Marine propulsion
Marine propulsion is the mechanism or system used to generate thrust to move a watercraft through water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electric motor or internal combustion engine driving a propeller, or less frequently, in pump-jets, an impeller. Marine engineering is the discipline concerned with the engineering design process of marine propulsion systems.
Human-powered paddles and oars, and later, sails were the first forms of marine propulsion. Rowed galleys, some equipped with sail, played an important early role in early human seafaring and warfare. The first advanced mechanical means of marine propulsion was the marine steam engine, introduced in the early 19th century. During the 20th century it was replaced by two-stroke or four-stroke diesel engines, outboard motors, and gas turbine engines on faster ships. Marine nuclear reactors, which appeared in the 1950s, produce steam to propel warships and icebreakers; commercial application, attempted late that decade, failed to catch on. Electric motors using battery packs have been used for propulsion on submarines and electric boats and have been proposed for energy-efficient propulsion.
Development in liquefied natural gas fueled engines are gaining recognition for their low emissions and cost advantages. Stirling engines, which are quieter, smoother running, propel a number of small submarines in order to run as quietly as possible. Its design is not used in civilian marine application due to lower total efficiency than internal combustion engines or power turbines.
History
Pre-mechanization
Until the application of the coal-fired steam engine to ships in the early 19th century, oars or the wind were the principal means of watercraft propulsion. Merchant ships predominantly used sail, but during periods when naval warfare depended on ships closing to ram or to fight hand-to-hand, galley were preferred for their manoeuvrability and speed. The Greek navies that fought in the Peloponnesian War used triremes, as did the Romans at the Battle of Actium. The development of naval gunnery from the 16th century onward vaulted broadside weight ahead of maneuverability; this led to the dominance of the sail-powered warship over the following three centuries.In modern times, human propulsion is found mainly on small boats or as auxiliary propulsion on sailboats. Human propulsion includes the push pole, rowing, and pedals.
Propulsion by sail generally consists of a sail hoisted on an erect mast, supported by stays, and controlled by lines made of rope. Sails were the dominant form of commercial propulsion until the late nineteenth century, and continued to be used well into the twentieth century on routes where wind was assured and coal was not available, such as in the South American nitrate trade. Sails are now generally used for recreation and racing, although innovative applications of kites/royals, turbosails, rotorsails, wingsails, windmills and SkySails's own kite buoy-system have been used on larger modern vessels for fuel savings.
Mechanized
In the second half of the 20th century, rising fuel costs almost led to the demise of the steam turbine. Most new ships since about 1960 have been built with diesel engines, both four- and two-stroke. The last major passenger ship built with steam turbines was 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 where the cargo can be used as bunker fuel.
Engines
Steam
power, which became prominent due to the First Industrial Revolution, has led to two types of steam engine for ships: reciprocating and turbine. The shaft power from each can either go directly to the propeller, pump jet or other mechanism, or it goes through some form of transmission; mechanical, electrical or hydraulic. In the latter half of the 19th century, steam was one of the main power sources for marine propulsion. In 1869, there was a large influx of steam ships as the steam engine underwent large advancements during the time period.Reciprocating
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, which gave way to screw propellers.The first commercial success accrued to Robert Fulton's North River Steamboat in US in 1807, followed in Europe by the of 1812. Steam propulsion progressed considerably over the rest of the 19th century. Notable developments include the steam surface condenser, which eliminated the use of sea water in the ship's boilers. This, along with improvements in boiler technology, permitted higher steam pressures, and thus the use of higher efficiency multiple expansion engines. As the means of transmitting the engine's power, paddle wheels gave way to more efficient screw propellers.
Multiple expansion steam engines became widespread in the late 19th century. These engines exhausted steam from a high pressure cylinder to a lower pressure cylinder, giving a large increase in efficiency.
Turbines
Steam turbines were powered by boilers burning, initially, coal, then fuel oil. 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 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 manpower by removal of the need for trimmers and stokers, and reduced space needed for fuel bunkers.
Nuclear-powered
Some steam turbine systems use nuclear power for their steam generation. In these vessels, the nuclear reactor heats water to create steam to drive the turbines. When first developed, very low prices of diesel oil limited nuclear propulsion's commercial attraction. The advantages of its fuel-price security, greater safety and low emissions were unable to overcome the higher initial costs of a nuclear powerplant. In 2019, nuclear propulsion is rare except in some Navy and specialist vessels such as icebreakers. In large aircraft carriers, the space formerly used for ship's bunkerage is 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 advantages. A few naval cruisers have also employed nuclear power; as of 2006, the only ones remaining in service are the Russian. An example of a non-military ship with nuclear marine propulsion is the with. In an ice-breaker, an advantage is fuel security and safety in demanding arctic conditions. The commercial experiment of the ended before the dramatic fuel price increases of the 1970s. The Savannah also suffered from an inefficient design, being partly for passengers and partly for cargo.In recent times, there is some renewed interest in commercial nuclear shipping. Fuel oil prices are now much higher. Nuclear-powered cargo ships could lower costs associated with carbon dioxide emissions and travel at higher cruise speeds than conventional diesel powered vessels.
Diesel
Most modern ships use a reciprocating diesel engine as their prime mover, due to their operating simplicity, robustness and fuel economy compared to most other prime mover mechanisms. The rotating crankshaft can be directly coupled to the propeller with slow speed engines, via a reduction gearbox for medium and high speed engines, or via an alternator and electric motor in diesel-electric vessels. The rotation of the crankshaft is connected to the camshaft or a hydraulic pump on an intelligent diesel.The reciprocating marine diesel engine first came into use in 1903 when the diesel electric rivertanker Vandal was put into service by Branobel. Diesel engines soon offered greater efficiency than the steam turbine, but for many years had an inferior power-to-space ratio. The advent of turbocharging however hastened their adoption, by permitting greater power densities.
Diesel engines today are broadly classified according to
- Their operating cycle: two-stroke engine or four-stroke engine
- Their construction: crosshead, trunk, or opposed piston
- Their speed
- *Slow speed: any engine with a maximum operating speed up to 300 revolutions per minute, although most large two-stroke slow speed diesel engines operate below 120 rpm. Some very long stroke engines have a maximum speed of around 80 rpm. 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 300–1000 rpm. Many modern four-stroke medium speed diesel engines have a maximum operating speed of around 500 rpm.
- *High speed: any engine with a maximum operating speed above 1000 rpm.
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 footprint required is smaller than that needed for equivalently rated four-stroke medium speed diesel engines. As space above the waterline is at a premium in passenger ships and ferries, 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 redundancy in the event of mechanical failure of one or more engines, and the potential for 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 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.