sentences:  The work had also diverted badly needed ships from fleet and convoy defence in order to provide escorts for the minelayers.There were no survivors to verify sinking of U-boats by the northern mine barrage; although it may have claimed several whose loss has not been otherwise explained, including in November 1940, in March 1942, in September 1942, in July 1943 and in September 1944.The Allies were unaware of these U-boat losses at the time, and attributed most of them to Allied ship and aircraft attacks after the Commander-in-Chief of the Home Fleet declared the Northern Barrage ""the least profitable voluntary major undertaking of the war"".