Mk.6 Contact & Magnetic Influence Exploder
    When discussing the performance of U.S. torpedos, in general one is discussing the performance of America's most secret "super weapon", the Mk.6 combined contact & magnetic influence exploder.  
The idea behind a magnetic exploder was quite simple. Normal torpedos impacted against the side of a ship, what brought with it several problems. First, most of the energy of a underwater explosion is directed upwards, with only a small percentage of the overall power being used to hole the ship. Second, if the torpedo struck armor, it might have no impact whatsoever.

    A magnetic influence detonator might change this. A torpedo would be fired to explode below the ship targeted, using the difference in earth's natural magnetic field created by the large mass of iron above it. The explosion venting upwards would break a ship's keel, that of a battleship with comparative ease as opposed to the difficulty of holing it slightly below the waterline.

    The U.S. Navy's torpedo facilities at Newport developed the first version of this detonator between 1922 and 1924, and used it succesfully in 1926 against a submarine. Astonishingly, after this one test, a large number of exploders were manufactured and locked up safely. Mk.5, a contact exploder of similiar weight and dimensions to the Mk.6 was substituted.

    When the bombs had stopped falling on Pearl Harbor, Mk.6s were distributed to the fleet in all bases, and made their first wartime journeys (It is maybe interesting to note that Albert Einstein, when shown the design, told the Navy it would not work, and proposed changes. The Navy refused.).

    It was disasterous. The Mk.6 failed to detonate over two thirds of the torpedoes that were fired. Numerous good shots were not able to bring down a significant number of Japanese ships, and at first, the problem was not traced to the Mk.6. June 1942 tests showed the Mk.14 for one was deep running, this problem was solved by ordering the submariners to fire ten meters higher than they normally would, but still expectations were far higher than results. by December 1942, surface engagements had been fought with far less than the usual number of hits. The magnetic exploder was first to be deactivated, in early 1943 (though attempts to revive it continued until December), but still, the problem was not entirely solved. In early 1943, fifteen torpedoes fired by a submarine on the Tonan Maru III failed to explode despite perfect conditions. The problem was, finally, traced to the contact expoders weak exploder pin, which was driven back to explode the explosives. This item was so weakly constructed that the sudden deceleration of an impact actually crushed it before it could fire the explosives. This problem was finally solved too, but by this time, 1944, many prospective sinkings had not been made.

    The reason for the Mk.6's failures can be attributed to a lack of testing on the U.S. Navy's side and to a basic misunderstanding of physics, as the fact that magnetic fields differ around the earth was not taken into account when the exploder was produced. It can only be assumed that if the USN had been allowed to fight off Newport, the Mk.6 would have worked.