The boundary between ironclads and later battleships

Central-battery vessels such as HMS Sultan show how older hulls could be reshaped with recognizable bridges, reduced masts, spotting positions, and new operational roles. HMS Achilles likewise illustrates why a well-built older ship could remain useful rather than becoming obsolete merely through age. Later steam battleships were no longer designed to depend on sail, yet they had not assembled the Royal Sovereign combination of Harvey armour, gun layout, hull form, and machinery. This distinction makes the technological boundary visible without pretending that every navy crossed it at the same moment.