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A jetboat is a
boat
A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats.
Small boats are typically found on inl ...
propelled by a jet of water ejected from the back of the craft. Unlike a
powerboat
A motorboat, speedboat or powerboat is a boat that is exclusively 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 gea ...
or
motorboat
A motorboat, speedboat or powerboat is a boat that is exclusively 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 gea ...
that uses an external
propeller in the water below or behind the boat, a jetboat draws the water from under the boat through an
intake
An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...
and into a
pump-jet
A pump-jet, hydrojet, or water jet is a marine system that produces a jet of water for propulsion. The mechanical arrangement may be a ducted propeller ( axial-flow pump), a centrifugal pump, or a mixed flow pump which is a combination of bot ...
inside the boat, before expelling it through a
nozzle at the
stern.
The modern jetboat was developed by
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
engineer
Sir William Hamilton in the mid-1950s. His goal was a boat to run up the fast-flowing rivers of
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
that were too shallow for propellers.
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's 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. The popularity of the jet unit and jetboat increased rapidly. It was found the waterjet was better than propellers for a wide range of vessel types, and waterjets are now used widely for many high-speed vessels including passenger ferries, rescue craft, patrol boats and offshore supply vessels.
Jetboats are highly manoeuvrable, and many can be reversed from full speed and brought to a stop within little more than their own length, in a manoeuvre known as a "crash stop". The well known ''Hamilton turn'' or "jet spin" is a high-speed manoeuvre 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 whether they are useful depends on the type of application. Classic prop-drives are generally more efficient and economical at low speeds, up to about , but as boat speed increases, the extra
hull resistance generated by struts,
rudder
A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adve ...
s, shafts and so on means waterjets are more efficient up to . For very large propellers turning at slow speeds, such as in
tugboat
A tugboat or tug is a marine vessel that manoeuvres other vessels by pushing or pulling them, with direct contact or a tow line. These boats typically tug ships in circumstances where they cannot or should not move under their own power, su ...
s, the equivalent size waterjet would be too big to be practical. The vast majority of waterjet units are therefore installed in high-speed vessels and in situations where shallow draught, maneuverability, and load flexibility are the main concerns.
The biggest jet-driven vessels are found in military use and the high-speed passenger and car ferry industry. South Africa's
s (approximately long) and the long United States
Littoral Combat Ship
The littoral combat ship (LCS) is either of two classes of relatively small surface vessels designed for operations near shore by the United States Navy. It was "envisioned to be a networked, agile, stealthy surface combatant capable of defeat ...
are among the biggest jet-propelled vessels . Even these 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
accelerating
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by th ...
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
Newton's
third law— 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
impeller
An impeller or impellor is a rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid.
In pumps
An impeller is a rotating componen ...
s and
stators – 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
transom 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
hydrofoils, which use underwater
wing
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is e ...
s or struts to lift the vessel clear of the water, standard jetboats use a conventional
planing hull
Planing may refer to:
* Planing (boat) or hydroplaning, a method by which a hull skims over the surface of the water
* Hydroplaning (tires), a loss of traction caused by a layer of water between the tires and the road surface
* Using a plane (t ...
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 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 an operator may 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 Zealand
Queenstown ( mi, Tāhuna) is a resort town in Otago in the south-west of New Zealand's South Island. It has an urban population of
The town is built around an inlet called Queenstown Bay on Lake Wakatipu, a long, thin, Z-shaped lake forme ...
, where jetboats are used extensively for
adventure tourism
Adventure travel is a type of niche tourism, involving exploration or travel with a certain degree of risk (real or perceived), and which may require special skills and physical exertion. In the United States, adventure tourism has grown in r ...
, 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
Excitor in the
Bay of Islands
The Bay of Islands is an area on the east coast of the Far North District of the North Island of New Zealand. It is one of the most popular fishing, sailing and tourist destinations in the country, and has been renowned internationally for it ...
.
Applications
![EFV hydroplaning](https://upload.wikimedia.org/wikipedia/commons/e/e7/EFV_hydroplaning.jpg)
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 rescue,
farming
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled peopl ...
,
fishing
Fishing is the activity of trying to catch fish. Fish are often caught as wildlife from the natural environment, but may also be caught from stocked bodies of water such as ponds, canals, park wetlands and reservoirs. Fishing techniques inclu ...
,
exploration
Exploration refers to the historical practice of discovering remote lands. It is studied by geographers and historians.
Two major eras of exploration occurred in human history: one of convergence, and one of divergence. The first, covering most ...
,
pleasure boating, and other water activities where motor boats are used. Jetboats can also be raced for sport, both on rivers
World Champion Jet Boat Marathonheld in Mexico, Canada, USA and New Zealand) and on specially designed racecourses known as sprint tracks. Recently there has been increasing use of jetboats in the form of
rigid-hulled inflatable boat
A rigid inflatable boat (RIB), also rigid-hull inflatable boat or rigid-hulled inflatable boat (RHIB), is a lightweight but high-performance and high-capacity boat constructed with a rigid hull bottom joined to side-forming air tubes that are i ...
s and as
luxury yacht tender
A yacht tender is a vessel used for servicing and providing support and entertainment to a private or charter yacht. They include utilitarian craft, powered by oar or outboard motor, and high-speed luxury craft, supporting superyachts, powered by ...
s. Many jetboats are small enough to be carried on a trailer and towed by car.
As jetboats have no external rotating parts they are safer for swimmers and
marine life, though they can be struck by the hull. The safety benefit itself can sometimes be reason enough to use this type of propulsion.
In 1977, Sir
Edmund Hillary
Sir Edmund Percival Hillary (20 July 1919 – 11 January 2008) was a New Zealand mountaineer, explorer, and philanthropist. On 29 May 1953, Hillary and Sherpa mountaineer Tenzing Norgay became the first climbers confirmed to have reache ...
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 jet unit's intake grill can have quite an adverse effect.
Another disadvantage of jetboats appears to be that they are more sensitive to engine/jet unit 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.
See also
*
Personal water craft
A personal watercraft (PWC), also called water scooter or jet ski, is a recreational watercraft that a rider sits or stands on, not within, as in a boat. PWCs have two style categories, first and most popular being a runabout or "sit down" whe ...
*
Sport
Sport pertains to any form of competitive physical activity or game that aims to use, maintain, or improve physical ability and skills while providing enjoyment to participants and, in some cases, entertainment to spectators. Sports can, ...
*
Jet sprint boat racing
References
External links
Hamilton waterjet history
Motorboats
Marine propulsion
New Zealand inventions