The switch to a water jet propulsion system for owners of inflatable vessels is often a decision dictated by the desire to protect themselves from damage to the screw against stones, snags or the bottom in shallow water. water-jet complex Many beginners, when they've mounted a system, find themselves in a paradoxical situation: a boat is less likely to sail than an outboard motor, or the engine is overheated, and the problem is not the power of the power plant, but the intricacies of the hydrodynamics and the correctness of the pairing of components.
The process of fine-tuning requires a systematic approach, taking into account the features of the soft board and the specifics of the work. impellerUnlike hard-hull boats, PVC boats are susceptible to deformities that affect the angle of water intake and the position of the suction hole. Ignoring these nuances leads to cavitation, thrust drops and even structural breakdown. In this article, we will discuss the key steps that will turn your boat into an efficient and safe floating craft.
And you can't do this successfully without understanding the physical processes inside the water duct, and you have to get the water to the impeller in a laminar flow, without vortices and air bubbles, and any violation of this rule reduces the pressure of the water. Efficiency of the system So the first step is always to look carefully at the geometry of the plant and the state of the transom.
Selection and preparation of components of water jet complex
The foundation of effective work is the right selection of equipment for a particular boat and motor, the main parameter here is the correspondence of the diameter of the suction hole and the output nozzle to the specifications of the engine. boat-engine power up to 15 hp, you will need a jet drive with a diameter of the impeller about 145-160 mm. the Use of too narrow aqueous on a powerful engine will lead to the engine in overload mode, and too wide - to loss of jet speed.
Material is also critical. Inflatable vessels, composite or aluminum water pipes with anti-cavitation plates are considered the best choice. Steel structures can be excessively heavy for light PVC boats, shifting the center of gravity. Particular attention should be paid to the quality of the treatment of the interior walls: roughness creates turbulence that “strangles” the flow. The ideal inner surface should be smooth as glass.
⚠️ Warning: Never use jet drives with sharp edges in the water inlet in combination with motors that have high thrust at low revs. This is guaranteed to cause cavitation bubbles to collapse directly on the blades, causing metal erosion and vibration.
When choosing impeller (wheel drive) Consider that PVC boats prefer a model with a reduced pitch of the screw, but increased diameter. This allows you to maintain traction when working on overgrown algae areas where standard screws quickly lose efficiency. Also check the presence of a bearing lubrication system, as working "dry" even for a few seconds can cause the knot to fail.
Installation of water intake and formation of a water conduit
Proper installation of the water intake is 80% of the success of the entire operation. The hole should be located strictly on the diametrical plane of the boat to prevent the vessel from moving away. For inflatable tubes, it is critical to use special reinforced linings (petaks) that distribute the load and prevent PVC tissue from being cut through by vibration. The attachment should be rigid, but without a drag that could deform the geometry of the suction pipe.
The angle of intake relative to the bottom requires precise adjustment. The optimal angle is the one at which the water flow enters the system without interruption from the walls. If the angle is too steep, vortices form that suck in the air. If it is too flat, there is friction resistance against the bottom. For most PVC boats, the optimal location is the inlet at a distance of 10-15 cm from the transom.
☑️ Inspection of water intake installation
The internal water duct connecting the intake to the pump should not have sharp bends. The radius of the pipe should be at least three diameters of the pipeline itself. Any narrowing of the section in this section is unacceptable, because it creates a zone of high pressure in front of the pump and dilution after it, which destabilises the operation of the entire system. Use smooth adapters and avoid corrugated hoses inside the high pressure circuit.
Balancing and centering of the propulsion system
After the physical attachment of the components, the fine-tuning of the motor relative to the water intake occurs. The gap between the impeller blades and the water conduit body (ring slit) is one of the most important parameters. For small jet drives, it is usually 0.3-0.5 mm. The increase in this gap due to improper alignment leads to water leakage and pressure drop at the outlet, which directly affects the speed of the boat.
The height of the diewood installation also requires adjustment. Unlike screw motors, the jet system is sensitive to submersion. Too high will lead to air capture from the surface, especially in wave or roll. Too low will increase water resistance and the risk of impacts on the bottom. Adjustment is done by moving the motor along the transom board or changing the angle of inclination of the transom.
| Parameter | Norma. | Symptom of deviation | Consequence |
|---|---|---|---|
| Annular gap | 0.3. 0.5 mm | Whistling, vibration. | Thrust drop to 30% |
| Deepening of the axis | Instructions. | Air capture | Cavitation, overheating |
| Water entrance angle | Parallel to the bottom | Noise in the waterway | Flow turbulence |
| Co-axiality of shafts | Maximum | Beating the shaft | Bearing destruction |
You can use a laser level or a strained thread to check for the alignment, and the axis of the motor shaft and the axis of the water duct should match perfectly, and even a slight skew will cause the bearings to wear unevenly and cause dangerous vibrations that will quickly break down the softboard anchorages.
How to check the gap without disassembly?
There's a method of checking the gap with wire, insert the soft copper wire between the blades and the body, turn the shaft by hand, then measure the thinning of the wire with a micrometer, and this will give an accurate picture of the uniformity of the gap in a circle.
Setup of the output nozzle and reverse
The outlet nozzle is the element that forms the jet stream. Its shape and cross-sectional area directly affect the speed and range of the jet. PVC skids often require the nozzle to narrow compared to standard factory parameters to increase the rate of water flow. But it is important not to overdo it: excessive narrowing will create excessive backpressure, strangling the motor.
The reverse steering system on jet drives works differently than on screw motors. There is no gear shift in the gearbox; the change in direction is by lowering the deflector, which redirects the jet forward. It is important to adjust the angle of lowering the deflector so that it overlaps the jet completely and symmetrically. Incomplete overlap will cause the boat to continue forward or sharply prowl to the side when the "reverse" is turned on.
⚠️ Attention: When adjusting the reverse, make sure that the control cables do not have backlashes. On the soft transom of the PVC boat, vibration can weaken the cable attachments, causing the deflector to spontaneously oscillate, causing the boat to yaw on the course.
To improve handling at high speeds, additional side rudders or flow stabilizers are recommended, which help compensate for the lack of a classic steering pen and make the boat's behavior more predictable when cornering.
To improve efficiency in shallow water, install a large removable net on the inlet, which will protect against large stones, but will not significantly reduce the flow of water, unlike fine-mesh filters, which instantly overgrow with mud.
Combating cavitation and strangulation
Cavitation is the main enemy of any water jet propulsion system, which is the formation of vapor bubbles in low-pressure areas that collapse with great force, destroying metal and creating noise. On PVC boats, the risk of cavitation is increased by the possible air sucking through the looseness of the water intake attachment points to the soft board. When moving, the fabric can "play" by letting air into the system.
To combat this phenomenon, you need to use high-quality sealants and reinforcement patches around the water intake, and also an effective method is to install jack-off deflectors in front of the inlet, which cut off the surface layer of water saturated with air bubbles. If you hear a characteristic crackling (“as if a fraction hit the pump”), then cavitation is already underway, and the settings require immediate correction.
Stifling often occurs when you start abruptly or when you're excited, and to minimize this effect, you can't do it. anticavitational It should be positioned strictly horizontally or with a minimum angle of attack, if the stove is upside down, the air will be actively sucked under it, and the angle of attack is adjusted by placing washers or changing the angle of inclination of the entire engine.
Testing and finalization on the water
Theoretical calculations and bench tests are mandatory, but the final adjustment can only be done on water. The first exit should be in calm water without strong currents. Go against the current and along the flow, comparing the tachometer readings and subjective thrust sensations. Pay attention to the behavior of the boat when going out for sleeve: if the nose is up too high, you may need to adjust the position of the center of gravity or the angle of the thrust.
When running, keep a close eye on the engine temperature. Water vents, unlike screws, require a constant and powerful flow of water to cool. If the engine starts to warm at low speeds, then the pump performance in these modes is insufficient, and an additional onboard pump or the engine cooling system is needed.
The final jet drive setting is always a trade-off between top speed and traction performance, and for fishing and cruising, it's better to sacrifice 10 percent of the speed for reliability and cross-country performance.
After a few hours of work, be sure to audit all the mounts. The vibration of the water jet complex is specific and can quickly weaken the bolts on the PVC transom of the boat. Use grover washers and thread locks. Only after all the defects are eliminated can the adjustment be considered complete.
Why does a jet drive on a PVC boat work worse than a screw?
Most often, the problem lies in the mismatch of the diameter of the water conduit of the engine power or the presence of an air sucker through the soft side. The screw is less sensitive to small flow disturbances, whereas the jet drive needs perfect tightness and geometry.
Do I need to clean the water before each exit?
If the design includes oilseeds or a water lubrication system, yes, it is a mandatory procedure. Dry movement of the ossels, even for a minute, can lead to their destruction and water entering the gearbox or bearings.
Can I put a jet drive on an electric motor?
Yes, it's a great combination. Electric motors have high torque at low rpm, which is ideal for water jet propulsion, but you have to make sure that the voltage and current match the pump performance.
How often should I clean the impeller from algae?
In overgrown bodies of water, after each exit, the algae that winds up disturbs the balance and can jam the shaft. jet drive designs usually allow you to quickly remove the protective grille for cleaning.