Inline Outboard Jet, or outboard tunnel?

Discussion in 'Jet Drives' started by Liopleurodon, Mar 23, 2017.

  1. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    This design has been discussed around my region for years (decades), the market for outboard pumps has been so small than no one wants to spend the money to develop the system and put it into production. It would be great if the military liked the final product so it would actually get built and then trickle down to the public. We need to mount 200 to 300 hp tiller outboard jets on boats just to get the performance of a prop motor at half the hp
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    The set-up illustrated in post #15 seems appropriate, I wonder why it was never adopted earlier.
     
  3. Yellowjacket
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    Yellowjacket Senior Member

    Likely the cost being higher and to get it to mount properly without a jack plate you're going to need a shorter tower, which also raises the cost.

    Compared to a prop it still isn't quite as efficient, but it's a lot better than the current centrifugal jets. the difference is that this can be made to have a low acoustic signature. That is something you can't do with a centrif pump or a typical prop, so the payoff to the military is real.

    Also remember that in a military operation not all of your enemies are crude or dumb. Some of them have the capability to listen under water and a quiet waterjet is an advantage. I've been under water and heard a prop a quarter of a mile away screaming and then when you come up the boat is so far away that you can't hear it at all and barely see it.
     
  4. Barry
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    Barry Senior Member

    The direction that the water exits the physical shape of the pump does not determine what type of pump that it is
    The shape of the impeller determines the type of pump
    Axial flow pumps are high volume low pressure pumps, Hamilton
    Mixed flow pumps are medium flow medium pressure pumps, American turbine
    A centrifugal pump, radial you call it, are high pressure low volume.
    As far as I have been involved with jets for 30 plus years, no one uses a centrifugal pump in a jet application
    The axial flow pump simplified say a couple of propellers. With stators. If you increase back pressure behind the propeller/impeller the impeller loses some thrust.
    Redesigning 2 or 3 stages, into a single impeller designed to increase bowl pressure without a loss of thrust, normally results in a single, simply stated, prop with a shroud. American turbine and outboard jet,mixed flow
    The Scroll, you refer to in this post, merely changes the flow direction for the out board jet

    A jet pump gains its thrust from changing the velocity of the water from the inlet side of the pump to the outlet nozzle of the pump.
    So two basic choices, for a given input shaft horsepower, you can move a lot of water at low pressure as an axial pump with low bowl pressure before the nozzle or move less water at a higher change of velocity with a mixed flow impeller

    John Hamilton first tried using a centrifugal pump but found them unsuitable.
     
    Last edited: May 11, 2017
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Not directly comparable no doubt, but some of the early jet plane engines had centrifugal pumps, which again were less apt to the task, but was successful enough in planes such as the MiG-15.
     
  6. Yellowjacket
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    Yellowjacket Senior Member

    Early gas turbines were centrifugal flow machines primarily because compressor technology wasn't very advanced at the time and that was pretty much the only way they could get the pressure ratio they wanted. As compressor technology advanced axial machines became predominant since they had a smaller frontal area and that was important to high speed aircraft. There's nothing wrong with a centrifugal machine, it does more work per stage than an axial machine, but by the time you put a diffuser on it the diameter gets pretty big. There are some modern engines (like the PW 150 series) that use a centrifugal stage at the end of the compressor to get a lot of compression in a short length. Centrifugal machines have very competitive efficiencies when compared to axial machines, so there isn't much to choose from in that regard. Stage efficiencies over 80% are very common in centrifugal machines and considering the pressure ratios available that's not much in the way of losses.
     

  7. OCB
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    OCB Junior Member

    Centrifugal machines have very competitive efficiencies when compared to axial machines, so there isn't much to choose from in that regard. Stage efficiencies over 80% are very common in centrifugal machines and considering the pressure ratios available that's not much in the way of losses.

    Yellowjacket, May 8, 2017 Report

    Very interesting tread! Anyone have any data on how much loss is from the spinning shaft the in the axial machine?

    OCB


    most efficient propulsion? https://www.boatdesign.net/threads/most-efficient-propulsion.23637/page-3
     
    Last edited: May 18, 2017
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