[jvanderbot]

I had never heard that theory before!

FWIW, everyone in the beginning of WWII had magnetic detonator / torpedo problems, so it couldn't be just that. They were difficult to depth-keep just right to pass under a ship but within detection, for one thing. The sub captain had to correctly identify the ship, look up the draft, and call down to manually set the depth keeping. (Good luck in the swells of north atlantic). Often it just didn't use that depth anyway, due again to issues with design/testing.

The contact detonators had their own issues, for one they couldn't explode at an oblique angle, instead needed near-right-angle impact - but even then had a high dud ratio.

So, in theory the magnetic ones were preferable, even though standard doctrine was to fire for right-angle impact regardless (it makes evasion much more difficult, for one thing).