outboard motor without a screw: what is called and how it works

Many novice water-engineers are faced with a situation when, when they approach the dock, they notice a boat with a working engine, but without the usual metal propeller on the lower unit. The question immediately arises: what is the name of the outboard motor without a screw and did it break? In fact, the absence of a classic propeller at the end of the shaft is not a failure, but a feature of the design of a certain type of propeller.

These units are most often called water-jet Or more broadly, impeller systems, which are very different from the propellers that we're used to, which pull the boat with the water's stop, and use the power of a jet stream that's ejected from the nozzle, and this engineering solution allows boats to speed up and, most importantly, to navigate safely through shallow waters where a normal propeller would instantly fail.

In this article, we will go into detail about the design of these motors, explain why they are chosen by professionals for difficult conditions, and answer the question of why they are sometimes mistakenly called "screwless motors." In fact, there is a screw there, but it is hidden inside the case and is called otherwise.

The correct terminology: jet drive or impeller?

When they talk about a outboard motor without a screw, they most often mean water-jetBut it's technically more accurate to call it an impeller, and the difference is the design of the wheel, and in a classic jet drive, you use a pump that sucks water in and throws it back violently, pushing the boat forward in a conservation of momentum. impeller The wheel is a similar to the screw, but hidden from the eye.

There is also another category of motors that really don’t have water contact at the bottom of the lower unit. propeller However, in the context of standard outboard motors mounted on a transom, it is jet drives that can be either hinged or built into the hull of the boat. The hinged versions often represent an upgrade of standard gasoline or electric units, where the standard lower unit is replaced by a water intake pipe.

It's important to understand that the term "screwless motor" is more of a colloquial simplification. Waterjet or Impeller DriveIt's the impeller that creates the pressure that you need, and its blades have a different geometry than the propeller, so they can work efficiently even if you're partially cavitated.

⚠️ Attention: Never try to run a water jet engine out of water or with a water intake closed. Unlike a propeller, the impeller rotates in a closed channel, and the absence of water flow will cause the seals to overheat instantly and the pump body to melt.

There are many different types of systems, and there are models where the water intake is in the bottom of the boat, and there are options where you draw water through special holes in the transom. Each type has its advantages depending on how deep you plan to walk. For shallow water, the type of water intake is critical so that it does not draw sand and grass from the bottom.

The principle of operation: how the boat moves

To understand the efficiency of this system, you need to look at the physics of the process, and the water is sucked in through the inlet (water intake) at the bottom of the housing. workwheel It spins, it gives the water high pressure and speed, and then the jet is ejected through the tapering nozzle in the aft.

The force that pushes the boat is called jet thrust, and it's directly proportional to the mass of water that's ejected per second and the speed of that jet, and you can control the direction of travel by turning the nozzle itself in different directions, which makes these boats incredibly maneuverable. reverse-bucketA jet that descends on the jet, redirecting it forward and causing the boat to brake or reverse.

One of the major problems with propellers is cavitation, which is the collapse of the vapor bubbles that come from a sharp drop in pressure, which destroys metal. In jet drives, this is also possible, but the design of the impeller and the rectifier minimizes the harmful effects, and the absence of protruding parts reduces the water's resistance to the movement of the body.

Advantages of water jet propellers before the screw

Why do boat owners choose units that are often more expensive and difficult to maintain? The answer lies in the unique performance specifications. The main advantage is that they are not the same. security And passability: The absence of rotating blades outside makes swimming near a running motor safe, and passing through grassy areas or rocky rolls possible.

Here are the main advantages that outweigh the disadvantages for many anglers and tourists:

• 🚤 Passage: The ability to walk at depths of only 10-15 cm, which is not available for screw motors.
• 🛡️ Safety: No risk of injuring people or animals with a rotating screw.
• 🌿 Protection from garbage: Water intake nets (streets) trap large grass and branches, although small fractions pass through the impeller.
• 🏎️ Speed: On planing jet drives often show the best results due to the lack of resistance of lower unit in the water.

Another important aspect is the durability of the boat's hull itself: underwater obstacles are hit by a solid water intake, not by the brittle propeller blades or the leg of a lower unit. For aluminum boats, which are often used on Siberian rivers with their complex bottom relief, this becomes a decisive factor.

However, it is worth noting that at low speeds and when trolling jet drives can be inferior to screw analogues in thrust. The efficiency of the system at low revs is lower, since jet thrust is effective only when a powerful jet is ejected. Therefore, quiet fishing at a minimum speed sometimes requires the development or use of electric motors.

Design features and types of impellers

The heart of the system is pump-pieceThis is where the magic of turning the shaft into the motion of water comes from: Impellers are axial (axial) and diagonal (propeller type) axial (propeller type) and they resemble a regular propeller, but with lots of blades and small pitch, and they are easier to make, but they are susceptible to cavitation.

Diagonal and radial-axis impellers have more complex geometries of blades that are bent at an angle. They create more pressure and work more efficiently at high revs. The materials used to make them are high-strength: special bronzes, titanium alloys or composites. Stainless steel It is also popular for its corrosion resistance.

An important element is the rectifier, the fixed blades behind the impeller, which removes the twisting of the flow that the screw creates, turning the chaotic rotation of water into a straight jet, which increases the overall efficiency of the installation by 10-15%.

Why are jet drives buzzing?

The characteristic high-frequency hum of the jet drive is caused by the impeller operating at high revs inside a closed volume. The sound spreads through the boat hull, so soundproofing the engine compartment is critical for crew comfort.

Compare the specifications of different types of working wheels in the table below:

Type impeller	Low-speed efficiency	High-speed efficiency	The tendency to cavitation	Application
Axial	Highly	Medium.	High.	Heavy boats, towing
Diagonal	Medium.	Highly	Medium	Speedboats
Radial-axial	Low.	Very tall.	Low.	Hydrocycles, sports
Regulated	Variable	maximum	Low.	Professional fleet

Service and typical problems

Despite the apparent simplicity (no external moving parts), water jet engines require careful maintenance. wear-and-wearSand that enters the pump with water acts like sandpaper, gradually breaking the gaps between the impeller and the chamber body, increasing the gaps leading to a drop in pressure and loss of traction.

Regular checks of bushings and seals are a must. If you hear a foreign noise or vibration that increases with revs, it is likely that the impeller's balancing is broken or a foreign object has entered the channel. Unlike a screw that can be inspected by taking the boat ashore, diagnosing jet drive often requires the lift of the vessel or diving.

Special attention should be paid to lubrication of the bearing unit. Water entering bearings quickly disables them. Modern systems have automatic lubrication systems, but control of the oil level in the gearbox and the tightness of the osteoils should be regular. Use only the recommended ones by the manufacturer. lubricantsresistant to emulsification.

⚠️ Attention: When cleaning the water intake from grass, never use sharp metal objects (knives, pins). Damage to the edges of the inlet will disrupt the laminarity of the flow, which will lead to reduced power and increased cavitation noise.

Comparison of cost and feasibility of purchase

Is it worth the effort? A water jet boat or a factory jet drive is always more expensive than a similarly powerful screwed sibling, the price difference can be anywhere from 30% to 100% or more, due to the complexity of manufacturing precision pump parts and smaller production volumes.

But when you add up the cost of repairing screws after encounters with snags, the loss of time to untangle nets and grass, and the opportunity to fish where others are afraid to go, the cost-effectiveness of jet drive becomes apparent. right choice.

For sea voyages in pure deep water, where maximum efficiency at cruising speeds is important, the classic propeller still holds the palm of the table. But technology is not standing still, and modern impeller systems are becoming more versatile and efficient.

Frequently Asked Questions (FAQ)

Can a conventional outboard motor be converted into a water jet?

Yes, there are special water jet nozzles that are installed instead of the standard lower unit leg, but such a rework requires fine alignment and often leads to a loss of engine power (up to 20-25%) due to hydraulic losses in the channel.

Why is a water jet engine louder than a screw engine?

The main source of noise is not the exhaust, but the hydrodynamic noise from the impeller and the cavitation inside the pump body. The sound is transmitted through the water and the design of the boat, creating a characteristic hum that can be heard even from a distance.

Does the fish get stuck in the jet drive?

Small fish can pass through the impeller without damage due to the flexibility of the tissues and the lack of cutting edges. Large fish can get stuck in the water intake grille, but this usually does not lead to engine failure, unlike falling into the propeller blades.

What is the maximum speed of a jet drive boat?

Modern sports models with water jet propulsion easily develop speeds over 80-90 km / h Record samples of seaplanes and racing boats reach speeds of more than 200 km / h, which is impossible for classical propellers due to cavitation.