Because you need to be able to have one boat in repair/refurbishment, one en route to deployment or returning from, and another being replenished and preparing

Because you need to be able to have one boat in repair/refurbishment, one en route to deployment or returning from, and another being replenished and preparing for its next deployment. Four is a minimum to get by and as you increase the number of boats you still only get an on-station percentage less than 50%.