Help me understand the limitations of a jet pump in a planing hull.

Discussion in 'Jet Drives' started by shaka, Dec 29, 2009.

  1. speedboats
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    speedboats Senior Member

    Baeckmo, sorry for the misnomer, yes turbulator is what I meant.


    While with an axial type pump you trim the outer diameter (or trailing edge) of the impeller, with an axial pump you play with impeller pitch and thrust orifice diameter (your thrust orifice isn't necessarly your nozzle, it will depend on your pump brand).

    With risk of looking like I'm doing an about face, you could try using a coarser pitch and larger orifice diameter. This would pass more water through the pump for every given rotation giving way to your reversion (overstuffing) issue, the down side is you can overdo it and ned up with excellent hole shot and acceleration, but limit your top end (this is due to higher volume flow rather than higher pressure/ velocity)


    You will still need to know what is happening along the roof of the tunnel. We use really low tech stuff. Tap a 1/8npt thread into various places along the tunnel roof, then attach a 3/16 air brake type nylon line to each fitting, and couple each hose to either a vac/boost gauge or oil pressure gauge. Definately use a vac/ boost gauge at the leading edge of the tunnel. Make a run and plot the results. I will admit it is easy in a boat where you have a navigator to make the plot and driver to do his job, perhaps not as easy when you need to hold on and make a plot. Perhaps someone can help with some sort of electronic data logging system, or set up a board with a camera, whatever, no doubt you could get creative...
     
  2. jim lee
    Joined: Feb 2007
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    jim lee Senior Member

    For logging data these were the most popular.

    http://home.pacbell.net/jmcclure/Qwikdata.html

    We used a much more complicated and expensive system on our machine. I think the Qwikdata, would have been a better choice in the end.

    -jim lee
     
  3. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    One of the reports on various boundary layer experiments here:

    Mununga: Control of Flow Separation in a WJ Using Experimental Techniques.
    Melbourne Graduate Fluids Conf 2001.

    It's not one of the better, f.i. it lacks relevant results on varying inlet velocity ratios, but I had it lying on my desk......

    When we deal with speeds from 30 m/s and up, cavitation around the lip profile is unavoidable. With racing (or big size hydrofoil/SES) speeds the whole throat region has to be designed for supercavitating flow. At low speed during acceleration the cavitation problem occurs in the impeller inlet and on the lip "topside" instead.

    The discussion on inlet problems is not per se specific for pump type; axial or diagonal does not matter, both have an axial flow impeller inlet. That's what counts, so quit the axial versus diagonal crap. Since cavitation is a major issue at high speeds, let's not have too much of guesswork before shaka has provided more tech info; he/she promised pictures, and I would be happy to see some main dimensions of the pump as well; inlet tip and hub diameter, outlet tip and hub dia, metering nozzle shape and dia.

    I use the term "metering" for the minimum exit area. As speedy notes, this is not necessarily the steering nozzle. Then there is the possibility of some basic pressure measurements as mentioned, but more on that when we have tech info!

    Until we have this, I have to go back to Submarine Tom, he is a jealous ******* and nagging as soon as he observes me in another thread..........see ya later!
     
  4. Submarine Tom

    Submarine Tom Previous Member

    LOL!

    Promises, promises...
     
  5. shaka
    Joined: Dec 2009
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    Location: Texas Gulf Coast

    shaka Junior Member

    LOL!

    This begs the question, is he a ******* by birth, or just another self made man?
     
  6. Submarine Tom

    Submarine Tom Previous Member

    shaka,

    Good one! LOL
     
  7. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    Perhaps the consideration of the inlet tunnel is indiscriminant to type of pump used, but the fixes to alter rpm and power consumption in the discharge end can be considerably different, hence the mention. Also, quite often a simple opening up of the other end of the pump can eleviate an overstuffing issue, sometimes at the cost of top end, but occasionaly with a benifit there...
     
  8. shaka
    Joined: Dec 2009
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    shaka Junior Member

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    Here are some of the basic pictures of my pump assembly, the intake which is molded into the hull, and other components such as the intake grate and the pump shoe.

    The last picture uses the level lazer to show how the hull would appear at speed. It is set at an approximate angle of attack of 3 degrees.

    Part of the keel is still, according to the lazer, in the water at high speeds, and this is what one would see if one were to take a picture of the boat as it traveled over 75mph. I am aware that many would comment that the keel could be part of the problem in that it could induce cavitation.

    Your comments are requested.
     
  9. shaka
    Joined: Dec 2009
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    shaka Junior Member



    Baeckmo, here is some of the information that you requested. I know that your technospeak is different from the PWC world, so at times I will be more descriptive.

    Pump inlet dia 155mm
    Venturi outlet dia. 85-87 mm (there are different sizes) style is round
    Wear ring Length 66mm (surrounds impeller)
    stator vein Length 33mm
    venturi Length 151mm
    steering nozzle dia. 88-90mm (swing bucket)
    total pump length 300mm (does not include swing bucket)
    Impeller dia. 155mm
    hub dia. (trailing) 78mm
    hub dia. (leading) 32mm
    Impeller pitch 14/20
    Distance from the (bottom of the) keel to the impeller (approx) 940mm
    Pump inlet to impeller 640mm

    Highlighted by the lazer shows where the hull begins to form into the inlet. The distance from the top of the keel to the inlet is 320mm. From the bottom of the keel to the inlet is 300mm. The keel slopes toward the bow.

    The pump inlet width starts at approx. 110mm and gradually opens to 120mm over 440mm into the tunnel. In the last 200mm of the inlet, the tunnel goes from 120 to 155mm in width where it meets the wear ring.

    Engine hp 165
    RPMs 7200
    Boat weight and rider approx 900-1000 lbs

    Present speed: approx. 72-73mph
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Thanx for this info, will first check possible operating point with regard to max flow. Two more q; the pitch numbers 14/20 would that be inlet/outlet pitch in inches? Or...? And in pic no 4 there is a "blocking plate" inserted in the roof of the ramp section; is this used for fine tuning?

    Unfortunately, we use different terminologies; I'm stuck with the commercial R&D "dictionary" since ages, so please bear with me if you have to translate my notes into "racing slang".
     
  11. TollyWally
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    TollyWally Senior Member

    This is fascinating. I think I understand the laser image. How do you the capture the correct planing waterline and attitude for the laser to simulate?
     
  12. shaka
    Joined: Dec 2009
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    shaka Junior Member

    In the word of an impeller manufactuer: "pitch does not apply to a PWC impeller the way it does a boat propeller. Where the pitch on a boat prop refers to the distance the unit moves forward per revolution, PWC impellers use numbers that reflect the outer blade angle, which identify how aggressive the impeller is, and also as stated in the question is a ãlabelä to distinct between other labeled impellers for ease of applying each impeller to a particular watercraft. The number of that angle cannot always be taken literally, because all of the impeller manufactureâs today have been known to slightly change an angle or length of the blade to optimize performance, and not change the ID of the impeller because the label of that impeller is already known to perform with certain characteristics on certain watercraft"

    On a more aggressive hole shot impeller there would be a lower number on the leading edge (more bite), where as for a higher top end you would want a larger number. The impeller on this craft is a 14/20 Dynafly from Solas which has a larger root angle and hub compared to the yamaha earlier (original) model impeller. Because the impeller hub was enlarged, the stator vein hub was also increased.


    As to your 2nd question, in a manner of speaking, yes I was suggesting that part of the roof could be blocked off. In an earlier post I was suggesting that perhaps another wing could be welded to mirror the existing wing and deflect any water that may flow toward the roof closer to the center of the impellar. In the picture, I used a large 25 thousandths machinist shim to give that effect. I just laid it on top of the mounting portion of the intake grate to get your opinion.

    The bold rectangular part of the intake grate follows the roof line of the intake tunnel, and the long black straight line shows where (and at what angle) high pressure water enters the intake tunnel. High pressure water would enter from this line and lower towards the leading edge of the front "wing" of the intake all the way down to the leading edge of the rear "rake" of the intake. You cannot see the rear "rake" in this photo, but it is the very aft part of the inlet hole. Other photos to help you see what I am discribing to follow.

    When the craft is at speed, high pressure water does not get close to the roof of the tunnel, according to the level lazer.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Fine, then I am getting the picture.
     
  14. shaka
    Joined: Dec 2009
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    shaka Junior Member

    Strickly low tech and not entirely accurate....but you get the point.

    To find the angle, I used a digital angle finder. The venturi (most PWC users call them thrust nozzles) on this craft is fixed at 3 degrees. I laid the angle finder on top of the shoe (flat pad) and adjusted the keel on the ride plate to 3 degrees to match the thrust nozzle Some peope call ride plates pump covers. The ride plate, if adjusted properly can be used like a giant trim tab and create lift.

    The level lazer was mounted on a tri-pod. You can see the lazer and tri-pod in an earlier picture located just in front of the bow in the back ground. The lazer height was adjusted so that at its LOWEST SETTING, the lazer light would touch both the most aft edge of the shoe and the most aft edge of the ride plate at the top of the keel. In order to simulate the wetted surface increasing and decreasing as the boat gains speed, you just raise or lower the heigth of the lazer.

    http://i900.photobucket.com/albums/ac208/shakadog/IMG_2307.jpg

    I took this picture with the camera right next to the level lazer to show how much of the inlet was exposed if the hull were truely riding at a 3 degree angle of attack. The inlet is very closed off at speed, as Baeckmo has stated earlier.


    http://i900.photobucket.com/albums/ac208/shakadog/IMG_2312.jpg


    In this photo, you see the wetted surface with the tripod in the background. The lazer clearly shows the keel of the hull in contact with the water at this simulated speed and ride angle. If the hull were in a 5 degree angle of attack, the keel would probably not be in the water.

    If you look closely, you can see the lip of the inlet tunnel, which is about 300mm from the keel.

    http://i900.photobucket.com/albums/ac208/shakadog/IMG_2311.jpg
     

  15. TollyWally
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    TollyWally Senior Member

    Interesting Shaka,
    I use a laser all the time, sometime a bit unconventionally, as a carpenter. I'm also involved in a slow motion project somewhat similar to yours learning about the hydrodynamics of my own very slow boat. In the past I've placed numbered vertical grids on my topsides and had a buddy video my boat running at speed from his boat pacing me. I then turned that into digital prints and measured off the pics to get some rough ideas of the trim and lift acting upon my boat at various speeds. I'd do a lot more of it if it wasn't such a hassle to coordinate with another guy.

    I'm impressed with the R&D you're doing, I hope you get a breakthrough in your mission to hit a hundy. :)
     
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