Modern warships could use armour around vital compartments

What forgotten naval-construction equipment would be most useful to recover?

Heavy armour-rolling equipment, because it could protect modern machinery and power compartments even without a full armoured belt.

The desired lost industrial capability is heavy armour manufacture, including the ability to roll plates fourteen inches thick or more. The proposal does not call for armouring an entire modern hull against every missile. Instead, selected internal compartments around engines, gas turbines, and electrical generation could receive substantial protection. Local citadels would limit the chance that fragments or blast disable the systems a ship needs to move, fight, and control damage. This makes armour an element of resilient internal layout rather than a return to the classic battleship scheme.