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For other uses, see Ship (disambiguation).
A ship is a large watercraft capable of offshore navigation. Ships may be operated by governments (military, rescue, research, transportation), private companies and institutions (transportation, offshore resources, research), and individuals (large yachts, research).
Italian Full rigged ship Amerigo Vespucci in New York Harbor, 1976
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A ship usually has enough size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb used is "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 (even though modern submarines could probably fit small boats aboard, tradition dictates they are always referred to as "boat"). Other types of large vessels which are traditionally called 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. Nautical means related to sailors, particularly customs and practices at sea. Naval is the adjective pertaining to ships, though in common usage it has come to be more particularly associated with the noun "navy."
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 and are used when describing merchant ships for the purpose of tolls, taxation, etc.
In Britain until Samuel Plimsoll\'s Merchant Shipping Act, 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 jail.
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 Line", exists on ships\' sides, and consists of a circle with a horizontal line through the centre. On the Great Lakes of North America 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. This is called the "freeboard mark" or "load line mark"in the marine industry.
Ships of the world in 1460, according to the Fra Mauro map.
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.
The development of piston-engined steamships was a complex process. Early steamships were fueled by wood, later ones by coal or fuel oil. Early ships 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 propulsion progressed considerably over the rest of the 19th century. Notable developments included the steam surface condenser, which eliminated the use of sea water (salt water) in the ship\'s boilers. This permits higher steam pressures, and thus the use of higher efficiency multiple expansion (compound) engines. As the means of transmitting the engine\'s power, paddle wheels gave way to more efficient screw propellers.
Steam turbines were fueled by coal or later, fuel oil, 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.
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. The financial returns on LNG are potentially greater than the cost of the marine grade fuel oil burnt in conventional diesel engines, so the re-liquefaction process is starting to be used on diesel engine propelled LNG carriers. Another factor driving the change from turbines to diesel engines for LNG carriers is the shortage of steam turbine qualified sea going engineers. With the lack of turbine powered ships in other shipping sectors, and the rapid rise in size of the worldwide LNG fleet, not enough have been trained to meet the demand. It may be that the days are numbered for the last stronghold for steam turbine propulsion systems, despite all but sixteen of the orders for new carriers at the end of 2004 being for steam turbine propelled ships. [1]
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. A few cruisers have also employed nuclear power; as of 2006, the only ones remaining in service are the Russian Kirov class. An example of a non-military ship with nuclear marine propulsion is the Arktika class icebreaker with 75,000 shaft horsepower. Commercial experiments such as the NS Savannah proved uneconomical compared with conventional propulsion.
About 99% of modern ships use diesel reciprocating engines[citation needed]. 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
Most modern larger merchant ships use either slow speed, two stroke, crosshead engines, or medium speed, four stroke, trunk engines. Some smaller vessels may use high speed diesel engines.
The size of the different types of engines is an important factor in selecting what will be installed in a new ship. Slow speed two-stroke engines are much taller, but the area needed, length and width, is smaller than that needed for four-stroke medium speed diesel engines. As space higher up in passenger ships and ferries is at a premium, these ships tend to use multiple medium speed engines resulting in a longer, lower engine room than that needed for two-stroke diesel engines. Multiple engine installations also give more redundancy in the event of mechanical failure of one or more engines and greater efficiency over a wider range of operating conditions.
As modern ships\' propellers are at their most efficient at the operating speed of most slow speed diesel engines, ships with these engines do not generally need gearboxes. Usually such propulsion systems consist of either one or two propeller shafts each with its own direct drive engine. Ships propelled by medium or high speed diesel engines may have one or two (sometimes more) propellers, commonly with one or more engines driving each propeller shaft through a gearbox. Where more than one engine is geared to a single shaft, each engine will most likely drive through a clutch, allowing engines not being used to be disconnected from the gearbox while others keep running. This arrangement lets maintenance be carried out while under way, even far from port.
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 at low power (cruising) output, 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 used the steam turbines to improve the efficiency of their gas turbines in a combined cycle, where wasted heat from a gas turbine exhaust is utilized to boil water and 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 these gas turbines is far more costly than that needed for the diesel engines, so the running costs are still higher.
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 a fleet of ___ sail (e.g., "a fleet of 40 sail"). Groups of submarines (particularly German U-boats in the 1940s) formerly hunted in wolf packs.
Semi-submersible MV Blue Marlin carrying the destroyer USS Cole
Research vessel RV Maria S. Merian
A two-masted schooner
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