waterjet propulsion

Discussion in 'Jet Drives' started by hawk, Feb 1, 2011.

  1. hawk
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    hawk New Member

    can someone tell me is the power will increased or decreased if we put that air inlet (on the picture)???
     

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  2. baeckmo
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    baeckmo Hydrodynamics

    It depends mainly on the velocity ratio: Vboat/Vinlet. In most practical cases, the static pressure at the impeller inlet at speed is above ambient, and at lower speeds and accelerating, there is an underpressure.

    This means that in certain speed regions, you are letting a gas into the impeller, which normally is a NONO, since pumps are designed for liquids; fans are for gases; the fanump is yet to be invented........ A typical jet pump will lose its pumping ability with about 5 % gas (by volume) in its inlet. Having a controlled leak opening in a jet inlet will function like a "clutch"; the jet looses thrust as soon as air is ingested. This little trick is used in order to stop a jet boat from moving around at idle.

    However, as always, there are exceptions to this rule. If the operation is aiming at a speed region, where cavitation is a problem, the impeller and the inlet can be designed for supercavitating operation. To establish a fixed cavity volume at all speeds, gas may be metered into the system by an inlet leak. Another, and better way of introducing gas into a supercavitating system, is to ventilate the outlet.

    Other cases, where you find an additional inlet, is when the jet is required to operate over a wide velocity range. In this case, the inlet area must be adjustable; big area at low speeds, smaller at high, since the flow has to be kept constant. The extra inlet scheme has been used on some jet-propelled hydrofoils, where both inlets are submerged and feeding the pump at low and medium speeds. When flying, the additional inlet is above surface, the main inlet is supplying the full flow and the static "ram" pressure is effectively stopping any air leaking into the propulsor via the extra inlet.
     
    Last edited: Feb 2, 2011
  3. hawk
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    hawk New Member

    ok. so without air intake...one more question, would i increase power if i put more than one impeler??
     
  4. MechaNik
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    MechaNik Senior Member

    Hawk, do you have a specific situation in mind or curious in general.
    If you had a poorly match installation were you couldn't get rpm's on the engineat low speed this could effectively increase your power.

    As said this will work by ventilating the impeller, decreasing the load on the engine and allow the engine to gain revs where it may generate more power. In a simple situation on a small boat the air supply could be controlled by a valve on the end of a hose.

    I couldn't imagine this being an ideal situation at cruising speed but maybe a fix to accelerate you there.
     
  5. hawk
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    hawk New Member

    oh sorry i forgot to say, it's a jet ski
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Aha, then you have a vessel that has to operate over a very wide speed range, resulting in a wide operating window for the inlet velocity ratio. In order to feed enough fluid to the pump during low speed acceleration, the inlet "mouth" (transverse area across the duct at the inlet lip station) needs to be big. But when bouncing around at top speed, this area is far too big, resulting in an unnecessary drag.

    One solution to this has been to open an "escape hole" in front of the impeller, where excess water is dumped. This practice of course means a loss of inlet energy that has to be compensated for by increased pump power, but in a few cases, the reduced drag may "overcompensate" for the loss. The dump opening should be directed straight aft.

    So, in your case, it is not an air inlet, it is a water outlet.
     
  7. Submarine Tom

    Submarine Tom Previous Member

    baeckmo,

    "...since pumps are designed for fluids; fans are for gases..."

    I realize English is likely not your first language, but I thought I should let you know, gases are fluids, as are liquids.

    -Tom
     
  8. MechaNik
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    MechaNik Senior Member

    In that case how do you determine if you have too much inlet pressure at max speed? Would a pressure reading by the shaft seal do or does it have to be on the impeller face (maybe through the inspection cover)?
     
  9. DCockey
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    DCockey Senior Member

    WARNING: I tried to open the illustration in the first message, and it appears to have downloaded software that was telling me my computer was infected but it would fix it.
     
  10. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    I've changed the image in the first post to a standard jpg attached; the image was linked from imageshack - I believe an ad from the imageshack site was generating the warning.
     

  11. baeckmo
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    baeckmo Hydrodynamics

    You're right Tom; should be "liquids". My post is now corrected.
     
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