Cavitation plate on the outboard motor: functions, malfunctions and repairs

Every owner of a small boat sooner or later faces a situation where the outboard motor suddenly loses traction, the speed jumps sharply, and the characteristic noise and whistle comes from under the water. This phenomenon is often called a "disruption", and in 90% of cases it is caused by a disruption of the hydrodynamic flow around the propeller, which is responsible for cavitation-plateThis seemingly simple metal sheet, positioned above the propeller, is a critical element of the outboard motor design.

Ignoring the problems associated with this part can lead not only to a slowdown, but also to serious engine failure due to overheating or mechanical damage to the screw. Understanding the principles of its operation will allow you to avoid expensive repairs in the service center and feel confident on the water. In this article, we will discuss in detail Construction of the anti-cavitation plate, the causes of its malfunction and methods of self-diagnosis.

Functional purpose of anticavitation plate

Main objective cavitation-stove It's a way to prevent air from being sucked from the surface of the water into the rotation zone of the propeller. When the boat moves, especially at high speeds or when it turns abruptly, there are vortices around the lower unit. Without a shield, air from the surface would easily penetrate the blades, creating vapor bubbles called cavitation. Anti-cavitation plate creates a barrier that cuts off surface air and directs the clean flow of water to the screw.

The second important function is to create a water bulge, and when you run the propeller in the foreground, you create a pressure zone in front of the propeller and a dilution zone behind it, and the plate helps stabilize that flow, improving the flow of the propeller. Efficiency of propellerIn addition, many modern motor models, such as Yamaha or TohatsuIn this stove is often built water intake for the cooling system, which makes it also an element of life support of the engine.

⚠️ Warning: Operation of the engine with a damaged or deformed plate can lead to cavitation destruction of the propeller blades and overheating of the engine due to a violation of cooling water intake.

Also worth noting is the safety feature: When hitting underwater obstacles, the plate takes the first hit, protecting the more vulnerable parts of the lower unit and the screw itself from direct contact with solid objects, which is why the condition of this part directly affects the resource of the entire suspension complex.

The principle of operation and physics of the cavitation process

To understand why. cavitation-plate It's important to understand the physics of the process. Cavitation is the formation of voids in a liquid filled with vapor. It happens when the local pressure in the fluid stream drops below the pressure of saturated vapor. On the propeller, it happens on the back of the blade, where the flow rate is highest. If there is air from the surface next to the propeller, the cavitation process is amplified many times over.

When the bubbles collapse, they create microscopic hydraulic shocks of enormous force, which can paint metal, break down the propeller blades and create a lot of noise, and a plate placed directly above the propeller forms a ceiling that prevents atmospheric air from mixing with water in the work area. hydrodynamic flow It becomes stable, which provides a smooth thrust.

There's also the concept of screw ventilation, which is often confused with cavitation, but in the context of a stove, it's actually a pump of air. If the stove is set too high or damaged, it creates a funnel that sends air to the screw, and as a result, the screw starts to work in an air-water mixture that is much lower in density than water, which causes the thrust to drop.

Typical malfunctions and their symptoms

The anti-cavitation plate problem can be identified by a number of specifications that occur during operation, most often owners experience mechanical damage or improper installation of the motor on the transom, symptoms usually increase gradually, but can also occur sharply after impact.

Among the main signs of malfunction are:

• 🚤 A sharp increase in engine speed without increasing the speed of the boat (the effect of "idle work").
• 💨 The appearance of air bubbles in the exhaust stream and around the screw when moving.
• 📉 Inability to bring the boat to planing even with full gas.
• 🌊 Unstable operation of the engine at idle speeds when moving in reverse.

A common cause of problems is deformation of the plate itself. If the motor hits the bottom or the log, a thin sheet of metal can bend up or down. The bending up will lead to air sucking, and the bending down will lead to increased water resistance and the risk of plate breaking. Also worth paying attention to the attachment sites: vibration can weaken the bolts, and the plate will start to walk, disrupting the flow.

The table below shows the main symptoms and their probable causes associated with the stove:

Adjustment of the height of the motor installation on the transom

One of the most common causes of cavitation problems is the wrong suspension height, and if the motor is attached too low, water resistance increases, which leads to fuel overruns. cavitation-plate The optimal position is considered when the stove is on the same level with the bottom of the boat or 10 to 25 mm lower for high-speed vessels.

The adjustment process requires precision and consistency. Before starting work, make sure that the boat is securely attached to the trailer or is on a flat keel. You will need to loosen the clamps or unscrew the engine mount bolts to the transom.

Checklist for height adjustment:

After changing the height, be sure to conduct a test run. Pay attention to the stream of water from the control hole (testimony of the pump) and the nature of the engine at full gas. If with a sharp addition of gas, the speed increases in spurts, and the boat does not accelerate, try lowering the engine to another half-sanitary.

Methods of repair and restoration of geometry

Recovery of a damaged anti-cavitation plate is possible at home, provided the damage is not catastrophic. Cracks or small dents can be repaired by returning the metal to its original shape. This will require removing the deadwater or dismantling the plate itself, if the design of the motor allows it (for example, on motors). Mercury or Suzuki with quick-release machinery).

If the plate is bent, it needs to be leveled, so use a wooden bar and a hammer to avoid damaging the paintwork even more, tap carefully on the deformed areas, controlling the flatness with a metal ruler or level, and it's important to restore not only the plane, but also the angle of inclination relative to the screw axis.

⚠️ Note: When aligning the plate, avoid strong local heating (such as a gas burner), as this can change the structure of the metal and cause it to further deform under load.

In the case of severe cracks or fractures of the edges, welding will be required. For aluminum alloys, which are most commonly used to make plates, argon-arc welding is the best fit (Arc welding is the most suitable).TIGAfter welding, the seams must be cleaned and treated with an anti-corrosion compound, if geometry cannot be restored, it is more advisable to purchase a new original spare part or make an analogue from a 3-5 mm thick dural sheet.

Upgrading and installing additional elements

Owners who want to make the most of their craft often resort to tuning this area. One popular solution is to install a hydrowing (stabilizer) over the regular floor. cavitation-stoveThis element, often referred to as "Doel-Fin" or simply "fin", helps the boat to go faster for planing and improves handling at high speeds.

The hydrofoil changes the flow dynamics, pressing the stern of the boat and preventing yaw, and the installation of such a element is simple: it is bolted directly to the standard stove through rubber pads. However, it is worth remembering that the installation of additional elements increases resistance, which can slightly reduce the maximum speed at full load.

There is also a practice of placing side cheeks (ear extensions) on the sides of the screw, which prevents air from sucking when the boat rolls, which is especially true for narrow boats and when performing sharp turns, and this solution effectively combats the "scrolling" of the screw in corners.