A ship's speed depends on horsepower, hull proportions and whether the structure can withstand the loads. Belfast provides a physical example through her forward hull construction, turbines and boilers. Machinery built beyond its ordinary operating requirement can last longer when used conservatively in peace. The same reserve can be used to demand more speed in war. The proposed endurance of that extra output is measured in hours, rather than only a few minutes.
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Hull strength and machinery reserve set practical cruiser speed limits
Extra boiler output helps only when the hull and propulsion machinery can sustain the resulting speed.