Clutch Slipping aka Cavitation aka Ventilation: AI's Take on the Whole Thing

[Ed Clark]

So, any of you that have ever driven a straight shift car with a worn out clutch will immediately recognize the analogy to a sudden loss of power on a jet drive system. To get a car with a clapped out clutch rolling, one must ever so gently ease the throttle on. Any abruptness in throttle application will result in the clutch going into full slip mode and the car refusing to move forward beyond a snail's pace even with the throttle pegged. Exactly the same phenomenon can occur with a jet driven ski or boat, although the jet drive has no clutch. What's up?

For starters, in my experience the exact same thing can happen with a severely damaged wear ring. This discussion assumes the wear ring has been ruled out as the source of slippage. I use the term "slippage" to simply indicate the effect whereby the engine speed increases (usually rapidly) but there's no corresponding increase in "push" (thrust) being generated on the ski/boat/whatever. Now, when slippage is encountered, what might be the cause?

It is commonly accepted that either cavitation or ventilation or both will result in slippage. However, what are the root causes of cavitation or ventilation? Before answering that, it's important to know the difference between the two. It is very common for the term "cavitation" to be universally assigned as the cause when referring to a slippage effect. Nothing really wrong with that because most everybody understands what is being referred to. Just for the record, and except for the racing community out there, cavitation will not rear its ugly head or result in appreciable slippage in our "daily drivers." What is really happening is "ventilation," or so I would propose to you.

Does it make any difference what we call it? Probably not although the stuff that causes ventilation is somewhat different than the stuff that causes cavitation.

To see what a "computer head" had to say about the subject, I queried a so-called artificial intelligence program (ChatGPT) that is commonly available on the Internet. I was more than a little impressed with the answers. Don't know if you will be, but I'm posting below the question/answers from the session.

Before I do, I'd like to point out that there's another thread in this forum on the topic of jet pumps in general that might behoove you to read if you're at all interested. Here's a link to that: How a Jet Pump Works

Question:
Please explain and discuss in detail the cause and effect of cavitation and ventilation in a water jet propulsion device such as is used on a "jet ski" or "jet boat" as the motive force of propulsion. Also, please compare "cavitation" vs "ventilation" in general and discuss the differences in same.

Answer:
Cavitation and ventilation are two phenomena that significantly affect the performance and efficiency of water jet propulsion devices, such as jet skis and jet boats. These two terms are often used interchangeably, but they refer to different issues, and their causes and effects are quite distinct. Let's break down both phenomena in detail and explore their differences.

Cavitation in Water Jet Propulsion​

Cavitation occurs when the local pressure in a fluid drops below the vapor pressure, causing the formation of vapor bubbles. In the context of a water jet propulsion system, cavitation is a serious concern because it can severely affect the efficiency, performance, and longevity of the system.

Cause of Cavitation​

In a water jet propulsion system, water is drawn into the pump and accelerated through a nozzle to generate thrust. When the pressure within the pump housing or around the impeller becomes too low (due to high rotational speed, insufficient intake water, or a blocked intake), it can cause small vapor pockets or bubbles to form. These bubbles then collapse when they encounter higher-pressure areas further along the flow path, causing shockwaves that can damage the components of the system.

Cavitation is typically caused by:

• Excessive pump speed: When the impeller is spinning too fast, it may create areas where the pressure is low enough for cavitation to occur.
• High angles of attack on the impeller blades: If the impeller blades are positioned at angles that disrupt smooth water flow, they may lead to localized low-pressure zones.
• High water temperatures: Warmer water has a lower vapor pressure, increasing the likelihood of cavitation.
• Insufficient water intake: If the water intake is restricted or if the boat is operating in shallow water, the pump may not be getting enough water to maintain proper pressure.

Effect of Cavitation​

Cavitation leads to several negative effects:

• Loss of performance: The presence of vapor bubbles reduces the efficiency of the water jet propulsion system, as the collapsed bubbles cause erratic water flow and inconsistent thrust.
• Damage to components: The implosion of the cavitation bubbles creates micro-shocks that can erode the metal surfaces of the pump, impeller blades, and nozzle, leading to long-term damage.
• Increased noise and vibration: Cavitation is often accompanied by a characteristic “gravel” or “marbles in a tin can” noise, and it causes vibrations that can reduce the comfort and stability of the vessel.

Ventilation in Water Jet Propulsion​

Ventilation, on the other hand, occurs when air enters the intake or pump system, disrupting the normal flow of water. Unlike cavitation, which involves the formation and collapse of vapor bubbles, ventilation is primarily concerned with the introduction of air into the water flow.

Cause of Ventilation​

Ventilation typically happens when the water jet propulsion system operates in conditions where air is drawn into the intake. This can happen when:

• The boat is operating in shallow water: If the boat is too close to the surface of the water, air can be drawn into the intake along with the water.
• Excessive trim angles or poor hull design: A hull that is angled too steeply, or one with an improperly designed intake, can lead to the intake drawing in air rather than water.
• High speeds: At very high speeds, the flow of water can become turbulent, and air may be entrained into the intake or the pump.
• Wake or wave interference: The boat may draw air from the wake or from waves if it is running too close to the water's surface.

Effect of Ventilation​

Ventilation can cause several problems for a water jet propulsion system:

• Loss of thrust: When air enters the intake, the pump is no longer able to create sufficient water pressure to generate effective thrust, leading to a loss of speed or propulsion.
• Erratic performance: The air in the intake disrupts the smooth flow of water, leading to inconsistent propulsion and a "cavitation-like" loss of performance.
• Damage to the pump: While ventilation doesn't create the physical damage associated with cavitation, prolonged ventilation can lead to strain on the pump and other components.

Cavitation vs. Ventilation: Key Differences​

Though both cavitation and ventilation can impair the function of a water jet propulsion device, they are caused by different factors and have different effects:

Cavitation​

• Cause: Low pressure in the water pump leads to the formation of vapor bubbles, which then collapse, creating shockwaves.
• Effect: Causes damage to components, reduces performance, and produces noise and vibration.
• Nature: Involves the formation and collapse of vapor bubbles in the water, typically leading to erosion of metal surfaces.

Ventilation​

• Cause: Air is drawn into the pump or intake, disrupting the normal water flow.
• Effect: Leads to a loss of thrust, inconsistent performance, and potential strain on the pump.
• Nature: Involves the physical introduction of air into the flow, rather than vapor bubbles in the water.

Conclusion​

In water jet propulsion systems like those used in jet skis and jet boats, both cavitation and ventilation are phenomena that can compromise performance. Cavitation, caused by low-pressure areas within the system, can lead to component damage, performance loss, and noise. Ventilation, on the other hand, occurs when air is drawn into the intake, disrupting the normal flow of water and causing a loss of thrust. While both can result in performance issues, they differ in their causes and effects, with cavitation being more directly damaging to the system and ventilation typically resulting in transient or temporary performance degradation.

Understanding the differences between these two phenomena is critical for optimizing water jet propulsion systems and ensuring that jet skis and jet boats operate efficiently and reliably.

 

[mikidymac]

Not sure if there was a question in there but this is what cavitation actually does to an impeller.



Cavitation is typically caused by a damaged impeller or wear-ring and is the most common on these skis.

Ventilation would be caused by a bad pump to hull seal or driveshaft seal and is less common than cavitation.

I like that you are getting into the technical aspects of these.
If you really want some amazing information and history start poking around Group K.
Air Leaks – Group K
Piston Seizures – Group K