[WernherVonTrapp]

In the beginning, the Germans didn't need to put more U-Boats to sea. They were sinking record numbers of vessels right off the American coast during 1942:

"America's first year in the war ended with the loss of 1,027 Allied ships to U-boat action. This was more than half of all the ships lost by all the Allies in the U-boat war, in all areas, all through the war years from 1939 to 1945."
-The Tenth Fleet pg.60

"The Strategic Situation in 1942:
This was a phenomenal and unprecedented episode in the whole history of warfare-a major and potentially decisive victory being scored by a tiny force of submarines...
...Doenitz's U-boats wrought havoc, not merely with the material strength of the Allies during this crucial period of their build-up, but indeed with their whole planning and the grand strategy of the war."
-The Tenth Fleet pg.61-62

"December, 1941, brought the crisis to a head in the U-boat arm. It came abruptly and dramatically in the wake of the first indubitable defeat of the U-boats: in a convoy battle west of Gibralter three British ships were sunk, but five U-boats were lost. Five more U-boats were sunk in other operations in waters around the Azores. Only twice before had Doenitz lost five boats in a single month and never ten in a thirty-day period. For the first time, defeatism swept the U-boat Command. Doenitz's staff openly voiced the opinion, and in no uncertain terms, that the U-boats had had it and were no longer capable of combatting the reinforced convoys."
-The Tenth Fleet pg.63