ship - Measuring ships, Propulsion, General terminology, Some types of ships and boats

A sea-going vessel of considerable size. The Egyptians built river boats around 3000 BC, but at the time of Queen Hatshepsut (c.1500 BC) an expedition to E Africa was mounted using vessels of about 20 m/60 ft in length. These are the first sea-going ships of which there are reliable pictorial records; they were steered by oars over the stern. Great strides were made in ship design by the Phoenicians, who traded throughout the Mediterranean, but they left no pictorial records. Chinese ships were making voyages of over a year by the 1st-c AD. The stern-mounted rudder was in use by AD 100 (Europe, 12th-c), and multiple-masted ships with water-tight bulkheads before 200 (Europe, 15th–18th-c). Greeks and Romans built galleys relying on oars for manoeuvrability and much of their propulsion, a square sail coming into use when the wind was favourable. Roman merchant vessels 28–56 m/90–180 ft long were propelled by a large square sail hung from a single mast, with a smaller sail mounted on a bowsprit to improve steering qualities; they were steered with two oars or sweeps mounted on the stern.

Viking ships varied in length from 20 m/70 ft to 40 m/120 ft, were propelled by a large square-rigged sail and oars, and steered with a massive sweep hung over the starboard side well aft. By the 15th-c the Portuguese had developed the lateen-rigged caravel into a three- or four-masted ocean-going craft. Out of this grew the carrack, where lateen sails gradually gave way to square sails and the forecastle developed a pronounced overlap at the stem. The stern-mounted rudder appears to have come into general use at about this time. In China, during this period, ships of up to 1500 tons were in use, including ironclads armed with cannon (documented from 1371). In Europe, the square rig increased in complexity and refinement until the last decade of the 19th-c. US influence dates from 1845, with the clipper ship. Their ships were generally larger than the British, and very often faster. The ultimate refinement in sail came with the British-built tea clippers, such as the Cutty Sark. These ships were immediately made obsolescent by the opening of the Suez Canal (1869) because they could not navigate in this area of light and fickle winds.

In 1802 the first viable steamship, the Charlotte Dundas, was used as a tug on the Forth–Clyde canal. The first steamship to cross the Atlantic was the US-built Savannah, but she steamed for only 3½ days out of her 25-day passage, sailing for the rest of the time. The first continuous steam crossing of the Atlantic was achieved by the British-owned packet vessel, the 700-tonne Sirius, in 1838. Brunel's Great Britain (1844) was the first screw-propelled, double-bottomed, iron-hulled transatlantic passenger ship. High pressure steam in 1860 heralded the end of the sailing ship's supremacy. Steam turbines were demonstrated at the Spithead Review in 1897, and powered the Mauretania when she took the Blue Riband in 1907. The diesel engine, patented in 1892, won ocean-going acceptance in the 5000-gross-tonne Selandia in 1912; but these engines, though cheaper to run and occupying less space, could not provide the power nor the reliability needed for passenger vessels until the late 1970s. In 1968 the Queen Elizabeth 2 was built with steam turbines, but in 1987 she was re-engined using diesel-electric propulsion, gaining power and speed with lower fuel costs. (By 1991, only 2·8% of the world's ships were steam-powered, but this represented 16·8% of world tonnage.) Nuclear-fuelled vessels provide an alternative means of generating the heat to provide steam to drive turbines. This method is used in government vessels such as ice-breakers, aircraft carriers, and missile-carrying submarines, but it has proved to be uneconomical for merchant vessels. In the 1970s very large tankers were built, powered by steam turbines. When fully laden they displace 600 000 tonnes, but steam at a relatively modest speed of 15 knots. Future developments will see no great overall increase in ship size, but there will be faster container ships and many large cruise ships, all diesel-engined.

A ship usually has sufficient 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". Consequently submarines are referred to as "boats", because early submarines were small enough to be carried aboard a ship in transit to distant waters. Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship. The five-masted Preussen was the outstanding example, but the big German ships and barques were built partly for prestige reasons. 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. most measure volume rather than weight, 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 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. 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.

Propulsion

Pre-mechanisation

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.

Most new ships since around 1960 have been built with diesel engines. Rising fuel costs have almost led to the demise of the steam turbine, with many ships being re-engined to improve fuel efficiency. 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 Liquefied 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. 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 diesel engines) on this gas has improved however, so these engines are beginning to appear in LNG carriers; 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 switch 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. At the same time it was the first diesel electric ship. 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.

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. 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

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. 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. 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 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 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, like the jetfoil. Because of 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 (like Arktika class icebreaker with 75,000 shaft horsepower), but this is not a separate form of propulsion; It has become usual only in large aircraft carriers, where the space previously used for ship's bunkerage could then be used instead to bunker aviation fuel, and in submarines, where the ability to run submerged at high speed and in relative quiet for long periods holds obvious advantage.

General terminology

Ships may occur collectively as fleets, squadrons or flotillas.

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.

Some types of ships and boats

Aircraft carrier Barge Bulk carrier Cable Layer Capital ship Cargo ship Catamaran Coaster Container ship Corvette Crane vessel Cruise ship Cruiser Cutter Destroyer Diving support vessel Drillship Dredger Ferry Frigate FPSO (Floating_Production_Storage_and_Offloading) Guided missile cruiser Hopper, Hopper barge, Split hopper barge Hovercraft Hydrofoil Icebreaker Jetfoil Junk Landing craft Lake freighter Livestock carrier LNG carrier Lugger Minesweeper Minehunter Ocean liner Packet ship Panamax Reefer (refrigerated ship) Research vessel RO-RO ship (roll on, roll off, Auto carrier) Sailing ship Selfdischargers Semi-submersible Sloop Submarine Supertanker Supply boat, Supply ship Survey Vessels Tanker Tender Train ferry Tugboat Yacht

Some historical types of ships and boats

Barque A sailing vessel with three or more masts, fore-and-aft rigged on only the aftermost. Caravel A much smaller, two, sometimes three-masted ship. Carrack Clipper A fast multiple-masted sailing ship, generally used by merchants because of their speed capablities. 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. Knarr 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.) Longship A Viking raiding ship Man of war A sailing warship. 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. Steamship A ship propelled by a steam engine. Ship of the line A sailing warship of first, second or third rate.