Ships get heavier as they age. The Chief of Naval Operatbns (CNO) has prescribed a requirement for "Service Life Allowance" of aircraft carriers of 7.5 percent weight margin (displacement.) The newest Nimitz carriers have approximately 1 percent margin remaining. They cannot incorporate design changes without identifying offsetting weight and stability changes. Additionally, the existing low stability margins can result in operational complexities. Increasing weight degraded speed; its distribution has also degraded stability and hence the ability to handle heavy seas and the sea conditions needed to launch and recover aircraft.

Sea-worthiness remains an important consideration, even for ships a large as aircraft carriers. The Nimitz class is generally limited to flight operations for deck excursions normal for sea-states below 5.

Investment cost 
     Ship acquisition cost                   $ 4,059
     Midlife modernization cost              $ 2,382
     Total investment cost                   $ 6,441 
Average annual investment cost               $   129
Operating and support cost
     Direct operating and support cost       $11,677
     Indirect operating and support cost     $ 3,205
     Total operating and support cost        $14,882
Average annual operating and support cost    $   298
Inactivation/disposal cost
     Inactivation/disposal cost              $   887
     Spent nuclear fuel storage cost         $    13
     Total inactivation/disposal cost        $   899
Average annual inactivation/disposal cost    $    18  

Total life-cycle cost                        $22,222
Average annual life-cycle cost               $   444  
[source: GAO]