Cavitation Causes and Remedies

Man in Hardhat Inspects Large Ship Propeller in Thrust Tunnel
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Next time you find yourself floating chest deep in water, try this little demonstration of cavitation in action while you are waiting for everyone to stop laughing and pull you out of the water.

Hold the palm of your hand vertically and pass it quickly back and forth through the water. You will see a stream of bubbles form opposite the direction of travel.

These bubbles are what is called cavitation. In the case of boats and ships, cavitation refers to a pocket, or cavity, of air forming on the backside of a prop or impeller blade.

What Is Cavitation? What Are Its Causes?

The most simple definition of cavitation is; an action that causes a void to form because of lower pressure.

As the definition above says, the condition of cavitation is caused by a low-pressure situation. When you moved your hand back and forth through the water you caused the pressure behind your hand to drop. That's where the bubbles formed. A prop with too much pitch or too much shaft speed will cause pockets to form on the back side of the blades or even at the tips.

The reason these voids form is boiling of the liquid. This is not boiling from heat, but boiling from the vacuum.

Physics experts tell us that a liquid will boil if heated to a certain temperature or if the pressure of the liquid is reduced. In the case of cavitation, the reason is lower pressure.

This cold boiling technique is good for many industrial uses, but it is not wanted near props or pump impellers. The collapsing bubbles are filled with very low-pressure water vapor and when they collapse damage is done to many surfaces.

Cavitation is a drag on efficiency because of the increased friction. The bubbles stick to surfaces and essentially increase the thickness of prop blades and more power is needed to increase or maintain speed.

Even worse, cavitation can cause vibration because of uneven prop loads and damage or break equipment. Even worse than vibration damage is pitting.

Pitting happens when bubbles collapse and all forces are focused on a tiny spot on the blade surface. Damage from vibration is very noticeable and usually preventable with modifications to operating style. Damage from pitting can be happening at a very subtle level and most of the affected components are out of sight in day-to-day operations.

An increase in power caused by a poorly adjusted governor can be enough to start minor cavitation near the prop tips and probably would not be noticed by most crews. Only at haul out would the damage to drive components to be noticed. Pitting increases surface area which causes corrosion and few anti-fouling coatings can withstand forces from collapsing bubbles that can eat into hardened steel.

This same set of conditions and the resulting damage can also happen inside things like pump housings and thruster tunnels. Cavitation is actually much easier to produce in an enclosed environment than in an open situation like a prop and shaft.

In an enclosed area, there is much less liquid volume to rush in and compress the vacuum bubbles that are forming. Cavitation inside pumps is a leading cause of failure. Turning a centrifugal pump too quickly causes the liquid in a pump chamber to boil from lack of pressure. This is even more of a problem if you are pumping a hot liquid like coolant or heavy fuel oil.

In a hot liquid situation, you are applying two sources of energy that will make the liquid boil. The first, heat, is external and is the better-understood form of boiling. The second is the mechanical vacuum caused by the impeller. The technical term for this second force is Net Positive Suction Head or NPSH.