Screw steamer

A screw steamer or screw steamship (abbreviated "SS") 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used in contradistinction to the paddle steamer, a still earlier form of steamship that was largely, but not entirely, superseded by the screw steamer.^[1]

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.^[2]