Hi John,

I don’t know if this allowed the mine that sank the Counsellor to go undetected all that time but the U-boat torpedo tube laid TMB & TMC mines had magnetic / acoustic detonators, not contact detonators. They didn’t float like contact mines so conventional sweeping techniques didn’t work for them. They were laid on the ocean floor in shallow water and they had two delay mechanisms.

First they could be set to not even arm themselves until a specified period had elapsed. This feature was always used to at least allow the U-boat get clear of the minefield before they armed but the delay could be much longer than just necessary for that as they had an 80 day clock. Second they could be set so that after arming they would not detonate until a specified number of ships had passed over them.

Typically a mix of settings was used. Some were set for longer delays to defeat the allied mine sweeping effort which (at least that early in the war) was simply for a wooden vessel to drag a metal barge over them in order to cause them to detonate. They would do a sweep and if no mine was detonated they would assume there was no mine there. Later after some number of ships passed over it the next ship that came along detonated the mine. They would then sweep again, find no more mines and later another ship would detonate another mine in the same field. At least that was the plan anyway.

Regards,

Ken Dunn