Atwin-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.[1]
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.[2]
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