What familiar readiness lesson still gets overlooked?
Short answerTwo ships do not guarantee one available, because an unexpected casualty can coincide with the other's maintenance.
NotesA recurring mistake is to assume that two ships guarantee one operational ship because only one needs scheduled maintenance at a time. That calculation leaves out damage or an unexpected defect affecting the other vessel. The comparison with owning a car breaks down because a replacement warship generally cannot be hired at short notice. A fleet plan therefore needs to consider unscheduled losses of availability as well as its intended maintenance cycle.