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  1. #1

    Is this Cavitaion? Could someone explain what it is in LAYMANS TERMS?

    So I have been testing a 1999 xl1200 Limited which I just rebuilt, and my experience is fairly minimal, involving the mechanical/technical side of PWCs. Anyways, so if I idling and give the ski full throttle, the engine seems to rev fine, but the ski kind of seems to hesitate, almost like it feels like the prop a spun bearing, which i know it is not. Then it kind of "catches" and does fine. Is this what is referred to as "cavitation"? I have searched the word in the forums, but everyone seems to give a scientific explanation of what causes cavitation, never really what the heck it is, a simple explanation of what I should experience were cavitation to occur. I am trying to learn, and this may be a dumb question, but it is easier if you know what terms actually mean, that way I can look in the right place for solutions. Or maybe there isn't one for this particular ski, maybe I should ease into the throttle more as opposed to just opening it up wide rather quickly.. Any responses are appreciated. Cheers!!


  2. #2
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    let's compare ski propulsion to automotive propulsion.

    Without sufficient friction between a tire and a road, a tire will spin as it tries to push the attached car.

    A bald tire spins on a wet surface more easily then a tire with tread remaining

    In the pwc world a poor jetpump condition means you have excessive space between the prop and the wear ring, so when the prop tries to push the water out of the pump, some of it escapes between the edges and the result in low pump pressure, the faster the pump spins, the more water is lost.

    Just as a spinning tire has little chance to regain traction on a wet surface, the faster a prop is spinning, the less chance you have of it establishing the water flow you need to generate significant thrust.

    You can get a car with bald tires to move on ice IF you slowly apply power..same on a pwc..apply power slowly and the prop will start to push water..once the ski is up on plane the load on the pump is far less and the excess space isn't as big of a factor.

    The phrase cavitation refers to a props ability to create a space within the water ( a cavity) with no air..it's actually a partial vacuum..when this partial vacuum collapses, it does so with great force and can actually damage your prop. The prop will look like it's been sandblasted at first and then it will look like it's rotting away.

    A jetpump needs a low clearance between the impeller and pump housing to operate properly and must not allow any air to lean into the pump area from the drive line seal. The area in front of the impeller has low pressure relative to the surrounding water as the impeller draws water thru the grate..if the drive line seal is questionable, air can leak into the water flow..and that will reduce the "traction" the impeller has in the water, which can also lead to cavitation.


    I suggest you try some more searches on cavitation, there are plenty of good examples out there, but this should give you a good start.

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  4. #3

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    Thats the best explanation I've read in awhile. I'm sure others will appriciate this. Nice job Pete.

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    Quote Originally Posted by DaddyT View Post
    Thats the best explanation I've read in awhile. I'm sure others will appriciate this. Nice job Pete.
    +1 GREAT EXPLANATION.....smokin ur tires then the wheel hop happens and u are trying to go but u arent

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    Don't forget that in a GPR, XLL, or XLT, the PRIMARY cause of cavitation is poor sealing of the areas around the pump intake, shoe, and pump itself. A pump seal/plug kit and 3M 5200 sealing up everything does wonders on boats that haven't had that done. The second major cause is the factory impeller is not the best design, a switch to a Solas Concord 12/18 on a XLL will do wonders if the pump is sealed. Pump to impeller clearance is not the highest cause on my list unless the boat is run up on the sand alot Just my experience.

  7. #6
    Quote Originally Posted by nmpeter View Post
    let's compare ski propulsion to automotive propulsion.

    Without sufficient friction between a tire and a road, a tire will spin as it tries to push the attached car.

    A bald tire spins on a wet surface more easily then a tire with tread remaining

    In the pwc world a poor jetpump condition means you have excessive space between the prop and the wear ring, so when the prop tries to push the water out of the pump, some of it escapes between the edges and the result in low pump pressure, the faster the pump spins, the more water is lost.

    Just as a spinning tire has little chance to regain traction on a wet surface, the faster a prop is spinning, the less chance you have of it establishing the water flow you need to generate significant thrust.

    You can get a car with bald tires to move on ice IF you slowly apply power..same on a pwc..apply power slowly and the prop will start to push water..once the ski is up on plane the load on the pump is far less and the excess space isn't as big of a factor.

    The phrase cavitation refers to a props ability to create a space within the water ( a cavity) with no air..it's actually a partial vacuum..when this partial vacuum collapses, it does so with great force and can actually damage your prop. The prop will look like it's been sandblasted at first and then it will look like it's rotting away.

    A jetpump needs a low clearance between the impeller and pump housing to operate properly and must not allow any air to lean into the pump area from the drive line seal. The area in front of the impeller has low pressure relative to the surrounding water as the impeller draws water thru the grate..if the drive line seal is questionable, air can leak into the water flow..and that will reduce the "traction" the impeller has in the water, which can also lead to cavitation.


    I suggest you try some more searches on cavitation, there are plenty of good examples out there, but this should give you a good start.
    Bravo Pete!

  8. #7
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Yamaha and kaws with their stainless wear rings often have other issues behind cavitation.

    Anything that prevents the expected water flow to the impeller be it excess impeller to ring clearance or assorted air leaks which can come from a number of places will cause it.

    One other possible issue..selection of an inappropriate prop for your application.

    In my neck of the woods towing tubes in one of the primary pwc activities. Put a prop designed for the highest speed on your ski then try to tow a tube with say 5 kids on it and you will cavitate.

    Sometimes your engine mods may produce enough power to cavitate the prop, even when everything else is on the spot.

    In some cases..cavitation is deliberate. Such as signalling a Russian sub that you are right on their ass....

    Use care when attempting to fix a cavitation problem by applying sealer. Seadoo for example specifies certain areas for sealer to be applied..typically not the entire ride plate, but that's a discussion for another thread.

    "tires spinning" and "clutch slippage" are the best ways I have to describe the issues to people who don't understand the physics of a jet pump.

  9. #8
    Thanks guys.. great info.

  10. #9
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    had one come in today for cavitation. There was an old bag from some beef jerkry wrapped around the prop..pretty tough plastic actually. He was complaining of cavitation which was reducing his top speed. Easy fix with some super long nose pliers.

    _anything_ that disrupts water flow in the pump will create cavitation.

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    Gotta seal everything up real good

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