U-boat force combat strength
What it all came down to: Numbers
The U-boat wars of 1914-1918 and 1939-1945 were very simple in terms of strategic goals: sink more shipping than the Allies could replace and thus force armistice or surrender through starvation.
Chief of U-boats Karl Dönitz estimated he needed 300 U-boats to defeat the Allied convoys and force Britain into submission. When war broke out on 3 Sept 1939 he had 26 ocean going U-boats. The 300 U-boat figure was calculated by him to mean that from this figure 100 boats would be in repairs, rest and working-up, 100 more on their way to and from operational areas - leaving on average 100 U-boats in actual combat areas. This meant about 200 U-boats would have been at sea at any one time, but with the French and Norwegian bases resulting in shorter transit times the ideal figure would be somewhat lower.
100 boats at sea for the first time
On 8 Aug 1942, nearly 3 years after the war began, the number of U-boats at sea reached 100 for the first time (some of these boats were inbound or outbound from bases). Dönitz finally had much of the striking power he had been insisting on.
For the next 11 months the number rarely dipped below 100, reaching as high as 159 on 29 April 1943. That peak was just in time for the horrific losses during May 1943 when 41 U-boats were lost to steadily improved Allied anti-submarine warfare (ASW).
With over 100 boats at sea the boats did inflict massive losses on Allied shipping but even more losses were avoided with skillful use of Enigma code breaking during much of the war. During most of 1942 the Allies were unable to penetrate the Enigma codes and the prior break in should have been obvious to the Germans as they all of a sudden would find convoys in greater numbers. This was largely offset by the German operations into US waters and the Caribbean, removing boats from the North Atlantic convoy lines.
Allied tonnage sunk by U-boats
In early June 1944 over 100 U-boats were at sea at the same time for 10 days, then the number again dipped below 90, never again reaching former strength.