Even submarines that can technically reach France through the Channel pay for the route in battery endurance, exposure, and operational time. The Type IIB figures are 3,100 nautical miles at eight knots on the surface and only 40 nautical miles at four knots submerged. A Type IXC extends surface range to 13,450 nautical miles at ten knots, yet its submerged endurance is still only 63 nautical miles at four knots. Currents, repeated crash dives, and patrol craft equipped with ASDIC turn the transit into a serious drain before a boat reaches its hunting area. Each dangerous outward and return passage reduces time on station and makes losses more costly to a still-small U-boat arm and its trained crews.
Chapter · 56:36-1:01:41
U-boat endurance makes the North Sea a strategic choke point
Comparing surfaced and submerged ranges shows why forcing U-boats through a heavily patrolled North Sea could sharply reduce their time on station and increase their losses.