In the navy, when prefixing a ship name, was used to mean the
ship is a steamer (Screw Steamer), in opposition with SV (Sailing Vesse)", or other
ship types.
(see
wikipedia's Ship_prefix list for more details)
In 1847, the revolutionary SS Great
Britain became the first iron-hulled screw-driven
ship to cross the Atlantic
The SS Savannah left the port of Savannah,
Georgia, on May 22, 1819