A ship is a large, sea-going watercraft, usually with multiple decks. A ship usually hassufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat can fit on a ship, but a ship can't fit on a boat". Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship. (Note that one refers to submarines as "boats", because early submarines were small enough to be carried aboard a ship in transit to distant waters.) Compare vessel.
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 aftsail 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. The same economic pressures which increased sizes to the point of carrying four or five masts, also introduced the fore and aft rig to larger vessels, so few ship-rigged vessels were built with more than three masts.
The five-masted Preussen was the outstanding example, but the big German ships and barques were built partly for prestige reasons.
Nautical means related to sailors, particularly customs and practices at sea. Naval is theadjective 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; most measure volume rather than weight, and are used when describing merchant ships for the purpose of tolls, taxation, etc.
In Britain until the Merchant Shipping Act of 1876, ship-owners could load their vesselsuntil 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 Mark", exists on ships' sides, and consists of a circle with ahorizontal line through the center. 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.
Propulsion
Pre-mechanisation
Until the application of the steam engine to ships in the early 19th century, oars propelledgalleys 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.
Steam propulsion
The development of the steamship became a complex process, the first commercial successaccruing 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 condenser, which reduced the requirement for fresh water, and the multiple expansion engine, which improvedefficiency. As the means of transmitting the engine's power, the paddle wheel gave way to the more efficient screw propeller. The marine steam turbine developed by Sir Charles Algernon Parsons, brought the power to weight ratio down. He had 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 out of date, in warships. Most new ships since around 1960 have been built with diesel engines. Rising fuel costs have almost lead to the demise of the steam turbine, with many ships being re-engined to improve fuel efficiency. One high profile example was the 1968 built Queen Elizabeth 2 which had her turbines replaced with a diesel-electric propulsion plant in 1986. The last major passenger ship built with steam turbines was the Fairsky, launched in 1984. Some specialised merchant ships have also been built with steam turbines since then, notably Liquified Natural Gas(LNG) and coal carriers where part of the cargo has been used as fuel for the boilers.
LNG Carriers
LNG carriers in particular have remained a stronghold for steam , and new ships continue to be built with steam turbines in this high growth area of shipping. This is because the Natural Gas is stored in a liquid state in cryogenic vessels onboard these ships. A small amount of "boil off" of gas is required 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 method of dealing with the gas. Technology to operate internal combustion engines (modified marine two stroke dieselengines) on this gas has improved however, so these engines are beginning to appear in LNG carriers; with their greater thermal efficiency, less gas is burnt. Also, developements have been made in the process of re-liquifying "boil off" gas, enabling it to 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-liquification process is starting to be used on diesel engine propelled LNG carriers. Another factor driving theswitch 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 increase in size of the worldwide LNG fleet, not enough have been trained to meet the demand. It may be that the days of the last stronghold for steam turbine propulsion systems are numbered, despite all but sixteen of the orders for new LNG carriers at the end of 2004 being for steam turbine propelled ships.
Diesel propulsion
The marine diesel engine first came into use around 1912: either the Vulcanus or the Selandia (depending upon who you talk to) first deployed it. It soon offered even greater efficiency than the steam turbine but for many years had an inferior power-to-space ratio.
About this period too, heavy fuel oil came into more general use and began to replace coalas the fuel of choice in steamships. Its great advantages were the convenience, the reduction in manning owing to the removal of the need for trimmers and stokers, and the reduction in space required for fuel bunkers. Diesel engines today are broadly classified according to their operating cycle (two-stroke or four-stroke), their construction (crosshead, trunk, or opposed piston) and their speed (slow speed, medium speed or high speed). Most modern larger merchant ships use either slow speed, two stroke, crosshead engines, or medium speed, four stroke, trunk engines. Some smaller vessels may operate high speed diesel engines. The operating ranges of the differant speed types are as follows:
* 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 ofaround 500 rpm.
* High speed- any engine with a maximum operating speed above 900 revs/ minute
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 require 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 continue to operate. This arrangement allows maintenance to be carried out while under way at sea. Diesel electric is another propulsion system that has been around for a long time, but is becoming more common. By having the engines drive alternators, which supply electricity to motors driving the propellers, gearboxes and clutches can be dispensed with and greater flexibility gained in the positioning of the engines, while still providing the step down in speed required for a medium speed engine to efficiently drive a ships propeller.
The size of the differant 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 foot print required- length and width- is smaller than that required 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 required for two stroke diesel engines. Multiple engine installations also gives greater redundancy in the event of mechanical failure of one or more engines and greater efficiency over a wider range of operating conditions.
Other propulsion systems
Many warships built since the 1960s have used gas turbines for propulsion, as have a few passenger ships. 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 required. Some warships and a few modern cruise ships have also utilised steam turbines to improve the efficiency of gas turbines in a combined cycle. In such a combined cycle, where waste heat from a gas turbine is used to create steam for driving a steam turbine, thermal efficiency can be the same or slightly greater than that of diesel engines. However, the grade of fuel required for gas turbines is much more expensive than that required for diesel engines so running costs are higher.
A few ships have used nuclear reactors, but this is not a separate form of propulsion; the reactor heats steam to drive the turbines. Nonetheless, it has caused concerns about safety and waste disposal. It has become usual only in large aircraft carriers and in submarines, where the ability to run submerged for long periods holds obvious advantage. In such long-endurance vessels, the resulting saving in bunkerage is an important consideration.
General terminology
Ships may occur collectively as fleets, squadrons or flotillas. Convoys of ships commonly occur. A collection of ships for military purposes may compose a navy or a task force. 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.
Shipboard terminology
The complexity of ships, particularly of sailing ships, led to the development of a rich and various vocabulary. Many of the following terms link to more detailed discussions of nautical terminology.
* Amidships - toward the middle of the vessel.
* Bow - strictly, one of the two curved structures where the hull broadens out from the stem (the pointed end). The bows is a term for the head of the vessel or front of the ship.
Compare prow, a more poetical term for the ship's head.
* Stern - the after end of the ship.
* Aft - towards the stern when the relationship is within the ship.
* Astern beyond the stern where the relationship is outside the vessel.
* Starboard - the side of the ship which lies to the right when an observer within the ship faces forward.
* Port - the side of the ship which lies to the left when an observer within the ship faces forward. (A mnemonic to distinguish port and starboard notes that left and port both have four letters. Another incorporates the navigation light: Is there any red port left?)
* (Navigation) Bridge - A structure above the weather deck, extending the full width of the vessel, which houses a command centre, itself called by association, the bridge. A bridge usually extends a little beyond the ship's side to enable observation of boats alongside, or the proximity of a dock or lock gate; these projections are called bridge wings. In big vessels, a docking bridge used to be found aft. (See Lord, Walter. A Night to Remember (1976) p.96). It enabled an officer to observe docking manoeuvres before giving orders. RMS Titanic had one but they have been superseded by Closed-circuit television cameras.
* Bulkheads - internal "walls" in a ship. Bulkheads are the vertical equivalent of decks. They have a structural function as well as dividing spaces. They serve to prevent collapse of the hull under stress, to maintain stability, in the event of flooding, and to contain fire. Many bulkheads feature watertight doors which, in the case of certain types of ships, the crew may close remotely. An internal "wall" that is not load-bearing is usually
referred to as a "partition". It is to a bulkhead as a flat is to a deck.
* Cabin - an enclosed room on a deck or flat.
* Capstan - a winch with a vertical axis.
* Coaming - Raised edges of hatches in decks for keeping water and articles free on the deck from falling into the hold.
* Decks - the structures forming the approximately horizontal surfaces in the ship's general structure. Unlike flats, they are a structural part of the ship.
* Deck Head - The under-side of the deck above. Sometimes panelled over to hide the pipe work. This panelling, like that lining the bottom and sides of the holds, is the ceiling.
* Draft - The vertical distance from the current waterline to the lowest point of theship or in the part of the ship under consideration.
* Figurehead - symbolic image at the head of a traditional sailing ship or early steamer.
* Forecastle - a partial deck, above the upper deck and at the head of the vesseltraditionally the sailors' living quarters.
* Freeboard - The vertical distance from the current waterline to the highest continuous watertight deck. This usually varies from one part to another.
* Galley - the kitchen of the ship
* Gunwale - Formerly a fabricated band placed for strengthening around the ship at the main or upper deck level to accommodate the stresses imposed by the use of artillery. In later use it is the angle between the ship’s side and upper deck. It remained as a structural member, in wooden boats where it was mounted inboard of the sheer strake regardless of the need for gunnery.
* Bulwark - the extension of the ship's side above the level of the weather deck.
* Hold - In earlier use, below the orlop deck, the lower part of the interior of a ship's hull, especially when considered as storage space, as for cargo. In later merchant vessels it extended up through the decks to the underside of the weather deck.
* Hull - the shell and framework of the basic flotation-oriented part of a ship
* Keel - the central structural basis of the hull
* Kelson - the timber immediately above the keel of a wooden ship.
* Mast - a spar (in a ship, a very heavy one stepped in the keelson) formerly designed for the support of one or more sails. In modern ships, it is a steel or aluminium fabrication which carries navigation lights, radar antennae etc.
* Prow - a poetical alternative term for bows.
* Scupper - a drainage waterway at the edge of a deck, is drained by a pipe or, on the weather deck, a small opening in the bulwarks, leading overboard. It is called a scupper which is distinct from larger openings with hinged covers on the bulwarks, designed for relieving the ship of large quantities of water in a seaway. These are called freeing ports or wash ports..
* Windlass - A winch mechanism, usually with a horizontal axis. used where mechanical advantage greater than that obtainable by block and tackle was needed.
* Weather deck - whichever deck is that exposed to the weather – usually either the main deck or, in larger vessels.
