twin-screw steamer
A twin-screw steamer (or steamship) (TSS) is a steam-powered vessel propelled by two screw propellers, one on either side of the plane of the keel.{{cite web|url=http://www.rina.org.uk/maritime_acronyms1.html|title=Maritime Acronyms and Abbreviations|website=Royal Institution of Naval Architects|id=TSS|url-status=dead|archive-url=https://web.archive.org/web/20160819043306/http://www.rina.org.uk/maritime_acronyms1.html|archive-date=19 August 2016}}
Arrangement
All propellers produce a transverse thrust, also called screwing effect or starting bias, which gives a tendency for end of ship to move sideways. In a twin-screw ships the port propeller is usually left-handed and the starboard right-handed, to cancel out the transverse thrust and avoid propeller walk.{{cite book|title=The Theory and Practice of Seamanship|section=The Principles of Ship Handling|pages=51–55|isbn=978-0415153720|date=31 October 1996|publisher=Routledge|last=Danton|first=Graham}}