Admiralty M-N Scheme

'The Admiralty M-N Scheme' (sometimes given as "Project M-N") was a World War I British plan to close the Strait of Dover in the English Channel to German U-Boats, by means of a chain of either eight or twelve massive towers linked by anti-submarine booms and nets. Only two towers had been constructed before the Armistice with Germany caused the cancellation of the project.

On 31 January 1917, it was announced to the German Reichstag that unrestricted submarine warfare would resume the next day, 1 February. The renewed U-boat campaign was initially a great success; nearly 500,000 tons of shipping being sunk in both February and March, and 860,000 tons in April, when Britain's reserve of wheat fell to 6 weeks' supply.

The British Admiralty redoubled its efforts to find effective remedy. Attention turned to the Strait of Dover, which was used by U-boats based in Bruges in occupied Belgium, to gain fast access to the Atlantic trade routes and the busy Western Approaches. If it could be successfully closed to them, they would have to take the much longer "Northern Route" around the north of Scotland, and therefore be able to spend less time on patrol. Existing countermeasures were the Dover Patrol of cruisers and destroyers, and the Dover Barrage, a line of steel anti-submarine nets supported by naval minefields stretching across the Strait, patrolled by destroyers, armed trawlers and drifters equipped with powerful searchlights, work on which had started in 1915. Although this had been improved by use of the new Mark H2 mine from mid-1917, it was not initially very successful as there was no agreement on the depth at which the mines should be tethered. Until December 1917, only two U-boats had been destroyed by the barrage. Furthermore, the patrolling vessels were vulnerable to attack by German torpedo boats, as in the action of 26–27 October 1916, and the action of 20 April 1917.

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