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Thread: OB Jet Cavitation on Holeshot

  1. #1

    Default OB Jet Cavitation on Holeshot

    Hey,

    First post here. Always came to this website for research or troubleshooting and finally joined. Lot of knowledge on here.

    Anyways, little 17' 4 degree sled with a 115 optimax. Just did a bunch of work on it and now I'm getting cavitation on the holeshot. It gets worse with a heavier load as well. The real strange thing is it really doesn't cavitate on hard sliding corners. It's more or less limited to a WOT or near WOT holeshot.

    It has 4 blade SS impeller. No dents/dings and I sharpened the leading edge, and flattened the trailing edge horizontal. Basically a mix of the OB Jets method and wooldridge method. Always worked great in past for me. Shimmed correctly.

    Added intake fins, UHMW splash plate, 2 degree transom wedge, and 'blueprinted' opened up the shoe to help flow and cut the bottom of the liner to smooth flow as well. Lastly.... I also raised the motor. My gut feeling is that ideal height is in between 2 holes. I went up one hole hoping it would work.

    From those with experience, it my motor height the main culprit of the cavitation or does something impeller or 'blueprinted' shoe sound off? It's literally just like the pump runs out of water at those WOT or near WOT holeshots.

    Thanks

  2. #2
    Member KantishnaCabin's Avatar
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    Simple things first. Sometimes when you add the wedge and plastic splash guard it blocks the outboard from getting full down trim. Check your outboard to see if it is hitting the plastic guard at full down trim. If so you just need to trim the plastic back a bit to allow your foot to get all the way down for your hole shot.

  3. #3

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    The UHMW is actually shimmed up so it allows the motor to be trimmed down all the way. I basically heat formed the UHMW around the shape of the jet shoe.

  4. #4
    Member KantishnaCabin's Avatar
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    If I'm reading your post correctly, you did not have this problem before you moved the outboard up a hole on the mount? If the splash guard is allowing the outboard to trim all the way down and you are still unable to feed enough water to the pump then you should drop it back to the previous setting. Once on step the outboard pump has sufficient water flow to maintain pump pressure. Although, I suspect that there is still a loss of pressure and you would notice an improvement. Holeshot is where the magic happens ant that's why its so noticeable. The pump has to draw more water than what is passing by the transom and it has to be "clean" water. Vortexes created by the transom are low pressure and not "clean", the pump drawing from this swirling water causes the cavitation. You need to get the leading edge of the pump housing below this "dirty" water.

  5. #5

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    I think you're exactly right. Had 4 people in it tonight and even going about 3/4 throttle, if I twisted it all the way it would spin out. I was just hung up on mounting it as high as possible, but that just doesn't work. Maybe I'll have to look at adding a tunnel to this if I rip a few shoes off.

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