A screw steamer or screw steamship is an old term for a steamship or steamboat powered by a steam engine, using one or more propellers (also known as screws) to propel it through the water. Such a ship was also known as an "iron screw steam ship".
In the 19th century, this designation was normally used in contradistinction to the paddle steamer, a still earlier form of steamship that was largely, but not entirely, superseded by the screw steamer.
Many famous ships were screw steamers, including the RMS Titanic and RMS Lusitania. These massive leviathans had three or four propellers. Ships under two hundred meters in length usually only had two or one propellers.
In 1844, Thomas Clyde partnered with Ericsson to apply his screw-propeller to steam vessels. After several experimental versions, Clyde launched the twin-screw propeller steamer John S. McKim making it the first screw steamer built in the United States for commercial use.