German mines were more effective because (1) they were at a fixed depth, and (2) in a fixed location. The earth's magnetic field varies all over the scale even in the same region, Bermuda triangle for example has many large deposits of iron ore under the seabed that makes compasses, magnetic anomaly detectors, and magnetic influence exploders go haywire. Even without degaussing though, the magnetic mines had a failure percentage that was pretty high, even if not as bad as the torpedo magnetic exploder.
Even in regions without large disturbances the magnetic field around a ship is smaller when it's running parallel to the earth's magnetic field than it is when running perpendicular - in other words a ship running north or south will need to have the torpedo set for a depth closer to the keel to have the magnetic influence exploder activate, one running east or west will have such a large field it will set off the warhead 100 yards away - the main reason why there were so many prematures that the captains claimed as hits, it looked like a hit and exploded about the right time after firing, must have been a hit. But it wasn't. The few that worked were a matter of luck and the right combination of target course, size, how recently it had been degaussed, distance from the north pole, local magnetic variations, etc. That was the whole problem with the theory itself, what worked in tests off Nantucket needed a different sensitivity setting in the North Sea (the magnetic north pole is actually in northern Canada) and the distance under keel that worked for a destroyer would be too close for a battleship, which has a larger magnetic field and would require more distance under the keel to avoid a premature. Assuming the same course, if the destroyer was going east and the battleship heading south the destroyer could have a LARGER magnetic field than the battleship. Move the whole smash to the South China Sea and all bets are off, you're much further from the north pole and the variation is completely different so the sensitivity of the exploder will need to be cranked way up. Which will cause a lot more premature explosions, numerous reports of the warhead going off immediately after the 400 yard arming run, set off by the magnetic field of the sub that fired the torpedo.
When my squadron (Helicopter AntiSubmarine Squadron 75) was deployed ashore at NAS Lakehurst, NJ, we did a lot of practice MAD runs on the wreck of the Andrea Doria. Large passenger ship, known location off New York harbor so easy to plot from landmarks, good training for new AX and AW rookies. Even so the needle spike on the MAD recorder varied according to the time of the year and the weather, and sometimes needed to tweak the sensitivity. Exercises down in the Bahamas against actual subs was a different story, constant false spikes and constant fiddling with the sensitivity of the detector.
AFAIK there was no adjustment for the Mark VI exploder for the Mark 14, and the whole thing was so top secret there were no manuals even for routine maintenance anyway.
|