Why two ships do not sustain a capability

What naval procurement decision can you not defend?

Reducing a capability to two units; three is the minimum, while four better guarantees one is always available.

A two-ship class may provide only one usable unit under ideal conditions and leaves little resilience for maintenance, defects, or strategic surprise. It therefore assumes a navy can choose when a crisis becomes a war. Three ships are a bare minimum for sustaining a capability, while four provide a more credible readiness cycle. The four-boat ballistic-missile-submarine force shows what it takes to guarantee one operational platform continuously at sea.