Fleet requirements must allow for ships unavailable through maintenance, damage or machinery wear. The illustrative ratios are one deployable ship from three in peacetime and one from five in wartime. Running engines at high speed adds to that wear. The Second World War example concerns Town- and Crown Colony-class cruisers keeping pace with an Atlantic liner carrying important cargo. In the example, machinery designed for cruising and short bursts at maximum speed needed repairs after sustaining the liner's pace across the Atlantic.
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Wartime availability and the cost of sustained speed
Fast liner escorts illustrate how sustained high speed can wear machinery and reduce the number of ships available for service.