A
jetboat is a
boatA boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...
propelled by a jet of water ejected from the back of the craft. Unlike a
powerboatA powerboat is another name for a motorboat. Powerboat may also refer to:* Powerboating* F1 Powerboat World Championship* Offshore powerboat racing...
or
motorboatA motorboat is a boat which is powered by an engine. Some motorboats are fitted with inboard engines, others have an outboard motor installed on the rear, containing the internal combustion engine, the gearbox and the propeller in one portable unit.An inboard/outboard contains a hybrid of a...
that uses a
propellerA propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
in the water below or behind the boat, a jetboat draws the water from under the boat into a
pumpA pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
inside the boat, then expels it through a nozzle at the
sternThe stern is the rear or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite of the bow, the foremost part of a ship. Originally, the term only referred to the aft port section...
.
Jetboats were originally designed by
Sir William HamiltonSir Charles William Feilden Hamilton , commonly known as Bill Hamilton, was a New Zealander who developed the modern jetboat, and founder of what is now the world's leading water jet manufacturing company - CWF Hamilton Ltd .Hamilton never claimed to have invented the jet boat. He once said "I do...
(who developed a waterjet in 1954) for operation in the fast-flowing and shallow rivers of
New ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, specifically to overcome the problem of propellers striking rocks in such waters.
Previous attempts at waterjet propulsion had very short lifetimes, generally due to the inefficient design of the units and the fact that they offered few advantages over conventional propellers. Unlike these previous waterjet developments, such as
Campini'sSecondo Campini was an Italian engineer and one of the pioneers of the jet engine.Campini was born at Bologna, Emilia-Romagna. In 1931 he wrote a proposal for the Italian Air Ministry on the value of jet propulsion and in 1932 demonstrated a jet-powered boat in Venice...
and the Hanley Hydrojet, Hamilton had a specific need for a propulsion system to operate in very-shallow water, and the waterjet proved to be the ideal solution. From this, the popularity of the jet unit and jetboat increased rapidly. Through further developments, it was found the waterjet offered several other advantages over propellers for a wide range of vessel types, and as such, waterjets are used widely today for many high-speed vessels, including passenger ferries, rescue craft, patrol boats and offshore supply vessels.
Jetboats are highly maneuverable, and many can, from full speed, be reversed and brought to a stop within little more than their own length, in a
maneuverManeuver, manoeuvre may be a synonym for strategy or tactic. It is the manipulation of a situation in order to gain some advantage.- Military or naval movement :* Military exercise...
known as a "crash stop". The well known
Hamilton turn or "jet spin" is a high-speed maneuvre where the boat's engine throttle is cut, the steering is turned sharply and the throttle opened again, causing the boat to spin quickly around with a large spray of water.
There is no engineering limit to the size of jetboats, though the validity of their use depends a lot on the type of application. Classic prop-drives are generally more efficient and economical at low speeds, up to about 20 knots (10.9 m/s), but as boat speed increases beyond this, the extra
hullA hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
resistance generated by struts,
rudderA rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
s, shafts (etc.) means waterjets are more efficient in the 20-50 knot range (up to 50 knot). Also, in situations with very large propellers turning at slow speeds (such as tug boats), the equivalent size waterjet would be too big to be practical. For these reasons, the vast majority of waterjet units are installed in high-speed vessels and in particular situations where shallow draught, maneuverability, and load flexibility are main concerns.
The biggest jet-driven vessels are found in military use or the high-speed passenger/car ferry industry. South Africa's s (approximately 120 metre long) are the biggest jet-propelled vessels so far. Even these German-built vessels are capable of performing "crash stops".
Function
A conventional screw propeller works within the body of water below a boat hull, effectively "screwing" through the water to drive a vessel forward by generating a difference in pressure between the forward and rear surfaces of the propeller blades and by
acceleratingIn physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
a mass of water rearward. By contrast, a waterjet unit delivers a high-pressure "push" from the stern of a vessel by accelerating a volume of water as it passes through a specialised pump mounted above the waterline inside the boat hull. Both methods yield thrust due to
NewtonSir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
's
third lawNewton's laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces...
— every action has an equal and opposite reaction.
In a jetboat, the waterjet draws water from beneath the hull, where it passes through a series of
impellerAn impeller is a rotor inside a tube or conduit used to increase the pressure and flow of a fluid.- Impellers in pumps :...
s and
statorThe stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
s - known as stages - which increase the velocity of the waterflow. Most modern jets are single-stage, while older waterjets may have as many as three stages. The tail section of the waterjet unit extends out through the
transomTransom may refer to:* Transom , a bar of wood or stone across the top of a door or window* Transom , one of the beams running athwart the ship's hull at the fashion timbers or the surface that forms the flat back panel of a stern of a vessel* Operation Transom, a major bombing raid* Transom knot,...
of the hull, above the waterline. This jetstream exits the unit through a small nozzle at high velocity to push the boat forward. Steering is accomplished by moving this nozzle to either side, or less commonly, by small gates on either side that deflect the jetstream. Because the jetboat relies on the flow of water through the nozzle for control, it is not possible to steer a conventional jetboat without the engine running.
Unlike conventional propeller systems where the rotation of the propeller is reversed to provide astern movement, a waterjet will continue to pump normally while a deflector is lowered into the jetstream after it leaves the outlet nozzle. This deflector redirects thrust forces forward to provide reverse thrust. Most highly-developed reverse deflectors redirect the jetstream down and to each side to prevent recirculation of the water through the jet again, which may cause aeration problems, or increase reverse thrust. Steering is still available with the reverse deflector lowered so the vessel will have full maneuverability. With the deflector lowered about halfway into the jetstream, forward and reverse thrust are equal so the boat maintains a fixed position, but steering is still available to allow the vessel to turn on the spot - something which is impossible with a conventional single propeller.
Unlike
hydrofoilA hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the...
s, which use underwater
wingA wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
s or struts to lift the vessel clear of the water, standard jetboats use a conventional
planing hullPlaning is the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift .-History:...
to ride across the water surface, with only the rear portion of the hull displacing any water. With the majority of the hull clear of the water, there is reduced drag, greatly enhancing speed and maneuverability, so jetboats are normally operated at
planing-Physical objects:* Aeroplane or airplane, a fixed-wing aircraft* Plane , a woodworking tool to smooth surfaces* Platanus, a genus of trees with the common name "plane"* Acer pseudoplatanus, a tree species sometimes called "plane"...
speed. At slower speeds with less water pumping through the jet unit, the jetboat will lose some steering control and maneuverability and will quickly slow down as the hull comes off its planing state and hull resistance is increased. However, loss of steering control at low speeds can be overcome by lowering the reverse deflector slightly and increasing throttle - so you increase thrust and thus control without increasing boat speed itself. A conventional river-going jetboat will have a shallow-angled (but not flat-bottomed) hull to improve its high-speed cornering control and stability, while also allowing it to traverse very shallow water. At speed, jetboats can be safely operated in less than 7.5 cm (3 inches) of water.
One of the most significant breakthroughs, in the development of the waterjet, was to change the design so it expelled the jetstream
above the water line, contrary to many people's intuition. Hamilton discovered early on that this greatly improved performance, compared to expelling below the waterline, while also providing a "clean" hull bottom (i.e. nothing protruding below the hull line) to allow the boat to skim through very shallow water. It makes no difference to the amount of thrust generated whether the outlet is above or below the waterline, but having it above the waterline reduces hull resistance and draught. Hamilton's first waterjet design had the outlet below the hull and actually in front of the inlet. This probably meant that disturbed water was entering the jet unit and reducing its performance, and the main reason why the change to above the waterline made such a difference.
Queenstown, New ZealandQueenstown is a resort town in Otago in the south-west of New Zealand's South Island. It is built around an inlet called Queenstown Bay on Lake Wakatipu, a long thin Z-shaped lake formed by glacial processes, and has spectacular views of nearby mountains....
, where jetboats are used extensively for adventure tourism, claims to be the jetboat capital of the world, and jetboats are very common for many coastal and riverine tourism activities in the country, such as the
ExcitorThe Excitor is a fast boat tourist experience in the Bay of Islands, New Zealand. It is used on a high-speed trip through the scenic bay out to Cape Brett, where the boat travels through the 'Hole in the Rock', a natural sea-tunnel...
in the
Bay of IslandsThe Bay of Islands is an area in the Northland Region of the North Island of New Zealand. Located 60 km north-west of Whangarei, it is close to the northern tip of the country....
.
Applications
Applications for jetboats include most activities where conventional propellers are also used, but in particular passenger ferry services, coastguard and police patrol, navy and military, adventure tourism (which is becoming increasingly popular around the globe), pilot boat operations,
surf rescueA rescue lifeboat is a boat rescue craft which is used to attend a vessel in distress, or its survivors, to rescue crewmen and passengers. It can be hand pulled, sail powered or powered by an engine...
, farming,
fishingFishing is the activity of trying to catch wild fish. Fish are normally caught in the wild. Techniques for catching fish include hand gathering, spearing, netting, angling and trapping....
,
explorationExploration is the act of searching or traveling around a terrain for the purpose of discovery of resources or information. Exploration occurs in all non-sessile animal species, including humans...
, pleasure boating, and other water activities where motor boats are used. Jetboats can also be raced for sport, both on rivers and on specially designed racecourses. Recently there has been increasing use of jetboats in the form of
rigid-hulled inflatable boatA rigid-hulled inflatable boat, or rigid-inflatable boat is a light-weight but high-performance and high-capacity boat constructed with a solid, shaped hull and flexible tubes at the gunwale. The design is stable and seaworthy...
s and as
luxury yacht tenderA luxury yacht tender is a transport used to service and to provide support and entertainment to a private or charter luxury yacht. Known commonly as the 'yacht's tender' a luxury yacht tender will often be a rigid-hulled inflatable boat which features cushioned inflatable rubber inner tubes...
s. Many jetboats are small enough to be carried on a trailer and towed by car.
One very important feature of the jetboat is the fact that it has no external rotating parts; it is thus safer for swimmers and marine life, though these can still be struck by the hull. The safety benefit itself can sometimes be reason enough to use this type of propulsion.
In 1977, Sir
Edmund HillarySir Edmund Percival Hillary, KG, ONZ, KBE , was a New Zealand mountaineer, explorer and philanthropist. On 29 May 1953 at the age of 33, he and Sherpa mountaineer Tenzing Norgay became the first climbers known to have reached the summit of Mount Everest – see Timeline of climbing Mount Everest...
led a jetboat expedition, titled "Ocean to Sky", from the mouth of the Ganges River to its source. One of the jetboats was sunk by a friend of Hillary.
Drawbacks
The fuel efficiency and performance of a jetboat can be affected by anything that disrupts the smooth flow of water through the jet unit. For example, a plastic bag sucked onto the jetunit's intake grill can have quite an adverse effect.
Another disadvantage of jetboats appears to be that they are more-sensitive to engine/jetunit mismatch, compared with the problem of engine/propeller mismatch in propeller-driven craft. If the jet-propulsion unit is not well-matched to the engine performance, inefficient fuel consumption and poor performance can result.
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