Propulsion
[edit] 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. 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.
[edit] 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.
[edit] 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.
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.
[edit] 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. [1]
[edit] 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.
[edit] Reciprocating diesel engines
About 99% of modern ships use diesel reciprocating engines[citation needed].
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.
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.
- Slow speed: any engine with a maximum operating speed up to 300
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.
[edit] 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.
[edit] 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.