Did you know that a Dowty led consortium won a £50m contract to build a UK Magnetic Treatment Facility for Submarines?
Submarines constructed of steel, have permanent magnetic fields surrounding the vessel; hence simple induction coils in both older mines and magnetometers in modern mines are equipped to detect these fields.
Techniques exist to reduce the permanent magnetism of vessels through “Deperming” by passing large currents through coils temporarily rigged over the exterior of the vessel while in dock.
Apparently, this project was not successful, as the current was to strong and the boat would have had to run its engines at quite a high level just to stay in the cradle.
So project was cancelled!