Cavitation

[Taylorman]

Whats the best way to tell if a jet pump is cavitating? I have a feeling mine does a little bit but i have only ridden in two other jet boats on a couple of occations so im not sure if mine is actually cavitating.

[1978 Rogers]

The pump looses it's bite on the water, so to speak. The engine does wind up some revs. It will wind up more if your sucking in air at the hand hole or where it sits on the base plate. A lot of cavitation will result in pitting of the impellar caused by the bubbles.
How did I do?

[berk]

Thats all true but i'm still waiting to get my A Hole tore out by a scientist :2purples: :D
LV jetboy thats your cue.

[UBFJ #454]

I'm going to wait until tomorrow night to "Tap The Keg" & "Pop The Pop Corn" cause jer isn't going to "Chime In" here on this subject until after 12:37 AM tommorrow morning.
Anyone what to Start a "Pool" on the Time he Appears?

[Squirtin Thunder]

11:58pm today !!!!
Jim

[corlish]

Dragboats -- no a-hole rippin' here -- but I don't think you gave one situation that would necessarily cause cavitation :smile: . Degraded performance; yes, but not necessarily cavitation and yes cavitation causes degraded performance.
The science: all centrifugal, axial or mixed flow pumps (most jets are the latter) will cavitate under certain conditions. When a pump manufacturer plots a performance curve they will typically determine a minimum value of pressure on the inlet side (when speaking centrifugal and mixed flow pumps this area is very near the front side of the impeller). Incidentally, the pressure on the inlet side is commonly referred to as NPSH for Net Positive Suction Head. When the inlet pressure falls below the min NPSH the pump will cavitate.
The practical: like everyone else has said; when the pump cavitates it can act much like a slipping clutch with a raise in rpms and no "noticeable" increase in speed. Some folks have monitored intake pressure with success. But the truth is monitoring cavitation is alot like monitoring detonation -- it is real easy to know when it is in full-effect but very difficult to know when it first begins to occur.
Monitoring intake pressure seems to be the best way to monitor what's going on. But what it should be can vary from setup to setup and ends up being a balancing act between optimum pressure, safety, and etc.
Corey

[MudPumper]

NPSH for Net Positive Suction Head.
Corey
Going to have to school the woman on this term. :)

[Taylorman]

What i don't understand is how do you get air bubbles during takeoff when the pump is always in the water. :confused:

[sdpm]

Sounds good to me! :argue:

[Zeug]

Suction cavitation
Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.
Discharge cavitation
Discharge cavitation occurs when the pump discharge is extremely high. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition due to the high pressure condition premature failure of the pump mechanical seal and bearings can be expected. Under extreme conditions this can break the impeller shaft.

[Zeug]

Think your impeller will stand up to a nuclear explosion??
OAK RIDGE, Tenn., March 4, 2002 — Researchers at Oak Ridge National Laboratory, Rensselaer Polytechnic Institute and the Russian Academy of Sciences have reported results that suggest the possibility of nuclear reactions during the explosive collapse of bubbles in liquid, a process known as cavitation.
The bubbles, which grow in the presence of sound waves, collapse to produce locally high pressures and temperatures. These pressures and temperatures can be sufficiently high to result in light emissions, called sonoluminescence, from the collapsing bubbles.
The full Story (http://www.ornl.gov/info/press_releases/get_press_release.cfm?ReleaseNumber=mr20020305-00)

[Zeug]

Dont fish get air from the water??????
Exactamundo!

[corlish]

Dragboats -- no offense intended. And I agreed with you on what it feels like just not what causes it. :boxed: Cavitation in our style of pump is caused from inlet pressure dropping below a critical level -- period dot. You mentioned that you were not sure on the science so I elaborated -- that's all. Sorry if it seemed like I was trying to poke you in the eye. Certainly if your intake is full of debris then cavitation may be induced if enough water cannot be supplied to the pump -- read as pressure on the front side of the impeller dropping below min NPSH. To me though, cavitation is indirectly related to debris in the intake.
To answer some the questions raised; the air bubbles form when the water drops below the vapor pressure for the given conditions. This usually very localized i.e. tips of a prop, eye of an impeller, and etc. Very similar to vapor trails from the wingtips of high-performance aircraft, there the fluid is air rather than water obviously.
As Zeug pointed out -- cavitation "burns" (really the wrong word to use) are not very well understood. It IS known that when the bubblles collapse it is very violent.
Corey

[Squirtin Thunder]

I can't believe he ( LVJetBoy) didn't chime in on this !!!!!!!!!!
Jim :hammerhea

[Cas]

Jer is away for the weekend, went to the lake to spend some quality time with his 2 passions :)

[flat broke]

Zeug,
I was reading the excerpt on the link you posted, and did you catch the mention of temerature of up to 10,000 Kelvin? Simply amazing. The implication for a nuclear fusion reaction source is profound if further testing supports the current theory/observations. Amazing that research that has implicatoins of such a larger magnitude has ties to our silly little boats.
Chris

[Zeug]

yep, crazy stuff.
Not teribly applicable to the discussion here, but fun reading none the less.
I guess I could put a "sightglass" in my bowl to watch for the tell tale glowing they spoke about :rolleyes: :rolleyes: .