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  1. #521
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by Palm70 View Post
    Hey guys
    Went and rode today with stage 1 plus mods. Including worx grate, pump seal kit.

    Solas 13/20.

    Pretty disappointed with the amount of cavitation out the hole. It's so much that I have to get going good before I can gas it.

    Any suggestions on getting the ski back to where their is little cavitation out the hole? Thanks!
    This is jet pump ventilation I suspect, not cavitation. Air bubbles being drawn into the intake water flow through the jet pump.

    How did you seal the intake grate rear to the transom plate?

    How did you seal the intake grate to the ride plate?

    Did you re-seal the pump sections where they mate together?

  2. #522

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    I didn't install it. I'm not sure. Are their specific instructions on how to do this?

  3. #523
    Click avatar for tech links/info K447's Avatar
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    Arrow GP1800 - Jet pump ventilation during acceleration, Worx & OEM intake grate sealing

    Quote Originally Posted by Palm70 View Post
    ... Are there specific instructions on how to do this?
    These two threads contain a bunch of info related to sealing of the transom plate and the intake grate.

    http://www.greenhulk.net/forums/show...xy#post2892529

    http://www.greenhulk.net/forums/show...=1#post2891441

    There are several things going on.



    The factory sealant between the GP1800 transom plate and the hull tends to not bond very well to the cast aluminum transom plate.



    This is compounded by the transom plate having large hollow spaces (voids) which limits the sealant to a narrow ridge of metal. The water forces inside the pump tunnel are quite high and can overwhelm the sealant. In my case it was allowing water to seep into the hull when running at speed on the water.



    This is corrected by filling the voids with West Systems epoxy. The new sealant will have a much wider surface area to bond with and should be able to stay in place regardless of the water flow and pressure inside the jet pump.

    The Worx WR246 intake grate (and the Yamaha OEM factory intake grate) have enormous void cavities right below where the intake grate upper edge is supposed to 'seal' to the transom plate. The voids extend down below the transom plate bottom.



    With the stock intake grate there is a rubber strip that is supposed to seal the intake grate to the ride plate and to the transom plate.



    To use the OEM rubber strip with the Worx intake grate requires that the rubber 'finger' chunks at each end be cleanly sliced away. If this is not done carefully then there can be an air gap in the seal, which can allow jet pump ventilation during acceleration from low speeds. Even a small air gap is enough to cause ventilation.



    Worx sells a kit of rubber pieces (WR0710) that fills most of the cavities inside the Worx intake grate. This requires ALL the rubber fingers to be neatly sliced away from the OEM rubber strip.



    I have two GP1800 with Worx intake grate. One has the Worx void seal kit installed, for the other I poured West System epoxy to fill the voids. Both methods work, but the epoxy fill method eliminates 100% of the voids space. The Worx filler blocks leave thin air pockets around the edges, which must be fully sealed to the transom plate to ensure no air leakage.



    Note that I also filled all the casting voids around the mounting bolt holes.



    This is required making a temporary dam with packing tape, to hold the liquid epoxy until it hardened. If there is a raised lip along the outer edge after the epoxy has cured, trim it away with a sharp knife.



    Rather than hacking all the rubber fingers off, I decided to omit the OEM rubber strip entirely and instead apply 3M 4200 Fast Cure sealant. My Worx intake grate now has all the voids filled, with sealant filling the area between the intake grate and the transom plate. Sealant where the intake grate bolt flanges mate to the hull above, and along the outer edges of the intake grate flanges.

    Sealant also between the front edge of the ride plate and the intake grate and along hull joint with ride plate front edge.

    I also applied sealant in the thin gap between the vertical sides of the intake grate and the hull tunnel. The idea is to force all the water to stay inside the active flow zone through the intake, with no unsealed cavities or hollow gaps/spaces where air pockets could form.



    Related note: the GP1800 hull seems at risk of stress cracking where the intake grate rear bolts to the hull.



    To avoid this risk, I reinforced the hull above the intake grate.




    In addition to all to all of the above (intake sealing and air ventilation prevention), there is an actual cavitation issue with the SVHO jet pump. The jet pump is 160mm diameter, but the transom plate the jet pump is bolted to is only 155mm diameter. This creates a 2.5mm ledge where the fast moving water flow must suddenly expand from 155mm to 160mm.

    When the impeller is spinning hard for acceleration that ledge can create cavitation (vacuum) bubbles. This reduces jet pump thrust.

    There are two ways to correct for this. R&D sells a 'cavitation fighter' tapered red ring which fits between the transom plate and the wear ring. Rather than a sharp ledge right in front of the impeller the water now flows along the tapered inside of the ring.

    The alternate is to taper the transom plate itself, going from 155mm where the transom plate meets the intake grate to 160 mm where the transom plate mates to the wear ring ahead of the impeller.

    I installed the R&D red ring on both my jet pumps.

    If I had to do it again I would install a tapered transom plate. These are available new or you can have your existing transom plate modified. You can also do it yourself.

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    Last edited by K447; 09-17-2017 at 11:00 AM.


  4. #524

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    Ok thanks for your help! How did your hull crack?

  5. #525

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    Jerry,
    did you have the same issues with your build?

  6. #526
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by Palm70 View Post
    ... How did your hull crack?
    My own GP1800 hulls did not crack. Photo of hull cracking is from an early report by another GH member.

    The reinforcement plates are a preventative measure, so the hull should never crack around the intake grate bolts.

    Not every GP1800 hull has or even will crack, but more than a few have. I want these hulls to last a long, long time. Early reinforcement at the intake grate rear bolts is part of that.

    I have about 50 hours on each GP1800. Intake grates were removed recently to reseal the transom plate (per my previous post) and the hulls showed no signs of stress around the intake grate bolt holes.

  7. #527

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  8. #528

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    I don't know if any of these pics help. It looks like it is sealed.

  9. #529
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by Palm70 View Post
    ... It looks like it is sealed.
    If the jet pump has an air leak, you will know the jet pump is ventilating when you hear the engine revving but the forward thrust is nowhere near as strong as it should be.

    If there is a gap in the sealing anywhere forward of the impeller, you will not see it from the outside.

    To find out where the air leak is generally requires removing the entire jet pump up to the transom plate. In my case I used an inspection mirror to examine the sealing between transom plate and intake grate, all the way around the transom plate rim. There was a visible gap in the sealant.

    I also had the ride plate and rubber strip removed which allowed daylight to be visible through the sealant gap.



    I suppose you could try peering at an inspection mirror inserted through the intake grate. I think that would be inconclusive.

  10. #530
    K447. If you have not already done this please do a post on exactly how you suggest tapering the transom plate. I don't need any information on the removal and the installation just on the actual tapering


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