Wooden ships patrolled the oceans on expeditions up until the late 18th century when ship builders began to experiment with iron. The British, Portuguese, Dutch and Spanish navies introduced cannons on their ships. But the weight of these increased stress on the wooden beams. So iron was used to reinforce a ship's internal frame. By the 19th century, the fourth phase of ship design occurred. Iron (and later steel) replaced wood as the preferred material for constructing 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.