Upgrading the outboard motor is not just a tribute to fashion, but a real way to improve the running qualities of the boat, especially in difficult operating conditions. Direct flow jet drive It's one of the most effective solutions for shallow water, algae-covered rivers and littered areas, and unlike the classic propeller, it eliminates the risk of grass winding onto a shaft, which often leads to engine overheating and water accidents.
The way this works is by taking water through the inlet and throwing it through a tapering nozzle under high pressure, and it creates a jet propulsion that pushes the boat forward, and many owners of PVC boats and metal kandaks are thinking about this, wanting to have a universal transport that can go where the screw motor immediately loses its efficiency or fails.
However, installation water-jet The wrong ratio of the impeller diameter to the power of the motor can lead to a drop in speed or, conversely, to overload the engine. In this article, we will discuss in detail the physical principle of operation, advantages over the screw and step-by-step process of self-installation of the system.
The principle of operation and the physical bases of jet thrust
The jet drive works by the law of conservation of momentum, and it gets sucked in through the intake hole at the bottom of the body, passes through the system of straightening blades, and hits a working wheel called a work wheel. impellerThe impeller, spinning at a high speed, gives the water energy, greatly increasing its pressure.
And then the flow goes into the narrowing channel. jet-nozzleIt's at this point that pressure is converted to speed, and water is released from the nozzle at a tremendous rate, creating a reactive force that is directed in the opposite direction from the jet, and that force is what pushes the boat forward.
The key difference between the slicing circuit and the axial one is the absence of complex curves inside the pump body, which minimizes hydraulic losses. Efficiency of direct flow jet drive It depends on the quality of the treatment of the inner walls of the channel and the accuracy of the impeller landing, and any vortex reduces the overall efficiency of the system.
⚠️ Attention: When running jet drive at low speeds, thrust is minimal. Effective operation begins only after the engine reaches certain speeds, when the impeller enters operating mode.
It's important to understand that jet drive does not create traction in place as efficiently as a propeller. It's a propeller, so you often use additional steering devices or rotary nozzle nozzle nozzles to maneuver.
Advantages and disadvantages of a propeller
The switch to water jet propulsion gives the owner a number of undeniable advantages, especially if the operation takes place in specific conditions, but the system has its limitations, which you need to know about before upgrading.
Among the main advantages are:
- 🌿 Full protection from overgrowth: algae and fishing nets are not wound on the shaft, as it is closed by the pump body.
- 🪨 Safety for the propeller: the ability to pass rocky rolls and shallow water without the risk of damaging the blades.
- 🛡️ Safety of people: The absence of open rotating parts reduces the risk of injury to bathers or animals.
- 📉 Less cavitation: at high speeds, the jet drive operates more stable than the screw, which can "break" the flow.
But there are defects in waterjet propulsionThe main one is the lower efficiency at low speeds compared to the propeller, and the system also requires more careful care of the filter elements of the water intake, so that large debris can damage the impeller does not get inside.
Another disadvantage is the difficulty of reverse, which requires a special flap to redirect the flow of water forward, and the reverse mechanism adds weight, and can create additional resistance when moving forward if it is structurally executed illiterately.
Selection of components: impeller, stator and materials
The heart of any water jet is the pump assembly, and the parameters of the pumping unit determine the final speed of the boat and the fuel consumption. squirt on the outboard motor you need to take into account the engine power and the expected load on the body.
The basic element is impellerIt can be semi-axial or diagonal. Small and medium power outboard motors (up to 15 hp) are most commonly used with high pitched propellers to provide the required head at relatively low revs. The fabrication material is also important: aluminum alloys are lighter, but plastic impellers are less susceptible to impacts on solid objects.
The stator (guide machine) is the directing force of the impeller, the quality stator turns the rotational motion of the water into translation, if it is made artisanal or in violation of geometry. Efficiency of the system It could drop by 20 to 30 percent.
Comparative table of components specifications for different capacities:
| Engine power (hp) | Impeller diameter (mm) | Case material | Optimum speed (km/h) |
|---|---|---|---|
| 5 - 9.9 | 90 - 110 | Glass-plastic/Aluminum | 25 - 35 |
| 15 - 20 | 115 - 130 | Aluminum D16T | 40 - 50 |
| 25 - 30 | 135 - 150 | Stainless steel/composite | 55 - 65 |
| 40+ | 160+ | Stainless steel | 70+ |
When choosing materials, you should prefer corrosion-resistant alloys or high-quality composites. Constant contact with water, especially salt water, requires increased protection against oxidation.
When making an impeller on your own, it is critically important to keep the gap between the blades and the body no more than 0.5 mm. The increase in the gap sharply reduces the pressure at the outlet.
Preparatory work and necessary tools
Before proceeding with the physical installation water-jetThe alteration of the outboard motor is irreversible or requires considerable effort to return to its original state, so a mistake in planning is unacceptable.
First of all, you need to take accurate measurements of the lower unit (legs) of the motor and the transom of the boat, the geometry of the water intake depends on this, and you should also check the condition of the flange of the mounting of the motor, since the installation of the jet drive can change the center of gravity and the load on the transom.
To work, you will need the following set of tools:
- 🛠️ Corner grinding machine (Bulgar) with metal discs.
- 🔩 A set of wrenches and heads for dismantling the gearbox.
- 📏 A rod and a ruler for accurate markings.
- 🔥 Gas burner or building hair dryer (for working with polymeric materials).
- 🧪 Degreasing agent and adhesive soil for sealing seams.
It is important to prepare a place in advance for the installation of water intake holes, which should be located in the areas with the least flow swirl when the boat is moving, usually the lower part of the lower unit or a special depression in the hull of the boat for an outboard motor.
☑️ Preparation for the installation of jet drive
Step-by-step instructions for installation of jet drive
The installation process begins with the removal of the standard gearbox and propeller, and the pump unit is installed in their place. If you use a ready-made kit, follow the manufacturer's instructions. If you build the system yourself, the key is to dock the engine shaft with the impeller shaft.
You need to have perfect alignment, and any shaft beat will cause the bearings and the coils to wear out quickly, and vibrations that can break the mounts. couplingwhich compensates for micro-disturbances.
Then you put a water intake into the water intake, and you cut a hole in the lower unit or the boat hull, and you press the receiving chamber to it, and you want the edges of the hole to be smooth, so you don't create turbulence at the inlet, and after you put the mechanical part in, you get a lot of sea sealant.
⚠️ Warning: Before starting, be sure to check the system for leaks on land (short-term) or when towing. Air entering the pump ("pump") will cause the flow to fail and thrust loss.
The final step is to install the control system. The gas cable must have a free ride to operate the reverse valve (if any), and the reverse switch cable now controls the rotary nozzle or deflector.
The secret of nozzle tuning
The diameter of the output nozzle can be adjusted with interchangeable rings. The reduction of the diameter increases the speed of the jet, but increases the load on the engine. Choose the optimal size experimentally, starting with a larger diameter.
Setup, testing and maintenance of the system
After installation, the test phase begins, and the first run is better done in shallow water, so that if you have problems, quickly shut down the engine. Direct flow jet drive can change the circulation of cooling water if the water intake for cooling is taken from the same source as for thrust.
When you test, check the reverse, the boat must confidently brake and reverse, and if the reverse turns on, the engine stalls or loses speed, then the deflector cross section area is too small or the mechanism is jammed.
jet drive maintenance is reduced to regular freshwater washing (especially after the sea) and visual inspection of the impeller. The presence of chipped or cracked blades requires immediate replacement or repair, as the balance is disturbed.
The condition should also be monitored. bearing-assembliesThe water that enters the pump carries abrasive particles, so the life of the bearings in the jet drive is usually lower than in the screw gear. Regular lubrication and replacement of the glands will prolong the life of the unit.
Properly tuned jet drive increases the boat's permeability by 80%, but requires regular checks of the tightness of the shaft seals.
Frequently Asked Questions (FAQ)
Will the maximum speed of the boat drop after installing the jet drive?
With proper calculation and selection of components, the speed drop is no more than 10-15% compared to the propeller, but in shallow water and grass, the jet drive will show a much higher result, since the propeller in such conditions loses up to 50% of the efficiency.
Can I install a jet drive on the engine without removal from the warranty?
Any interference with the design of the diewood and gearbox will usually void the manufacturer's warranty. To maintain the warranty, there are hinged water jet kits that do not require drilling and cutting of the engine components.
How often should the water intake be cleaned of garbage?
The frequency of cleaning depends on the operating conditions. In clean water, inspection is carried out once a season. In overgrown water bodies, it is recommended to check the water intake net after each release to the water or even in the process if you feel a drop in thrust.
Is the jet drive suitable for sea trips?
However, you need to use materials with high corrosion resistance (stainless steel, titanium, special plastics) and be sure to wash the system with fresh water after each outing.