(Nonfiction/MilHist) The Terrible Horrible Mark 14 Torpedo

Throughout the first half of the Pacific War, the USN's submarine force was hamstrung by the abysmal performance of its vaunted Mark 14 torpedo. Though developed in high secrecy and employing the latest technology, the Mark 14 was in many ways a step down from its WWI-era predecessor, and its name remains a black mark on the history of the USN Bureau of Ordnance (BuOrd).

The Mark 14's design was a break from traditional torpedoes, which work by directly hitting the target ship underneath the waterline and opening a hole in the hull. Instead, the Mark 14 was supposed to pass -underneath- the ship and explode below its keel using a magnetic detonator. Because water does not compress, the energy of the explosion would theoretically be transferred upward to lift up a small part of the ship, causing the target to break under its own weight. In practice, the Mark 14 ran -too- far underneath its targets, often running ten feet deeper than it was set. This was because the Mark 14 was tested using a dummy warhead that was lighter than the actual combat payload. Although this saved on development costs by allowing the torpedo to float and be recovered after test firing, it meant that Mark 14s fitted with real warheads ran deeper than the test runs anticipated. Furthermore, the Mark 14's hydrostatic depth sensor was poorly placed and gave inaccurate readings.

The Mark 14 also tended to explode prematurely. Again, this was because of poor testing procedure: the Mark 14 was tested only in the waters off of Rhode Island, thousands of miles away from where it would actually be needed. Anywhere outside of the North Atlantic, the Earth's magnetic field was different enough that the Mark 14 would explode long before it could do any damage. For example, USS Tunny (SS-282) once fired ten torpedoes at the Japanese light carriers Hiyou, Junyou, and Taiyou; seven exploded, but they did not even slow the carrier group down. Worse, BuOrd refused to believe that the painstakingly developed magnetic detonator could suffer from such an obvious defect. BuOrd did not launch an investigation until mid-1943, and its investigator purposely sabotaged one of the trial torpedoes (by reversing its gyroscope so it would not run straight) and blamed the maintenance crew.

USN submarine commanders soon learned to turn off the magnetic detonator and aim for direct hits using the traditional contact detonator. This was of little help, because the contact detonator was -also- defective. On a textbook straight-on hit, the firing pin would usually bend or jam under the mechanical stress, instead of setting off the explosive. In one test, ten Mark 14s were dropped onto the ground from 90 feet up--seven of them failed to explode. In another infamous incident, USS Tinosa (SS-283) crippled the whaling ship Tonan Maru III, then closed to point-blank range and hit the ship with nine torpedoes. None of them detonated.

When the Mark 14 did explode, its warhead was relatively weak--weighing about half as much as its Japanese counterpart, the Long Lance. If the Mark 14 actually worked, this would not be a problem, because its "back-breaking" mechanism used the target's own weight against it--no amount of armor could defend against it. As it was, a direct hit from the Mark 14 was inadequate against the armor belt of a capital ship.

On occasion, the Mark 14 would get lucky and obtain a kill... -on the submarine that fired it!- The Mark 14 included a gyroscope, allowing the torpedo to turn after it was fired. Sometimes the gyroscope would fail to end the turn--leading the torpedo in a large circle back to the point where it came from. As submarines are much slower than torpedoes when submerged, the USS Tullibee (SS-284) ended up sinking itself with its own torpedo, and the USS Sargo (SS-188) came close to suffering the same fate.

Eventually, the major bugs with the Mark 14 were worked out and the weapon redeemed itself somewhat, achieving an overall hit percentage of 36 percent by the end of the war. In the end, the USN was able to exact a terrible toll on Japanese warships and merchantmen, achieving the sterling kill ratio of 20 ships sunk for every submarine lost. But this was largely in spite of, not because of, the Mark 14.

Sources:
http://pwencycl.kgbudge.com/U/s/US_Mark_14_torpedo.htm
http://en.wikipedia.org/wiki/Mark_14_torpedo
http://www.ibiblio.org/hyperwar/USN/Admin-Hist/BuOrd/BuOrd-6.html