For the first two years of the Pacific War after Pearl Harbor, the US Navy suffered from an awkward problem–its torpedoes did not work. And efforts to fix the problems were hampered by the Navy’s denial that anything was wrong.
Mark XIV Torpedo. Armed Forces History Museum, St Petersburg, Florida.
In the years before the Second World War, the US Navy believed that its fleet of battleships was the most powerful sea force in the world, and could defeat any potential threat. But when the battleships were destroyed by air attack at Pearl Harbor, the Navy had to develop a brand new strategy, utilizing the only forces it had remaining after the strike–the carriers and the submarine fleet. As it turned out, this was exactly what was needed anyway: carrier air power was the key to pushing Japan out of its island fortresses, and the submarine fleet systematically destroyed Japan’s merchant shipping, removing its ability to transport supplies and materials and preventing Japan from replacing its losses of ships and planes.
Immediately after the declaration of war, the US Navy ordered its submarine fleet, operating both from Pearl Harbor and from bases in Australia, to begin “unrestricted warfare” against Japanese military and merchant shipping. But disturbing reports soon came in: submarine captains complained that their new Mark XIV torpedoes would often go awry, either running off-course, detonating prematurely before reaching the target, or ineffectually bouncing off the side of the target ship and failing to detonate. In response to the complaints, the Navy’s Bureau of Ordnance did some tests and found nothing wrong with the torpedo, concluding that the inexperienced American submarine crews were simply not using them properly. Rear Admiral Ralph Christie, who was in charge of the submarine fleet, had himself helped to design the Mark XIV torpedo, and accepted the Ordnance Bureau’s conclusions. But the torpedoes continued to malfunction. In the first four months of the war, US submarines fired 300 torpedoes, but got only 10 hits. In the entire year of 1942, 1442 torpedoes were used, but only 211 hits were reported (and postwar Japanese records showed that almost half of these were actually premature detonations that did no damage). The Navy blamed the submarine crews, and replaced most of the Captains.
It wasn’t until Rear Admiral Christie was killed in a plane crash and was replaced by Rear Admiral Charles Lockwood, however, that anyone began to take the claims of the submarine captains seriously and look into the possibility that something might be wrong with the torpedoes. Lockwood was a former submariner himself, and carried out some tests by firing some torpedoes at an undersea net. He found that there was a design flaw in the apparatus that set the depth at which the torpedo would run, which caused it to run about ten feet below its set depth–enough to make it run harmlessly underneath an attacked ship. At first, the Ordnance Bureau insisted there was no problem, but soon its own series of tests showed otherwise.
The flaw was traced to an improvement that had been made during the torpedo’s development. The Mark XIV was originally designed to carry a warhead containing 600 pounds of TNT. But to give it a greater punch, this was increased to 660 pounds, and the TNT was replaced by a new explosive called Torpex. This increased the weight of the torpedo and also slightly changed its balance–but the corresponding changes were, for some reason, not made in the practice dummy-warhead versions that were used in all the test-firings (for budgetary reasons, the Navy never did any live-fire testing of an actual armed Mark XIV torpedo). As a result, no one realized that the actual armed torpedo ran about ten feet deeper than the practice version. The Ordnance Bureau now fixed this error and re-calibrated the torpedo’s depth gauge.
But the misses continued, and there were increased reports of torpedoes that either exploded before reaching their target, or failed to explode at all. Lockwood suspected that the problem lay with the super-secret “magnetic pistol” detonator used in the Mark XIV. The Ordnance Bureau once again insisted that there was nothing wrong with the torpedo’s detonators, and again concluded that the submarine crews were simply not using them correctly.
It took a lot of testing to finally discover the flaw. The torpedoes were designed to explode only when they reached their target ship. In most torpedoes of the time, the detonator (referred to as the “pistol”) was set to go off when the nose of the torpedo physically ran into the steel plates on the side of the ship, setting off the warhead and blowing a hole that would sink the target. The new Mark XIV torpedo carried a standard American “contact pistol” that could be set to fire on impact. But the Mark XIV also carried a new “pistol” that used an electromagnet which could detect the changes in the Earth’s magnetic field produced by a large mass of metal (such as a ship). The new magnetic-pistol detonator was designed to run underneath a target ship rather than into it, and detonate there, creating a huge cavity in the water that would temporarily lift the target ship out of the sea and cause its own weight to shatter the ship’s keel, breaking her back and causing catastrophic structural damage, sinking her. The magnetic pistol was so secret that it had not even been issued to the submarine crews until after the war had begun. And like the other Mark XIV tests, all of the new detonator’s testing was done with the inaccurately-running practice versions.
During the war, submarine captains had been ordered to run their torpedoes using the magnetic detonator rather than the contact detonators, which allowed the relatively small American warhead (the Japanese “Long Lance” torpedo carried over twice as much explosive as the American Mark XIV did) to do far more damage.
What the Navy designers did not know, however, was that the Earth’s magnetic field itself was not uniform, but had lots of local variations–and the new detonator was sensitive enough to pick these up. In areas where the Earth’s magnetic field was stronger than average (which included much of the Pacific), this would cause the torpedo’s detonator to go off before it had gotten close enough to the target to do any damage, resulting in premature detonations. There was no way to fix this flaw with 1940’s technology, and as a result, in 1943 submarine commanders were ordered to switch off their magnetic-pistol detonators and use only the contact-pistol detonators instead.
But once again, problems still remained. Reports began streaming in complaining about duds with the contact detonators–submarine commanders began reporting that they would get good solid hits straight into the side of enemy ships, but the torpedo would often fail to detonate on contact. Oddly, it seemed that the torpedoes were more likely to work properly when they made impact with a badly-aimed shot at an angle to the target ship rather than straight-on. Navy technicians again declared that there was no technical problem with the detonators.
To solve the problem, the Navy fired a number of Mark XIV torpedoes at the side of an underwater cliff, then recovered the duds for examination. It was found that the firing pins used in the contact detonators were too weak, and they often bent on impact, preventing them from striking the primer charge and setting off the torpedo (torpedoes that hit at an angle, though, generated less bending forces and the firing pin was able to reach the primer). The dud rate was as high as 70%. This time, however, the fix was a simple one–stronger firing pins were swapped in. At last, after almost two years of war, the Navy subs had a reliable torpedo that could accurately aimed and would reliably detonate when it hit the target.
Armed at last with a workable weapon, US submarines began wreaking havoc on Japanese shipping. In the last two years of the war, American submarines sunk 4.9 million tons of Japanese shipping, including 1,178 merchant cargo ships and 214 Japanese Navy combat ships, including 8 aircraft carriers, 1 battleship, and 11 heavy cruisers–about 60% of all Japanese ships lost during the war. By 1944, Japan’s shipping losses were twice that of her construction. Japan started the war with 6 million tons of cargo shipping, and ended it with less than 3 million. Japanese imports of iron, aluminum ore, cotton, rubber, and lumber all fell by over 90% during the war. The loss of gas and oil tankers was so severe that the Japanese were making desperate plans to build huge oil-tanker submarines and aircraft that could deliver supplies to factories in Japan without exposing themselves to submarine attack.