Some things I learned today.
I was aware of the Rule of Three. One ship in refit, one ship returning/working up and one ship available (not necessarily sailing).
But there are other "rules".
Rule of Four: If you have four of the same class one will be at sea at all times. So not just available but sailing.
Rule of Five: If you have five of the same class two will be available at all times and one of those will be at sea.
This increase in availability is caused by more overlapping availability schedules and maintenance routines. And it explains why the RCN is looking at fleet sizes of 15 (RCD) and 12 (CPSP). 15 means that there will be 6 ships available, and one task group (min 3 RCD) at sea, at all times. 12 subs means that there will always be three submarines at sea.
I did see one commentator directly point to the RCN as a good example for the RN regarding class size as they estimated that with the full 15 RCD based on historical averages there will be up to 8 ships available on short notice for taskings. His point was that the RN should build 8, 12 or 15 as class sizes because destroyer and frigate availability with their current sizes will be challenged.
Interesting stuff.