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