Steam Steam power, which became prominent due to the
First Industrial Revolution, has led to two types of
steam engine for ships:
reciprocating (with steam driving
reciprocating pistons connected to a
crankshaft) and
turbine (with steam driving blades attached radially to a rotating shaft). 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 (often called
Clermont) 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 (compound) 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 cargo ship. 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 aboard a cargo ship 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 (rpm), 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. 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 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 (especially ones with a car deck), 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 (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, has had gas turbines installed in addition to
diesel engines. Because of 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. However, in the case of passenger ships the main reason for installing gas turbines has been to allow a reduction of emissions in sensitive environmental areas or while in port. Some warships, and a few modern cruise ships have also used steam turbines to improve the efficiency of their gas turbines in a
combined cycle, where
waste 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. Some private yachts, such as the
Aga Khan's
Alamshar, also have gas turbine propulsion (Pratt and Whitney ST40M), which enables top speeds of up to 70 knots, which is unique for a 50-meter yacht.
LNG Engines Shipping companies are required to comply with the
International Maritime Organization (IMO) and the
International Convention for the Prevention of Pollution from Ships emissions rules. Dual fuel engines are fueled by either marine grade diesel, heavy fuel oil, or liquefied natural gas (LNG). A
Marine LNG Engine has multiple fuel options, allowing vessels to transit without relying on one type of fuel. Studies show that LNG is the most efficient of fuels, although limited access to LNG fueling stations limits the production of such engines. Vessels providing services in the LNG industry have been retrofitted with dual-fuel engines, and have been proved to be extremely effective. Benefits of dual-fuel engines include fuel and operational flexibility, high efficiency, low emissions, and operational cost advantages. Liquefied natural gas engines offer the marine transportation industry with an environmentally friendly alternative to provide power to vessels. In 2010, STX Finland and Viking Line signed an agreement to begin construction on what would be the largest environmentally friendly cruise ferry. Construction of NB 1376 will be completed in 2013. According to Viking Line, vessel NB 1376 will primarily be fueled by liquefied natural gas. Vessel NB 1376 nitrogen oxide emissions will be almost zero, and sulphur oxide emissions will be at least 80% below the International Maritime Organization's (IMO) standards. Company profits from tax cuts and operational cost advantages has led to the gradual growth of LNG fuel use in engines.
LPG Engines As environmental sustainability becomes a paramount concern, the maritime industry is exploring cleaner propulsion technologies. LPG (Liquid Petroleum Gas) is another fuel alternative that brings operational, economics and environmental benefits. Studies have shown that using LPG reduces sulfur oxide emissions by 97% and particulate matter by 90%. Similar to LNG, many LPG vessels have been retrofitted with dual-fuel engines which are extremely effective. Using LPG as fuel also makes the process of transporting LPG easier. First, LPG deck tanks are filled together with the LPG cargo tanks using the cargo system during loading. LPG is then drawn from the deck tanks into a fuel gas supply system and piped to the engine. This increases operational and economic efficiency, especially during long-haul shipping. In 2020, BW LPG pioneered the world’s first Very Large Gas Carrier (VLGC) that was retrofitted with LPG dual-fuel propulsion technology and the company has the largest VLGC fleet that has been retrofitted with LPG dual fuel propulsion technology. This technology works towards reductions in emissions and a step closer to achieving
carbon-neutral shipping. Stirling Since the late 1980s, Swedish shipbuilder
Kockums has built a number of successful Stirling engine powered submarines. The submarines store compressed oxygen to allow more efficient and cleaner external fuel combustion when submerged, providing heat for the Stirling engine's operation. The engines are currently used on submarines of the and classes and the Japanese submarine. These are the first submarines to feature Stirling
air-independent propulsion (AIP), which extends the underwater endurance from a few days to several weeks. While hydrogen is a promising fuel, it has a few disadvantages. Hydrogen is far more flammable than other fuels such as diesel, so precautions must be taken. It is also not very energy dense, so it has to be heavily compressed to increase its energy density enough for it to be practical, similar to methane and LNG.
Electric Battery-electric propulsion first appeared in the latter part of the 19th century, powering small lake boats. These relied entirely on
lead-acid batteries for electric current to power their propellers.
Elco (the Electric Launch Company) evolved into the industry leader, later expanding into other forms of vessel, including the iconic World War II
PT boat. In the early part of the 20th century electric propulsion was adapted to use in
submarines. As underwater propulsion driven exclusively by heavy batteries was both slow and of limited range and timespan, rechargeable battery banks were developed. Submarines were primarily powered by combined
diesel-electric systems on the surface, which were much faster and allowed for dramatically expanded range, charging their battery systems as necessary for still limited subsurface action and duration. The experimental
Holland V submarine led to the adoption of this system by the
U.S. Navy, followed by the
British Royal Navy. To expand the range and duration of the submarine during World War II the German
Kriegsmarine developed a
snorkel system, which allowed the diesel-electric system to be utilized while the submarine was all but completely submerged. Finally, in 1952, was launched, the world's first nuclear powered submarine, which eliminated the restrictions of both diesel fuel and limited duration battery propulsion.
Several short-range ships are built as (or converted to) pure electric vessels. This includes some powered by batteries which are recharged from shore, and some
shore-powered by
electrical cables,
either overhead or
submerged (no batteries). On November 12, 2017
Guangzhou Shipyard International (GSI) launched what may be the world's first all-electric, battery-powered inland coal carrier. The 2,000 dwt vessel will carry bulk cargo for up to 40 nautical miles per charge. The ship carries
lithium ion batteries rated at 2,400 kilowatt-hours, about the same amount as 30
Tesla Model S electric sedans.
Diesel-electric The
diesel-electric transmission of power from the engine to the propeller affords flexibility in distribution of machinery within the vessel at a higher first cost than direct-drive propulsion. It is a preferred solution for vessels that employ pod-mounted propellers for precision positioning or reducing general vibrations by highly flexible couplings. Diesel-electric provides flexibility to assign power output to applications on board, other than propulsion. The first diesel electric ship was the Russian tanker , launched in 1903.
Turbo-electric Turbo-electric transmission uses electric generators to convert the mechanical energy of a turbine (steam or gas) into electric energy and electric motors to convert it back into mechanical energy to power the driveshafts. An advantage of turbo-electric transmission is that it allows the combination of high-speed turbines with slow turning propellers or wheels, without requiring a gearbox. It can also provide electricity for other electrical systems, such as lighting, computers, radar, and communications equipment.
Transmission of power To transmit the rotational force of the shaft into thrust, propellers are most commonly used in today's merchant vessels. The developed thrust from the propeller is transferred to the hull via a thrust bearing. ==Propulsion types==