You are very right . The proposed strength was based on the rule of three. Like with kit , one on, one spare, and one in the laundry. we recommended 3 crews for each ship with spares been held in sea-going replacements. The outcome was 1 1/2 crews which derails when you run PNCO courses and train recruits, allow for special leave, sickness , overseas attachments etc., etc . Specialists who are singletons on board like Captains, Coxswains, CERA, RRM , SBA,SPO always face early relief duties.