 Admiral Karl Dönitz, commander of the U-boat fleet, had planned a maximum submarine effort for the first month of the war, with almost all the available U-boats out on patrol in September.That level of deployment could not be sustained; the boats needed to return to harbour to refuel, re-arm, re-stock supplies, and refit.The harsh winter of 1939–40, which froze over many of the Baltic ports, seriously hampered the German offensive by trapping several new U-boats in the ice.