When the war started, the submarine force was immediately sent into action, with the order to wage "unrestricted submarine warfare" against Japan. However it would be 18 months before this really happened, and most of the problem during that time was torpedo related. The American problems with their torpedoes paralleled that of the Kriegsmarine, though the German depth keeping problem came from a leaky seal on a balance tank while the American problem originated with an engineer who based the settings on production torpedoes with data obtained from practice torpedoes with warheads that weighed 200 pounds less than the production version. Thus American torpedoes had a depth keeping problem. A torpedo set to run at fifteen feet would actually run as much as ten feet deeper. This problem was compounded by the blunt statement from the Bureau of Ordnance that there was nothing wrong with the depth keeping mechanism, and the commanders were obviously just missing their targets.
Eventually, it was no longer possible to ignore the commanders' complaints. Tests were run, using the relatively simple expedient of firing torpedoes at a fish net, and it was confirmed that the Mark-14 torpedo was running ten feet deeper than set. After this, BofO finally did their own tests, at last conceding that there was a depth problem. The commanders were ordered to adjust the depth settings to compensate for the error, and new torpedoes were modified to fix the problem. This having been accomplished, it was presumed that the success rate would now soar. It didn't.
Commanders were complaining that, even with the corrected depth settings, and perfect shots, the magnetic exploders were either, detonating prematurely, which only served to warn the target and alert the escorts, or they were passing under the target and not exploding at all.
Once again American torpedo problems ran along the same lines as German torpedo problems. The U-boats had experienced the same problems with prematures and failures, using their own magnetic exploders. The Germans quickly recognized the problem, ordered the magnetic exploders deactivated, and went back to blowing up targets. The Americans, on the other hand, insisted that the exploder worked, and that the problem had to be in the people using it.
Unfortunately for the Americans the Mark-6 magnetic feature was, based on a pair of false premises. First, that the earth's magnetic field was essentially the same everywhere and, second, that a steel-hulled ship is going to disturb that field. In fact, the earth's magnetic field varies considerably. An exploder that worked unfailingly off Newport could fail miserably in the Pacific. And it's a relatively simple process to degauss a ship's hull—something that was done routinely to warships and others going into combat areas once magnetic mines were introduced.
The American problem was compounded by two Rear Admirals, Robert English at Pearl Harbor and Ralph Christie, in Australia. Christie had worked on the Mark-6 exploder at Newport, and was convinced that it worked. He presumed that any problems came from poor maintenance or other user error. And it wasn't until English died in a California plane crash, and Lockwood took over at Pearl, that anyone would really listen to the commanders. Lockwood, like the Germans before him, allowed the magnetic exploders to be deactivated on Pearl Harbor boats, unfortunately Christie persisted in mandating their use.
But there was a third part of the problem that had been hidden by the magnetic exploder problem. Because the captains had been under orders to use the magnetic exploder, and had been setting their torpedoes to run the required five feet under their targets, few of them had had the opportunity to realize that that contact exploder was also defective.
Time after time, a perfect shot would send a torpedo squarely into the side of a target, only to have it fail to explode. It might punch a hole in the side of a freighter, but most likely not something that couldn't be repaired at sea. And with a warship, made of thicker steel, it might do nothing more than cause a small dent. At the same time, bad shots, made at extreme angles, where the torpedo hit the target at an oblique angle instead of square, resulted in the warhead detonating and the target going down.
Lockwood ordered more tests. Live torpedoes were fired at the cliffs on Kahoolawe. Three torpedoes were fired from U.S.S. Muskallunge, with the third failing to explode. Navy divers found the dud and it was returned to Pearl Harbor, where it was taken apart.
It was found that the contact mechanism, built to essentially the same standards as that in the slower Mark-10 torpedo, had failed under the greater impact of the much faster Mark-14. Instead of striking the primer, the firing pin had bent and jammed in the guides, which had also distorted. More tests were made, this time by dropping dummy warheads fitted with live exploders onto steel plates from a height of 90 feet, confirming the diagnosis. Once understood, the problem was fairly easy to fix. New firing pins were machined from a light, high-strength aluminum alloy (the metal reportedly came from the propellers of Japanese fighters shot down during the Pearl Harbor raid) and the guides were strengthened, so that they would hold up long enough for the firing pin to strike the primer and detonate the warhead. This "Pearl Harbor Modification" was fitted to all the torpedoes in the inventory, and the changes incorporated into new production. After that, the Mark-14 torpedo suddenly became a model of reliability, and sinkings finally did soar.