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

Main parts of ship. 1: Chimney; 2: Stern; 3: Propeller; 4: Portside; 5: Anchor; 6: Bulbous bow; 7: Bow; 8: Deck; 9: Superstructure.

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."

Measuring ships                                                 

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 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 jail.

Plimsoll mark 

Samuel Plimsoll, a member of Parliament, realized 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 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 mechanization, 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 Peloponnese 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

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

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.

Steam turbine

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

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.

Nuclear-powered steam turbines

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.

Reciprocating diesel engines

About 99% of modern ships use diesel reciprocating engines. 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.

Reciprocating diesel engines

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.

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.

Gas turbines

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.

Gas turbine

Group terminology

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.

Some types of ships and boats
Semi-submersible MV Blue Marlin carrying the destroyer USS Cole

Semi-submersible Zhen Hua 1 in Astoria, Oregon

F221 Hessen, a Sachsen-class frigate of the German Navy

Some historical types of ships and boats
A two-masted schooner
A sailing vessel with three or more masts, fore-and-aft rigged on only the aftermost.
A sailing vessel with three or more masts, square-rigged only on the foremast.
A lightly-armoured battleship.
A large, heavily-armoured and heavily-gunned warship. A term which generally post-dates sailing warships.
An ancient vessel, propelled by two banks of oars.
A cargo vessel used for trade between Eastern India and Indochina.
Blockade runner
A ship whose current business is to slip past a blockade.
A two-masted, square-rigged vessel.
A two-masted vessel, square-rigged on the foremast and fore-and-aft rigged on the main.
A much smaller, two, sometimes three-masted ship.
A fast multiple-masted sailing ship, generally used by merchants because of their speed capablities.
A vessel designed for the coal trade.
An early twentieth century type of battleship.
Precursors to galleys.
East Indiaman
An armed merchantman belonging to one of the East India companies (Dutch, British etc.)
Fire ship
A vessel of any sort, set on fire and sent into an anchorage with the aim of causing consternation and destruction. The idea is generally that of forcing an enemy fleet to put to sea in a confused, therefore vulnerable state.
A Dutch-made vessel from the Golden Age of Sail. It had multiple decks and usually three square-rigged masts. It was usually used for merchant purposes.
A sailing and rowing warship, equally well suited to sailing and rowing.
A sixteenth century sailing warship.
A warship propelled by oars with a sail for use in a favourable wind.
A wooden warship with external iron plating.
A type of Viking trade ship
Liberty ship
An American merchant ship of the late Second World War period, designed for rapid building in large numbers. (The earliest class of welded ships.)
A Viking raiding ship
A sailing warship.
A small, very heavily gunned warship with shallow draft. Designed for coastal operations.
Paddle steamer
A steam-propelled, paddle-driven vessel, a name commonly applied to nineteenth century excursion steamers.
A Dutch ironclad. By the end of the nineteenth century, the name was applied to a heavy gunboat designed for colonial service.
An ancient warship propelled by 50 oars, 25 on each side.
A small dinghy, originally of a clinker construction and called in English, as in Danish, a praam. The Danish orthography has changed so that it would now be a prĂ¥m in its original language. It has a transom at both ends, the forward one usually small and steeply raked in the traditional design.
A type of battleship of the late 19th century to early 20th century, characterized by having a mixed offensive battery, in contrast to the "all-big-gun" Dreadnought type battleships.
A commerce raider camouflaged as a merchant vessel.
An ancient warship propelled by three banks of oars. On the upper row three rowers hold one oar, on the middle row - two rowers, and on the lower row - one man to an oar.
A fore and aft-rigged vessel with two or more masts of which the foremast is shorter than the main.
A large, heavily built, sixteenth century boat. Fore and aft rigged. More recently it has been a poetically frail open boat.
Slave ship
A cargo boat specially converted to transport slaves.
Small Waterplane Area Twin Hull (SWATH)
A modern ship design used for Research Vessels and other purposes needing a steady ship in rough seas.
A ship propelled by a steam engine.
Ship of the line
A sailing warship of first, second or third rate. That is, with 64 or more guns. Before the late eighteenth century, fourth rates (50-60 guns) also served in the line of battle.
Torpedo boat
A small, fast surface vessel designed for launching torpedoes.
Tramp steamer
A steamer which takes on cargo when and where it can find it.
An ancient warship propelled by three banks of oars.
Victory ship