Jet pump moved forward in hull

Discussion in 'Jet Drives' started by yachtnetwork, Sep 22, 2014.

  1. yachtnetwork
    Joined: Apr 2012
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    yachtnetwork Junior Member

    Guys, we are working on a small jet tender project for a customer right now and the short of it is we are considering moving the jet pump/engine package a bit farther forward in a 10'10' inflatable jet tender that we already manufacture. Currently the old (outdated) jet nozzle ends up about 4 inches inside the transom at its farthest point back and has plenty of room to send the thrust from port to starboard.

    We are thinking of using a shorter driveline and engine package from another company and it would in turn move the pump nozzle about another 8 inches towards the bow. Farther recessed if that makes sense. The transom pump opening on the boat would be opened a bit wider to accommodate for the flow/thrust to freely exit the back end without obstruction.

    Has anyone come across in the past a system that has been successful recessed like this? Center or gravity is not a problem with engine etc. however we are wondering what we can expect with the more inset pump. Any input or predictions would be welcome.
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Hi
    Does the intake have to be moved forward as well?
    Can you send some info or picts of what you build now?
    While we have not built 11 foot inboard jets, I have ran across a few limited one off
    builds that had a lot of issues at higher speeds
    It appears that as the center of lift moves backwards as the speed increases, that when the higher pressure stagnation line gets close to the intake, the hull loses lift as the water moves into the intake and the bow of the boat dives hard into the water.

    The two that I have seen were a 12 and 14 foot aluminum hull, inflatable.

    I have ridden in a 20foot aluminum jet, 18 degree mono ,454 stroked, twin turbo'd, intercooled, at around 75 mph, the boat seemed to lose lift and the bow dove downward violently, skewed about several times until it stopped.
     
  3. yachtnetwork
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    yachtnetwork Junior Member

    barry, shoot me a pm with your email and I will forward what we have for you. All in all the pump, intake etc. have been forward of where they used to be about 6-8 inches. however on an 11' boat that is a lot.
     
  4. Barry
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    Barry Senior Member

    hi
    I prefer not to post my email address, if you want send me yours
    Thanks
    Barry
     
  5. yachtnetwork
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    yachtnetwork Junior Member

    i prefer not to post mine either. I cant send you a direct pm as you have it blocked it says. Feel free to drop me a pm
     
  6. Nick_Sinev
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    Nick_Sinev Junior Member

    It must be really wide and relatively short, otherwise you'll organize a thing called "ejector".

    Potentially, in case of narrow and long “openings” (cone shape with small angle) you could get the following problems.
    1) The symmetrical variant of jet stream would become unstable. It will be deflected to the wall of the cone.
    2) You will organize a zone of low pressure inside the cone.

    [​IMG]
     
  7. Barry
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    Barry Senior Member

    yachtnetwork,
    try the pm now, I think that I am unblocked now
     
  8. baeckmo
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    baeckmo Hydrodynamics

    From the jet propulsive efficiency point of view, the inlet should be as far back as possible. The more of the available bottom boundary layer that can be ingested, the better, since this part of the flow includes part of the dynamic energy that has been converted to pressure.

    If there is a bottom surface aft of the inlet lip, then there must be a new boundary layer created, which takes energy from the flow, ie the drag is increasing.

    In addition to this, the further back the working fluid is taken, the less aerated it will be. It is also deeper in the water, which means that it is working at a slightly higher static pressure, which is good for cavitation performance.

    So: a jet should be installed with its inlet at the aftmost position possible! All other arrangements onboard should be contemplated with this in mind.
     
  9. yachtnetwork
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    yachtnetwork Junior Member

    thanks for all the input from everyone. I am hoping we found a happy medium from where our old pump used to be to where the new pump will be. The old beat up picture is how the setup used to be. With the pump removed in the demo hull you can see from the photos it seems pretty far forward. However once the intake grate and all rigging is installed its not as dramatic visually. Unfortunately in small boat like this we only have so much interior and under deck space to work with and lots of rigging in the driveline so without redesigning from scratch this is what we came up with. When building these in the past on other models some with and some without inflatable collars and tubes we have had porpoising problems resulting in having large hp to weigh ratio on small 10 and 11 jet tenders. That was with mounting the pump exit at the transom line or even a bit beyond it. We have had to counter react that with an extended skid plate underneath that in turn hinders the reverse on the boat.

    In theory we are thinking that the fact there is now some hull past the end of the jet exit will help keep the bow down (probably slow some mph but the boat we can afford that) and make the boat more controllable. More controllable at slow speeds as well with less counter steer. We have a feeling the boat will skid a bit in turns so we will have to work on a small keel system or two I think. We have also added a few degrees of downward hook at the end of the hull to help with this. It seems that a lot of our big boat jet tender practices don't seem to apply to stuff this small as they all behave much different. Also the reverse gate is removed and that adds another 3 to 4 inches in length to the total drive line. The gate directs the water under the boat to either port or starboard rather than directly port or starboard like some others do so it should not thrust directly into the pump housing. Also the engine is in the exact position that it was previously. However its smaller and lighter than older engine.
     

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  10. Steve W
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    Steve W Senior Member

    Im not sure i understand why you are mounting it so far forward, I don't have any experience in this area but I just bought a 13ft Sea doo jet rib today so im curious to learn more about them.

    Steve.
     
  11. yachtnetwork
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    yachtnetwork Junior Member

    It because of a new engine that is out. Has a shorter driveline and lighter weight. Less expensive etc. That being said we have to accommodate the pump at the end of the shaft. And that falls shorter than the older kind so unless we want our engine housing starting at your feet and taking up room inside the transom we have to go this route. Also your sea doo is most likely the straddle seat kind. You are sitting on the actual engine if I am not mistaken. We have to have the engine under the center console and that kind of dictates our position for the jet pump unless we start making custom shafts etc.
     
  12. Barry
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    Barry Senior Member

    Porpoising occurs on flat water when the center of lift gets very close to the center of the dynamic center of gravity.
    As the speed of the boat increases, the center of lift moves rearward, if it goes beyond the center of dynamic gravity, the bow will move downward, as the bow moves downward, the center of lift moves forward (more lift due to the increase in wetted surface) pushing the bow up, then the center of lift moves behind the center of gravity, and the bow drops again and so the cycle continues.

    The highest planing pressure occurs near the center of the boat about 1/4 to 1/3 aft of the start of the wetted surface. The stagnation line. ( Many variables here, deadrise, length of the boat, trim angle etc)

    So the area around the stagnation line creates more lift per square inch than say than the area around the transom.

    In a jet, if the intake is far forward, and the stagnation line moves into the area of the intake, this lift pressure will drop as the water moves into the intake, causing the bow to drop.

    This can occur quite violently.

    As you increase the length of the boat sternward of the intake, you increase the chance of the water near the stagnation line/center of lift going into the intake.

    Certainly, for an outboard powered boat, increasing the length of the boat, will reduce porpoising, ( for a given speed) which you have done, I would be concerned that losing the lift around the intake might produce some unwanted characteristics.

    Do you power this with 143 horsepower? How do you get the boat certified as I thought that the maximum horsepower rating of a boat relates to width, freeboard, length and probably a few other parameters.

    I have never seen a 10' 10" boat rated at 143 hp. That would be a rush
     
  13. yachtnetwork
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    yachtnetwork Junior Member

    Barry, Thanks for the input. She will be powered with 70hp and 90hp power plants depending on our fuel mapping when we are done. Certification as you discussed only applies to outboard motors in terms of LOA, WIdth formula etc. Since the boat is technically inboard power there are no parameters needed to follow. We could put 300hp in it and still get it certified. However our insurance company would stop answering our calls I think. If this was an outboard boat 30hp would probable be the max hp based on the specs.

    I see what you mean about the lift and we had thought of that going into this project. Pretty much the water underneath the boat "falls" into the intake and the boat falls down a bit as you said. We choose a much smaller intake for this project than normal hoping to avoid that problem. We are also hoping that it limits the speed on the boat a bit too. We have done a few others in the past that once we fixed the porpoising issue with a 110hp Yamaha and an extended trim plate the boats ran 55mph easily. Way to fast for consumer use so we had to limit the throttle. We are hoping this boat comes in around 35-42mph once its all said and done given the power plant. This is a video of the original boat with an 80hp Yamaha two stroke engine and pump. Boat ran around 40mph. Fast forward to about 45 seconds into it.
    http://www.youtube.com/watch?v=62ywIV4iGJI&list=UU1tGiTGi-fmFqZ-TFUb8_Kw


    On past models I have done (without) tubesets on them the pump exit has been very far aft however the boat tended to bounce. Here is a video with no extended ride plate.
    http://www.youtube.com/watch?v=B9iCC7neHhI&list=UU1tGiTGi-fmFqZ-TFUb8_Kw

    Once the extended ride plate was installed she ran true and really fast.

    http://www.youtube.com/watch?v=boj9w2i8fkI&list=UU1tGiTGi-fmFqZ-TFUb8_Kw
     
  14. Barry
    Joined: Mar 2002
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    Barry Senior Member

    great videos
    The second one with the 200 pound live ballast in the front moves the dynamic center of gravity ahead and as the wetted surface, center of lift moves rearward, you can begin to see the onset of porpoising

    The third video lacks the 200 pound live ballast moving the center of lift sternward plus your changes to inhibit porpoising.



    re the smaller intake,
    With the smaller intake, the velocity into the intake will be higher, ie flow rate equals area times velocity, so if you drop the cross sectional area the intake velocity increases ( or speed if you assume the direction of the water is at 90 degrees to the cross section) .

    The impellor attack angle of the jets vanes ( as well as the trailing edge angle) will have been designed to operate efficiently with a given range of inlet speeds.

    Not sure how much impact this would have if the change in intake cross section is small. Baekmo might comment as he has jets dialed in

    This could work in your favor as it would lower thrust and speed, something that you might want. But it might also allow the engine to turn at a faster rpm,
     

  15. yachtnetwork
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    yachtnetwork Junior Member

    thanks barry.

    re jet vanes
    this package is actually designed to work together. The intake size and pump/impeller etc are all from same manufacturer and sold as a package so they should work in harmony pending they are installed at the same rate.

    We can actually change the rpm and power band in this engine ecu once we put her on the water to test. I am thinking we can probably get her dialed in pretty well once we see what the stock set up rides like.
     
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