In 1942 production of the Type 92 electric 21in torpedo for submarines was restarted. It was slower than the oxygen-fuelled Type 95 but still capable of a respectable 30 knots out to 7000m (7655yds).This would form the basis of the Type 10 Kaiten human torpedo, following the example of the use of the Type 93 power unit in the Type 1 Kaiten already described in Chapter 2.
The Type 93 was redesigned to start on an oxygen-kerosene fuel mixture, inhibited from exploding by the injection of carbon tetrachloride. The internal rearrangement, which omitted the first air bottle, allowed the fitting of a much larger warhead with 780kg (1716lbs) of explosive. Luckily for the Allies, none of these Type 93 Model 3 torpedoes would go into service before the war ended. Examples of the extensive damage caused by the warhead of a standard Type 93 torpedo can be seen in Part IV.
Compared with the successes scored by the surface ships, the results obtained by the Japanese submarine force were disappointing, not from any fault with the torpedoes, but because of their tactical deployment. Japanese submariners were ordered to sink heavily escorted warships, in order to reduce Japan’s numerical inferiority, instead of concentrating on troop transports and supply ships. Despite valiant efforts, and crippling losses, during the entire Pacific conflict the large fleet of Japanese submarines succeeded in sinking only two carriers (one of which was the already crippled Yorktown), one light carrier and one escort carrier, one heavy cruiser (the unescorted Indianapolis) and one light cruiser, two destroyers and two destroyer escorts, and four submarines. These relatively modest results were doomed to make little impression on the overwhelming US Navy forces deployed against Japan.
At the same time, the development of homing torpedoes and magnetic influence exploders, both of which would have been extremely valuable in combating escort vessels, was begun but abandoned. As with their ambitious aircraft production plans, the Japanese empire was simply not producing sufficient numbers of trained engineers to carry these projects through to completion. By the time they realised that their tactics of prioritising attacks on warships had allowed the Americans to tighten the noose around the home islands, it was too late to do anything but prepare to die heroically in suicide aircraft, exploding motor boats and human torpedoes.
When the USA entered the Second World War there was a severe shortage of modern torpedoes for submarines. Even in 1942 many boats sailed from Pearl outfitted with old Mark 10 torpedoes, or with destroyer Mark 15s fitted with shorter warheads and muzzle-loaded into their rear torpedo tubes.
As a reaction to the attack on Pearl Harbor the US submarine fleet was immediately launched into an unrestricted war on Japanese warships and merchant ships. The sixty-six live torpedoes fired by US submarines during the month of December 1941 alone compares with the eleven fired against U-boats in the Great War and a very small number expended in live test firings. The latter was one of the causes of the severe problems which would plague the US submarine fleet well into the summer of 1943.
Submarine commanders returning from patrols began to report that their Mark 14 (and even the older Mark 10) torpedoes were clearly seen to be running straight and true, but they had passed well below the target vessel, meaning they were running much deeper than the depths set into the control mechanism. The Mark 14s with their top-secret influence exploder more often than not failed to go off, even when the torpedo passed beneath the target’s keel, and some even exploded prematurely.
Even more disconcerting were the reports that when a torpedo ran straight and true and struck the target, far too often the warhead would fail to explode. The worst example was an attack by the USS Tinosa on a Japanese whaler, the Tonan Maru 3. Torpedo after torpedo was launched, up to fifteen in total, and all were observed to hit, but not one exploded. Some were seen to rebound from the target’s hull and leap into the air. It is estimated that of all US torpedoes fired up to the summer of 1943, 70 per cent were duds.
When such disastrous results were passed on up the line, the reaction from the ordnance establishment (BuOrd) was that there was no problem with the torpedoes; therefore, the fault had to lie with the submarine crews. This blinkered attitude contributed to the disappointing performance of US submarines during the whole of 1942 and the first half of 1943. If the 70 per cent of duds had actually exploded, the rapid advance of the Japanese invasion forces might have been slowed or curtailed, and the Pacific War significantly shortened.
The first problem was with depth control. On 5 January BuOrd advised that the Mark 10 torpedo, dating from the Great War period and still in use in ‘S’-class boats, appeared to be running some 4ft deeper than set. It seems that this discrepancy was caused by the difference in weight between practice heads and warheads, an unbelievable situation when one thinks that all practice heads used for training and, in particular, calibrating the running of service torpedoes should have had the exact same hydrodynamic characteristics as the warhead which would replace them in action. Otherwise the training exercise was meaningless. In addition, it is true that the age of the Mark 10s, the oldest of which had entered service in 1915, might have been a contributing factor, with the springs in the depth mechanisms losing tension over the years. However, realistic testing, including the use of catcher nets, would quickly have identified such problems. Net testing was never carried out prior to the Second World War.
The higher speed Mark 14 suffered from a separate problem, in that the positioning of the water inlet of the hydrostatic depth-keeper valve in the torpedo meant it was adversely affected by the high-speed water flow over the torpedo casing. To make things worse, the depth recorder device was affected by the same problem, and registered a correct depth even when the torpedo ran too low. When finally this problem had been identified, the hydrostatic valve was repositioned inside the free-flooding rear balance chamber, which resolved the problem. Again, if the problem had been accepted in early 1942, rapid redesign measures could have been implemented.
The BuOrd began by accepting that perhaps the Mark 14s were running 4ft lower than set, based on their findings with the old Mark 10. Submarine commanders continued to report deep runners passing under their target, and it was not until the operational units carried out their own impromptu tests with catcher nets at Frenchman’s Bay in Australia on 20 June 1942 that it was proved the Mark 14s were running as much as 11ft deeper than set, not 4ft. The BuOrd still refused to accept these results, and it took the personal intervention of Admiral King in August 1942 to oblige BuOrd to investigate, when they found the Mark 14 ran 10ft deeper than planned. Modification kits were sent out, and the torpedoes so modified were identified by the suffix ‘A’, as in the Mark 14 Mod 3A.
The deep running having been identified and action having been initiated to rectify it, the next problem to be examined concerned the Mark 6 magnetic influence exploder. The British and German navies had already experienced difficulties with this type of exploder, and both had deactivated them. The design object was to initiate warhead explosion beneath the keel of the target ship by using the difference in the Earth’s magnetic field caused by the ferrous metal of the ship overhead. The difficulty arose because these differences varied according to latitude, and also because warships in particular were being degaussed following the introduction of the magnetic mine early in the Second World War. Because the Commander of Submarines for the Southwest Pacific, Captain Ralph Christie, had been involved with the development of the Mark 6 exploder and implicitly believed in its advantages, it was not until late 1943 that instructions were sent out to deactivate the magnetic influence aspect of the exploder.