Why more U-boats were not a simple substitute for battleships

The proposal that Germany should have built more U-boats instead of Bismarck and Tirpitz looks simple on paper. A notional exchange of one battleship for thirty submarines also means generating thirty commanding officers, executive officers, chiefs, and fully trained crews. Britain could build and crew Flower-class corvettes more easily, and a sunk surface escort offered a better chance of recovering trained personnel than a lost U-boat. More submarines would remain dangerous, but they would concentrate the German threat into an anti-submarine problem for which convoys already carried capable escorts. Force structure must therefore be measured in people and enemy adaptation as well as construction tonnage.