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  1. #31
    Click avatar for tech links/info K447's Avatar
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    Arrow Resealing the jet pump sections during reassembly?

    Quote Originally Posted by mjh3ides View Post
    ... The biggest benefit of sealing it is to prevent the pump from sucking air on the holeshot & causing cavitation...
    Well, air leaks cause pump ventilation, not cavitation. Different causes, similar effects

    From the factory the GP1800 SVHO pump has black sealant between the pump sections. Wear ring to transom plate, wear ring to pump stator, stator to pump nozzle.

    Question: When reassembling the pump sections, do you:

    Leave the factory sealant as is and just bolt the pump back together?

    Add a thin layer of fresh sealant (what kind?) and bolt it back together?

    Remove the factory sealant and reassemble it as bare metal?

    Remove the black factory sealant and apply fresh sealant?
    Last edited by K447; 11-21-2017 at 10:39 AM.


  2. #32
    BLASTER 1's Avatar
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    I have always just bolted my pumps back together, weather it was a Yamaha or a Kawasaki and never had any issues

  3. #33
    YAMA HAMA's Avatar
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    Quote Originally Posted by K447 View Post
    Question: When reassembling the pump sections, do you:

    Leave the factory sealant as is and just bolt the pump back together?

    Add a thin layer of fresh sealant (what kind?) and bolt it back together?

    Remove the factory sealant and reassemble it as bare metal?

    Remove the black factory sealant and apply fresh sealant?
    I've had all these same questions. The 2 times so far that I've pulled my pump assembly for mods I've cleaned all mating surfaces of the old sealer, lightly scuffed them and applied new sealant to each. Figured it couldn't be wrong but was it necessary? I mean when somebody is dialing In their ski wth different props and/or pitches are they doing all this each time?
    Last edited by K447; 09-28-2017 at 11:28 AM.

  4. #34
    mjh3ides's Avatar
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    Quote Originally Posted by K447 View Post
    Well, air leaks cause pump ventilation, not cavitation. Different causes, similar effects

    From the factory the GP1800 SVHO pump has black sealant between the pump sections. Wear ring to transom plate, wear ring to pump stator, stator to pump nozzle.

    Question: When reassembling the pump sections, do you:

    Leave the factory sealant as is and just bolt the pump back together?

    Add a thin layer of fresh sealant (what kind?) and bolt it back together?

    Remove the factory sealant and reassemble it as bare metal?

    Remove the black factory sealant and apply fresh sealant?
    No, it's the same thing. The dictionary definition of cavitation is the creation of empty spaces(air bubbles) in water. If the pump shoe section of the grate is not sealed, vacuum pressure pulls even more air in...more air = more bubbles = more cavitation. It's important to seal the vacuum side of the pump where the wear ring mates to the transom. Sealing wear ring to pump & pump to nozzle is optional. I rarely do on my race boat because I'm commonly swapping setups. On my rec skis I'll wait to seal the pressure side until I'm done tweaking the setup(which may be never). I do recommend sealing everything for people who ride in salt because it helps stop corrosion that tends to fuse parts together & make it a real bitch to disassemble. I prefer a lightweight silicone like 1211 or UltraBlack. It's also what the service manual recommends.
    Last edited by mjh3ides; 09-28-2017 at 11:27 AM.

  5. #35
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by mjh3ides View Post
    ... it's the same thing. The dictionary definition of cavitation is the creation of empty spaces (air bubbles) in water...
    My understanding is that cavitation in a jet pump is the creation of vacuum ‘bubbles’ in the liquid. There is literally nothing in the cavitation bubbles, no air, no water. Just empty spaces (a genuine vacuum). Cavitation tends to occur on the low pressure side of the impeller blades and often near the blade outer edges and trailing edges.

    Cavitation tends to damage adjacent metal surfaces, when the vacuum ‘bubbles’ rapidly collapse in the pressure zones they do so with violence, the cause of jet pump and impeller ‘cavitation burns’ and pitting. When the cavitation bubbles collapse, they are gone. No air. Perhaps some steam from the sonic impulses.

    Cavitation vacuum bubbles are effectively gaps in the water, reducing the volume of water actually moving through the jet pump.


    Ventilation bubbles contain air. The air is drawn into the water flow by the reduced pressure (suction) in the water compared to the air pressure, plus a leakage path for the air to get into the water. Air bubbles are compressible, which is why they reduce thrust in the jet pump. The air bubbles will be there all the way through the exit nozzle.

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  7. #36
    mjh3ides's Avatar
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    You're overthinking it a bit, dude. Whether the bubbles contain gases or not, the effect is the same. It reduces the load on the impeller & causes rpm to increase rapidly. Basically the same thing as spinning tires in a car. The result is a loss of traction.

  8. #37
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by mjh3ides View Post
    You're overthinking it a bit, dude. Whether the bubbles contain gases or not, the effect is the same. It reduces the load on the impeller & causes rpm to increase rapidly. Basically the same thing as spinning tires in a car. The result is a loss of traction.
    But the fix is different. Ventilation is fixed with sealing.

    Cavitation is fixed by other means.

  9. #38
    mjh3ides's Avatar
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    Quote Originally Posted by K447 View Post
    But the fix is different. Ventilation is fixed with sealing.

    Cavitation is fixed by other means.
    It's a fix for the same problem. The only difference is WHERE the cavitation is coming from. Sealing the pump shoe & intake grate selection help load the pump in front of the prop. Using a pump with more vanes, a larger cone or a smaller reduction nozzle all increase load behind the prop.

  10. #39
    Like a chimp with a ball-peen hammer smackwave's Avatar
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    Quote Originally Posted by K447 View Post
    My understanding is that cavitation in a jet pump is the creation of vacuum ‘bubbles’ in the liquid. There is literally nothing in the cavitation bubbles, no air, no water. Just empty spaces (a genuine vacuum). Cavitation tends to occur on the low pressure side of the impeller blades and often near the blade outer edges and trailing edges.
    Well... there is something in the bubbles, it's water vapor. The pressure in water doesn't get low enough to create a true vacuum, because it reaches the vapor pressure first and literately boils the water from liquid to gas. The density in the gas bubbles is in the neighborhood of 1000X less than the density in the surrounding liquid water, but it's still not a complete vacuum.

    But yeah, cavitation is a specific physical process where voids are created internal to a substance. Ventilation is a different process where outside air is introduced. Both will cause reduction of thrust, but cavitation is more destructive because of the bubble collapse and corresponding sonic shock wave once the local static pressure returns to above the vapor pressure.

  11. #40
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    Yup - think we're all principally saying the say thing . . .
    We all want the impeller to stay as fluid-bound as possible. #1 common source is going to be surface air ingestion, and it has a huge performance-hit, aka high rpms, poor hole-shot, poor max speed, hookup, etc . . . Those that might be just tuning into this thread, just need know that all the many seams and cracks between the transom plate/hull, to include the pump shoe/housing can all potentially allow this surface air into the induction side of the jet pump. And "no", the oem isn't always good at insuring this air-tight desire - hence the value of this thread.

    In contrast, true cavitation (by definition) is fortunately a much harder phenomenon to achieve, as it takes a significant vacuum to cause dissolved/suspended gas molecules in the water to spontaneously create bubbles (degasification - Henry’s Law). We might expect this to likely occur in overly pitched props, very high rpms, high torque, and in a cold water columns as cooler water can absorb a greater amount of dissolved gasses. Thankfully, there’s also less of a performance-hit as compared to air ingestion, yet some performance degradation nonetheless. Perhaps the worst, is the physical damage as pitting and sculling of the blades.

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