Armour narrows the ranges at which shells can penetrate

Armour did not make a warship invulnerable. Its value was to create ranges at which an incoming shell was unlikely to retain enough energy to penetrate. Four and a half inches offered a stronger starting point against four-inch fire and against larger six- or eight-inch shells. Those larger guns could defeat two inches across a much wider range band. Thicker armour therefore reduced tactical exposure even when it could not stop every hit, a distinction that mattered especially for Spain's intended flagships.