"...the German naval staff were confident that the mine would be extremely effective..."

"...the pattern of shipping activity in the Bristol Channel made it an obvious choice for mine operations..."

The first magnetic mines were developed by the Royal Navy and laid off the Belgian coast in August and September 1918. They were not considered a success, many detonated soon after being laid and further British interest in developing the weapon waned with the armistice in November 1918. However, the 1920s did see a certain amount of progress with the development of a moored magnetic mine for defensive purposes.

German interest in magnetic mines dated from the earliest days of rearmament in 1932. The German Navy worked closely with Rheinmetall-Borsig who were also heavily engaged in developing a series of electromechanical bomb fuses for the German Air Force. An effective magnetic mine presented a series of challenging technical problems and a satisfactory solution was not achieved until late in 1936. The German Navy planned to lay their magnetic mines by surface craft and U-boats, building on their experiences of the 1914-1918 war, when covert mine laying proved very successful in disrupting patterns of commerce for both sides.

By 1939, the German navy possessed an extremely effective magnetic ground mine which unlike conventional moored contact mines could be laid very accurately in shipping lanes and approaches to ports and which did not have an effective set of countermeasures. The British were unaware that Rheinmetall had solved the technical problems of an effective magnetic fuse and the German naval staff were confident that the mine would be extremely effective in a trade war with a maritime power. Furthermore, effective countermeasures would take a long time to develop. In the early part of 1939 there was considerable debate in the U-boat command over how to use this new weapon effectively.

U-boat command realised that the mine weapon could only be used effectively in a specific set of circumstances. The mine would only be effective in depths of 25 to 30 metres (i.e. coastal waters). It was best suited to narrow approach channels and areas of high activity, which would presumably also be subject to intense anti-submarine patrols. The submarine launched mines could be laid accurately and quickly with the minimum of exposure by the U-boat. It was also believed that winter nights were the best time to lay mines as bad weather and poor visibility would often prejudice attacks by torpedo, mine laying thus providing the best value for U-boat sorties. Most submarine laid mines were fitted with an 80 day clock fuse. At the end of this period the mine rendered itself inert thus enabling subsequent U-boats to enter mined areas for further missions. One further element of effective mine laying was a detailed understanding of the nature of coastal traffic. To that end, interviews with German merchant crews were supplemented by careful observation by U-boat captains; particularly in the opening months of hostilities. However, it should be remembered that both navies were intimately familiar with the shipping and trade patterns of each other as a consequence of the 1914-1918 war.

The specific requirements of the magnetic mine and the pattern of shipping activity in the Bristol Channel made it an obvious choice for mine operations in the closing months of 1939. Accordingly, several U-boats were given missions to both lay mines and operate with torpedoes in the Channel. U 28, U 29, and U 33 each carrying 12 mines and six torpedoes were despatched to mine Swansea, Milford Haven and The Foreland in December. The magnetic mine used for these early operations was the TMB mine (Admiralty designation ‘Type GS’). This mine contained 1221 lb. of explosive in a cylindrical case just over seven feet in length. Although Type VII U-boats could theoretically carry 33 TMB mines, it was always considered prudent to carry a small number of torpedoes, thus a mix of 12 mines and six torpedoes is a typical mission load. Operational experience with the TMB mine revealed that it was not effective at depths greater than 25 metres which put greater pressure on U-boat commanders to place them in the correct position and at the correct depth. It is possible that the loss of U 33 in the Firth of Clyde in February 1940 was due to the need for the boat to move as close as possible to the mouth of the Clyde to lay her mines at the required depth and hence being caught in shallow water by an anti-submarine patrol.

Above: A contemporary British drawing of the type of TMB magnetic mine used to mine the approaches to Swansea in late 1939.

Later developments in the mine war takes the story further.