As far as magnetic exploders, every navy that tried to develop one had the same problem. Everyone assumed the Earth's magnetic field was constant, but it isn't. Torpedoes were all developed and tested near each navy's torpedo facility. Even just a few hundred miles away, the Earth's magnetic field is sufficiently different to trip the exploder whether there's a ship present or not. In the 1940s, no one had yet mapped the Earth's magnetic field.It is interesting that the Germans encountered the same problems with torpedoes as the US Navy (depth setting, magnetic and contact exploders, unresponsive ordnance organizations). Was it because tight budgets in the 20's and 30's prevented adequate testing?