jetboat is a boat driven by a jet of water released from behind the plane. Unlike a powerboat or a motor boat that uses an external propeller in the water below or behind the boat, a jet boat pulls water from underneath the ship through the intake and into the pump jet in the boat, before removing it through a nozzle at the stern.
The jet boat was originally designed by Sir William Hamilton (who developed a waterjet in 1954) for operations in New Zealand's rushing and shallow streams, particularly to overcome the problem of propellers crashing into the rocks in the waters.
Previous attempts at waterjet propulsion have very short durability, generally due to inefficient unit design and the fact that they offer several advantages over conventional propellers. Unlike previous waterjet developments, such as Campini and Hanley Hydrojet, Hamilton has a special need for propulsion systems to operate in very shallow water, and waterjet proves to be the ideal solution. From here, the popularity of jet and jetboat units is increasing rapidly. Through further developments, waterjet offers several other advantages over propellers for different types of vessels, and as such, waterjets are widely used today for many high-speed vessels, including passenger ferries, rescue boats, patrol boats and offshore. ship supplies.
Jetboat is very maneuverable, and many can, from full speed, be reversed and blocked in time a little longer than its length, in a maneuver known as "crash-stop". The famous Hamilton's turn or jet spin is a high-speed maneuver in which the boat's engine throttle is cut, steering is rotated sharply and the throttle is open again, causing the boat to spin rapidly with a large spray of water.
There is no technical limit to the size of jet vessels, although the validity of their use is highly dependent on the type of application. Classic prop-drives are generally more efficient and economical at low speeds, up to about 20 knots (37 km/h, 23 mph), but as ship speed increases beyond this, extra hull resistance generated by struts, rudders, shafts (etc.) meaning waterjets are more efficient in the range of 20-50 knots (up to 90 km/h, 60 mph). Also, in situations with very large propellers spinning at a slow speed (like a tugboat), the equivalent size waterjet would be too large to be practical. For this reason, most of the waterjet units are installed in high speed vessels and in certain situations where shallow drafts, maneuverability, and load flexibility are of primary concern.
The largest jet-driven vessels are found in military or passenger fleet/high-speed passenger car use. South African Valor frigates (about 120 meters or 390 feet long) and United States Combat Ships (127 meters or 417 feet long) are among the largest jet-propelled vessels so far. Even these ships are capable of "crash stops".
Video Jetboat
Function
A conventional screw blades work inside a water body under the hull of the boat, effectively "disrupting" through water to propel the vessel forward by generating a pressure difference between the front and rear surface of the propeller blades and by accelerating the rear water masses. Instead, the waterjet unit sends a high pressure "pressure" from the stern of the vessel by accelerating the volume of water as it passes through a special pump mounted above the water level inside the boat's hull. Both methods generate encouragement because of Newton's third law - every action has the same and opposite reaction.
In a jetboat, the waterjet draws water from beneath the hull, where it passes through a series of impellers and stators - known as stages - that increase the velocity of water flow. Most modern jets are single stage, while older waterjets may have three stages. The tail of the waterjet unit extends outward through the gastric transom, above the water level. The jetstream comes out of the unit through a small nozzle at high speed to push the ship forward. Steering is done by moving this nozzle to one side, or more rarely, by a small gate on either side that deflects the jet stream. Because jetboat depends on the water flow through the nozzle for control, it is not possible to drive a conventional jet boat without a running engine.
Unlike conventional vane systems where rotation of the blades is reversed to provide an astern movement, the waterjet will continue to pump normally while the deflector is lowered to jetstream after leaving the outlet nozzle. This deflector redirects the force forward to give a back impulse. Most highly reversed deflectors direct jet flow down and to each side to prevent water recirculation through the jet again, which may cause aeration problems, or increase back pressure. The steering is still available with the reverse deflector lowered so that the ship will have full maneuverability. With the deflector lowering about half way down the jetstream, the forward and backward thrust are the same so the boat maintains a fixed position, but the steering is still available to allow the vessel to ignite the spot - something impossible with conventional propellers.
Unlike hydrofoils, which use underwater wings or struts to lift clean vessels from water, standard jet boats use conventional hull planning to rise on the water surface, with only the back of the hull replacing any water. With most of the stomach from the water, there is a drag reduction, greatly improving the speed and maneuverability, so jetboat is usually operated at planing speed. At lower speeds with less water pumping through the jet unit, the jetboat loses steering control and maneuverability and will rapidly slow down when the stomach comes from its planning conditions and gastric resistance increases. However, the loss of steering control at low speeds can be overcome by lowering the reverse deflector slightly and increasing the speed - so the operator can increase the thrust and thus control it without increasing the speed of the boat itself. Conventional jet boats will have shallow (but not flat-bottomed) hulls to improve control and high-speed cornering stability, while also allowing to cross very shallow water. At speed, the jetboat can be safely operated in water less than 7.5 cm (3 inches).
One of the most significant breakthroughs, in the development of waterjet, is to change the design so as to expel jetstream above the waterline, contrary to the intuition of many people. Hamilton found early on that this greatly improved performance, compared to expelling beneath the surface of the water, while also providing a "clean" stomach base (ie nothing that stood out below the line of the stomach) to allow the boat to pull over through very shallow water. It makes no difference to the amount of thrust generated whether the outlet is above or below the water surface, but afterwards above the water level reduces gastric and draft resistance. Hamilton's first waterjet design has an outlet under the hull and is actually in front of the inlet. This may mean that disturbed water enters the jet unit and reduces its performance, and the main reason why changes above the water level make a big difference.
Queenstown, New Zealand, where jetboat is widely used for adventure tourism, claims to be the jetboat capital of the world, and jet boats are very common for many of the country's coastal and river tourism activities, such as the Excitor in the Bay of Islands.
Maps Jetboat
Apps
Applications for jetboats cover most of the activities where conventional propellers are also used, but in certain passenger ferry services, coast guard and police patrols, navies and military, adventure tours (which are becoming increasingly popular around the world), pilot vessel operations, surfing safer, farming, fishing, exploration, boating pleasure, and other water activities where motor boats are used. Jetboats can also be driven for sport, both on the river (World Champion Jet Boat Marathon held in Mexico, Canada, US and New Zealand) and in specially designed racecourses known as sprint tracks. Recently there has been an increase in the use of jet boats in the form of rigid hardened rubber boats and as the tender of luxury yachts. Many jet boats are small enough to carry with trailers and towed in cars.
One very important feature of jetboat is the fact that it has no external spinning part; thus safer for swimmers and marine life, although this can still be hit by the hull. The security benefits themselves can sometimes be reason enough to use this type of propulsion.
In 1977, Sir Edmund Hillary led a jetboat expedition, entitled "Ocean to Sky", from the mouth of the Ganges River to its source. One of the jets was drowned by a friend of Hillary's.
Weakness
The fuel efficiency and performance of a jetboat can be affected by anything that interferes with the smooth flow of water through the jet unit. For example, a plastic bag that is sucked into a jetunit intake grill can have an adverse effect.
Another disadvantage of jetboat seems to be that they are more sensitive to engine/jetunit mismatches, compared to engine mismatch problems/propellers on propeller driven aircraft. If the jet-propulsion unit is incompatible with engine performance, inefficient fuel consumption and poor performance may occur.
See also
- Personal Water Craft
- Sports
- Jet sprint boat racing
References
External links
- Find brochures for many jet boat brands and models from the early '60s and beyond...
- Official website for jet boating in Australia also historical photos of rare junior hamilton boats
- Waterjet history of Hamilton
- The origins and history of jet boats
- Turbine-drive jetboat project
- ThunderJet Boat
- A place to discuss all things Jet Boat
Source of the article : Wikipedia
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