A ship that is notionally available differs from a capability sustained on task. Three hulls normally divide between deployment, work-up or return, and maintenance. Four are needed in this model to guarantee one continuously at sea, as illustrated by British and French ballistic-missile-submarine forces. Two ships may provide one dockside hull theoretically able to sail within about twenty-four hours, depending on its crew and supplies and assuming no accident or defect. The shorthand is that one hull offers an occasional asset, two uncertain availability, three a capability, and four a guaranteed capability through the operating cycle.
Chapter · 6:10-8:46
From ship availability to a dependable capability
The rule of three and the four-boat deterrent cycle show why buying two ships does not guarantee that one will be at sea.