Defence against mines
How degaussing cable worked. Illustration from Hurd's 'Britain's Merchant Navy'
From the early months of the war all main British ports were regularly swept for mines by Royal Navy minesweepers. A wide range of vessels was used for this highly dangerous work. Former Fleetwood trawlers based at Wallasey dock, Birkenhead, for example, patrolled the Mersey and its approaches.
From 1940 onwards the hulls of most large British ships were fitted with degaussing cable. This greatly reduced the threat from German magnetic mines. Degaussing was a method of neutralising a ship's magnetism by passing an electric current through a cable fitted around the top of a ship's hull.