When a warship needs a new hull rather than new engines

At what point is building a new hull better than replacing a ship's engines?

When hull wear limits performance or leaves too little life to repay the machinery investment, although political and fleet-gap pressures can alter the decision.

A hull that can no longer safely use full power has reached an obvious replacement point. Installing new machinery also makes little sense when the hull is expected to reach that condition within five to ten years, unless the ship will be needed for a war before then. The rule of thumb offered is that a naval ship receives its last major engine overhaul about eleven years before leaving service, followed by replacement parts. Warspite, Renown, Queen Elizabeth, and Valiant illustrate how extensive rebuilding could also depend on politics. Repair money counted as a running cost, while a new ship required approval for a capital expense. The available budget category could therefore favor a rebuild even when replacement deserved consideration.