Scapa Flow shows the operational value of hull numbers outside ocean convoys. A force of perhaps sixteen thousand-ton patrol ships could maintain far more continuous coverage around the anchorage. Active ASDIC patrols would complicate a submarine's approach and exploit its vulnerability rather than treating it as an invincible weapon. The counterfactual suggests that denser layered defense might have prevented the penetration that sank HMS Royal Oak.
Notable Point · 28:47-30:56
More patrol craft at Scapa Flow
A dense force of ASDIC-equipped patrol vessels could have made the submarine penetration of Scapa Flow far more difficult and perhaps protected HMS Royal Oak.