As Davw said-a commission was set up in mid-April to investigate the case thoroughly. The commission came out with a comprehensive report in late July, which placed a considerable blame on the Torpedo Department. The TD, it was found, had supplied the boats with the new magnetic firing pistol with four-blade propellers before it had undergone the necessary trials. Consequently, the personnel of the Torpedo Experimental Institute responsible for that SNAFU were court-martialed and sentenced to prison terms.
Although the negligence of the Institute had been established, it was not until February 1942 that the U-Bootwaffe got to the heart of the matter.
On January 30, the crew of U-94 made a little extra effort and conducted an on-board examination of their torpedoes amidst the Atlantic. They thus discovered an excess pressure in the torpedoes' balance chambers, where the hydrostatic valve controlling the depth at which the 'fish' ran was located. When they radioed back their findings, the Inspector of the Torpedo Department ordered a check on board all submarines in port. Half of the torpedoes were found to have the same problem, and the mystery of the torpedoes' deeper-than-set-depth run was finally fathomed. The results of this and later investigations were summed up into a Memorandum by Grand Admiral Raeder on Feb 9, 1942.
But what was this higher pressure in the chambers due to? According to Dönitz's memoirs, the higher than normative pressure (the normative was atmospheric) in the chambers was due to the "frequent releases of compressed air which are essential when [the boat] [is] proceeding submerged" (92). During long periods of submersion, a considerable excess pressure thus accumulated. One can't help asking, hadn't the constructors foreseen such a problem? They had, indeed. Normally, no excess pressure should have accrued. However, many balance chambers were found not to be airtight - they had leaks. This discovery, added to magnetic influence from the Norwegian fjords, largely explained the stupendous failure of the boats during Operation Hartmuth, for many of them had remained submerged for as long as 20 hours daily. After the final computations were made, it was found that between 30 and 35 % of the torpedo attacks during the Norwegian campaign had been failures.
In December 1942, well into the war, a new, improved magnetic pistol was introduced which also functioned on contact. It proved very efficient. Until then, writes Dönitz, "the effectiveness of our torpedoes was no greater than it had been during the First World War" (94). Indeed, inter-war naval doctrine called for a single torpedo that would be able to break the back of even a battleship. In fact, far more money was spent on big gunnery research than on torpedo improvement, with the result that when the war broke out, the torpedo that the Navy required existed only in the minds of the complacent Torpedo Department and the Torpedo Experimental Establishment - the monopoly producer and tester of the torpedoes, respectively. Had they had more competition from private industry, we would have had a better torpedo, to put it in Dönitz's words.
Later into the war, in an analysis of torpedo performance in the period January-June 1942 (Paukenschlag and the apex of U-boat activity in American waters - the richest single harvest of the whole war), it was estimated that only 40% of the ships had been sunk by a single torpedo during that period, while the rest had either required two or more, or had escaped after one or multiple hits. In light of the more than two million tons of shipping actually sunk (a third were tankers), it is easy to imagine what could have been the outcome had the U-boats been armed with the weapon they really needed.
Together with the introduction of the new magnetic pistol torpedo, two more innovations were pioneered into the Kriegsmarine, which effectively surpassed any previous German torpedo as well as any Allied countermeasures. The first innovation in U-boat weaponry was a torpedo that, after running a specified distance, began circling. This was quite useful, especially against convoys since the circling run increased the torpedo's chances to pick up another ship if it missed its original target.
The second novelty, delivered to the Navy in September 1942, was the acoustic torpedo, which steered directly at the noise of the target's propellers. It was an extremely useful weapon, because it could be fired from any angle, now matter how narrow a broadside the target presented. This also meant that the U-boats finally had a reliable weapon to fight destroyers, and indeed a number of successes against the latter were eventually scored.
The torpedo crisis was thus ultimately brought to an end, though by that time it was too late. In July 1942, the Allied shipbuilding capacity for the first time surpassed the U-boats' rate of sinking (which was then particularly high). The U-Bootwaffe never caught up again.