Propeller Cavitation Explained
Enjoying time out on the water can be a great experience. For boating enthusiasts, though, some of the focus will likely be on the quality of their boat, the engine they are using, and the propeller that drives them through the water. This is especially true for anyone who wants to race their boat, or who is looking to get the most power and efficiency from their engine and propeller combination. Cavitation is one aspect of the propeller that can make a difference. Here is what to know about it.
How a Marine Propeller Works
When a marine propeller turns, the blade moves through the water based on the torque the propeller absorbs from the engine. This torque comes from the revolutions of the engine, which creates horsepower that is transferred, ultimately, to the propeller blade. It is converted to thrust, and that pushes the vessel through the water. The positive pressure on the blade's face, and negative pressure on the back side of that same blade, are big parts of creating that thrust, based on how they work together.
What many people do not realize, though, is that there can be damage to the propeller blades from this process. The negative pressure at the back of the blade causes any gas that is in the water to form into bubbles. This is similar to the bubbles found in champagne, when the bottle is opened. The bubbles collapse as the propeller moves through the water and that causes serious impact loads on the blades. These can be similar to hammer blows and, in time, will cause visible damage.
The Power and Thrust Ratio Must be Within Specific Values
If either the power or the thrust are not within ranges that are specific to the propeller type, the depth at which it is being used, and the flow where it works properly, the flow pattern of water over the blades will be incorrect. As that pattern breaks down, propeller cavitation occurs. Cavitation is the term used for the actual breakdown of the flow pattern, which results in both propeller damage and loss of thrust at a level which is noticeable. The cavitating flow cannot produce proper thrust for the blades.
Cavitation bubbles are a serious concern for anyone with a conventional propeller, because significant damage to the overall hydrofoil can occur from the vapor bubbles that are collecting in the water and striking the surface of the blades. With the right flow, cavitation ceases to become an issue. Not only can the cavitation that occurs with improper flow cause harm to the blades, but a cavitating propeller can also result in harm to the rudder, and even to the steelwork that is part of the vessel's hull.
What to Look for When Detecting Cavitation
Among the most important things to do, to reduce or eliminate cavitation issues with a ship's propeller, is to discover the problem early. The faster it is found, the faster it can be corrected before it does a lot of damage. A small amount of cavitation may not be enough to put a propeller at risk, but a large amount certainly could be. Add to that the lack of thrust that comes with cavitation, and a ship can quickly become far less fun and a lot more work.
When searching for cavitation, the most obvious thing to look for is the actual, physical damage to the blade. The cavitation itself is a process, or an occurrence, and difficult to see while it is happening. But the results of that cavitation appear in the pitting of the blade and the harm that comes to it over time. The kind of cavitation that is taking place results in different types of damage, which can help a boat's owner determine exactly what might be taking place when the propeller is in the water.
For example, bubble cavitation typically causes damage mid-chord, while sheet cavitation is seen across the entire blade. If these kinds of issues are discovered during the design and creation stages of propeller development, changes can be made to the propeller itself. There are options to adjust the propeller in order to reduce concerns.
How to Solve the Issue, and Protect the Propeller
By adjusting the propeller pitch and flow velocity, bubble cavitation may be reduced. That can eliminate the development of gas bubbles, which are the cause of the pitting issues on a boat propeller. Sheet cavitation and vortex cavitation can also occur, as the swirling of the water around the prop blade and propeller shaft, as well as the blade tip, can all lead to a cavitation tunnel that funnels the gas bubbles right into the blades. The trailing edge of the prop must smoothly move through the water to avoid this.
Propeller design is another area to consider, when working to reduce risk to the blade surface from cavitation damage. A boat propeller is solid and strong, but does have ways in which it can be put at risk. The diameter of the boat propeller, the engine RPM, and the static pressure around the blades as they move through water can all add up to cause concerns with vibration, corrosion, and cavitation. Catching these types of problems means correcting them so the damage to the blades does not continue.
By addressing the propeller diameter, blade shape, blade size and blade pitch, it is possible to reduce the chances that a cavitation burn will start to harm the propeller efficiency. The fluctuation of the pressure field around a cavitating propeller can spell disaster for the blades, and can also mean the vessel will operate at a much lower level of thrust than it should, given the size of the engine and the available power and torque offered. The right prop can make a difference.