You change your boat every 2 years in F18, not because it is worn out, but because it is a development class and new stuff comes along.

As the F16 boys will tell you, F18s are almost over built and the hulls will go on and on.

It sounds extravagant to expect to change your boat every 2 years, but it means that the boat is passed on to someone with lower expectations and the fleet grows. In fact the opposite of what you have found in n20 where the same guys keep refurbishing the same boats and there is no fleet expansion and it eventually falls on its butt!