Germany entered the war with 21 available U-boats against Karl Dönitz's desired force of 300. His calculation assumed that only one third could be on patrol, with another third in transit and the rest refitting or working up. The 1939 force was therefore far short of the 100 boats he wanted at sea for the campaign. Allied radar, Enigma intelligence, ASDIC, depth charges, and ahead-throwing weapons further reduced what each boat could achieve. Early depth-charge attacks often required one escort to hold ASDIC contact while another crossed the target, which explains the value of later weapons that could fire ahead.
Chapter · 1:27-3:26
Dönitz's 300-submarine force calculation
Why Dönitz wanted 300 submarines, how the one-third availability rule worked, and why 21 boats could not execute his campaign plan.