Why finding a minefield matters as much as sweeping it

How effective were minesweepers based at Milford Haven at protecting shipping?

They were effective when defenders knew where mines had been laid, but unobserved submarine or aircraft minelaying could leave the danger undiscovered until a patrol or ship encountered it.

Mine clearance was partly an information problem as well as a test of sweeping equipment. If defenders knew roughly where mines were laid, they could close the area, route traffic around it, and send sweepers to a bounded search. Unobserved minelaying forced patrols to discover the danger before a merchant ship did. Reconnaissance, reporting, and traffic control were essential parts of protecting approaches such as Milford Haven.