The speed figures can be applied directly to a Gibraltar transit. A seven-knot U-boat facing a four-knot current might make only three knots of progress, while a stronger current or weak battery could force it to surface. Italian designers had studied the strait and sought boats able to sustain enough submerged speed for the passage. One account has HMS Nelson detecting a surfaced submarine near Gibraltar and using radar-directed 16-inch gunfire before aircraft could arrive. The episode shows how a geographic choke point could turn an older submarine's operating limits into opportunities for very different anti-submarine weapons.
Notable Point · 2:00:47-2:02:13
Gibraltar could force a U-boat back to the surface
Current and battery limits could expose a submerged U-boat in the Strait of Gibraltar, where even HMS Nelson's heavy guns might join the anti-submarine fight.