Shortly before the Second World War broke out, the United Kingdom began to look to its naval defences. Haunted by the German U-boat campaigns against Allied shipping during the First World War, many in the British establishment (including Winston Churchill) believed that enemy attacks on merchant shipping could again be a serious danger.
Owing to German rearmament during the 1930s, updated U-boats were far more advanced than the Royal Navy’s arsenal of anti-submarine countermeasures. As a result, in July 1939 the British decided to lay
