What is cavitation?
- Thread starter Sabah
- Start date
- Location
- new jersey
When your jet pump has a leak and is sucking air every time the ski jumps outta water- another term is porpoising
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What affect does that have on the ski?
- Location
- new jersey
Your ski is not hooked up to the surface of water , it's jumping in and out like a porpoise.
Having a good pump seal and still water u should be locked , in a bay with chop it's not as easy
Having a good pump seal and still water u should be locked , in a bay with chop it's not as easy
Vumad
Super Hero, with a cape!
- Location
- St. Pete, FL
That is not entirely correct. Porpoising can cause cavitation but it is not another term for the same thing.
Any fluid pump can cavitate. Your electric bilge pump can cavitate.
Wiki article on cavitation. https://en.m.wikipedia.org/wiki/Cavitation
So, anything that airates a fluid causes cavitation. In the case of our pumps, it often caused by bad pump shoe deal, mid shaft seal, wear ring gap, bend prop or unloading the intake track.
Oh okay
- Location
- Granbury, TX
Being a pump engineer I'll try and put it in simple terms....
When your pump demands more water than the incoming water can supply you get cavitation. Basically, the pressure (or lack thereof) on the water in your pump drops below its vapor pressure. Bits of the water instantly turn to vapor (boil) and those vapor bubbles then instantly implode. The implosion of the vapor is as powerful as an explosive charge and can actually eat away bits of metal (known as cavitation "pitting"). Air leaks exacerbate this problem immensely. Your pump is trying to shove a massive amount of water through and the water is turning to vapor instead of pure water thrust.
Porpoising is not cavitation, it is poor ski control and has NOTHING to do with cavitation.
When your pump demands more water than the incoming water can supply you get cavitation. Basically, the pressure (or lack thereof) on the water in your pump drops below its vapor pressure. Bits of the water instantly turn to vapor (boil) and those vapor bubbles then instantly implode. The implosion of the vapor is as powerful as an explosive charge and can actually eat away bits of metal (known as cavitation "pitting"). Air leaks exacerbate this problem immensely. Your pump is trying to shove a massive amount of water through and the water is turning to vapor instead of pure water thrust.
Porpoising is not cavitation, it is poor ski control and has NOTHING to do with cavitation.
This thread is a lot more informative than I thought it would be.
- Location
- SW Michigan
Like smoofers said, cavitation occurs when you do not have enough inlet pressure to fill the void left by the spinning pump element as it goes by. The pressure required on the inlet is sometimes referred to Gear Tip Velocity Head (GTVH). It is essentially dependent on the pump diameter and rotational speed. The greater the pump diameter and/or the greater the rotational speed, the more pressure you need on the inlet to the pump to fill the pump and prevent cavitation. At higher watercraft speeds the inlet pressure is generally higher, so at a given rpm, at a higher craft speed, you fill the pump better. Also you will find a loader intake grade will help with cavitation – because it will increase the pump intake pressure.
How air enters the pump also can play a role. An air leak, drawing in air under vacuum on the inlet will act very much like cavitation. With larger air leaks, not only do you lose inlet pressure on the pump (which causes a harder time to fill the pump), but you are also introducing air (under vacuum) right into the pump. These vacuum bubbles will implode when the reach the high pressure side of the pump. The higher the outlet pressure of the pump is, the more severe the implosion. A higher pressure collapses the vacuum bubble much faster.
Now on the contrary, if the pump is just gulping air (like the pump is out of the water), you are dealing with air ingestion. Air coming into the pump at atmospheric pressure will compress on the outlet. It does not damage components like an imploding vacuum bubble.
Cavitation damage can be pretty crazy. I had a stock superjet aluminum impeller almost eaten through near the root on all three blades. Around the edges it was eroded somewhat, but then on the blade surface just looked frosted from the pitting. Obviously the aluminum impeller didn’t help the situation being so soft.
How air enters the pump also can play a role. An air leak, drawing in air under vacuum on the inlet will act very much like cavitation. With larger air leaks, not only do you lose inlet pressure on the pump (which causes a harder time to fill the pump), but you are also introducing air (under vacuum) right into the pump. These vacuum bubbles will implode when the reach the high pressure side of the pump. The higher the outlet pressure of the pump is, the more severe the implosion. A higher pressure collapses the vacuum bubble much faster.
Now on the contrary, if the pump is just gulping air (like the pump is out of the water), you are dealing with air ingestion. Air coming into the pump at atmospheric pressure will compress on the outlet. It does not damage components like an imploding vacuum bubble.
Cavitation damage can be pretty crazy. I had a stock superjet aluminum impeller almost eaten through near the root on all three blades. Around the edges it was eroded somewhat, but then on the blade surface just looked frosted from the pitting. Obviously the aluminum impeller didn’t help the situation being so soft.
Attachments
- Location
- new jersey
I shoulda left it to the experts , much more than I knew. Good info ^^^
- Location
- New Jersey
Wow. Some pretty serious info. Pretty cool! Thanks for sharing
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Sent from my iPhone using Tapatalk
Vumad
Super Hero, with a cape!
- Location
- St. Pete, FL
But I have an awesome visual in my head of someone jumping onto the front bumper of a fire engine while the pump is cavitating shouting "Help me guys, it's porpoising!"
bored&stroked
Urban redneck
- Location
- AZ
Everyone is leaving the easy answer on the table. Cavitation is......bad. Mmmmm k?
Wow, I thought I would get a one sentence answer at most. This thread is starting to look more like a research paper.