Choosing a propulsion system for an inflatable vessel is always about finding a compromise between speed, passability and comfort. PVC Often faced with the dilemma of leaving the classic propeller or switching to a water jet propeller, each of these options has its own strengths, which become apparent only in specific conditions of use.
At first glance, it may seem that jet drive is the perfect solution for shallow water, but its installation on low-power engines often leads to disappointment. Efficiency This system is much lower at speeds of up to 20 km/h than an open impeller, which is why understanding the physics of the process and the technical specifications of your boat is critical before buying expensive equipment.
In this article, we will explore the nuances of both types of propulsion, explaining why jet drive works wonders in shallow water, and the screw loses in deep water, and also the safety, reliability and impact of propulsion type on the handling of the vessel in different weather conditions.
Principle of work and design features
Classical. propeller It's powered by throwing water back, creating jet propulsion, and the propeller blades, spinning in the water, create a dilution zone at the front and a pressure zone at the back, which pushes the boat forward, and the design is as simple as possible: the shaft that connects the motor to the propeller, and the impeller itself, and it's simplicity that makes this type of propulsion the most common in the world.
Unlike the screw, water-coat It takes water through the intake hole into the bottom or the transom, drives it through the impeller, and throws it out in a powerful jet through the nozzle in the aft. Inside the hull, the water passes through a rectifier that converts the rotational motion of the flow into translation, and this creates thrust without protruding parts under the bottom of the boat.
⚠️ Attention: In water jet propulsion, the absence of foreign objects in the water is critical, and even a small stone in the impeller can lead to the destruction of the blades and expensive repair of the pump part.
The jet drive is more complex because it requires the coordination of the internal combustion engine with the hydraulic part, and it requires a certain amount of water to run through the system per unit of time to work efficiently, and at low speeds, the efficiency of the pump drops, which is its main technical feature.
Historical background
The first water jet propulsion was described in the 17th century, but it was widely used on small craft only with the advent of powerful and light gasoline engines that can provide the necessary water pressure.
Comparison of performance and speed specifications
When it comes to maximum speedThe propeller almost always wins over the jet drive on PVC boats. The open propeller has a higher efficiency (efficiency) at planing speeds. The jet drive loses some of the energy to frictify the water against the walls of the conduit and the hydraulic resistance inside the hull.
However, if we consider low-speedThe water jet can develop a good traction force from low turns, which is convenient when trolling or going against a strong current, and the screw at low turns can work unstable, especially if the diameter is not matched correctly.
The difference in speed can be as high as 10 to 15 percent in favor of the propeller at the same power. For example, a boat with a 9.9 hp engine on the propeller will go faster than the same boat with a jet drive of the same power, but the jet drive will provide a smoother acceleration and no vibrations characteristic of the propeller at the limit of capabilities.
The propeller is more efficient at high speeds and at full gas, whereas the jet drive is better at medium and low revs, providing stable traction.
Weight is also important, because the water-gun nozzle adds extra weight to the transom, which can be critical for lightweight inflatable boats, and shifting the center of gravity backward requires proper weighting of passengers and cargo.
Passability and safety of operation
The main trump card of the jet drive is his passabilityThe absence of protruding parts under the bottom allows you to boldly go out to silted shores, pass through reed thickets and move down to depths where the screw would have long ago wound the grass or hit the bottom. For fishermen and hunters, this is often the decisive argument.
Question safety The jet drive has no open blades that rotate in the water, which minimizes the risk of injury to people and animals near the boat, and it is much safer to swim next to a running motor with a jet drive, although it is not recommended to do so in the immediate vicinity of the nozzle.
- 🌿 The ability to move in shallow water depths of up to 10-15 cm without the risk of damaging the propulsion system.
- 🛡️ No risk of winding algae and fishing nets on the shaft (in the presence of a protective grid).
- 🚫 Inability to get hit by a propeller blade when falling overboard or swimming next to a boat.
- 🏖️ Safe approach to the gently shore for disembarking passengers without stopping the engine.
However, the propeller also has its advantages in terms of field serviceability: Replacing the cut studs or even the screw on the shore is easier than disassembling the water jet nozzle, which often requires special tools and skills to maintain.
Economy and fuel consumption
Fuel consumption is directly dependent on propulsion efficiencyBecause the jet has a lower planing efficiency, it will require more engine speeds to achieve the same speed, and therefore more gasoline, a difference that can range from 10 to 25 percent depending on the design of the nozzle and the condition of the boat.
In trolling or slow motion modes, jet drive can be even more economical, since the engine operates in a more comfortable range of revolutions, without requiring "gas" to maintain traction, as is the case with an improperly selected propeller. fuel jet drive is always higher.
It's also important to note that the engine's life is very low, and the engine's jet drive is often more gentle in vibration, but with a constant high load, and the propeller, especially when you go to the plane, creates jerk loads, and the right screw pitch allows the engine to operate at an optimal range of revs, which prolongs its life.
⚠️ Attention: Installing jet drive on engines with a power of less than 5 hp makes little sense. Loss of speed will be catastrophic, and you will not notice fuel economy due to the low efficiency of the system at low power.
To calculate the savings, you should take into account not only the price of a liter of fuel, but also the cost of the equipment itself. Water-gun nozzles are more expensive than good screws, and the return on fuel savings in the case of jet drive should not be considered as an argument at all - it will not.
Boat handling and maneuverability
PVC boats with jet drive have a unique maneuverabilityThe jet of water ejected from the nozzle allows the boat to be turned almost in place if the design involves a rotary nozzle, which is especially valuable in narrow ducts or when mooring in difficult conditions.
Screwboats also boast good handling, especially if a motor with gas distribution or a rotary rotor is used. However, the inertia of the rotor creates a gyroscope effect that can slightly complicate sharp steering at high speeds.
The thing about the jet is that it doesn't have a backrest, so if the propeller, when you turn the back on, immediately starts pulling the boat back, the jet first has to stop the jet from ejecting forward, turn it around, and then start moving, which makes reverse maneuvering less intuitive and addictive.
To improve the handling of the boat with jet drive at high speeds, it is recommended to install additional zygomatic linings, which will help compensate for the change in the center of pressure of the water on the bottom.
Water jet boats are softer in the wave, the lack of propeller cavitation and vibration makes the ride more comfortable for passengers, and the propeller on the wave can periodically "grab" air, leading to short-term engine speed surges and loss of speed.
specifications comparison table
To organize the information and help you make the final choice, we compare the key parameters in the table: the data are averaged for PVC boats with a length of 3.5-4.2 meters with engines 9.9-15 hp.
| Parameter | The propeller | Water-guide |
|---|---|---|
| Maximum speed | High (baseline) | Medium (10-15% lower) |
| Passage (shallow water) | Limited (risk of impact) | Excellent (up to 10 cm) |
| Safety of people | It requires caution. | Maximum |
| Fuel consumption | Optimal. | Up 10-20%. |
| Noise and vibration | Medium level | Minimum |
And you can see from the table that the choices are based on priorities, and if you want speed and economy, you want your propellers, and if you want children's safety, shallow fishing and cross-country travel, you should consider jet drive.
Practical recommendations for selection
Before buying, determine the main use case. river-fishing With overgrown shores, jet drive is a lifeline. For cruising through large reservoirs, where speed and the ability to quickly cover distance are important, it is better to stay on the screw.
Notice the power of your motor. As mentioned, a jet drive will require power of 5-6 hp or higher. On weak engines, the loss of speed will be too noticeable. Also important is the design of the boat: the rigid transom must withstand the additional weight and load from the water jet nozzle.
☑️ Criteria for the selection of the propulsion
Don't forget to maintain it. The water jet requires regular sand washing and the impeller to be checked for wear, the screw needs a lower unit lubricant and antifouling coating, both of which require attention, but the nature of the work varies.
Frequently Asked Questions (FAQ)
Can I install a jet drive on a 5 hp engine?
Technically, it's possible, but it's not practical. A motor of this power won't be able to generate enough water pressure to plane a jet drive efficiently, and the speed will drop critically, and the boat may not even go into the plane with one person.
How much easier is it to maintain a jet drive compared to a screw?
The jet drive service takes longer, and you need to regularly disassemble the case to clean up debris and check for impeller gaps, and the screw can be removed, inspected for bullies and lubricated when you replace it.
Does the jet drive affect handling in a strong side wind?
Yes, it does. Because of the lower speed, the jet drive boat is more susceptible to wind demolition. However, the lack of sailing of the lower unit (it protrudes less into the water) may partially compensate for this effect, but in general, the sailing of the jet drive boat is higher due to the lower speed.
Can I use a jet drive tow a water skier?
You can, but the thrust will be less than that of the propeller. For a comfortable towing will require a motor of greater power than recommended for this boat with a propeller to compensate for the loss of efficiency of the water jet propulsion.