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  1. #1

    Jet Jon Cavitation question

    I don't know if this is the appropriate place to post this, but here it goes. I recently put a 93 sl750 motor and jet pump in a 14' Lowe Jon boat with modified V. It works great on calm water, but when it gets choppy it cavitates really bad. I expected some cavitation and I beleive it is caused by the difference in hull shape from the ski to the Jon boat. I was hopeful that someone could give me some advice on how I might reduce the cavitation. The motor and jet pump are completely stock. The impeller looks to be in decent shape but the piece behind it(stator?) is a little chewed up. Not sure if this would play into it. Before the project the jet ski performed awesome, no cavitation issues. Thank you for any help.

  2. #2
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    What does the jet pump intake area look like?

    Is the hull flat bottomed?

    It sounds like what you are experiencing is jet pump ventilation rather than cavitation.

    If air mixed with water is getting pushed into the jet pump intake then you will have greatly reduced thrust.

  3. #3
    Attachment 353852

    Attachment 353853

    Here's a pic before it was finished. I did remove about 3 feet of the center rib In front of the intake. The bottom is flat.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by K447; 03-19-2015 at 08:59 PM.

  4. #4
    Sorry about the pics. I'm gone for work can't take any better ones at this time.

  5. #5
    Removing 3 feet of that rib down the center seemed to improve my issue.

  6. #6

    Join Date
    Apr 2006
    Marble Falls TX
    Ice chest full of beverages toward the front of the boat will help....

  7. #7
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Quote Originally Posted by Trevo22 View Post
    Removing 3 feet of that rib down the center seemed to improve my issue.
    Flat bottom hull has a lot of lift when on plane. The jet pump intake is probably not very far below the water surface when running at speed.

    Trim tabs on the transom might help push the bow down and reduce the amount of air entrained water getting to the jet pump.

    If the salvaged hull section from the Polaris had been mounted an inch or three lower than the flat bottom hull surface the water flow into the jet pump might have had a cleaner water feed.

    Try the trim tabs, or just moving weight forward in the hull.

  8. #8
    You are correct. It does help to put some weight up there.

  9. #9
    I agree with you about setting the hull a little deeper. This is my first build and I was focusing on the bottom being flat and not having any fiberglass exposed. You beleive that the flat bottom is the whole issue? Messing with the pump won't help?

  10. #10
    You are also correct about the pump not being very deep. On plane it will run through and inch or two of water.

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