When used as a nautical vessel designation, the letters SS stand for steam ship or screw steamer. It was originally used to distinguish a screw steamer from a paddle steamer, which was called a PS.
After paddle steamers started to decline in popularity, SS became an acronym for steam ship instead of just screw steamer. Screw steamers were developed after paddle steamers. Paddle steamers had performance issues if the ship was lowered due to added weight. Screw steamers were more effective because the screw-propeller they used performed consistently regardless of depth. As of 2014, ships are identified by an identification code on the side called a hull number.