When you look at the nature of the environment the enclosed sea space , the surrounding land on which to base aircraft , the HF/DF activity to fix and radio locate radio traffic , the entrances to the sea , the convoy routes , the limitations of the submarine technology at that exact moment in time and you can appreciate the essence of the U-Boat's need to take advantage of any opportunity to exploit an Achilles heel as it appeared.
From the diagrams the bottle necks are apparent , the clusters of sinkings , the submarine losses , the extreme range from base indicates the need for long range resupply at sea - the "Milk Cow" , or the pragmatic and occasionally desperate chain of events which led to one boat resupplying another when the supply system failed or when enemy action resulted in the loss of a vital supply unit.
Consider the war waged off Americas Eastern Seaboard in 1942 - and the disastrous interference of Hitler who in sending boats to the Mediterranean and to Norway and a little of the background conflict of interests , the over stretched resources of the navy and the demands of other areas come into play - the need to crush Malta , support an Italian surface fleet which could not match the British , counter Allied supply runs to Russia , all too much for a U-Boat arm which itself lacked the resources to bring about a decision in any one area yet was expected to deliver on all fronts.
( In a similar way German efforts in Russia were little more than an illusion which foundered at Stalingrad under the misdirection of a Hitler who increasingly failed to take advice and reasonable steps to avoid impending disasters).