What if Dönitz began the war with 300 U-boats?

How differently would the war have gone if Dönitz had the 300 seagoing U-boats he requested?

Germany would pose a larger early threat, but years of conspicuous construction would trigger a major British and imperial escort response and leave Germany with many short-ranged boats and fewer surface ships.

The estimate allows only about fifty of the three hundred boats to patrol at once, with the fleet rotating through six watches. A program begun early enough to reach that total by 1939 would likely mass-produce many shorter-ranged designs while creating continuing costs for crews, torpedoes, maintenance, and replacement boats. Such conspicuous construction would alert the Royal Navy, Foreign Office, and Treasury years before war. Britain could respond with destroyers, Black Swan-type sloops, corvettes, and possibly small escort carriers. German concentration on submarines would also reduce its balanced surface threat. The counterfactual therefore turns on industrial timing and enemy reaction, not on adding three hundred mature ocean-going boats to the historical war without consequences.