A barge is a flat-bottomed boat, built mainly for river and canal transport of heavy goods.
Most barges are not self-propelled and need to be moved by tugboats towing or towboats
pushing them. Barges on canals (towed by draft animals on an adjacent towpath) contended
with the railway in the early industrial revolution but were outcompeted in the carriage of
high value items due to the higher speed, falling costs, and route flexibility of rail
transport. Barges are still used today for low value bulk items, as the cost of hauling
goods by barge is very low. Barges are also used for very heavy or bulky items; a typical
barge measures 195 feet by 35 feet (59.4 meters by 10.6 meters), and can carry up to 1500
tons of cargo.
Self propelled barges may be used as such when traveling downstream or upstream in placid
waters and operated as an unpowered barge with the assistance of a tugboat when traveling
upstream in faster waters.
Types of barges:
* Barracks barge (living quarters)
* Company barge
* Dry bulk cargo barge (coal, rock, grain, etc.)
* Jackup barge, mainly used inshore for a stationary stable platform for civils diving
or drilling operations.
* Lighter
* Liquid cargo barge (fresh water, finished petroleum products)
* Pleasure barge- providing a floating bedroom, dance floor, or viewing platform
* Railcar barge (with tracks and using special loading/offloading facilities such as a
barge slip)
* Royal barge (ceremonial)
* Row barge
* Sand barge
* Severn trow
* Vehicular barge, often used to transport vehicles to natural shorelines such as
beaches
* Ware barge
* West country barge
On the UK canal system, the term barge is used to describe a boat wider than a narrowboat.
The people who move barges are often known as lightermen.
In the U.S. deckhands perform the labor and are supervised by a leadman and or the mate. The
Captain and Pilot steer the towboat. The towboat pushes one or more barges that are held
together with rigging and is called collectively the tow. The crew live aboard the towboat
as it travels along the inland river system and or the intracoastal waterways. These
towboats travel between ports and are also called line haul boats.
Poles are used on barges to fend off the barge as it nears other vessels or a wharf, often
called pike poles, and on shallow canals for example in the UK long punt poles are used to
manoeuvre or propel the barge.
[edit]
Etymology
Barge is attested from 1300, from Old French barge, from Vulgar Latin barga. The word
originally could refer to any small boat, the modern meaning arose around 1480. Bark "small
ship" is attested from 1420, from Old French barque, from Vulgar Latin barca (400 AD). The
more precise meaning "three-masted ship" arose in the 17th century, and often takes the
French spelling for disambiguation.
Both are probably derived from a Latin *barica, from Greek baris "Egyptian boat", ultimately
from m Coptic bari "small boat."
By extension, the term "embark" literally means to board the kind of boat called a "barque".
The long poles used to manoeuvre or propel a barge have given rise to the saying, "I
wouldn't touch that (subject/thing) with a barge pole." This is a variation on the phrase "I
wouldn't touch that with a (insert length) pole." It appears that the association with barge
poles came after the phrase was in use. Modern useage uses a ten foot pole, but the earliest
instances in print involve a forty foot pole[1], which is improbably long for operating a
barge.
Weigh bridge
A Weigh bridge is a device for weighing loads carried by road or rail wagons. It is a very heavy duty weighing scale which can weigh the vehicle both empty and when loaded and thus calculate the load carried by the vehicle. In earlier versions the bridge is installed over a rectangular pit that contains levers that ultimately connect to a balance mechanism. The most complex portion of this type is the arrangement of levers underneath the weigh bridge since the response of the scale must be independent of the distribution of the load. Modern devices use multiple strain gauges that connect to electronic equipment to totalize the sensor inputs. In either type of semi-permanant scale the weight readings are typically recorded in a nearby hut or office.
For many uses (such as at police over the road truck weigh stations or temporary road intercepts) weigh bridges have been largely supplanted by simple and thin electronic weigh cells, over which a vehicle is slowly driven. A computer records the output of the cell and accumulates the total vehicle weight. By weighing the force of each axle it can be assured that the vehicle is within statutory limits, which typically will impose a total vehicle weight, a maximum weight within an axle span limit and an individual axle limit. The former two limits ensure the safety of bridges while the latter protects the road surface.
For many uses (such as at police over the road truck weigh stations or temporary road intercepts) weigh bridges have been largely supplanted by simple and thin electronic weigh cells, over which a vehicle is slowly driven. A computer records the output of the cell and accumulates the total vehicle weight. By weighing the force of each axle it can be assured that the vehicle is within statutory limits, which typically will impose a total vehicle weight, a maximum weight within an axle span limit and an individual axle limit. The former two limits ensure the safety of bridges while the latter protects the road surface.
Helicopter
A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors.
Helicopters are classified as rotary-wing aircraft to distinguish them from conventional
fixed-wing aircraft. The word helicopter is derived from the Greek words helix (spiral) and
pteron (wing). The first single-rotor, fully-controllable helicopter to enter large
full-scale production was made by Igor Sikorsky in 1942.
Compared to conventional fixed-wing aircraft, helicopters are much more complex, more
expensive to buy and operate, and are more limited in speed, range, and payload. The
compensating advantage is maneuverability: helicopters can hover in place, reverse, and
above all take off and land vertically. Subject only to refueling facilities and
load/altitude limitations, a helicopter can travel to any location, and land anywhere with
enough space (approximately twice the area of the rotor disk).
Compared to other vertical lift aircraft like tiltrotors (V-22 Osprey for example) and
vectored thrust airplanes (AV-8 Harrier for example), helicopters are very efficient,
carrying more than twice the payload, consuming less fuel in hover and costing considerably
less to buy and operate. However these other configurations have a much higher cruise speed
than a helicopter (270 km/h for a helicopter, 460 km/h for a tiltrotor, 900+ km/h for a
vectored thrust airplane).
Generating lift
In conventional aircraft, the wing profile (called airfoil) is designed to deflect air
efficiently downward. This downward deflection causes an opposite lifting force on the wing
(described by Newton's third law) and a lower pressure on the upper surface, higher pressure
on the lower surface. This pressure difference integrated over the airfoil area causes a net
lift. However, the more the lift of the airfoil, the more drag that is caused (induced drag
by creating wingtip vortices). A helicopter makes use of the same principle, except that
instead of moving the entire aircraft, only the wings themselves are moved in a circular
motion. The helicopter's rotor can simply be regarded as rotating wings, from where the
military name of "rotary wing aircraft" originates.
Conventional layout
There are several possible layouts for arranging a helicopter's rotors. The most common
design is the Sikorsky-layout, which is used by approximately 95% of all helicopters
manufactured. Turning the rotor generates lift but it also applies a reverse torque to the
vehicle, which would spin the helicopter fuselage in the opposite direction to the rotor if
no counter-acting force was applied. At low speeds, the most common way to counteract this
torque is to have a smaller vertical propeller mounted at the rear of the aircraft called a
tail rotor. This rotor creates thrust which is in the opposite direction from the torque
generated by the main rotor. When the thrust from the tail rotor is sufficient to cancel out
the torque from the main rotor, the helicopter will not rotate around the main rotor shaft.
The world's largest and smallest series-produced helicopters follow this Sikorsky layout.
The Mil Mi-26 can lift 27 metric tons, the Robinson R22 has a crew of two and a gross weight
of 1300 lb (590 kg). Almost all civilian helicopters have the main rotor and tail rotor
system.
Sometimes the blades of a tail rotor are not separated by the same angle, but laid out in an
X-shape, which is supposed to reduce the noise levels for military use (e.g. AH-64 Apache).
The primary reason is to make the arrangement of the pitch controls simpler. If the tail
rotor is shrouded (i.e., a fan embedded in the vertical tail) it is called a fenestron. The
fenestron rotor system on the model EC120 helicopter uses a shaft driven system and gearbox
to turn the fan. It is less efficient but the advantages are that less noise is generated,
it is safer for people that may walk near it and there is less chance of the blades being
damaged by objects because it is shrouded, unlike the traditional tail rotor.
The amount of power required to prevent a helicopter from spinning is significant. A tail
rotor typically uses about 5 to 6% of the engine's power, and this power does not help the
helicopter produce lift or forward motion. To reduce this waste during cruise, the vertical
stabilizer is often angled to produce a force which helps counter the main rotor torque. At
high speeds, it is possible for the vertical stabilizer to counteract the entire torque,
leaving more power available for forward flight. This is commonly known as slip-streaming
and can make hovering turns difficult on windy days. Another reason for the angled vertical
stabilizer is to make it possible to stage a successful high-speed, run-on landing, in case
of the tail rotor failure or damage.
Many military helicopters, especially attack types, have short wings called stub wings to
add lift during forward motion. They are also used as external mounts for weapons. In
extreme cases, such as that of the Mil Mi-24, the wings are large enough to obstruct airflow
down from the rotors, making the helicopter all but unable to hover.
Alternative layouts
There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor.
Such Coaxial rotor designs use two main rotors which turn in opposite directions, or
contra-rotate, so that the torques from each rotor cancel each other out. These methods
introduce even more mechanical complexity to the design and are usually relegated to
specialized helicopter types.
The co-axial design, where rotors are mounted on top of each other at the top of the
fuselage and share a common main axle complex, was first built by Theodore von Karman and
Asbóth Oszkár in 1918 and later became the hallmark of soviet Kamov design bureau (see for
example the Kamov Ka-50 "Hokum"). Co-axial helicopters in flight are highly resistant to
side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing
system. Another example is the Kamov Ka-26, a successful crop duster aircraft. See Coaxial
rotor.
The slightly different system of intermeshing rotors, also called a synchropter, which was
developed in Nazi Germany for a small anti-submarine warfare helicopter, the Flettner Fl 282
Kolibri, features two main rotors on separate, obliquely mounted axles. The counter-rotating
rotors are on top of the fuselage, close to each other. During the Cold War the American
Kaman company started to produce similar helicopters for USAF firefighting purposes. Kamans
have high stability and powerful lifting capability. The latest Kaman K-Max model is a
dedicated sky crane design, used for construction works.
In the flying-wagon or tandem rotor system (sometimes called "flying banana" for the
peculiar shape of early U.S. examples), the two main rotors are located at the front and
rear extremity of a long, boxy fuselage that resembles a railway wagon. A prime example is
the Boeing CH-47 Chinook, that can carry 14 tons of payload. Wagon helicopters are practical
for military logistical purposes, because entry and unloading is easy via the unobstructed
front and rear ramps. The rotors and turbines are located very high on top of the fuselage,
making them less sensitive to damage and dirt. The main drawback of a tandem rotor is
limited agility in air and the need for a highly trained crew, as the large main rotors have
long outreach beyond the fuselage and may easily hit nearby obstacles. In 2001, while on
live TV, a South Korean Army CH-47 Chinook crashed into a bridge for this reason.
A helicopter built by Juan de la Cierva had three main rotors. These were placed at the
corners of an equilateral triangle and all turned the same direction.
In the cross system, the rotary wing aircraft resembles a traditional fixed-wing airplane,
with the two main rotors mounted at the extremities of its wings. Such helicopters are rare,
because structural integrity of the wings is difficult to maintain against the amplified
resonance of far off-board rotor-turbine units. The 1930s German FW-61 helicopter was built
to such design. The world's largest ever helicopter, the Soviet Mil-V-12 prototype, was a
cross of two Mil Mi-6 turbine-rotor units built onto a modified Antonov cargo plane. The
U.S. V-22 Osprey tilting rotorcraft is similar, although its nacelles can be rotated, and
shares some of the inherent technical problems of a cross system.
A recent development in helicopter technology is the NOTAR system, which stands for N'O'
TAil Rotor. The NOTAR eliminates the tail rotor by conducting high-velocity air through the
tail boom, using the Coandă effect to produce forces to counter the torque. NOTARs adjust
thrust by opening and closing a sliding circular cover near the end of the tail boom. The
NOTAR system was developed in the United States and is used exclusively by McDonnell Douglas
Helicopters.
The most unusual design is the roto-rocket principle, where the single main rotor draws
power not from the shaft, but from its own wingtip jet nozzles, which are either pressurized
from a fuselage-mounted gas turbine or have their own pulsejet combustion chambers. Although
this method is simple and eliminates precession, development of such helicopters ceased
because their extreme noise levels preclude both military and civilian use.
[edit]
Controlling flight
Useful flight requires that an aircraft be controlled in all three dimensions (see flight
dynamics). In a fixed-wing aircraft, this is easy: small movable surfaces are adjusted to
change the aircraft's shape so that the air rushing past pushes it in the desired direction.
In a helicopter, however, there is often not enough speed for this method to be practical.
For rotation about the vertical axis (yaw) the anti-torque system is used. Varying the pitch
of the tail rotor alters the sideways thrust produced. Dual-rotor helicopters have a
differential between the two rotor transmissions that can be adjusted by an electric or
hydraulic motor to transmit differential torque and thus turn the helicopter. Yaw controls
are usually operated with anti-torque pedals, on the floor in the same place as a fixed-wing
aircraft's rudder pedals.
For pitch (tilting forward and back) or roll (tilting sideways) the angle of attack of the
main rotor blades is altered or cycled during the rotation creating a differential of lift
at different points of the rotary wing. More lift at the rear of the rotary wing will cause
the aircraft to pitch forward, an increase on the left will cause a roll to the right and so
on.
Helicopters maneuver with three flight controls besides the pedals. The collective pitch
control lever controls the collective pitch, or angle of attack, of the helicopter blades
altogether, that is, equally throughout the 360 degree plane-of-rotation of the main rotor
system. When the angle of attack is increased, the blade produces more lift. The collective
control is usually a lever at the pilot's left side. Simultaneously increasing the
collective and adding power with the throttle causes a helicopter to rise.
The throttle controls the absolute power produced by the engine that is connected to the
rotor by a transmission. The throttle control is a twist grip on the collective control. RPM
control is critical to proper operation for several reasons. Helicopter rotors are designed
to operate at a specific RPM. However, for each weight and speed there would be an ideal RPM
(design-rpm). In practice, a single (higher) RPM is used in order to minimize resonance
design requirements and add a safety margin to rotor stall RPM. Usually only in autorotation
are different RPMs used to increase rotor efficiency, which can be crucial in the case of an
emergency without engine power.
If the RPM becomes too low, the rotor blades stall. This suddenly increases drag and slows
the rotor down further. The centrifugal forces are then not able to straighten the rotor
blades any more, excessive coning ("tuliping") develops and a catastrophic accident is
certain.
If the RPM is too high, damage to the main rotor hub, power transmission and engine from
excessive forces could result. In general, RPM must be maintained within a tight tolerance,
usually a few percent. In many piston-powered helicopters, the pilot must manage the engine
and rotor RPM. The pilot manipulates the throttle to maintain rotor RPM and therefore
regulates the effect of drag on the rotor system. Turbine engined helicopters, and some
piston helicopters, use a servo-feedback loop, otherwise known as a governor, in their
engine controls to maintain rotor RPM and relieves the pilot of routine responsibility for
that task.
The cyclic changes the pitch of the blades cyclically, that is, during the rotation of the
blades around each complete circle (2 pi radians). This causes the lift to vary across the
plane of the rotor disk. This variation in lift causes the rotor disk to tilt and the
helicopter to move during hover flight or change attitude in forward flight. The cyclic is
similar to a joystick and is usually positioned in front of the pilot. The cyclic controls
the angle of the stationary section of the swashplate, which in turn controls the angle of
the rotating section of the swashplate. The rotating section rotates with the rotor and is
connected to blade pitch horns through pitch links, one link for each blade. When the
swashplate is not tilted, the blades are all at the collective angle. When it is tilted, the
links give a pitch-up at some azimuthal angle and a pitch-down at the opposite angle, hence
creating a sinusoidal variation in blade angle of attack. This causes the helicopter to tilt
in the same direction as the cyclic. If the pilot pushes the cyclic forward, then the rotor
disc tilts forward, and the rotor produces a thrust in the forward direction.
As a helicopter moves forward, the rotor blades on one side move at rotor tip speed plus the
aircraft speed and is called the advancing blade. As the blade swings to the other side of
the helicopter, it moves at rotor tip speed minus aircraft speed and is called the
retreating blade. To compensate for the added lift on the advancing blade and the decreased
lift on the retreating blade, the angle of attack of the blades is regulated as the blade
spins around the helicopter. The angle of attack is increased on the retreating blade to
produce more lift, compensating for the slower airspeed over the blade. And the angle of
attack is decreased on the advancing blade to produce less lift, compensating for the faster
airspeed over the blade.
If the angle of attack of any wing, including rotor blades, is too high, the airflow above
the wing separates causing instant loss of lift and increase in drag. This condition is
called aerodynamic stall. On a helicopter, this can happen in any of four ways.
1. As helicopter speed increases, airflow over the advancing blades approaches the speed
of sound and generates shock waves that disrupt the airflow over the blade causing loss of
lift.
2. As helicopter speeds increase, the retreating blade experiences lower relative
airspeeds and the controls compensate with higher angle of attack. With a low enough
relative airspeed and a high enough angle of attack, aerodynamic stall is inevitable. This
is called retreating blade stall. See dissymetry of lift for a fuller treatment of cases 1
and 2 together in a single analysis.
3. Any low rotor RPM flight condition accompanied by increasing collective pitch
application will cause aerodynamic stall.
4. Unique to helicopters is the vortex ring state (also known as settling with power)
which is when a helicopter in a hover or descent comes into contact with its own down wash
causing immense turbulence and loss of lift.
Helicopters are powered aircraft but they can still fly without power by using the momentum
in the rotors and using downward motion to force air through the rotors. The main rotor acts
like a "windmill" and turns. This technique is known as autorotation. A transmission
connects the main rotor to the tail rotor so that all flight controls are available after
engine failure. Autorotation can allow a pilot to make an emergency landing if the engine
failure occurs while the helicopter is traveling high enough or fast enough. (see
Height-velocity diagram).
Helicopters are classified as rotary-wing aircraft to distinguish them from conventional
fixed-wing aircraft. The word helicopter is derived from the Greek words helix (spiral) and
pteron (wing). The first single-rotor, fully-controllable helicopter to enter large
full-scale production was made by Igor Sikorsky in 1942.
Compared to conventional fixed-wing aircraft, helicopters are much more complex, more
expensive to buy and operate, and are more limited in speed, range, and payload. The
compensating advantage is maneuverability: helicopters can hover in place, reverse, and
above all take off and land vertically. Subject only to refueling facilities and
load/altitude limitations, a helicopter can travel to any location, and land anywhere with
enough space (approximately twice the area of the rotor disk).
Compared to other vertical lift aircraft like tiltrotors (V-22 Osprey for example) and
vectored thrust airplanes (AV-8 Harrier for example), helicopters are very efficient,
carrying more than twice the payload, consuming less fuel in hover and costing considerably
less to buy and operate. However these other configurations have a much higher cruise speed
than a helicopter (270 km/h for a helicopter, 460 km/h for a tiltrotor, 900+ km/h for a
vectored thrust airplane).
Generating lift
In conventional aircraft, the wing profile (called airfoil) is designed to deflect air
efficiently downward. This downward deflection causes an opposite lifting force on the wing
(described by Newton's third law) and a lower pressure on the upper surface, higher pressure
on the lower surface. This pressure difference integrated over the airfoil area causes a net
lift. However, the more the lift of the airfoil, the more drag that is caused (induced drag
by creating wingtip vortices). A helicopter makes use of the same principle, except that
instead of moving the entire aircraft, only the wings themselves are moved in a circular
motion. The helicopter's rotor can simply be regarded as rotating wings, from where the
military name of "rotary wing aircraft" originates.
Conventional layout
There are several possible layouts for arranging a helicopter's rotors. The most common
design is the Sikorsky-layout, which is used by approximately 95% of all helicopters
manufactured. Turning the rotor generates lift but it also applies a reverse torque to the
vehicle, which would spin the helicopter fuselage in the opposite direction to the rotor if
no counter-acting force was applied. At low speeds, the most common way to counteract this
torque is to have a smaller vertical propeller mounted at the rear of the aircraft called a
tail rotor. This rotor creates thrust which is in the opposite direction from the torque
generated by the main rotor. When the thrust from the tail rotor is sufficient to cancel out
the torque from the main rotor, the helicopter will not rotate around the main rotor shaft.
The world's largest and smallest series-produced helicopters follow this Sikorsky layout.
The Mil Mi-26 can lift 27 metric tons, the Robinson R22 has a crew of two and a gross weight
of 1300 lb (590 kg). Almost all civilian helicopters have the main rotor and tail rotor
system.
Sometimes the blades of a tail rotor are not separated by the same angle, but laid out in an
X-shape, which is supposed to reduce the noise levels for military use (e.g. AH-64 Apache).
The primary reason is to make the arrangement of the pitch controls simpler. If the tail
rotor is shrouded (i.e., a fan embedded in the vertical tail) it is called a fenestron. The
fenestron rotor system on the model EC120 helicopter uses a shaft driven system and gearbox
to turn the fan. It is less efficient but the advantages are that less noise is generated,
it is safer for people that may walk near it and there is less chance of the blades being
damaged by objects because it is shrouded, unlike the traditional tail rotor.
The amount of power required to prevent a helicopter from spinning is significant. A tail
rotor typically uses about 5 to 6% of the engine's power, and this power does not help the
helicopter produce lift or forward motion. To reduce this waste during cruise, the vertical
stabilizer is often angled to produce a force which helps counter the main rotor torque. At
high speeds, it is possible for the vertical stabilizer to counteract the entire torque,
leaving more power available for forward flight. This is commonly known as slip-streaming
and can make hovering turns difficult on windy days. Another reason for the angled vertical
stabilizer is to make it possible to stage a successful high-speed, run-on landing, in case
of the tail rotor failure or damage.
Many military helicopters, especially attack types, have short wings called stub wings to
add lift during forward motion. They are also used as external mounts for weapons. In
extreme cases, such as that of the Mil Mi-24, the wings are large enough to obstruct airflow
down from the rotors, making the helicopter all but unable to hover.
Alternative layouts
There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor.
Such Coaxial rotor designs use two main rotors which turn in opposite directions, or
contra-rotate, so that the torques from each rotor cancel each other out. These methods
introduce even more mechanical complexity to the design and are usually relegated to
specialized helicopter types.
The co-axial design, where rotors are mounted on top of each other at the top of the
fuselage and share a common main axle complex, was first built by Theodore von Karman and
Asbóth Oszkár in 1918 and later became the hallmark of soviet Kamov design bureau (see for
example the Kamov Ka-50 "Hokum"). Co-axial helicopters in flight are highly resistant to
side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing
system. Another example is the Kamov Ka-26, a successful crop duster aircraft. See Coaxial
rotor.
The slightly different system of intermeshing rotors, also called a synchropter, which was
developed in Nazi Germany for a small anti-submarine warfare helicopter, the Flettner Fl 282
Kolibri, features two main rotors on separate, obliquely mounted axles. The counter-rotating
rotors are on top of the fuselage, close to each other. During the Cold War the American
Kaman company started to produce similar helicopters for USAF firefighting purposes. Kamans
have high stability and powerful lifting capability. The latest Kaman K-Max model is a
dedicated sky crane design, used for construction works.
In the flying-wagon or tandem rotor system (sometimes called "flying banana" for the
peculiar shape of early U.S. examples), the two main rotors are located at the front and
rear extremity of a long, boxy fuselage that resembles a railway wagon. A prime example is
the Boeing CH-47 Chinook, that can carry 14 tons of payload. Wagon helicopters are practical
for military logistical purposes, because entry and unloading is easy via the unobstructed
front and rear ramps. The rotors and turbines are located very high on top of the fuselage,
making them less sensitive to damage and dirt. The main drawback of a tandem rotor is
limited agility in air and the need for a highly trained crew, as the large main rotors have
long outreach beyond the fuselage and may easily hit nearby obstacles. In 2001, while on
live TV, a South Korean Army CH-47 Chinook crashed into a bridge for this reason.
A helicopter built by Juan de la Cierva had three main rotors. These were placed at the
corners of an equilateral triangle and all turned the same direction.
In the cross system, the rotary wing aircraft resembles a traditional fixed-wing airplane,
with the two main rotors mounted at the extremities of its wings. Such helicopters are rare,
because structural integrity of the wings is difficult to maintain against the amplified
resonance of far off-board rotor-turbine units. The 1930s German FW-61 helicopter was built
to such design. The world's largest ever helicopter, the Soviet Mil-V-12 prototype, was a
cross of two Mil Mi-6 turbine-rotor units built onto a modified Antonov cargo plane. The
U.S. V-22 Osprey tilting rotorcraft is similar, although its nacelles can be rotated, and
shares some of the inherent technical problems of a cross system.
A recent development in helicopter technology is the NOTAR system, which stands for N'O'
TAil Rotor. The NOTAR eliminates the tail rotor by conducting high-velocity air through the
tail boom, using the Coandă effect to produce forces to counter the torque. NOTARs adjust
thrust by opening and closing a sliding circular cover near the end of the tail boom. The
NOTAR system was developed in the United States and is used exclusively by McDonnell Douglas
Helicopters.
The most unusual design is the roto-rocket principle, where the single main rotor draws
power not from the shaft, but from its own wingtip jet nozzles, which are either pressurized
from a fuselage-mounted gas turbine or have their own pulsejet combustion chambers. Although
this method is simple and eliminates precession, development of such helicopters ceased
because their extreme noise levels preclude both military and civilian use.
[edit]
Controlling flight
Useful flight requires that an aircraft be controlled in all three dimensions (see flight
dynamics). In a fixed-wing aircraft, this is easy: small movable surfaces are adjusted to
change the aircraft's shape so that the air rushing past pushes it in the desired direction.
In a helicopter, however, there is often not enough speed for this method to be practical.
For rotation about the vertical axis (yaw) the anti-torque system is used. Varying the pitch
of the tail rotor alters the sideways thrust produced. Dual-rotor helicopters have a
differential between the two rotor transmissions that can be adjusted by an electric or
hydraulic motor to transmit differential torque and thus turn the helicopter. Yaw controls
are usually operated with anti-torque pedals, on the floor in the same place as a fixed-wing
aircraft's rudder pedals.
For pitch (tilting forward and back) or roll (tilting sideways) the angle of attack of the
main rotor blades is altered or cycled during the rotation creating a differential of lift
at different points of the rotary wing. More lift at the rear of the rotary wing will cause
the aircraft to pitch forward, an increase on the left will cause a roll to the right and so
on.
Helicopters maneuver with three flight controls besides the pedals. The collective pitch
control lever controls the collective pitch, or angle of attack, of the helicopter blades
altogether, that is, equally throughout the 360 degree plane-of-rotation of the main rotor
system. When the angle of attack is increased, the blade produces more lift. The collective
control is usually a lever at the pilot's left side. Simultaneously increasing the
collective and adding power with the throttle causes a helicopter to rise.
The throttle controls the absolute power produced by the engine that is connected to the
rotor by a transmission. The throttle control is a twist grip on the collective control. RPM
control is critical to proper operation for several reasons. Helicopter rotors are designed
to operate at a specific RPM. However, for each weight and speed there would be an ideal RPM
(design-rpm). In practice, a single (higher) RPM is used in order to minimize resonance
design requirements and add a safety margin to rotor stall RPM. Usually only in autorotation
are different RPMs used to increase rotor efficiency, which can be crucial in the case of an
emergency without engine power.
If the RPM becomes too low, the rotor blades stall. This suddenly increases drag and slows
the rotor down further. The centrifugal forces are then not able to straighten the rotor
blades any more, excessive coning ("tuliping") develops and a catastrophic accident is
certain.
If the RPM is too high, damage to the main rotor hub, power transmission and engine from
excessive forces could result. In general, RPM must be maintained within a tight tolerance,
usually a few percent. In many piston-powered helicopters, the pilot must manage the engine
and rotor RPM. The pilot manipulates the throttle to maintain rotor RPM and therefore
regulates the effect of drag on the rotor system. Turbine engined helicopters, and some
piston helicopters, use a servo-feedback loop, otherwise known as a governor, in their
engine controls to maintain rotor RPM and relieves the pilot of routine responsibility for
that task.
The cyclic changes the pitch of the blades cyclically, that is, during the rotation of the
blades around each complete circle (2 pi radians). This causes the lift to vary across the
plane of the rotor disk. This variation in lift causes the rotor disk to tilt and the
helicopter to move during hover flight or change attitude in forward flight. The cyclic is
similar to a joystick and is usually positioned in front of the pilot. The cyclic controls
the angle of the stationary section of the swashplate, which in turn controls the angle of
the rotating section of the swashplate. The rotating section rotates with the rotor and is
connected to blade pitch horns through pitch links, one link for each blade. When the
swashplate is not tilted, the blades are all at the collective angle. When it is tilted, the
links give a pitch-up at some azimuthal angle and a pitch-down at the opposite angle, hence
creating a sinusoidal variation in blade angle of attack. This causes the helicopter to tilt
in the same direction as the cyclic. If the pilot pushes the cyclic forward, then the rotor
disc tilts forward, and the rotor produces a thrust in the forward direction.
As a helicopter moves forward, the rotor blades on one side move at rotor tip speed plus the
aircraft speed and is called the advancing blade. As the blade swings to the other side of
the helicopter, it moves at rotor tip speed minus aircraft speed and is called the
retreating blade. To compensate for the added lift on the advancing blade and the decreased
lift on the retreating blade, the angle of attack of the blades is regulated as the blade
spins around the helicopter. The angle of attack is increased on the retreating blade to
produce more lift, compensating for the slower airspeed over the blade. And the angle of
attack is decreased on the advancing blade to produce less lift, compensating for the faster
airspeed over the blade.
If the angle of attack of any wing, including rotor blades, is too high, the airflow above
the wing separates causing instant loss of lift and increase in drag. This condition is
called aerodynamic stall. On a helicopter, this can happen in any of four ways.
1. As helicopter speed increases, airflow over the advancing blades approaches the speed
of sound and generates shock waves that disrupt the airflow over the blade causing loss of
lift.
2. As helicopter speeds increase, the retreating blade experiences lower relative
airspeeds and the controls compensate with higher angle of attack. With a low enough
relative airspeed and a high enough angle of attack, aerodynamic stall is inevitable. This
is called retreating blade stall. See dissymetry of lift for a fuller treatment of cases 1
and 2 together in a single analysis.
3. Any low rotor RPM flight condition accompanied by increasing collective pitch
application will cause aerodynamic stall.
4. Unique to helicopters is the vortex ring state (also known as settling with power)
which is when a helicopter in a hover or descent comes into contact with its own down wash
causing immense turbulence and loss of lift.
Helicopters are powered aircraft but they can still fly without power by using the momentum
in the rotors and using downward motion to force air through the rotors. The main rotor acts
like a "windmill" and turns. This technique is known as autorotation. A transmission
connects the main rotor to the tail rotor so that all flight controls are available after
engine failure. Autorotation can allow a pilot to make an emergency landing if the engine
failure occurs while the helicopter is traveling high enough or fast enough. (see
Height-velocity diagram).
Ships
A ship is a large, sea-going watercraft, usually with multiple decks. 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". 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 aft
sail 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 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. 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 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 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 a
horizontal 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.
[edit]
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]
Steam propulsion
The development of the steamship became a complex process, the first commercial success
accruing 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 improved
efficiency. 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.
[edit]
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 diesel
engines) 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 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. [1]
[edit]
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 coal
as 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 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. 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.
[edit]
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.
[edit]
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.
[edit]
Shipboard terminology
See also: Glossary of nautical terms. 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 the
ship 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 vessel;
traditionally 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, the upper deck.
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". 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 aft
sail 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 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. 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 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 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 a
horizontal 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.
[edit]
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]
Steam propulsion
The development of the steamship became a complex process, the first commercial success
accruing 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 improved
efficiency. 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.
[edit]
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 diesel
engines) 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 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. [1]
[edit]
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 coal
as 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 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. 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.
[edit]
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.
[edit]
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.
[edit]
Shipboard terminology
See also: Glossary of nautical terms. 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 the
ship 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 vessel;
traditionally 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, the upper deck.
All-terrain vehicle
The term "all-terrain vehicle" is used in a general sense to describe any of a number of
small open motorised buggies and tricycles designed for off-road use. However, the American
National Standards Institute (ANSI) defines an ATV as a vehicle that travels on low pressure
tires, with a seat that is straddled by the operator, and with handlebars for steering
control. By the ANSI definition, it is intended for use by a single operator. The 4-wheeled
versions are most commonly called "quads," "four-wheelers" or "ATVs" in the United States
and Canada, and "quad bikes" or "quad cycles" in other English-speaking countries. Models
with 3 wheels are typically known as ATCs (though this is a Honda trademark) and
"three-wheelers," and less commonly "all-terrain cycles" and "trikes." 6- and 8-wheel models
exist for specialized applications. The rider sits on these models just like on a
motorcycle, but the extra wheels make them more stable at slow speeds. ATVs can also be
considered Off Highway Vehicles (OHV) or Off Road Vehicles (ORV), along with motorcycles,
Jeeps and other off-road capable machines.
Engine sizes of ATVs currently for sale in the United States (as of 2006) range from 50cc to
800cc. They range in price from about $2000 to nearly $8000.
Safety Issues
Since the expiration of the consent decrees between the major manufacturers and CPSC in
April of 1998, the manufacturers have entered into "voluntary action plans" that mimic the
previously mandatory consent decrees. However, despite the move from 3-wheel to 4-wheel
models and the action plans, some deaths and injuries still occur. Statistics released by
CPSC show that in 2004, there were an estimated 136,100 injuries associated with ATVs
treated in US hospital emergency rooms -- more than double the number of injuries treated in
the last year of the consent decrees. In 2003, the latest year for which estimates are
available, 740 people died in ATV-associated incidents.
The action plans in place with CPSC cover only certain manufacturers of ATVs. Other
manufacturers that have entered the market since the expiration of the consent decrees are
not covered by the action plans and so are not bound by the rules governing things such as
labelling and safe marketing practices, and what ages a distributor may recommend a
particular sized ATV for. These manufacturers and distributors, most of whom originate from
Asia and Italy, are completely exempt of government oversight.
Focus has shifted since the consent decrees ended to attention to machine size balanced with
rider age. Many states have enacted legislation specifically governing the usage of ATVs on
state run land categorized by age ranges and engine displacements - in line with the consent
decrees. ATVs are mandated to be labelled from the manufacturer that the use of machines
greater than 90cc by riders under the age of 16 is prohibited. Critics point out that
blanket policies concerning age are not sufficient and often use as example that early teen
male children are physically larger and stronger than many adult women riders. Some
localities have either banned minors (typically those under 16 years of age) from using ATVs
or are considering such legislation. Advocates of ATVs argue that starting younger improves
safety. They recommend that children can develop the necessary expertise by starting as
young as 6 years of age instead of waiting until age 16. The U.S. Consumer Product Safety
Commission approved the sale of sub-50cc ATVs for use by youngsters as young as age 6.
In 1988, the All-terrain Vehicle Safety Institute (ASI) was formed to provide training and
education for ATV riders. The cost of attending the training is minimal and is free for
purchasers of new machines. Successful completion of training such as provided here is in
many states a minimum requirement for minor-age children to be granted permission to ride on
state lands.
[edit]
EPA Concerns
[edit]
Emissions
Due to the lack of emission controlling hardware and software, for year 2000 all
recreational spark ignited (SI) non-road vehicles (of which ATVs are a subset) contributed
8% of HC, .16% of NOx, 5% of CO and .8% of PM emissions for the entire non-road EPA family.
The entire range of non-road emissions accounted for 49% of engine produced emissions of all
types. (Source: EPA 1) While recreational SI vehicles (of which ATVs are a subset) produce
an aggregate of <4% of all HC emissions in the US, based on the relatively small population
of ATVs (<1.2M) and small annual usage (<350 hrs), EPA emission regulations now include such
engines starting with model year 2006. (source: EPA 2)
[edit]
Fuel Economy
The EPA estimates that each ATV consumes less than 59 gallons of fuel per year and obtains
between 40 and 50 mpg, making them not likely to fall under future fuel economy regulations.
(Ibid. EPA 1)
[edit]
Land Usage
Some ATV riders cross privately owned property in rural areas and travel overland where
their use is explicitly limitied to trails. Further, environmentalists criticize ATV riders
for excessive use in areas they consider biologically sensitive, especially wetlands and
sand dunes. While the deep treads on some ATV tires are effective for navigating rocky,
muddy, and root covered terrain, these treads also dig channels that may drain boggy areas,
increase sedimentation in streams at crossings and damage groomed snowmobile trails. Studies
have also shown that ATVs may help in the spread of invasive species such as knapweed. While
there is much scientific evidence regarding the impact of ATVs, its credibility often comes
under great scrutiny from ATV users who believe them to be overtly biased against ATVs.
To address these land usage concerns, well funded ATV advocacy groups have been organized to
purchase property and/or obtain permission of landowners, build and maintain trails suitable
for ATV riding and educate ATV riders about responsible riding. Many states have also formed
separate governing bodies that license ATVs separately than other ORVs. The monies from
these registrations are used to secure trails to ride and perform grooming and maintenance.
Unfortunately, the image of the great majority of responsible riders is often tainted by the
actions of some who ride off designated trails, on private land without permission, and
under the influence of alcohol or drugs. Additionally, self regulation has proven
particularly difficult considering that the main public complaint against ATVs is excessive
noise. Although the majority of ATVs comply with noise regulations, there are those whose
intentional violation can disturb the activities of other recreational users for miles
across open landscapes. Tampering with an ATVs exhaust silencer and spark arrestor is
illegal on all federal lands and most state lands, however enforcement is spotty. It is also
possible to install aftermarket exhaust systems that do not have silencers and spark
arrestors.
Fellow outdoor recreationists who have expressed concern about irresponsible ATV use are
snowmobile users who resent improper use of exclusive snowmobile trails, ATV trail riders
whose trails have been damaged by improper use and hunters whose game has been driven off by
those riding during prime hunting times.
Nationally, the US Forest Service considers managed ATV use to be a legitmate activity in
national forests, yet it also lists their unregulated use as one of the four greatest
threats to long term forest management. The US Forest Service recently released a national
travel management plan designed to minimize the negative environmental impacts of ATVs while
providing a safe, sustainable and enjoyable opportunity for ATV users.
small open motorised buggies and tricycles designed for off-road use. However, the American
National Standards Institute (ANSI) defines an ATV as a vehicle that travels on low pressure
tires, with a seat that is straddled by the operator, and with handlebars for steering
control. By the ANSI definition, it is intended for use by a single operator. The 4-wheeled
versions are most commonly called "quads," "four-wheelers" or "ATVs" in the United States
and Canada, and "quad bikes" or "quad cycles" in other English-speaking countries. Models
with 3 wheels are typically known as ATCs (though this is a Honda trademark) and
"three-wheelers," and less commonly "all-terrain cycles" and "trikes." 6- and 8-wheel models
exist for specialized applications. The rider sits on these models just like on a
motorcycle, but the extra wheels make them more stable at slow speeds. ATVs can also be
considered Off Highway Vehicles (OHV) or Off Road Vehicles (ORV), along with motorcycles,
Jeeps and other off-road capable machines.
Engine sizes of ATVs currently for sale in the United States (as of 2006) range from 50cc to
800cc. They range in price from about $2000 to nearly $8000.
Safety Issues
Since the expiration of the consent decrees between the major manufacturers and CPSC in
April of 1998, the manufacturers have entered into "voluntary action plans" that mimic the
previously mandatory consent decrees. However, despite the move from 3-wheel to 4-wheel
models and the action plans, some deaths and injuries still occur. Statistics released by
CPSC show that in 2004, there were an estimated 136,100 injuries associated with ATVs
treated in US hospital emergency rooms -- more than double the number of injuries treated in
the last year of the consent decrees. In 2003, the latest year for which estimates are
available, 740 people died in ATV-associated incidents.
The action plans in place with CPSC cover only certain manufacturers of ATVs. Other
manufacturers that have entered the market since the expiration of the consent decrees are
not covered by the action plans and so are not bound by the rules governing things such as
labelling and safe marketing practices, and what ages a distributor may recommend a
particular sized ATV for. These manufacturers and distributors, most of whom originate from
Asia and Italy, are completely exempt of government oversight.
Focus has shifted since the consent decrees ended to attention to machine size balanced with
rider age. Many states have enacted legislation specifically governing the usage of ATVs on
state run land categorized by age ranges and engine displacements - in line with the consent
decrees. ATVs are mandated to be labelled from the manufacturer that the use of machines
greater than 90cc by riders under the age of 16 is prohibited. Critics point out that
blanket policies concerning age are not sufficient and often use as example that early teen
male children are physically larger and stronger than many adult women riders. Some
localities have either banned minors (typically those under 16 years of age) from using ATVs
or are considering such legislation. Advocates of ATVs argue that starting younger improves
safety. They recommend that children can develop the necessary expertise by starting as
young as 6 years of age instead of waiting until age 16. The U.S. Consumer Product Safety
Commission approved the sale of sub-50cc ATVs for use by youngsters as young as age 6.
In 1988, the All-terrain Vehicle Safety Institute (ASI) was formed to provide training and
education for ATV riders. The cost of attending the training is minimal and is free for
purchasers of new machines. Successful completion of training such as provided here is in
many states a minimum requirement for minor-age children to be granted permission to ride on
state lands.
[edit]
EPA Concerns
[edit]
Emissions
Due to the lack of emission controlling hardware and software, for year 2000 all
recreational spark ignited (SI) non-road vehicles (of which ATVs are a subset) contributed
8% of HC, .16% of NOx, 5% of CO and .8% of PM emissions for the entire non-road EPA family.
The entire range of non-road emissions accounted for 49% of engine produced emissions of all
types. (Source: EPA 1) While recreational SI vehicles (of which ATVs are a subset) produce
an aggregate of <4% of all HC emissions in the US, based on the relatively small population
of ATVs (<1.2M) and small annual usage (<350 hrs), EPA emission regulations now include such
engines starting with model year 2006. (source: EPA 2)
[edit]
Fuel Economy
The EPA estimates that each ATV consumes less than 59 gallons of fuel per year and obtains
between 40 and 50 mpg, making them not likely to fall under future fuel economy regulations.
(Ibid. EPA 1)
[edit]
Land Usage
Some ATV riders cross privately owned property in rural areas and travel overland where
their use is explicitly limitied to trails. Further, environmentalists criticize ATV riders
for excessive use in areas they consider biologically sensitive, especially wetlands and
sand dunes. While the deep treads on some ATV tires are effective for navigating rocky,
muddy, and root covered terrain, these treads also dig channels that may drain boggy areas,
increase sedimentation in streams at crossings and damage groomed snowmobile trails. Studies
have also shown that ATVs may help in the spread of invasive species such as knapweed. While
there is much scientific evidence regarding the impact of ATVs, its credibility often comes
under great scrutiny from ATV users who believe them to be overtly biased against ATVs.
To address these land usage concerns, well funded ATV advocacy groups have been organized to
purchase property and/or obtain permission of landowners, build and maintain trails suitable
for ATV riding and educate ATV riders about responsible riding. Many states have also formed
separate governing bodies that license ATVs separately than other ORVs. The monies from
these registrations are used to secure trails to ride and perform grooming and maintenance.
Unfortunately, the image of the great majority of responsible riders is often tainted by the
actions of some who ride off designated trails, on private land without permission, and
under the influence of alcohol or drugs. Additionally, self regulation has proven
particularly difficult considering that the main public complaint against ATVs is excessive
noise. Although the majority of ATVs comply with noise regulations, there are those whose
intentional violation can disturb the activities of other recreational users for miles
across open landscapes. Tampering with an ATVs exhaust silencer and spark arrestor is
illegal on all federal lands and most state lands, however enforcement is spotty. It is also
possible to install aftermarket exhaust systems that do not have silencers and spark
arrestors.
Fellow outdoor recreationists who have expressed concern about irresponsible ATV use are
snowmobile users who resent improper use of exclusive snowmobile trails, ATV trail riders
whose trails have been damaged by improper use and hunters whose game has been driven off by
those riding during prime hunting times.
Nationally, the US Forest Service considers managed ATV use to be a legitmate activity in
national forests, yet it also lists their unregulated use as one of the four greatest
threats to long term forest management. The US Forest Service recently released a national
travel management plan designed to minimize the negative environmental impacts of ATVs while
providing a safe, sustainable and enjoyable opportunity for ATV users.
SUV
A sport utility vehicle, or SUV, is a type of passenger vehicle which combines the
load-hauling and versatility of a pickup truck with the passenger-carrying space of a van or
station wagon. Most SUVs are designed with a roughly square cross-section, an engine
compartment, a combined passenger and cargo compartment, and no dedicated trunk. Most
mid-size and full-size SUVs have 5 or more seats, and a cargo area directly behind the last
row of seats. Mini SUVs, such as the Jeep Wrangler, may have fewer seats.
It is known in some countries as an off-roader or four wheel drive, often abbreviated to 4WD
or 4x4, and pronounced "four-by-four". More recently, SUVs designed primarily for driving on
roads have grown in popularity. A new category, the crossover SUV uses car components for
lighter weight and better fuel economy.
Design characteristics
SUVs were traditionally derived from light truck platforms, but several SUVs and crossover
SUVs are based on platforms of unibody construction.[1].
SUVs typically have high seating and most can be equipped with four wheel drive, providing
an advantage in low traction environments. The design also allows for a large engine
compartment, which allows for a wide variety of engine choices, both gasoline and diesel.
Popularity
SUVs became popular in the United States, Canada, and Australia in the 1990s and early 2000s
for a variety of reasons. Buyers became drawn to their large cabins, higher ride height, and
perceived safety when in the market for a new vehicle. Additionally, most full-size SUVs
have far greater towing capacities than conventional cars, allowing owners to tow RVs,
trailers, and boats with relative ease, adding to the utilitarian image.
A large growth in SUV popularity and sales is due to advertisement targeted towards women.
Women constitute more than half of SUV drivers, and SUVs are the most popular vehicle choice
of women in the United States. [citation needed]
In Australia, a unique situation resulted in the growth in popularity of SUVs. There, SUVs
have a much lower import duty compared with cars. This means a typical SUV has a significant
price advantage over a similarly-equipped, imported sedan. However, in recent years, the
import duty has been lowered for cars as well, and is currently at 10% (compared with 5% for
SUVs).
A common reason for SUV popularity cited by owners was their perceived safety advantage in a
collision with regular cars. For instance, the higher profile allows for better visability
and anticipation of danger. The enhanced weight helped reduce the risk of injury by a third
in children under the age of 16, though the roll-over fatality risk is much higher in SUVs
than cars negating the advantage.[citation needed] Some of their success could also be
attributed to their "utilitarian" image. In the late 1990s and early 2000s, vehicle
manufacturers sold SUVs very effectively, with per-vehicle profits substantially higher than
other automobiles. Historically, their simpler designs often made the vehicles cheaper to
make than comparably-priced cars.
In the mid 2000s, however, their popularity has waned, due to higher gasoline
prices[citation needed], rollover accident fatalities[citation needed] and higher relative
pollution.[citation needed] As of the spring of 2006, some of the larger SUVs now require
over 100USD per fillup, making thier everyday use more cost-prohibitive.[citation needed]
Current model SUVs (crossovers) take into account that 98% of SUV owners never
offroad[citation needed]. As such, SUVs now have lower ground clearance and suspension
designed primarily for paved road usage.
[edit]
SUVs in remote areas
SUVs are often used in places such as the Australian Outback, Africa, the Middle East,
Alaska, Northern Canada and most of Asia, which have limited paved roads and require the
vehicle to have all-terrain handling, increased range, and storage capacity. The low
availablity of spare parts and the need to carry out repairs quickly allow model vehicles
with the bare minimum of electric and hydraulic systems to predominate. Typical examples are
the Land Rover, the Toyota Land Cruiser and the Lada Niva.
SUVs targeted for use in civilization have traditionally originated from their more rugged
all-terrain counterparts. For example the Hummer H1 is derived from the HMMWV, originally
developed for the US Armed Forces.
[edit]
Other names
Outside of North America and India, these vehicles are known simply as four-wheel-drives,
often abbreviated to "4WD" or "4x4". They are classified as cars in countries such as the UK
where the U.S. distinction between cars and 'light trucks' is not used. In Australia, the
automotive industry and press have recently adopted the term SUV in place of four wheel
drive in the description of vehicles and market segments. "Utility" or "ute" refers to an
automobile with a flatbed rear or pick-up, typically seating two passengers and is often
used by tradesmen, and is typically not a 4WD vehicle.
[edit]
SUVs in recreation and motorsport
SUVs are also used to explore off-road places otherwise unreachable by vehicle or for the
sheer enjoyment of the driving. In Australia, China, Europe, South Africa and the U.S. at
least, many 4WD clubs have been formed for this purpose. Modified SUVs also take part in
races, most famously in the Paris-Dakar Rally, and the Australian Safari.
With the increasing urbanisation of the world, SUVs are also becoming more of a requirement
for those seeking unmodified landscapes and isolation, especially in nations with large
wilderness areas through which a viable road network could not be maintained without
excessive costs. Of course, roads are rarely constructed with scenic purposes foremost in
mind, instead trying to utilise the shortest and most economical length in order to reach a
specific destination and in many cases this means many natural features of interest are
inaccessible to cars. To travel with the absence of this infrastructure (which often leads
to settlements being built) serves to add to the appeal of SUV ownership due to a sense of
independence this invokes in many people, an ability to appreciate natural landscapes upon
their own terms.
The recreation value of SUVs also brings with it a pro environmentalist agenda which is
often overlooked in debates over their overall merits. By allowing owners to go off road,
SUVs promote a greater value being applied to wilderness areas, an attachment difficult to
gain through reading or simply seeing things on television. SUV clubs often promote this
ideal and a commercial manifestation of this can be seen in the number of tourism operators
dependent on SUVs for their activities, Australia being a strong example. Sensible off road
driving can promote a greater physical connection between people and the pristine
environment, something which has decreased with ever growing urban areas
Fuel economy
The recent popularity of SUVs is one reason the U.S. population consumes more gasoline than
in previous years. SUVs are as a class much less fuel efficient than comparable passenger
vehicles. The main reason is that SUVs are classified by the U.S. government as light
trucks, and thus are subject to the less strict light truck standard under the Corporate
Average Fuel Economy (CAFE) regulations. The CAFE requirement for light trucks is an average
of 20.7 mpg (US), versus 27.5 mpg (US) for passenger cars (8.6 and 11.4 km/L, respectively).
As there is little incentive to change the design, SUVs have numerous fuel-inefficient
features. The high profile of SUVs increases wind resistance. Heavier suspensions and larger
engines increase vehicle weight. Some SUVs also often come with tires designed for off-road
traction rather than low rolling resistance.
The low fuel economy is caused by
* high parasitic masses (compared to the average load) causing high energy demand in
transitional operation (in the cities) {P_{accel}= m_{vehicle} \cdot a \cdot v } where P
stands for power, mvehicle for the vehicle mass, a for acceleration and v for the vehicle
velocity.
* high cross-sectional area causing very high drag losses especially when driven at high
speed {P_{drag}= A_{cross} \cdot cw_{vehicle} \cdot \frac {v_{air}^3 \rho_{air}} {2} } where
P stands for the power, Across for the cross-sectional area of the vehicle, ρair for the
density of the air and vair for the relative velocity of the air (incl. wind)
* high rolling resistance due to all-terrain tires (even worse if low pressure is needed
offroad) and high vehicle mass driving the rolling resistance {P_{roll}= \mu_{roll} \cdot
m_{vehicle} \cdot v } where μroll stands for the rolling resistance factor and mvehicle for
the vehicle mass.
Average data for vehicle types sold in the U.S.A. (source theautochannel.com):
load-hauling and versatility of a pickup truck with the passenger-carrying space of a van or
station wagon. Most SUVs are designed with a roughly square cross-section, an engine
compartment, a combined passenger and cargo compartment, and no dedicated trunk. Most
mid-size and full-size SUVs have 5 or more seats, and a cargo area directly behind the last
row of seats. Mini SUVs, such as the Jeep Wrangler, may have fewer seats.
It is known in some countries as an off-roader or four wheel drive, often abbreviated to 4WD
or 4x4, and pronounced "four-by-four". More recently, SUVs designed primarily for driving on
roads have grown in popularity. A new category, the crossover SUV uses car components for
lighter weight and better fuel economy.
Design characteristics
SUVs were traditionally derived from light truck platforms, but several SUVs and crossover
SUVs are based on platforms of unibody construction.[1].
SUVs typically have high seating and most can be equipped with four wheel drive, providing
an advantage in low traction environments. The design also allows for a large engine
compartment, which allows for a wide variety of engine choices, both gasoline and diesel.
Popularity
SUVs became popular in the United States, Canada, and Australia in the 1990s and early 2000s
for a variety of reasons. Buyers became drawn to their large cabins, higher ride height, and
perceived safety when in the market for a new vehicle. Additionally, most full-size SUVs
have far greater towing capacities than conventional cars, allowing owners to tow RVs,
trailers, and boats with relative ease, adding to the utilitarian image.
A large growth in SUV popularity and sales is due to advertisement targeted towards women.
Women constitute more than half of SUV drivers, and SUVs are the most popular vehicle choice
of women in the United States. [citation needed]
In Australia, a unique situation resulted in the growth in popularity of SUVs. There, SUVs
have a much lower import duty compared with cars. This means a typical SUV has a significant
price advantage over a similarly-equipped, imported sedan. However, in recent years, the
import duty has been lowered for cars as well, and is currently at 10% (compared with 5% for
SUVs).
A common reason for SUV popularity cited by owners was their perceived safety advantage in a
collision with regular cars. For instance, the higher profile allows for better visability
and anticipation of danger. The enhanced weight helped reduce the risk of injury by a third
in children under the age of 16, though the roll-over fatality risk is much higher in SUVs
than cars negating the advantage.[citation needed] Some of their success could also be
attributed to their "utilitarian" image. In the late 1990s and early 2000s, vehicle
manufacturers sold SUVs very effectively, with per-vehicle profits substantially higher than
other automobiles. Historically, their simpler designs often made the vehicles cheaper to
make than comparably-priced cars.
In the mid 2000s, however, their popularity has waned, due to higher gasoline
prices[citation needed], rollover accident fatalities[citation needed] and higher relative
pollution.[citation needed] As of the spring of 2006, some of the larger SUVs now require
over 100USD per fillup, making thier everyday use more cost-prohibitive.[citation needed]
Current model SUVs (crossovers) take into account that 98% of SUV owners never
offroad[citation needed]. As such, SUVs now have lower ground clearance and suspension
designed primarily for paved road usage.
[edit]
SUVs in remote areas
SUVs are often used in places such as the Australian Outback, Africa, the Middle East,
Alaska, Northern Canada and most of Asia, which have limited paved roads and require the
vehicle to have all-terrain handling, increased range, and storage capacity. The low
availablity of spare parts and the need to carry out repairs quickly allow model vehicles
with the bare minimum of electric and hydraulic systems to predominate. Typical examples are
the Land Rover, the Toyota Land Cruiser and the Lada Niva.
SUVs targeted for use in civilization have traditionally originated from their more rugged
all-terrain counterparts. For example the Hummer H1 is derived from the HMMWV, originally
developed for the US Armed Forces.
[edit]
Other names
Outside of North America and India, these vehicles are known simply as four-wheel-drives,
often abbreviated to "4WD" or "4x4". They are classified as cars in countries such as the UK
where the U.S. distinction between cars and 'light trucks' is not used. In Australia, the
automotive industry and press have recently adopted the term SUV in place of four wheel
drive in the description of vehicles and market segments. "Utility" or "ute" refers to an
automobile with a flatbed rear or pick-up, typically seating two passengers and is often
used by tradesmen, and is typically not a 4WD vehicle.
[edit]
SUVs in recreation and motorsport
SUVs are also used to explore off-road places otherwise unreachable by vehicle or for the
sheer enjoyment of the driving. In Australia, China, Europe, South Africa and the U.S. at
least, many 4WD clubs have been formed for this purpose. Modified SUVs also take part in
races, most famously in the Paris-Dakar Rally, and the Australian Safari.
With the increasing urbanisation of the world, SUVs are also becoming more of a requirement
for those seeking unmodified landscapes and isolation, especially in nations with large
wilderness areas through which a viable road network could not be maintained without
excessive costs. Of course, roads are rarely constructed with scenic purposes foremost in
mind, instead trying to utilise the shortest and most economical length in order to reach a
specific destination and in many cases this means many natural features of interest are
inaccessible to cars. To travel with the absence of this infrastructure (which often leads
to settlements being built) serves to add to the appeal of SUV ownership due to a sense of
independence this invokes in many people, an ability to appreciate natural landscapes upon
their own terms.
The recreation value of SUVs also brings with it a pro environmentalist agenda which is
often overlooked in debates over their overall merits. By allowing owners to go off road,
SUVs promote a greater value being applied to wilderness areas, an attachment difficult to
gain through reading or simply seeing things on television. SUV clubs often promote this
ideal and a commercial manifestation of this can be seen in the number of tourism operators
dependent on SUVs for their activities, Australia being a strong example. Sensible off road
driving can promote a greater physical connection between people and the pristine
environment, something which has decreased with ever growing urban areas
Fuel economy
The recent popularity of SUVs is one reason the U.S. population consumes more gasoline than
in previous years. SUVs are as a class much less fuel efficient than comparable passenger
vehicles. The main reason is that SUVs are classified by the U.S. government as light
trucks, and thus are subject to the less strict light truck standard under the Corporate
Average Fuel Economy (CAFE) regulations. The CAFE requirement for light trucks is an average
of 20.7 mpg (US), versus 27.5 mpg (US) for passenger cars (8.6 and 11.4 km/L, respectively).
As there is little incentive to change the design, SUVs have numerous fuel-inefficient
features. The high profile of SUVs increases wind resistance. Heavier suspensions and larger
engines increase vehicle weight. Some SUVs also often come with tires designed for off-road
traction rather than low rolling resistance.
The low fuel economy is caused by
* high parasitic masses (compared to the average load) causing high energy demand in
transitional operation (in the cities) {P_{accel}= m_{vehicle} \cdot a \cdot v } where P
stands for power, mvehicle for the vehicle mass, a for acceleration and v for the vehicle
velocity.
* high cross-sectional area causing very high drag losses especially when driven at high
speed {P_{drag}= A_{cross} \cdot cw_{vehicle} \cdot \frac {v_{air}^3 \rho_{air}} {2} } where
P stands for the power, Across for the cross-sectional area of the vehicle, ρair for the
density of the air and vair for the relative velocity of the air (incl. wind)
* high rolling resistance due to all-terrain tires (even worse if low pressure is needed
offroad) and high vehicle mass driving the rolling resistance {P_{roll}= \mu_{roll} \cdot
m_{vehicle} \cdot v } where μroll stands for the rolling resistance factor and mvehicle for
the vehicle mass.
Average data for vehicle types sold in the U.S.A. (source theautochannel.com):
Hovercraft
A hovercraft, or air-cushion vehicle (ACV), is a vehicle or craft that can be supported by a
cushion of air ejected downwards against a surface close below it, and can in principle
travel over any relatively smooth surface, such as gently sloping land, water, or marshland,
while having no substantial contact with it.
The first recorded design for a vehicle which could be termed a Hovercraft was in 1716 by
Emanuel Swedenborg, a Swedish designer, philosopher and theologian. His man-powered air
cushion platform resembled an upside-down boat with a cockpit in the center and manually
operated oar-like scoops to push air under the vehicle on each downward stroke. No vehicle
was ever built, no doubt because it was realised that human effort could not have generated
enough lift.
In the mid-1870s, the British engineer Sir John Isaac Thornycroft built a number of ground
effect machine test models based on his idea of using air between the hull of a boat and the
water to reduce drag. Although he filed a number of patents involving air-lubricated hulls
in 1877, no practical applications were found. Over the years, various other people had
tried various methods of using air to reduce the drag on ships.
Early development of the modern "hovercraft" began with a design of American inventor
Charles J. Fletcher, who designed his "Glidemobile" while in the United States Navy during
World War II. The design worked on the principle of trapping a constant airflow against a
uniform surface (either the ground or water), providing anywhere from ten inches to two feet
of lift to free it from the surface, and control of the craft would be achieved by the
measured release of air. Shortly after being tested on Beezer's Pond in Fletcher's hometown
of Sparta Township, New Jersey, the design was immediately appropriated by the United States
Department of Defense and classified, denying Fletcher the opportunity to patent his
creation. Fletcher's claim as the original inventor was substantiated during the case of
British Hovercraft Ltd v. United States, in which the British corporation which maintained
the rights to Sir Christopher Cockerell's patent unsuccessfully sought to win $104,000,000
in lost royalties.
Col. Melville W. Beardsley (1913-1998), an American inventor and aeronautical engineer,
along with Dr. W. Bertelsen worked on developing early ACV's in the USA.
In 1952 the British inventor Christopher Cockerell designed a vehicle based on his
'hovercraft principle'. He was knighted for his services to engineering in 1969. Sir
Christopher invented the word 'Hovercraft' to describe his invention.
Cockerell used simple experiments involving a vacuum cleaner motor and two cylindrical cans.
He proved the workable principle of a vehicle suspended on a cushion of air blown out under
pressure, making the vehicle easily mobile over most surfaces. The supporting air cushion
would enable it to operate over soft mud, water, and marshes and swamps as well as on firm
ground.
The British aircraft manufacturer Saunders Roe which had aeronautical expertise developed
the first practical man-carrying hovercraft, the SR-N1, which carried out several test
programmes in 1959 to 1961 (the first public demonstration in 1959), including a
cross-channel run. The SR-N1 was powered by one (piston) engine, driven by expelled air, and
could carry little more than its own weight and two men,and did not have any skirt at first
trials. It was found that the craft's lift was improved by the addition of a 'skirt' of
flexible fabric or rubber around the hovering surface, to contain the air. The skirt was an
independent invention made by a Royal Navy officer who worked with Sir Christopher to
develop the idea further.
The first true passenger-carrying hovercraft was the Vickers VA-3, which in the summer of
1961 carried passengers regularly along the North Wales Coast from Wallasey to Rhyl. It was
powered by two turboprop aero-engines and driven by propellers. During the 1960s Saunders
Roe developed several larger designs which could carry passengers, including the SR-N2,
which operated across the Solent in 1962 and later the SR-N6, which operated across the
Solent from Southsea to Ryde on the Isle of Wight for many years. Operations commenced on
24th July 1965 using the SR-N6 which carried just 38 passengers. Two modern 98 seat AP1-88
hovercraft now ply this route, and over 20 million passengers have used the service as of
2004.
Bell licenced and sold the SRN-5 as the Bell SK-5. There were deployed on trial to the
Vietnam War by the Navy as PACV patrol craft in the Mekong Delta where their mobility and
speed was unique. Advanced AACVs were developed with automated turrets and slab sides, but
use was eventually abandoned. Experience led to the proposed Bell SK-10 which was the basis
for the LCAC now deployed.
As well as Saunders Roe and Vickers (which combined in 1966 to form the British Hovercraft
Corporation), other commercial craft were developed during the 1960s in the UK by
Cushioncraft (part of the Britten-Norman Group) and Hovermarine (the latter being 'sidewall'
type hovercraft, where the sides of the hull projected down into the water to trap the
cushion of air).
In the late 1960s and early 1970s, Jean Bertin developed a hovercraft train dubbed the
Aérotrain in France. His I-80 prototype established the world speed record for overland air
cushion vehicles with a mean speed of 417.6 km/h (260 mp/h) and a top speed of 430 km/h (267
mp/h).
By 1970 the largest British hovercraft were in service, the Mountbatten class SR-N4 model,
each powered by four Rolls-Royce Proteus engines, regularly carrying cars and passengers
across the English Channel from Dover or Ramsgate to Calais. This service ceased in 2000
after years of competition with traditional ferries, catamarans, and the opening of the
Channel tunnel.
In 1998, the US Postal Service began using the British built Hoverwork AP.1-88 to haul mail,
freight, and passengers from Bethel, Alaska to and from eight small villages along the
Kuskokwim River. Bethel is far removed from the Alaska road system, thus making the
hovercraft an attractive alternative to the air based delivery methods used prior to
introduction of the hovercraft service. Hovercraft service is suspended for several weeks
each year while the river is beginning to freeze to minimize damage to the river ice
surface. The hovercraft is perfectly able to operate during the freeze-up period, however,
it could potentially break the ice creating hazards for the villagers using their
snowmobiles for transportation along the river during the early winter.
The commercial success of hovercraft suffered from rapid rises in fuel prices during the
late 1960s and 1970s following conflict in the Middle East. Alternative over-water vehicles
such as wave-piercing catamarans (marketed as the Seacat in Britain) use less fuel and can
perform most of the hovercraft's marine tasks. Although developed elsewhere in the world for
both civil and military purposes, except for the Solent Ryde to Southsea crossing,
hovercraft disappeared from the coastline of Britain until a range of Griffon Hovercraft
were bought by the Royal National Lifeboat Institution.
There are an increasing number of small homebuilt and kit-built hovercraft used for fun and
racing purposes, mainly on inland lakes and rivers but also in marshy areas and in some
estuaries.
Hovercraft typically have two (or more) separate engines (some craft, such as the SR-N6,
have one engine with a drive split through a gearbox). One engine drives the fan (aka the
impeller) which is responsible for lifting the vehicle by forcing air under the craft. One
or more additional engines are used to provide thrust in order to propel the craft in the
desired direction. Some hovercraft utilise ducting to allow one engine to perform both tasks
by directing some of the air to the skirt, the rest of the air passing out of the back to
push the craft forward.
cushion of air ejected downwards against a surface close below it, and can in principle
travel over any relatively smooth surface, such as gently sloping land, water, or marshland,
while having no substantial contact with it.
The first recorded design for a vehicle which could be termed a Hovercraft was in 1716 by
Emanuel Swedenborg, a Swedish designer, philosopher and theologian. His man-powered air
cushion platform resembled an upside-down boat with a cockpit in the center and manually
operated oar-like scoops to push air under the vehicle on each downward stroke. No vehicle
was ever built, no doubt because it was realised that human effort could not have generated
enough lift.
In the mid-1870s, the British engineer Sir John Isaac Thornycroft built a number of ground
effect machine test models based on his idea of using air between the hull of a boat and the
water to reduce drag. Although he filed a number of patents involving air-lubricated hulls
in 1877, no practical applications were found. Over the years, various other people had
tried various methods of using air to reduce the drag on ships.
Early development of the modern "hovercraft" began with a design of American inventor
Charles J. Fletcher, who designed his "Glidemobile" while in the United States Navy during
World War II. The design worked on the principle of trapping a constant airflow against a
uniform surface (either the ground or water), providing anywhere from ten inches to two feet
of lift to free it from the surface, and control of the craft would be achieved by the
measured release of air. Shortly after being tested on Beezer's Pond in Fletcher's hometown
of Sparta Township, New Jersey, the design was immediately appropriated by the United States
Department of Defense and classified, denying Fletcher the opportunity to patent his
creation. Fletcher's claim as the original inventor was substantiated during the case of
British Hovercraft Ltd v. United States, in which the British corporation which maintained
the rights to Sir Christopher Cockerell's patent unsuccessfully sought to win $104,000,000
in lost royalties.
Col. Melville W. Beardsley (1913-1998), an American inventor and aeronautical engineer,
along with Dr. W. Bertelsen worked on developing early ACV's in the USA.
In 1952 the British inventor Christopher Cockerell designed a vehicle based on his
'hovercraft principle'. He was knighted for his services to engineering in 1969. Sir
Christopher invented the word 'Hovercraft' to describe his invention.
Cockerell used simple experiments involving a vacuum cleaner motor and two cylindrical cans.
He proved the workable principle of a vehicle suspended on a cushion of air blown out under
pressure, making the vehicle easily mobile over most surfaces. The supporting air cushion
would enable it to operate over soft mud, water, and marshes and swamps as well as on firm
ground.
The British aircraft manufacturer Saunders Roe which had aeronautical expertise developed
the first practical man-carrying hovercraft, the SR-N1, which carried out several test
programmes in 1959 to 1961 (the first public demonstration in 1959), including a
cross-channel run. The SR-N1 was powered by one (piston) engine, driven by expelled air, and
could carry little more than its own weight and two men,and did not have any skirt at first
trials. It was found that the craft's lift was improved by the addition of a 'skirt' of
flexible fabric or rubber around the hovering surface, to contain the air. The skirt was an
independent invention made by a Royal Navy officer who worked with Sir Christopher to
develop the idea further.
The first true passenger-carrying hovercraft was the Vickers VA-3, which in the summer of
1961 carried passengers regularly along the North Wales Coast from Wallasey to Rhyl. It was
powered by two turboprop aero-engines and driven by propellers. During the 1960s Saunders
Roe developed several larger designs which could carry passengers, including the SR-N2,
which operated across the Solent in 1962 and later the SR-N6, which operated across the
Solent from Southsea to Ryde on the Isle of Wight for many years. Operations commenced on
24th July 1965 using the SR-N6 which carried just 38 passengers. Two modern 98 seat AP1-88
hovercraft now ply this route, and over 20 million passengers have used the service as of
2004.
Bell licenced and sold the SRN-5 as the Bell SK-5. There were deployed on trial to the
Vietnam War by the Navy as PACV patrol craft in the Mekong Delta where their mobility and
speed was unique. Advanced AACVs were developed with automated turrets and slab sides, but
use was eventually abandoned. Experience led to the proposed Bell SK-10 which was the basis
for the LCAC now deployed.
As well as Saunders Roe and Vickers (which combined in 1966 to form the British Hovercraft
Corporation), other commercial craft were developed during the 1960s in the UK by
Cushioncraft (part of the Britten-Norman Group) and Hovermarine (the latter being 'sidewall'
type hovercraft, where the sides of the hull projected down into the water to trap the
cushion of air).
In the late 1960s and early 1970s, Jean Bertin developed a hovercraft train dubbed the
Aérotrain in France. His I-80 prototype established the world speed record for overland air
cushion vehicles with a mean speed of 417.6 km/h (260 mp/h) and a top speed of 430 km/h (267
mp/h).
By 1970 the largest British hovercraft were in service, the Mountbatten class SR-N4 model,
each powered by four Rolls-Royce Proteus engines, regularly carrying cars and passengers
across the English Channel from Dover or Ramsgate to Calais. This service ceased in 2000
after years of competition with traditional ferries, catamarans, and the opening of the
Channel tunnel.
In 1998, the US Postal Service began using the British built Hoverwork AP.1-88 to haul mail,
freight, and passengers from Bethel, Alaska to and from eight small villages along the
Kuskokwim River. Bethel is far removed from the Alaska road system, thus making the
hovercraft an attractive alternative to the air based delivery methods used prior to
introduction of the hovercraft service. Hovercraft service is suspended for several weeks
each year while the river is beginning to freeze to minimize damage to the river ice
surface. The hovercraft is perfectly able to operate during the freeze-up period, however,
it could potentially break the ice creating hazards for the villagers using their
snowmobiles for transportation along the river during the early winter.
The commercial success of hovercraft suffered from rapid rises in fuel prices during the
late 1960s and 1970s following conflict in the Middle East. Alternative over-water vehicles
such as wave-piercing catamarans (marketed as the Seacat in Britain) use less fuel and can
perform most of the hovercraft's marine tasks. Although developed elsewhere in the world for
both civil and military purposes, except for the Solent Ryde to Southsea crossing,
hovercraft disappeared from the coastline of Britain until a range of Griffon Hovercraft
were bought by the Royal National Lifeboat Institution.
There are an increasing number of small homebuilt and kit-built hovercraft used for fun and
racing purposes, mainly on inland lakes and rivers but also in marshy areas and in some
estuaries.
Hovercraft typically have two (or more) separate engines (some craft, such as the SR-N6,
have one engine with a drive split through a gearbox). One engine drives the fan (aka the
impeller) which is responsible for lifting the vehicle by forcing air under the craft. One
or more additional engines are used to provide thrust in order to propel the craft in the
desired direction. Some hovercraft utilise ducting to allow one engine to perform both tasks
by directing some of the air to the skirt, the rest of the air passing out of the back to
push the craft forward.
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