Over the period 14 May 1942 to 1 April 1944 German U-Boats planted a total of 338 mines in the approaches to Western Atlantic ports running from St. Johns, Newfoundland to Panama. Of these, 130 were planted in the approaches to U.S. East Coast ports inflicting 9 ship casualties and closing ports for a combined total of 30 days (New York – 2 days; Chesapeake Bay – 3 days; Jacksonville, Charleston and Savannah – 3 days; Wilmington and Charleston – 8 days).
In their attempt to stem the flow of critical materials to England and then Europe, the Germans elected to concentrate on torpedo attacks against East Coast shipping and the Atlantic convoys. And yet, with only 11 submarine sorties and the expenditure of 120 mines they inflicted 9 ship casualties and interrupted the vital flow of war materials for a total of 30 days. What would have been the impact on the war had they concentrated their growing U-Boat fleet on mining East Coast and Gulf Coast ports, and their submarines and Luftwaffe on mining the approaches to English harbors rather than bombing English cities?
Source: Project Nimrod, Chapter II, A History of Mine Warfare, pages 43-44, Mine Advisory Committee, National Academy of Sciences-National Research Council, 1969.