Exhelo: Interesting figures but here is what Clay Blair – a WWII American submariner himself and author of the definitive works on both the German U-boat and American Pacific submarine campaigns (they are each two volumes, and each volume is about 900 pages long) says.

Referring to the period after September 1942 – “Contrary to the accepted wisdom or mythology:
U-boats never even came close at any time to cutting the vital North Atlantic lifeline to the British Isles.”

As for the period between the start of the war and the entry of the U.S., he points out that, despite the numerous sinkings, the British actually were able to increase their tonnage of cargo ships and tankers afloat. During this time the British sailed over 900 convoys and only 19 of them suffered losses of 6 or more ships due to U-boats. The convoys over this time period consisted of a total of 12,057 ships and 98% of them arrived in the British Isles safely. And of course, this is before Kaiser began turning out Liberty ships like popcorn.

And he further states “At the close of 1941 German U-boats were nowhere close to isolating and strangling Great Britain.”