Every owner of a water transport, whether a fast plane boat or a heavy boat, sooner or later faces an unpleasant hum and loss of traction. boat-motorIt can not only ruin the pleasure of cruising, but also cause serious mechanical damage to the power plant, based on the physics of the formation of vapor bubbles in areas of local water thinning, which, when they collapse, create shock waves of enormous force.
Many beginners confuse cavitation with conventional air suction, but the effects of these phenomena are different, although the symptoms may be similar. Cavitation erosion It can literally eat a stainless steel screw blade during a season of active use, not to mention softer aluminum alloys. Understanding the nature of this process is the first step to conserving your life. propeller and a gearbox.
In this article, we will discuss in detail the mechanics of the occurrence of cavitation bubbles, methods of their visual and acoustic diagnosis, as well as practical ways to combat this enemy of speed. slab-stove And you can choose your propeller to keep your motor running at optimal speed, giving you maximum efficiency without the risk of damaging vibrations.
Physics of the process: why boils cold water
To effectively combat this problem, you need to understand what's going on in the water column around the rotating blades. propeller It rotates at high speeds, creates high pressure zones in front of the blade and a sharp vacuum immediately behind it, and according to the laws of physics, when the pressure drops strongly, the boiling point of the liquid drops.
Water, even cold water, boils instantly in these areas of thinning, turning into steam, forming microscopic bubbles that flow into the normal pressure zone, where they collapse instantly (condensed). cavitationIt occurs at a tremendous speed and generates a shock wave directed at the surface of the blade.
Millions of these micro-explosion per second create the same characteristic high-frequency noise, resembling the sound of a jet flying or the roar of rubble in a concrete mixer, the energy of which is so great that it is able to rip microscopic metal particles from the surface of the screw, creating craters and roughness that only increase the vortexes and exacerbate the problem.
⚠️ Attention: Prolonged operation of the motor in the mode of strong cavitation leads not only to the destruction of the screw, but also to the erosion of the sealing ossils of lower unit, which can cause water to enter the gearbox.
The intensity of the process depends on the speed of rotation and the shape of the blade. The sharper the edge and higher the revolutions, the higher the risk of occurrence of steam-cavitationBut there's another type of vortex cavitation, which is caused at the ends of the blades by the flow of water from one side of the blade to the other, forming powerful vortex harnesses.
Basic signs and methods of diagnosis
You can tell if there's cavitation by a number of indirect and direct signs that a skilled motorist will notice right away. The first signal is usually a change in the sound background of the engine, and the motor starts to "roar" or "buzz" in high tones, and that sound is different from the usual exhaust noise or gearbox.
The second thing is the loss of traction, and you can see that the engine speeds are increasing, but the boat speed is not increasing or even falling, because the propeller starts to rotate in a combined cycle mixture that is much lower than the density of water, and the repulsion efficiency from the aquatic environment is drastically reduced, an effect often called "breakdown" of the propeller.
For accurate diagnosis, you need to conduct a visual inspection of the screw after several hours of operation at high speeds.
- 🔍 Matte, rough spots on the back of the blades (the thinning zone), which to the touch resemble sandpaper.
- 🕳️ Small craters or ulcers, mainly at the edges of the blades and at the tips (tips).
- 🌪️ Traces of cavitation harnesses that can leave spiraling traces of erosion on the surface of the metal.
Also worth noting is the vibration of the boat hull, because the strong cavitation causes an uneven distribution of loads on the shaft, which is transmitted to the lower unit and the transom, causing a noticeable shudder. If you feel that the engine is "uneven" at full speed, although the idle is stable, this is an occasion for a thorough inspection of the underwater part.
Causes of cavitation on the outboard motor
There are many factors that cause the formation of vapor bubbles, most often the fault of the wrong engine installation relative to the bottom of the boat. anticavitational (the horizontal plane above the propeller) is set too high, the propeller captures air from the surface, forming an air cushion, and if it's too low, water resistance increases and flow is disrupted.
The second common cause is damage to the propeller itself. Even a small chip on the edge, a dent or a deformation of the step disrupts the laminar flow of water. At the site of the defect, vortices form, the pressure drops, and the steaming process begins. Using a screw with an inappropriate pitch can also drive the engine into cavitation mode, since the engine will not be able to develop the desired speeds or, conversely, will go into "spread".
The third factor is the state of the bottom of the boat and the transom. The growths of algae, seashells or damaged gelcoat in the area of lower unit create turbulent flows even before the water reaches the screw. The uneven flow coming to the blades is guaranteed to cause cavitation. In addition, the speed is affected by the weight of the boat: the overload of the vessel causes it to go deeper into the water, changing the angle of attack of the screw.
Effects of Water Temperature on Cavitation
Warm water has a higher saturated vapor pressure than cold water, which means that in hot weather or warm water bodies, cavitation occurs more easily and at lower speeds of rotation of the screw, and in summer the risk of damage to the screw by cavitation erosion increases by 15-20%.
A design feature worth mentioning is that cheap screws with rough surface treatment and sharp angles tend to cavitate more than screws with polished surfaces and rounded edges. Flow turbulence - The main enemy of smooth running.
Setting up the anti-cavitation plate and installation height
The correct height of the motor on the transom is the foundation of the fight against cavitation, the optimal position is considered when the lower plane of the anti-cavitation plate is on the same line with the bottom line of the boat or slightly lower (by 1-2 cm for high-speed boats), which ensures a stable flow of water to the propeller without trapping air.
If the plate is set too high, the screw will start to "grab" air, especially when the boat is rolling or on the wave, which causes sharp surges in speed and loss of traction. If it is too low, the drag of the lower unit increases, which can also lead to vortices and cavitation, especially at low speeds, and the adjustment of height is carried out by moving the motor along the fastening holes on the transom.
It is also important to check the parallelity of the plate of the water surface when moving, using adjustment washers (trim tabs) between the motor and the transom. Incorrect angle of the trim can cause one side of the screw to be submerged deeper than the other, causing one-way cavitation and the boat to move away.
☑️ Diagnostics of motor installation
Remember that different types of boats (planing, displacement) may have different installation requirements. For planing, it is important that the propeller does not pick up air when entering mode, so often you need to install the engine higher than for the displacement mode.
Selection and maintenance of propeller
The choice of propeller is always a trade-off between thrust and speed. High pitch allows for higher speed, but requires a powerful motor and ideal conditions. If the pitch is too big for your engine, it won't be able to spin the propeller to speed, which will lead to overloading and increased cavitation processes at the edges.
The material of the screw also plays a role. Aluminum screws are more flexible and when hit by an underwater obstacle, they often bend, which immediately changes geometry and causes cavitation. Stainless steel screws hold their shape better, but they transmit the blows to the lower unit more rigidly. It is important to check the screw balancing regularly: the imbalance causes the beat that destroys the laminar flow.
Rotor maintenance involves regular polishing of the surface. Rawnesses left over from casting or during operation work as vapor centers. Pollinating the back of the blades to mirror shine significantly reduces the risk of cavitation.
| Type of screw | Materials | Risk of cavitation | Recommended application |
|---|---|---|---|
| Standard. | aluminum | Medium. | cruising boats, low speeds |
| High Five / Vortex | Nerzh. | Low. | Sports boats, high loads |
| Herb-bread | Composite | Minimum | Heavy boats, trolling |
| With adjustable step | Nerzh. | Depends on the setting. | Racing cars, special equipment |
There are special "anti-cavitation" screws, which are characterized by the increased area of the blades and the special shape of the rear edge, which minimizes the thinning, and these models often have the "Anti-Cav" marking or labeling in the manufacturers' catalogs.
Methods of elimination and protection against erosion
If cavitation is already detected, the first step is to eliminate the cause, the damaged screw must be repaired (brewed and sanded) or replaced. floorboard (Extra plate) on the lower unit, which equalizes the flow of water in front of the screw.
To protect metal surfaces from the destructive effects of shock waves, special epoxy coatings containing ceramic or polymeric fillers are applied to the screw and create a shock-absorbing layer that takes on the energy of the collapsing bubbles.
⚠️ Warning: Never use conventional paints to protect the screw, they lack the necessary elasticity and strength, will quickly climb under the influence of cavitation and only aggravate the roughness of the surface.
One of the modern solutions is to install air or water supply devices into the screw zone (PVS systems - Performance Vent System), which artificially saturate the flow with gas bubbles. This may seem paradoxical, but a controlled gas cushion changes the specifications of the environment and prevents the formation of destructive vapor bubbles near the metal surface.
When polishing the screw, use abrasives with a gradation of at least P2000, and finishing with a felt circle with a polished paste for metal. A smooth surface is the best protection against cavitation.
Regular monitoring of the rotor and timely replacement of the anodic protection (which can also suffer from cavitation) will help to extend the life of your propulsion system. Remember that cavitation is not just noise, it is a signal that the system is operating in extreme mode.
A properly selected and polished screw, mounted at the optimum height, eliminates 90% of cavitation problems without the need for expensive improvements.
Frequently Asked Questions (FAQ)
Can cavitation completely destroy the screw?
Yes, with prolonged operation in the mode of strong cavitation (especially on soft aluminum screws), erosion can thin the blades to such an extent that they will collapse under mechanical stress or simply fall off.
Why is cavitation increasing in shallow water?
In shallow water, the propeller operates in the area of turbulent flows raised from the bottom and closer to the surface of the water, where the pressure is lower, a combination of factors that dramatically increases the likelihood of vaporization.
Does the quality of gasoline affect cavitation?
Indirectly, yes. Bad fuel can cause detonation or unstable engine operation, leading to propeller thrusts, and sudden changes in rotational speed disrupt hydrodynamics and can trigger cavitation failure.
How to distinguish cavitation from screw ventilation?
Ventilation is the capture of air from the surface (bubbles are big, noise gurgling). Cavitation is the boiling of water itself (bubbles are microscopic, high-frequency, like a chirp or whistle). Visually, when cavitation is done, the screw can be hidden by whitish vapor foam.
Should I change the screw if it shows the first signs of erosion?
If the erosion is superficial (matted spots), polishing is enough, and if deep craters (holes more than 0.5 mm deep) or sharp edges are formed, the screw is better replaced or professionally restored, since these defects will work as centers of increased cavitation.