Naval warfare was transformed by the development of reliable steam engines and the technology to build ships of iron and steel. Ships no longer depended on the wind and could manoeuvre freely; cannon could fire explosive shells instead of cannon balls, and no sailing ship could stand against them. The 'ironclad' revolution continued throughout the 19th century, a period of technological progress almost as rapid as today's IT revolution. Ship designs were rendered obsolete every few years. New weapons suggested new tactics and one old one - the ram - was revived. A fascinating era with all manner of lessons for today as navies again wrestle with continuous changes in technology.