During World War Two, Germany became the only country in history to operate submarine tankers, a circumstance forced by the need to refuel U-boats in distant operational areas reached only by passing through an Allied-controlled ocean. Conventional surface tankers had been too easy to locate by Allied aircraft and naval patrols, but the submarine tankers were undetectable once submerged and sailing towards their secret refuelling rendezvous.
The submarine tankers, known affectionately as 'milk cows', gave the attack U-boats the ability to double or sometimes even treble active operational time in their patrol area. Thus both Germans and Allies regarded the cows as the most important units of the U-boat fleet, and Allied forces had explicit orders to attack the tankers first, whenever a choice existed between U-boat targets.
The rendezvous of a tanker with other U-boats was arranged by coded W/T messages. But in 1941, and then from 1943 onwards, the German ciphers were secretly broken by the Allies. In their early days of operation (1942) the German milk cows were able to carry out their intended tasks under conditions of almost peace-time normality, refuelling up to two-dozen U -boats in quick succession before returning straight to base for more fuel. Later, conditions degenerated to the extent that by 1944 the surviving cows carried out essential, individual refuelling while guarded by special 'flak' U-boats. During this period, members of the tanker crews were dying of heart attacks and other stress-related illnesses.
The increasing impact of the Allied decryption of German U-boat ciphers is stressed repeatedly throughout the book, creating an unwinding tale of courage, suspense and horror. Tanker crews had to lie stopped in mid-ocean with hatches open, fuel hoses connected to the suckling U-boat, guns manned and always fearfully awaiting a sudden air attack from which they could not escape by diving. The story is told against the background of changing U-boat fortunes in the North Atlantic. By the end of the war, virtually every tanker had been sunk.
'The Milk Cows' is a substantially revised and expanded version of the author's 1998 book 'U-boat Tankers', drawing on reader feedback and fresh research, especially into Allied decryptions of German ciphers. German-speaking readers should follow this link: U-Boot Tanker 1941-45.
Last updated 20 June 2009.