but the weigh of these increased stress on the wooden beams . So iron was used to reinforce a ship is internal frame . By the 19th century. the fourth phase of ship design occurred. Iron ( and later steel ) replace wood as the preferred material for construction ships . To prevent sinking , especially during maritime battles ,่ship designers built watertight bulkhead compartments . These were separate boxlike sections made of iron. If a ship had a hole in it, water would only fill up the one bulkhead