The one-in-three rule changes fleet arithmetic

Salt water, machinery failures, upgrades, battle damage, training, transit, and planned refits all reduce the number of ships available at one moment. A one-in-three wartime rule of thumb shows why a fleet cannot safely surge every hull indefinitely without creating a later readiness collapse. The Type 45 example explains how delaying maintenance can produce short-term deployments followed by years with too few ships available. Repeated crisis deployments can consume tomorrow's availability when governments buy too few hulls for the standing demand.