A Ship is a large watercraft capable of offshore navigation. It is usually designed for government (including military), research, or commercial use. A ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it does not always apply) goes: "a boat can fit on a ship, but a ship can't fit on a boat". Consequently submarines are referred to as "boats", because early submarines were small enough to be carried aboard a ship in transit to distant waters. Other types of large vessels which are traditionally called a boats are the Great Lakes freighter, the riverboat, and the ferryboat. Though large enough to carry their own boats and/or heavy cargoes, these examples are designed for operation on inland or protected coastal waters. Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship. During the age of sail, ship signified a ship-rigged vessel, that is, one with three or more masts, usually three, all square-rigged. Such a vessel would normally have one fore and aft sail on her aftermost mast which was usually the mizzen. Almost invariably she would also have a bowsprit but this was not part of the definition........

Measuring ships Information
One can measure ships in terms of overall length, length of the waterline, beam (breadth), depth (distance between the crown of the weather deck and the top of the keelson), draft (distance between the highest waterline and the bottom of the ship) and tonnage. A number of different tonnage definitions exist. In Britain until the Samuel Plimsoll Merchant Shipping Act of 1876, ship-owners could load their vessels until their decks were almost awash, resulting in a dangerously unstable condition. Additionally, anyone who signed onto such a ship for a voyage and, upon realizing the danger, chose to leave the ship, could end up in gaol. Samuel Plimsoll, a member of Parliament, realised the problem and engaged some engineers to derive a fairly simple formula to determine the position of a line on the side of any specific ship's hull which, when it reached the surface of the water during loading of cargo, meant the ship had reached its maximum safe loading level. To this day, that mark, called the "Plimsoll Mark", exists on ships' sides, and consists of a circle with a horizontal line through the centre. On the Great Lakes the circle is replaced with a diamond. Because different types of water, (summer, fresh, tropical fresh, winter north Atlantic) have different densities, subsequent regulations required painting a group of lines forward of the Plimsoll mark to indicate the safe depth (or freeboard above the surface) to which a specific ship could load in water of various densities. Hence the "ladder" of lines seen forward of the Plimsoll mark to this day. Or what we call for "freeboard mark" or "load line mark"in the marine industry.

Pre-mechanization Information
Until the application of the steam engine to ships in the early 19th century, oars propelled galleys or the wind propelled sailing ships. Before mechanisation, merchant ships always used sail, but as long as naval warfare depended on ships closing to ram or to fight hand-to-hand, galleys dominated in marine conflicts because of their maneuverability and speed. The Greek navies that fought in the Peloponnesian War used triremes, as did the Romans contesting the Battle of Actium. The use of large numbers of cannon from the 16th century meant that maneuverability took second place to broadside weight; this led to the dominance of the sail-powered warship.

Reciprocating steam engines Information
The development of piston-engined steamships was a complex process. Early ones were fueled by wood, later ones by coal or diesel. Early ones used stern or side paddle wheels, later ones used screw propellers. The first commercial success accrued to Robert Fulton's North River Steamboat (often called Clermont) in the US in 1807, followed in Europe by the 45-foot Comet of 1812.

Steam turbines Information
Steam turbines were fueled by coal or later, diesel or nuclear power. The marine steam turbine developed by Sir Charles Algernon Parsons, raised the power to weight ratio. He achieved publicity by demonstrating it unofficially in the 100-foot Turbinia at the Spithead naval review in 1897. This facilitated a generation of high-speed liners in the first half of the 20th century and rendered the reciprocating steam engine obsolete, first in warships, and later in merchant vessels. In the early 20th century, heavy fuel oil came into more general use and began to replace coal as the fuel of choice in steamships. Its great advantages were convenience, reduced manning due to removing the need for trimmers and stokers, and reduced space needed for fuel bunkers. In the second half of the 20th century, rising fuel costs almost led to the demise of the steam turbine. Most new ships since around 1960 have been built with diesel engines. The last major passenger ship built with steam turbines was the Fairsky, launched in 1984. Similarly, many steam ships were re-engined to improve fuel efficiency. One high profile example was the 1968 built Queen Elizabeth 2 which had her steam turbines replaced with a diesel-electric propulsion plant in 1986. Most new-build ships with steam turbines are specialist vessels such as nuclear-powered vessels, and certain merchant vessels (notably Liquefied Natural Gas (LNG) and coal carriers) where the cargo can be used as bunker fuel.

LNG carriers Information
New LNG carriers (a high growth area of shipping) continue to be built with steam turbines. The natural gas is stored in a liquid state in cryogenic vessels aboard these ships, and a small amount of 'boil off' gas is needed to maintain the pressure and temperature inside the vessels, to within operating limits. The 'boil off' gas provides the fuel for the ship's boilers, which provide steam for the turbines, the simplest way to deal with the gas. Technology to operate internal combustion engines (modified marine two stroke diesel engines) on this gas has improved however, so such engines are starting to appear in LNG carriers; with their greater thermal efficiency, less gas is burnt. Also, developments have been made in the process of re-liquefying 'boil off' gas, letting it be returned to the cryogenic tanks.

Nuclear-powered steam turbines Information
In these vessels, the reactor heats steam to drive the turbines. Partly due to concerns about safety and waste disposal, nuclear propulsion has become usual only in specialist vessels. In large aircraft carriers, the space formerly used for ship's bunkerage could be used instead to bunker aviation fuel. In submarines, the ability to run submerged at high speed and in relative quiet for long periods holds obvious advantage.

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Reciprocating diesel engines Information
The rotating crankshaft can power the propeller directly (with slow speed engines), via a gearbox (with medium and high speed engines) or via an alternator and electric motor (in diesel-electric vessels). The reciprocating marine diesel engine first came into use in 1903 when the diesel electric rivertanker Vandal was put in service by Branobel. Diesel engines soon offered greater efficiency than the steam turbine, but for many years had an inferior power-to-space ratio. Diesel engines today are broadly classified according to Their operating cycle: two-stroke or four-stroke. Their construction: Crosshead, trunk, or opposed piston. Their speed. Slow speed: any engine with a maximum operating speed up to 300 revs/minute, although most large 2-stroke slow speed diesel engines operate below 120 revs/minute. Some very long stroke engines have a maximum speed of around 80 revs/minute. The largest, most powerful engines in the world are slow speed, two stroke, crosshead diesels. Medium speed: any engine with a maximum operating speed in the range 300-900 revs/minute. Many modern 4-stroke medium speed diesel engines have a maximum operating speed of around 500 rpm. High speed: any engine with a maximum operating speed above 900 revs/minute.

Gas turbines Information
Many warships built since the 1960s have used gas turbines for propulsion, as have a few passenger ships, like the jetfoil. Gas turbines are commonly used in combination with other types of engine. Most recently, the Queen Mary 2 has had gas turbines installed in addition to diesel engines. Due to their poor thermal efficiency, it is common for ships using them to have diesel engines for cruising, with gas turbines reserved for when higher speeds are needed. Some warships and a few modern cruise ships have also use steam turbines to improve the efficiency of gas turbines in a combined cycle, where waste heat from a gas turbine is used to create steam for driving a steam turbine. In such combined cycles, thermal efficiency can be the same or slightly greater than that of diesel engines alone. However, the grade of fuel needed for gas turbines is far more costly than that needed for diesel engines, so running costs are higher.

Group terminology Information
Ships may occur collectively as fleets, squadrons, flotillas, or Convoys. A collection of ships for military purposes may compose a navy, task force, or an armada. In the past, people counting or grouping disparate types of ship may refer to the individual vessels as bottoms, but this generally refers only to merchant vessels. Groups of sailing ships could constitute, say, a fleet of 40 sail. Groups of submarines (particularly German U-boats in the 1940s) hunt in wolf packs.

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