Cavitation on a marine propeller causes a decrease in efficiency and thrust, generating excessive noise and vibration. It can lead to erosion and damage on the propeller blades and compromise the propeller’s structural integrity. Additionally, cavitation affects the boat’s maneuverability due to unbalanced thrust distribution, potentially reducing its overall performance and longevity
What is Propeller Cavitation
Cavitation is bubbles caused by excessive propeller speed or loading. The water vaporizes or boils due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the boat propeller’s blade surface due to the collapse of microscopic bubbles on the blade.
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Cause of Propeller Cavitation
High Speeds: As the rotational speed of the propeller increases, the likelihood of cavitation occurring also rises. At high speeds, the pressure difference between the low-pressure side and the high-pressure side of the propeller blade can be significant, leading to the formation of vapor-filled cavities.
Insufficient Submergence: When the propeller is not fully submerged in water, such as when operating in shallow waters or during vessel acceleration, the water pressure above the propeller blades decreases. This pressure drop can trigger cavitation.
Improper Blade Design: Inadequate propeller blade design, including excessive camber or thickness, can contribute to cavitation. Poorly designed blades may not handle the water flow smoothly, resulting in local areas of low pressure that encourage cavitation.
Rough Water Conditions: Rough seas or turbulent water can create irregular flow patterns around the propeller, increasing the likelihood of cavitation. Sudden changes in water pressure and flow disturbances can promote the formation of vapor-filled cavities.
Damage or Fouling: Damage to the propeller blades, such as dents, nicks, or surface irregularities, can disrupt the smooth flow of water, leading to cavitation. Similarly, the presence of marine growth or fouling on the propeller can disturb the water flow and trigger cavitation.
Types of Cavitation
The cavitation “sheet” or “laminar” takes the form of a thin stationary sheet, usually starting at the leading edge of the blades.
Sheet or Laminar Cavitation
Under certain conditions, the cavitation of the web breaks up behind the blade into a shape known as a “fog” or “cloud” cavitation.
‘Bubble‘ cavitation, as the name implies, is the formation of distinctive bubble cavities.
Bubble Cavitation
The swirl cavitation is in the form of a swirled rope and can be present on either the blade tips or the boss.
Vortex Cavitation
Cavitation increases with higher blade loading, i.e., with a larger angle of incidence or with different blade cross-section shapes.
Means of Avoiding Cavitation
1.reduce the angle of the propeller blade and the angle of descent can be achieved by adjusting the diameter of the propeller
2.To prevent the back side of the propeller blade loading surface is too high, can be used more uniform pressure interface shape
3.To avoid the excessive peak of the propeller front section, the angle of camber and the shape of the air inlet can be adjusted appropriately
4.Adjust the speed of the propeller. Proper adjustment of speed can reduce cavitation but also loss of speed
An outboard propeller is said to be fully cavitating when the whole of the back is covered in sheet cavitation. This phenomenon is also called supercavitation. After the back of the section has become completely denuded of water, the increase in revolutions per minute cannot reduce
pressure there anymore, and so no additional lift can be generated by the back. On the face, however, pressure continues to increase with higher revolutions, and so does the total thrust, although at a slower rate than before cavitation began.
An introduction to propeller cavitation
When the propeller turns, the engine generates horsepower at a constant speed, which is converted into thrust and propels the propeller forward. Positive pressure is generated along with negative pressure.
The negative pressure causes the gases in the solution to become bubbles, which explode and generate shock loads that can damage the blades of the propeller.
The ratio of the input power or thrust delivered to the total area of the propeller blades is called thrust power and thrust load, respectively.
If a certain value is exceeded, then the propeller’s water delivery mode collapses, and the resulting thrust is severely lost, causing physical damage to the propeller surface as well as to the rudder and the local steel structure of the hull. This flow disturbance is known as cavitation.
The cavitation attack phenomenon is generally determined by the type of cavitation attack, the proximity of the water surface, and the rate of change of the cavity volume. Cavitation can have different effects on different metal materials.
Figure 1 shows the various types of propeller cavitation.
The material of the propeller is an important factor affecting the size of the cavitation. The material of the propeller is compacted under the pressure of the water temperature, thus creating an orange-peel effect. That is, the surface is deformed to form an orange peel-like surface under pressure to be applied.
The cavitation attack is a test of the hardness of the propeller because the blade surface of the propeller becomes hard and brittle under the cavitation attack, which is related to the material and manufacturing process of the propeller.
Different patterns of propeller cavitation can occur on a marine propeller, as illustrated in Figure 1, and these are usually grouped as follows:
• tip vortex cavitation
• sheet cavitation
• cloud cavitation
• bubble cavitation
• root cavitation
• face cavitation
• boss vortex cavitation
Some of these forms are relatively benign, but others can be very aggressive in their effect on the propeller material.
The level of propeller displacement will also produce cavitation in different situations. These additional vortices collide with the normal vortices, and these collisions cause severe erosion and abrasion on the leading edge of the rudder or rudder angle. Located at the blade tip.
Blade cavitation occurs when a large suction pressure builds up near the leading edge of the blade, causing a band of bubbles to cover the back of the blade, and is highly dependent on the angle of impact of the blade along with the various wakefields generated by the shovel.
Propeller rotation. The greatest pressure drop occurs at the back of the blade, where most leaf and bubble cavitation occurs, and high tip speeds increase the likelihood of such cavitation. If the leaf is relatively stable, blade breakage is less likely to occur than if the leaf is unstable in some way.
Cloud cavitation is often located close to sheet cavitation fault areas and is very aggressive due to the destructive effects of large numbers of exploding bubbles and should always be treated with care and eliminated if possible.
Bubble cavitation usually occurs in the middle of a chord and is usually due to too high curvature or camber of the blade sections. It can be eliminated if its presence is suspected at the design stage.
Figure shows a typical pressure regime around a marine propeller blade section.
Root cavitation can occur at different times of propeller rotation if the circulation around the root is strong enough and can be aggressive enough to cause erosion or boss damage. When the root vortices pass downstream behind the propeller, they merge into a main vortex and often appear as a filament with several filaments equal to the number of blades.
If, as is usual on individual propeller vessels, the rudder is immediately behind the propeller, the string of bubbles can collapse, causing serious damage to the leading edge of the rudder or rudder post.
The propeller is said to swing completely when the entire back is covered with sheet cavitation. This phenomenon is also called supercavitation and is a whole new ball game.
Performance
The effect of cavitation on performance can be significant. Cavitation usually starts at the blade tips and gradually spreads throughout the blades as propeller loading increases. If cavitation has expanded to about 0.75 radii, a significant loss of thrust is detected, followed by a decrease in torque, which in practice, means a significant increase in RPM for a given power. Since the thrust rupture occurs faster than the change in torque, this can result in a significant reduction in efficiency.
