A steam launch (or steamboat) is a steam-powered boat, usually one that travels on lakes or rivers. The word ‘launch’ refers to a motorised boat or barge. An ocean-going steam-powered boat is called a steamship and its name will usually be prefixed with S.S. A paddle-steamer is indicated by the prefix P.S. and those powered with a steam turbine with T.S. Steam-powered boats were invented in the 18th century and were used until the second half of the 20th century, when diesel-power took over.