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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]
A screw steamer or screw steamship (abbreviated "SS") is an old term for a steamship or steamboat powered by a steam engine, using one or more propellers (also known as screws) to propel it through the water. Such a ship was also known as an "iron screw steam ship".
Steam-powered ships were named with a prefix designating their propeller configuration i.e. single, twin, triple-screw. Single-screw Steamship SS, Twin-Screw Steamship TSS, Triple-Screw Steamship TrSS. Steam turbine-driven ships had the prefix TS. In the UK the prefix RMS for Royal Mail Steamship overruled the screw configuration prefix. [11]
A screw turning through a solid will have zero "slip"; but as a propeller screw operates in a fluid (either air or water), there will be some losses. The most efficient propellers are large-diameter, slow-turning screws, such as on large ships; the least efficient are small-diameter and fast-turning (such as on an outboard motor).
Mechanical (obsolete) – This type is like the radial davit, but both arms are moved out at the same time using a screw system; uses manila rope falls. An example is the Welin Quadrant davit type used on RMS Titanic. [6] Gravity (industry standard) – There are multiple forms; one man can operate; uses wire falls. [6]
The picket boat of the Royal Navy, (also sometimes called a P1000) was a twin screw boat in use at Britannia Royal Naval College in Dartmouth primarily to train officer cadets in boat handling and seamanship.
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TSS Earnslaw is a 1912 Edwardian twin screw steamer based at Lake Wakatipu in New Zealand. She is one of the oldest tourist attractions in Central Otago , and the only remaining commercial passenger-carrying coal-fired steamship in the southern hemisphere.