As a rule, people who decide to associate a type of activity (be it a hobby or a profession) with water bodies like rivers or lakes, sooner or later encounter the problem of choosing a boat and the type of mover for it. Motor-jet or still screw? Each has its pros and cons. How to choose what to pay attention to? And is it worth it to make a choice between a water jet and a classic open-rotor motor?
Jet propulsion
A water jet is an engine that provides the vessel with the help of the force created by the ejection of a water stream.
The mover consists of a screw with a shaft (impeller), a water tube, a straightener and a steering device.
The principle of operation is the flow of water through the impeller into the water intake compartment, and then the liquid is ejected through a conical tube, the outlet of which is smaller in diameter than the inlet. This creates a jet that provides the movement of a motor boat. Using the steering device, the direction of the jet is changed by turning the propulsion device in a horizontal plane, which provides the vessel with turns, and blocking the outlet opening creates a reverse flow, providing the boat with reverse gear.
People who are often forced to overcome cluttered or rapid ponds usually tend to choose water cannons. A conventional screw motor in these conditions risks becoming unusable due to the high risk of winding mud on a screw in shallow water or the usual ingress of large debris. In such situations, it is water-jet propellers that provide high speed, maneuverability and safety.
Do not limit yourself to the opinions of participants in various forums. After all, not every review allows you to compose a complete picture. A water cannon is not only a fairly complex design, it can not be suitable for any model of a vessel. If the beginner is satisfied with the idea of using a vessel with a water jet, you should stop at the finished version of the vessel with a water jet in the factory equipment. Moreover, it is advisable to choose a manufacturer engaged in the production of these movers for a long time.
Advantages and disadvantages
The device of the water jet, especially in that all the most important moving parts are “hidden” inside the body. If the boat is aground, the bottom is touched by the hull. This design feature allows you to protect parts from damage, which can not be said about outboard motors with a “bare” screw. The water jet is not afraid of meeting with underwater debris.
When a motor boat moves in shallow water with a depth approximately equal to the hull landing (about 20 centimeters), the water cannon allows you to overcome littered areas, as well as places with obstacles protruding from the water, due to its maneuverability.
If you run into an obstacle at a depth of about 30 centimeters, the bottom of the boat will take the blow, not the water jet, since the mover does not have protruding parts, not to mention the outboard, where the propeller blades take the blow .
Sometimes jet propellers are also used on pleasure boats because of the soft work of the power transmission (transmission) and the lack of vibration.
The advantages also include the lack of additional resistance to water, characteristic of engines with an open propeller (the rotor blades create additional resistance). In addition, they distinguish high inertia indicators, more convenient controllability at high speed (both in forward gear and in reverse). Of considerable importance is the low noise range: the outboard water jet is noticeably quieter than a motor with a propeller.
Nevertheless, the negative side should be noted: when driving in shallow water, there is a big risk that stones, sand and debris from the bottom will be dragged into the engine, because the water washer works on the principle of a pump-action pump. This can lead to damage to the impeller, failure of the cooling system and incorrect operation of the drain nozzle.
Another negative side is friction. It is due to the high speed of movement of water inside the pipe. Do not forget about the cost of installation. Outboard motors for motors are about twice as expensive as conventional outboard propellers. Because of this, boats with a jet propulsion unit in the kit significantly add to their value and are perceived by customers as a whim or inadmissible luxury.
Unusual for fans of classic screw motors is a water-jet control system. The problem is that the classic propeller with an open propeller has a single lever control system. Water-jet propulsion devices have a multi-link reversing-steering device. Some manufacturers manage to produce boats with a built-in water jet with a single lever control system. On the one hand, it helps to master the water cannon, on the other hand, it rather entails trouble than benefits:
- Firstly, the beginner has a wrong idea about the operation of the jet propulsion. This is due to the lack of such a gearbox, which allows you to translate the gear lever into a neutral position. The gearbox can either engage the clutch or off. The water-jet propulsion is gaining speed smoothly when turned on, an instant reaction in the form of a jerk should not be expected.
- Secondly, for a better understanding of the principles of the water jet, it is recommended to take the appropriate training course. The trick to controlling a jet propulsion device is to use the throttle lever (to increase travel speed) only in an open reservoir. When driving along a rapids, it is better not to do so.
- The third important minus characteristic of any type of water transport is overgrowing. This problem is especially acute with a water jet, since all moving parts are inside. With constant use of the mover, there are no problems. However, if the boat is not used for a long time, the insides are overgrown. In particular, the fouling of the insides of the drainage system leads to a decrease in the speed of movement up to 10%. The problem is solved by disassembling the water jet and cleaning by hand, but if the motor boat has been idle for a very long time, you will have to go to the workshop and look for suitable spare parts for boat motors. The use of a special coloring composition will solve this problem, but not for long: the constant movement of water will quickly wash off this paint.
A water cannon is safe!
Of course, the safety of the jet engine is the main positive feature. Since the impeller is inside, the water cannon is not dangerous for a person who is in the water. Such devices are used on jet skis and boats when towing water skiers and surfers.
The structural features of the jet propeller allow the motor boat to make turns almost on the spot thanks to the reversing-steering device (RRU), which provides a change in the direction (reverse) of the outgoing stream.
Spare parts of the water cannon are easily accessible, and repair does not cause any special difficulties. If the jet engine is out of order, you should use the services of any car repair shop where it can be easily repaired or replaced with a new one. Everything will depend on the degree of damage. It may be necessary to refine the fastening, cooling and exhaust systems in the case of a replacement.
The jet propulsion has a number of nuances that should not be forgotten. One of these: maneuver should be at high speeds, you should not reset those before the end of the maneuver, whether it is a turn, turn or reverse.
Like the outboard motor, the jet engine is at risk of winding algae on the impeller, which in turn can jam. To prevent damage to the engine in the case of winding algae on the shaft, a special key is provided that can be cut. It is also easy to get rid of algae only by opening the hatch. Stone protection is provided - grill.
How to choose a water cannon
Conventional outboard motors with an open propeller have a coefficient of performance ( COP ) of 0.65–0.75 when driving at moderate speeds. For a water jet, the efficiency is about 0.55 at a speed of 40–55 km / h. With an increase in speed to 100 km / h it is already 0.60–0.65. Proper design of all elements of water-jet propulsion gives an efficiency of about 0.70. In this case, it should be considered not only the water cannon, but also the acceleration of the boat with the installed structure of the water cannon.
It is recommended that you study a number of rules that allow you to correctly select water cannons, the main emphasis of which relates to design and manufacturing technology. It should be noted that it is necessary to pay attention to the shape of the nozzle. The drainage system must have a round or ellipsoidal cross-sectional shape. Less desirable options are square drain pipes and in the form of a rectangle with rounded corners.
An important aspect is the angle of inclination of the axis of the intake. You should choose according to the principle "speed is higher - the slope is less." Jet boats develop a speed of movement within 55–65 km / h, which is achieved using an angle of 35–39 degrees. To achieve higher speeds, the angle should be reduced to 25 degrees. In this case, the angle of inclination of the axis of the propeller shaft is selected in the range from zero to five degrees.
Installation
Install a water-jet engine should be on light high-speed vessels, called "gliding". Such boats are designed for speeds above 60 km / h. However, sometimes the water cannon is also installed on medium-sized motor boats with an angle of inclination of the bottom (deadlift) of 10 to 30 degrees.
During installation, the mass of the mover should be taken into account, because the water constantly inside the water adds a substantial proportion of the weight of the vessel. Therefore, when calculating the “propulsion” of a vessel, one should take into account this important nuance. But if you consider the whole picture, an installed water cannon on a boat is a more compact option than an engine with an angular column. The extra weight is easily compensated for by the lack of a gearbox, which is replaced by a reverse-steering gear. It is worth noting that experts recommend installing a special coupling between the engine and the water jet. This ensures the isolated functioning of the motor, regardless of the operating mode of the jet propulsion.
Relevance
The genuine interest in jet propellers from commercial organizations has arisen relatively recently. Thanks to the experiments of shipbuilding companies in the form of installing water cannons on high-speed sea ferries, military and commercial vessels, this type of ship propulsion has gained popularity.
Successful operating experience has shown many hidden advantages, in addition to the obvious superiority that jet boats in shallow water have won.
Thus, one of the Italian shipbuilding companies, advertising a yacht with a jet propulsion, indicates such features as a high degree of adaptability to changes in the load on the vessel (which can often change), as well as more significant efficiency at speeds from 60 to 95 km / h .
Water cannons outside shallow water
These characteristics play an important role for a high-speed yacht, since on a classic propeller with an open propeller, the speed of the ship directly depends on the speed of rotation of the propellers. Changes in weather conditions, contributing to the slowdown of the vessel, makes it difficult to maintain a constant speed. This negatively affects engine performance due to the inability to develop a higher number of revolutions. Due to the peculiarity of the water jet, consisting in the impossibility of overloading the engine, it became possible to develop a large number of revolutions, regardless of the speed of the vessel. That is, the number of revolutions will not fall, the engine load will remain the same, fuel consumption per unit time will remain unchanged, but fuel consumption per unit path will increase.
In addition, the maneuverability of water-jet propulsors allows yachts to moor in the conditions of close river moorings, rather than specialized ports. In addition, an important aspect due to which water cannons have gained recognition in the field of application on cruise yachts is their noiselessness.
Motor yachts begin to experience difficulties when driving above 50 km / h on the high seas. This is due to the increasing resistance of water to the blades (even of a rotating) of the propeller. When using a water jet, the resistance is almost zero due to the design of the mover, which ensures constant flow around the body.
Cruise yachts rarely sail at high speeds, the need for water jets is caused more by the desire for rational and safe functioning of the engine. But there are also instances that provide for an increase in speed on the high seas due to power - that is, the installation of several engines.
Market share
The reliability of water cannons in littered water bodies does not cause any complaints. It is known that ships overcome such polluted places as the English Channel without a single breakdown.
Despite its usefulness, jet propulsors are used in diametrically opposite areas of shipbuilding: either on multi-engine cruise yachts, or on small high-speed boats or water motorcycles. Moreover, for the latter, a water cannon is the only possible option. The lion's share of the market is made up of boats of various sizes with classic propellers. About the extremely small number of boats that go off the assembly line, already having a built-in water cannon, it’s not worth mentioning.
In total, about 11% (according to experts) of the market belongs to jet propellers. But this figure can significantly decrease, if we do not fully consider the market of propulsive devices, excluding jet skis, where the water jet is an integral part of the design.
According to the forecasts of companies producing engines, there is a probability of an increase of this figure to 45% due to the disclosure of the potential of the market of jet propellers.
Reviews about motors and manufacturers
Many fishing enthusiasts prefer to use Rotan 240M boats in conjunction with a Yamaha 40 water cannon.
According to their opinions, claims against the manufacturer Yamaha are very minor. Basically, they are connected with mistakes “out of habit”, because the reviews were compiled for the most part by those who recently changed the water cannon from one to another. It does not immediately jerk from a place after a smooth feed of the lever, then deeply fails the back of the boat.
Negative reviews have been reported to Tohatsu. Firstly, fishermen complain about the frequent acquisition of defective products. Secondly, the Tohatsu 40 model received the nickname "dishonest magpie", as the engine does not give out 40 horsepower. Often replaced with Tohatsu 50, but this model quickly overheats.
Expediency of using a propeller
The installation of a classic outboard mover is advisable if it is necessary to use the most simplified design and move around water areas with a depth greater than average at low speeds (up to 50 km / h).
Well proven Yamaha engines. There are three main types:
- Median exhaust blade. The peculiarity of the structure is that the outlet through which the fuel is released, as well as the release of combustion energy, is located in the middle - in the center of the axis on which the blades are mounted.
- Screw constructions with an exhaust hole over the axis.
- Systems with two exhaust openings located above and below the axle.
Conclusion
In general, water-jet outboard motors bring many advantages to the design of the boat, but the disadvantages described above should not be discounted. Otherwise, an expensive mover can become a burden.
Foreign manufacturers produce boat water jets with a very high efficiency and relatively small size. For example, a Yamaha water cannon measuring 350x560x300 mm and weighing 19 kg has a cost of about 75,000 rubles in the domestic market.
The Mercury ME JET 25 ml water jet (made in the USA) is more bulky: it has a length of 508 mm along the upper part of the body (horizontal), weight is about 60 kg, engine displacement is 420 cm 3 , the impeller revolutions per minute reaches 5000, fully manual control. The cost in the domestic market is already 263,500 rubles.
The Japanese Tohatsu M25JET water jet with similar characteristics (it differs only in the number of revolutions: 5200–5600 per minute) costs 287,500 rubles.
For comparison, a classic screw motor can be found at a price of 30,000 rubles or more.
Not surprisingly, due to such a difference in cost, few decide to buy a water cannon. The price is considerable, and not everyone can afford such chic. It is hoped that over time, pricing will stabilize, as foreign manufacturers predict. Then jet propellers will win a significant market share.
It should also be noted that the growing popularity of the use of water jets is provided by the popularity of manufacturers. The most famous companies:
- Yamaha (Japan);
- Suzuki (Japan);
- Tohatsu (Japan);
- Honda (Japan);
- Mercury (USA);
The products of each company are characterized by their high quality, equipment and productivity ratio.
The question of the need to procure spare parts for outboard motors is not urgently raised. It's all about the fame of these brands. Details are available both for purchase on order and for sale in specialized stores or auto repair shops. Repair of water cannons is not a big problem.
Manufacturers spend an enormous amount of time checking each drawing, part, and every numerical metric before putting the model on stream. Samples are being improved: water cannons are produced with a large number of cylinders, with a large number of screws, with shafts of left and right rotation. Particular attention is also paid to noise-insulating materials in the manufacture of appropriate gaskets in order not only to muffle the sound of a running engine, but also to reduce vibration. Exclusive steering systems are available for installation complete with a water jet. Devices to reduce the so-called torque in order to make the control of the ship the most comfortable.
There are samples with a fully lever control that does not involve the use of a cable system, which significantly reduces the likelihood of unpleasant situations that can suddenly bring an exploding cable.
Given all of the above, the choice of a water jet should be treated very carefully. Considerable funds invested in the acquisition should pay off in the future, and the difference between the use of screw and jet engines should be noticeable.