Understanding Propeller Cavitation – Principle, Causes, Effects and Prevention
Propeller cavitation is a destructive phenomenon that can damage the propeller and the components around it such as the hull, rudder, and any other appendages. While the first instances of a working screw propeller go back to 200 BC, the theory of cavitation has been understood relatively recently. Since then, it has been instrumental in understanding the causes and effects of cavitation.
In this article, we explore propeller cavitation to understand how it affects our systems and what can be done to avoid it. Let’s begin.
Propeller cavitation working principle
An increase in the pressure of a fluid also increases its boiling temperature. We use this concept in many day-to-day applications. For instance, in a pressure cooker, by not allowing the steam to escape, we cause it to build up pressure inside the cooker. The increased pressure increases the boiling temperature of the water inside the cooker by as much as 20°C. This allows us to cook the food at a higher temperature and reduce the cooking time considerably.
The same phenomenon occurs during cavitation. The shape of the propeller blade is such that it creates a low-pressure area on one side and a high-pressure area on the other side. If the pressure on the low-pressure side is sufficiently low, the surrounding water starts evaporating at significantly lower temperatures such as 20 °C.
The evaporation leads to the formation of vapor bubbles. These vapor bubbles grow in size as they get closer to the blade. However, as soon as they cross the blade edge and enter the high-pressure area, the surrounding water rushes onto the vapor bubbles from all sides at high velocity. The bubbles collapse into themselves almost immediately. This effect, known as implosion, has a high impact energy and if it takes place near or on a blade, it erodes the material at the closest point forming pits and craters.
The implosion also creates high local temperatures. The temperatures can go as high as 400 °C at cavitation sites causing tempering of the propeller blade. The same can be identified through discoloration or Heat Affected Zones on the propeller face.
Over time, cavitation can start to eat away at the blade material so severely that the blade will become non-functional. The problem of cavitation is more serious in high-speed propellers as the faster a propeller cuts through the water, the lower the pressure on the suction side.
So What Causes Propeller Cavitation?
Propeller cavitation can be caused by several reasons. Some of the most common ones are as follows:
High speed of the propeller
A higher speed can increase the speed of water flow over the propeller blades. A faster water flow leads to lower suction pressures (Bernoulli’s law) on the blade surfaces and tips. The lower pressures increase propeller cavitation and exacerbate material erosion.
Breakdown of water flow pattern
The ratio of absorbed power to the propeller blade area is known as the power loading of the propeller. Similarly, the ratio of delivered thrust to the propeller blade area is known as the thrust loading. If the value of any of these ratios crosses a certain threshold, it leads to the breakdown of the water flow pattern and results in cavitation.
Any damage to the boat propeller can also induce cavitation in a previously non-cavitating propeller. Similarly, inadequate clearance between the propeller and the hull or the skeg can also break the water flow pattern and result in cavitation.
Incorrect pitch distribution
If the pitch is not uniformly distributed along the length of the propeller blade, it can lead to cavitation. Most propellers suffer cavitation on the back of the propeller blade in a radial fashion. However, incorrect pitch distribution can enable cavitation bubbles to also impact the driving face.
Most ship propellers have a constant pitch across the blade length but on high-speed boats, a variable pitch propeller is a necessity to endure the excessive thrust loading. The consequence is an increased cavitation tendency in these boat propellers.
The effects of cavitation can be severe and permanent. They can also serve as an indication of the degree and spread of cavitation in the stern of the ship. Some of these are as follows:
Cavitation can lead to severe material erosion of the propeller and the surrounding areas. It reveals itself as very heavy pitting on the propeller blades and other areas such as the hull and the rudder. Periodic inspections can enable the stakeholders to identify cavitation and take proactive steps to prevent further damage.
A second indicator of propeller cavitation is the reduced performance of the ship. Cavitation damages the propeller. It cannot produce sufficient thrust resulting in a loss of speed and performance.
High vibration is also an effect of excessive cavitation. Cavitation vibration can damage the shaft components, bearings, seals, and hull resulting in a shorter life of these expensive components.
The implosion of water vapor bubbles on the blade surface also produces a lot of noise. This noise can be heard over the engine noise and is a clear indicator of boat propeller cavitation.
Prevention of cavitation
The quest to prevent cavitation starts from a very early stage. Marine propellers must be matched with other factors such as the application, craft speed, material of construction, and so on. Let us understand the two ways to prevent propeller cavitation: Design and Operation.
Prevention through design
There are several ways to prevent cavitation through intelligent design. Let us see the considerations at the design stage that are crucial to our objective.
Improve propeller design
An improved propeller design with suitable diameter, engine RPM, and static pressure around the blades can go a long way in preventing marine propeller cavitation.
Cavitation occurs also due to inappropriate blade shape, size, and pitch and hence, they also need optimization to avoid a cavitation burn. For instance, a higher blade area can provide greater thrust at low revolutions. The pitch can also be varied over the blade length to reduce excessive loading in vital regions.
When the propeller is designed for operation under 35 knots, a flat or low-loaded profile with a low angle of attack works best.
Suitable material of construction
The resistance to cavitation attack also depends on the metal grade. Some metals have higher resistance and must be preferred during the material selection of the ship’s propeller.
Using stronger and cavitation-resistant materials also allows us to reduce the blade thickness.
The stern must also be designed to provide as uniform a wake as possible.
Prevention through operation
Despite all of the above measures, if severe cavitation should occur, there are several practical solutions to it.
Increase the aft draft
Net Positive Suction Head (NPSH) is a crucial factor in maintaining a cavitation-free operation. Cavitation bubbles form when the available NPSH falls below the required NPSH. By increasing the water column directly above the propeller, we can increase the available NPSH and reduce cavitation.
Reduce the engine RPM and speed
Reducing the engine RPM also affects how much cavitation occurs. Bernoulli’s theorem states that an increase in the velocity of a fluid leads to reduced pressure. By reducing the RPM and the speed of the vessel, we can reduce the absolute velocity of the fluid flowing over the blade section shapes. Reduced drop in pressure due to the slower velocity and engine RPM helps us avoid cavitation and associated negative effects.
Propeller cavitation is a very serious issue that can put a ship out of operation. We must monitor it vigilantly and take any action required to mitigate it in the early stages.