it could stand for either steam ship or either sail ship.
This is what those terms stand for. SS stands for Steam Ship. USS stand for United State Ship. And HMS stands for His or Her Majesty's Ship.
twin screw motor ship
Training Ship
USS - United States Ship
Her Majesty's Ship or His Majesty's Ship (depending on the monarch at the time)
If your talking about ship names then it would be either HIS Majesty's Ship or HER Majesty's Ship
motor tanker
landing ship tank
Her Majesty
her majestys ship
Motor Ship (nautical vessel designation; also see SS for steam ship)