Researching material for a history-inspired novel I am writing about the Battle of the Atlantic, I came across the fascinating story of the Type XIV Milchkuh (milk cow) submarine tanker.
In early 1942, German U-Boats hunting Allied ships along the U.S. East Coast, Gulf of Mexico and Caribbean had to sail all the way back to port in occupied France to be re-supplied with fuel, armament, food and other necessary supplies. However, with deployment of the Milchkühe in April 1942, U-Boats could be resupplied in Mid-Atlantic.
But the Milchkuh success was short-lived. By early 1943, with increasing awareness of their presence and advances in radar technology, the Allies began systematically sinking the Milchkühe. All ten of the Type XIV submarine tankers put into operation were sunk by 1944.
Here is a nice article about the Milchkühe in the excellent site boat.net: