Cavitation... Bad Cavitation
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Cavitation... Bad Cavitation

  1. #1
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    Default Cavitation... Bad Cavitation

    I have a Rogers, 5.3 LS powered.


    According to the impeller chart im making somewhere in the neighborhood of 400-425 hp on motor @ 5200 rpm, and about 600 on a 150hp shot spinning about 6200 rpm or so with a B- AT impeller.


    The boat runs 70 on motor (repeatably have seen higher a couple times) and 85 on the bottle on GPS.


    Heres where my Cavitation questions come in.


    The Pump is a JC suction housing, Dominator Bowl, AT aluminum impeller. No inducer, no stuffer, everything is stock. Has a loader by GS i dont know what kind, and i believe a Backcut shoe. No Diverter, Full droop with a set of adjustable wedges. Wedges are setup at 3*up and i also have a rideplate. I dont know the angle of the rideplate somewhere around -3.75 degrees


    Before the intake there is a pretty bad transition, think a Flat intake in a rounded hull. And not blended in. I have corrected this, but have not taken the boat out since.


    I just put a brand new AT impeller in that was cut down for me to a B-, other than that the impeller is stock and non-detailed. I put a brand new non shouldered stainless wear ring in with this impeller. Clearance measured out to about .024" total. I did not check for runout. I should also mention, this was an issue before the new impeller.


    What happens?


    If i flat foot it from idle, the boat will literally bounce off the rev limiter as long as i keep my foot flat (rev limiter is 7200). The boat does move and accelerate)


    If i Flat foot it from idle, and i hear it come "unhooked" and back off the throttle, hold a steady throttle about half or so. Then stab it, its fine and pulls to my max RPM and holds


    If i dont flat foot it, but get on it spiritedly wait til the boat planes then stab it she loads well and takes off pretty nicely. Will pull hard and run to WOT no problem


    If i'm cruising from plane (say 3-3500 rpm or 45mph or so) and stab the throttle. The boat pulls to WOT and holds hard and accelerates very nicely


    If i wait until the boat planes, and grab the Nitrous it hits hard as hell and pulls to the max RPM and the boat accelerates as expected.


    Basically the boat blows the pump out from idle, but once im moving its fine.


    I have a couple of thoughts, but id like to hear your ideas.


    1. With no diverter, i go so bow up (from the droop/up angle) that it uncovers the jet intake, causing it to suck air
    2. I need a stuffer
    3. I need to buy a diverter
    4. The transition into the intake was the issue, everything will be fixed when I take it out.
    5. Im an idiot, and doing everything wrong
    6. LS engines wont work in a jet boat

    I have posted this on sjcb as well. And received some great ideas form them. I know a lot more people frequent this site so i wanted to hear your ideas also.

    One person suggested it may be sucking air from the rope seals, they are new, but it amy be loose. The hand hole o-ring is brand new and tight
    Last edited by steves86ta; 03-04-2016 at 03:42 PM.

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  3. #2
    LP-25.com Infomaniac's Avatar
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    Why do you have down ride plate?

    Packing is my first guess. No water getting in the boat?

    The shape of the keel will do that also. Had a delam once at a race. Put a couple layers of glass cloth over it in front of the intake and it wouldnt load a lick of water.

    A stick or small rock will make a pump malfunction

    Pick One
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    Quote Originally Posted by Infomaniac View Post
    Why do you have down ride plate?

    Packing is my first guess. No water getting in the boat?

    The shape of the keel will do that also. Had a delam once at a race. Put a couple layers of glass cloth over it in front of the intake and it wouldnt load a lick of water.

    A stick or small rock will make a pump malfunction

    Pick One
    Yes I have water in the boat, but its from my transom plates, I'm not the best at fabricating.

    But the packing does leak about a drop every 30 seconds to a minute. It was suggested to tighten this up a bit and try again

    Ride plate isn't down its up. I was told that's considered a negative number.

    I'm really leaning towards the disruption before the intake as my issue. I blended and filled it after the last time out but haven't gotten it wet yet since

    Pumps clean inside. All fresh I installed the impeller then took it out. I should say chances are its clean. I didn't see anything when I was under it blending the intake in.

    I'm also not against the diverter idea. I go very bow up on the launch. I'll get a video shortly

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    Here's a video of one of my nitrous runs.

    The launch in this video was one of the ones where I launched the boat and once it came unhooked. I lifted a little bit and held it for a second then stabbed it and laid into the bottle
    https://youtu.be/fvvhRQgJjuw

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    I'm not an expert but I'll chime in any way.
    First off, that hull design is not very conducive to going fast. Be that as it may, my '76 Kona did the very same thing with my last 650 horse motor.
    Secondly, that aluminum impellor won't last very long either, and a blown up impellor will definitely ruin an otherwise delightful day. Go to, at least, a bronze impellor, or even better, a stainless one.
    It's my understanding of fluid dynamics that tells me that we are simply exceeding the ability of the pump to keep itself loaded because we are simply feeding it too much horsepower for the ability of the induction side of the pump to keep up with. I suggest an inducer and a proper shoe (and a different impellor material).
    The cut of the impellor will be dictated by what RPM range your particular engine is built to make it's torque peak. These jet engines are always under load unless they are at idle or shut off. There is no light load/part throttle cruise situation like there is in an automotive application.
    My 650 horse engine would run right at it's calculated torque peak, 5,550 RPM with the A cut impellor. If you're LS engine peaks it's torque curve higher (which it sounds like you've got a handle on) then it requires a 'slipperier" cut impellor.
    Rob

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    Senior Member jetboatperformance's Avatar
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    Not that its the answer to your issues but like Ron, would question the -3.75* plate ? was this a base line setting or an attempt to correct ?additionally you mentioned transom plates is this jet set back ?

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    Quote Originally Posted by jetboatperformance View Post
    Not that its the answer to your issues but like Ron, would question the -3.75* plate ? was this a base line setting or an attempt to correct ?additionally you mentioned transom plates is this jet set back ?
    I had the plate at 4 degrees up and it porpoises a little bit. Dropped it down to 3. 75 up and the porpoising seemed to get a lot better.

    Jet is "half" back. Hand hole is halfway out halfway in. Factory location for this hull.

    I think the negative is wrong. I was told with the plate "up" it'd read as a negative number.

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    Quote Originally Posted by White Lightning View Post
    I'm not an expert but I'll chime in any way.
    First off, that hull design is not very conducive to going fast. Be that as it may, my '76 Kona did the very same thing with my last 650 horse motor.
    Secondly, that aluminum impellor won't last very long either, and a blown up impellor will definitely ruin an otherwise delightful day. Go to, at least, a bronze impellor, or even better, a stainless one.
    It's my understanding of fluid dynamics that tells me that we are simply exceeding the ability of the pump to keep itself loaded because we are simply feeding it too much horsepower for the ability of the induction side of the pump to keep up with. I suggest an inducer and a proper shoe (and a different impellor material).
    The cut of the impellor will be dictated by what RPM range your particular engine is built to make it's torque peak. These jet engines are always under load unless they are at idle or shut off. There is no light load/part throttle cruise situation like there is in an automotive application.
    My 650 horse engine would run right at it's calculated torque peak, 5,550 RPM with the A cut impellor. If you're LS engine peaks it's torque curve higher (which it sounds like you've got a handle on) then it requires a 'slipperier" cut impellor.
    Rob
    Thank you for the information

    That's definitely a possibility. This thing builds power very quickly and comes up in revs fast.

    In a car, with my cam the recommended shift point is about 7500. You can feel the power band hit almost like a 2 stroke.

    I'd go with a smaller impeller. But I like the ability to cruise around 3k.

    And I plan on bumping the nitrous up more once all this is figured out.

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    Do you have a nozzle bushing? that can help if you don't have one have you ever measured the nozzle angle if its go a bunch of up in it that may be the problem. also 3.090 on the bushing is a good starting point Nozzle angle about 5-6 off the keel

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    The clearance sounded good but also if it has shouldered wear ring the clearance between the shoulder and impellor is critical

  13. #11
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    your moon launch and repeated bouncing is washing out the area around the intake, which is allowing air into the suction piece. get a place diverter that's hydraulically operated with the up/down button on the steering wheel. a pd nozzle that's machined for nozzle inserts (what brady called a bushing). a stock nozzle diameter is not helping maintain suction.
    the next thing to do is get a cavitation reducer. between smoothing out the launch with a pd, and the cavitation reducer, your problem should be close to eliminated.
    maybe it's just me, but it's kind of hard to believe the engine picks up 1000 rpm with a 150 shot, if the pump is effectively loaded. I don't pick up half that with a 150 shot, and that's all I've ever run.

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    Quote Originally Posted by bp298 View Post
    your moon launch and repeated bouncing is washing out the area around the intake, which is allowing air into the suction piece. get a place diverter that's hydraulically operated with the up/down button on the steering wheel. a pd nozzle that's machined for nozzle inserts (what brady called a bushing). a stock nozzle diameter is not helping maintain suction.
    the next thing to do is get a cavitation reducer. between smoothing out the launch with a pd, and the cavitation reducer, your problem should be close to eliminated.
    maybe it's just me, but it's kind of hard to believe the engine picks up 1000 rpm with a 150 shot, if the pump is effectively loaded. I don't pick up half that with a 150 shot, and that's all I've ever run.
    Yes. I think a place diverter will help big time. And that's what I'm saving up for

    as far as the 1k on a 150 shot. I would normally agree with you.

    Here's my thought on why I gain so much. I'm simply not making power I at 5k like I am at 6k.

    What I'm trying to say is I am rally getting 150 or so out of the shot. But the engine is making much more power there that the nitrous allows it to make because of the rpms

    a very similar engine build in one of the magazines dynoed approx 415 or so at 5k but about 500 or so at 6k.
    I think it's allowing my over cammed combo to breathe more or less if that makes sense

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    Quote Originally Posted by bradyte083 View Post
    The clearance sounded good but also if it has shouldered wear ring the clearance between the shoulder and impellor is critical
    Its a non shouldered wear ring

  16. #14
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    Quote Originally Posted by steves86ta View Post
    Yes. I think a place diverter will help big time. And that's what I'm saving up for

    as far as the 1k on a 150 shot. I would normally agree with you.

    Here's my thought on why I gain so much. I'm simply not making power I at 5k like I am at 6k.

    What I'm trying to say is I am rally getting 150 or so out of the shot. But the engine is making much more power there that the nitrous allows it to make because of the rpms

    a very similar engine build in one of the magazines dynoed approx 415 or so at 5k but about 500 or so at 6k.
    I think it's allowing my over cammed combo to breathe more or less if that makes sense
    most people running small shots like this, me included, is normally running rpm several hundred, if not a thousand below peak hp. so as rpm climbs, the engine is going to be making more n/a hp. a given.

    what I believe you're seeing is, when you are on the bottle, pump suction is not keeping the pump fully loaded. this is a guess, but I have seen the effect before.

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