What is Cavitation?
Search for cavitation and marine propeller on the Internet and you will be bombarded with descriptions of flow phenomena, pump operations, vapor pressure, noise, vibration and other such good terms. Most people leave scratching their heads as to what is going on and why, never really understanding just what it is.
The What and Why of Cavitation?
Several key people figured out the problem of cavitation, the first was Euler in 1754 working with water wheels and their efficiency. Eulers work lead the way for turbo-machinery and pump equations that are in use today. Also, a colleague of Euler, Daniel Bernoulli, developed equations to explain why the pressure drops when the flow is increased. The Bernoulli equations are famous in ship hydrodynamics and are part of the explanation as to why cavitation occurs. The “what” however was discovered in 1895 by R.E. Froude who correctly identified cavitation on a propeller; the phenomena has since been found in pumps, spill gates on dams, and also in the human body. When you crack your knuckles you are forcing the synovial fluid in the joint to cavitate – amazing! Froude discovered cavitation when he was investigating a boat called ss Turbinia that was not making power. The new steam turbine engine in the boat was delivering power to the shaft, but the propeller was not putting the power down – like car wheels spinning due to too much torque. Froude was able to determine, through experimentation, that the cause was not enough blade area on the propeller and took the boat from 19 kts to 32 kts, the predicted speed and coincidentally, became the forerunner of the planing craft.
So what is cavitation? In simple terms cavitation is cold boiling of the water due to low pressure. This explains the pops and bubbles a cavitating propeller makes, just like boiling a pot of water on the stove. When water reaches its boiling point on the stove, the pressure that keeps water in it’s liquid phase is low enough to allow the boiling water to change phase become steam. This process can occur by using heat to lower the vaporization pressure to make steam, or if the ambient pressure is low enough, water cannot be held together by pressure and the same process occurs – it begins to change phase.
How does cavitation happen?
As explained above, cavitation is a low pressure boiling which occurs in areas of very fast flow, that is the key fact. It is therefore seen when flow is forced to speed up, over or around an object. SO despite cavitation being discovered on a ship’s propeller, you will find it in many places. Pumps are a big source of cavitation, spill gates on dams where the water turns sharply often sees cavitation damage, it can happen in the body, and even certain types of shrimp use cavitation bubbles when they clip their claws to stun prey. So how does it do it? There we come back to Bernoulli. If you put your thumb on a garden hose and when you block it the hose swells up (low flow gives an increase in pressure). But take your thumb off the hose and the pressure drops, that is the key part. So when you have a propeller blade at an angle of attack there is a limit as to how fast the flow can go over the blade . The faster the flow goes the lower the pressure, keep going and the pressure drops to a point where the water is not held together any more and it boils pulling all the gases out. The water will bubble and boil at low pressure just without the heat.
The reason certain types of cavitation are bad for propellers is that the cavitation bubbles are really tears in the water, mini bubbles of dissolved gases (the stuff the fish breathe) they are actually at such low pressure they are vacuum bubbles. When they leave the propeller and get to an area of higher pressure they collapse and return to normal. This collapse can be 600 psi which causes damage to the material surrounding it, not to mention noise as it collapses and vibration as it hits the surface.
All propellers can cavitate
It is important to remember that all propellers can cavitate depending upon the loading condition and not all cavitation is bad. For most ships their propellers will show tip vortex type of cavitation as they get to their design speed. It is when the bubbles collapse on or near the blades (such as bubble or mist cavitation) that damage can occur. Some boats even take advantage of the reduced drag due to the cavitation on the propeller. SuperCavitating propellers deliberately operate inside a cavitation bubble, but the cavity collapses down stream of the blade away from where it can do any damage. So next time you are on a ship and you hear a sound like a couple of Quarters bouncing around in a tumble dryer, you are listening to full scale cavitation.