see Chapter 22). The Type 97 Model 2 was used from 1942 with an important modification. Because of the impossibility of recharging the first air bottles when the torpedoes were loaded in the tubes of the midget submarine, the torpedoes were modified to run on oxygen-enriched air, with 38 per cent oxygen instead of 100 per cent. This meant that the problematic first air bottles could be eliminated, the 38 per cent oxygen mixture being safe to use for engine starting. A minor advantage was that the engine slide valves could now be coated in lubricant to prolong their working lives. In all the pure oxygen models no lubrication could be used on the valves, which typically had to be changed after only three practice runs. The modified Type 97s went into action against Allied ships in Madagascar and in Sydney Harbour. Examination of a Type 97 which had missed its target at Sydney revealed the old problem of maintenance when loaded externally — the gimbals of the gyro had rusted and caused the torpedo to deviate off course.