Why Do Boats Cavitate? (Explained For Beginners)

Cavitation is a word to describe the creation of gas bubbles underwater that results from a fast propeller in low-pressure environments.

At certain low pressures, a vacuum can cause water to “boil” at low temperatures, thereby causing the bubbles that we see with increased propeller speed.

These bubbles then pop, creating noise and small implosions and shockwaves against your propeller’s metal.

Why do boat propellers cavitate?

The faster a boat propeller goes, the more bubbles are produced, which is the cause of cavitation. Slowing down or reducing surface pressure will reduce cavitation and therefore save your propeller from unnecessary abuse.

Cavitation is detrimental to your boat and propeller and should not occur with your boat very often, so consider these facts and information in order to prevent this from happening to you:

Here is Why a Boat Propeller Cavitates:

As you can see, a boat propeller causes cavitation when it operates at high speeds in certain conditions.

Pressure, mixed with speed, causes the water behind the propeller to form a vacuum, thereby creating small bubbles or “cavities.”

Then, the bubbles implode or ‘pop’ in the water, and those implosions can have a lot of negative effects on the materials used in the propeller.

In fact, propellers often require maintenance if they are operating too fast all the time, as the implosion of the bubbles and overworking of the materials will cause a lot of wear and tear.

Cavitation can be prevented with a few steps and attention to detail, so consider these options below:

How Do you Reduce Cavitation in Boats?

Sometimes, cavitation can be caused by fouling of the propeller or the lower unit.  Make sure that that is not a problem first.   Then, cavitation can be reduced by simply slowing down your speed or redesigning your propeller.

Slowing Down Your Speed:

A reduction in speed is the simplest way to prevent cavitation from occurring.

The faster you go, the more bubbles you are likely to create.

This is because the speed of the propeller creates a vacuum that pulls the bubbles out of the water.  The popping of those “boiling” bubbles is a huge part of cavitation and can have lasting effects such as noise pollution and damage to your propeller.  It can also cause the motor to speed up excessively while not moving faster through the water.

Slowing down prevents more bubbles from occurring, and will improve the efficiency of your boat.   Therefore, a slower engine speed may not cause you to go much slower through the water.  At some point, running your engine slower will reduce your speed and thrust. This still means that you’ll have a quieter and more efficient ride, but maybe a slower one.

However, this can be solved by redesigning a propeller that can maintain thrust while reducing cavitation, as they describe in the video above.

Redesigning Your Propeller:

As the video explained, the surface of the blade is where the lowest pressure occurs, and therefore the cavitation and air bubble popping.

If your propeller blade has a large surface and a low pressure, you are more likely to cause cavitation.

However, like it was also explained in the video, creating smaller surfaces for low-pressure points to occur reduces the number of bubbles and therefore, sound, damage, and problems that cavitation can have on a propeller and its metal structure.

The smaller surface of the blades will create the same kind of thrust that you are looking for without reducing your speed, simultaneously reducing your cavitation and therefore enhancing your experience.

How Loud is the Noise from Cavitation in Boats?

Not only are cavities, or bubbles, damaging, but they are also loud.

When a gas bubble implodes underwater, it gives off a small shockwave that results from its change of phase from liquid to gas.  

Not only does this give off the noise, but it also can cause damage to a propeller over time.

The popping of gas bubbles in the water is actually quite loud. Most submarine operators make sure that their cavitation is as quiet as possible in order to prevent themselves from being detected by enemy sonar or anything that they might want to sneak up on or sneak past.

Although these shockwaves are very small, when there are hundreds of bubbles popping every second, the noise is actually really easy to pick up on sonar.

Furthermore, this can create a lot of stress and erosion on the metal of your propeller.

When scientists began to consider the possibilities of renewable energy with tidal stream turbines, this problem arose as one of the challenges that would have to be overcome in order to try and create consistent energy without creating too much noise or damaging pumps and propellers.

Personal Boat Cavitation:

Because your propeller on a regular boat should be selected for the boat and motor characteristics, it is highly unlikely that you will experience severe cavitation.

This is because your boat propeller should be designed to match the boat and the motor performance, so they will work together without causing cavitation.   

Furthermore, it is highly unlikely that your propeller will suffer great injury from use.

However, if you do experience this, it is recommended that you either slow down when traveling, or you should consider investing in a better model of propeller.  This will require consultation with a marine motor professional.

Final Thoughts:

Boats will take a lot of abuse in their time, especially if you run them fast and over rough waters.

While cavitation is much more common with submarine propellers in deep waters, cavitation can happen to any boat owner.

However, if you find that your boat propeller is cavitating more than it should, consider investing in a better-designed propeller or slowing down.

This will reduce your noise pollution, damage to your propeller, and will ultimately make your trip a much smoother one.

References

Cavitation – Facts & Wiki

Cavitation! explained HD – YouTube

Cavitation Causes and Remedies