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

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

  1. 2xm3
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    2xm3 New Member

    anyone else ever continue this discussion ??
  2. drmiller100
    Joined: Feb 2009
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    drmiller100 Junior Member

    For every action, equal and opposite reaction.

    As the PWC is cruising along, there is 100 plus horsepower thrusting water out the back of the boat.
    The faster you go, the more pressure there is in the intake of the pump. EVENTUALLY you reach a point where the water stagnates - it slows down, and eventually the water is not going very fast UP into the pump.

    When the water is not going UP into the pump, the PWC is not being pulled DOWN to the water, and the PWC actually rides the bubble of water on the intake.

    Which causes cavitation, which causes water to stack up even more under the intake.

    So now we have a very short boat going 100 mph and the back of the boat lifts up a tad, causing even more cavitation.

    At about the time the back of the boat lifts UP, the front of the boat touches DOWN. The front of the boat used to be up in the air with no drag, but with the front touching down, teh front sucks down HARD into the water (bernoulli) and a LOT of drag occurs at the front of the boat, pulling the boat down hard into the water.

    Center of gravity is higher than water level, so the back of the boat lifts even higher. Wimpy rider cannot hang on to an instantaneous 5 or 10 G braking, and flies over handlebars.

    Answer? smaller intake.
  3. Haje74
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    Haje74 New Member


    Shaka (and all others contributing),

    I know the last relevant post dates from 5 years ago but what an amazing thread! Very informative; learning heaps...
    Has there ever been any progress made with testing that setup? Very curious as to what results may have been gathered...


    Last edited: Feb 12, 2015
  4. Mudcrab
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    Mudcrab Waterjet4Fun

    Hi Baekmo, if you are still out there somewhere, do you recall the origin of the software used to produce the info?
    Paul ;)
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The shown performance data come from a series of tests made "in house" on various pump configurations. Pump differential pressure, inlet pressure, flow and power were recorded both in an ordinary pump test tank and in a planing hull. At the time, the Curtis-Wright PB1 pump (9 inch tip diameter, design speed 3000 rpm) was used as a benchmark for further developments in order to increase flow capacity and improve cavitation performance, and the diagram shows a similar size axial pump performance.

    Some of the info has since gone into calculation sheets and CFD modeling for validation of empirical data, such as typical inlet losses, limiting suction specific speeds a.s.o. The figures cited are thus results as tested, not "computerized dreams....".
    Last edited: Jul 5, 2015

  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    This does not make a lot of sense. As the pump increases in revolutions, it pumps more water to create more thrust.

    Hence the velocity in the intake must increase as the boat speed increases. As the velocity increases the STATIC pressure drops, ( the dynamic pressure increases but dynamic pressure is just measuring the velocity of the water coming into the intake, which varies across its cross section)

    The definition of stagnation is when the speed is zero not
    "it slows down"

    When a planning hull is planning there is a high pressure line about 1/4 to 1/3 back from the front of the water wetted surface that exhibits the highest pounds per square inch of pressure.

    As the boat speeds up, this line moves rearward due the fact that the boats wetted surface is reduced as speed increases.

    In the case of a prop, behind the transom, this high pressure line can move a long way back without issues. When the center of lift gets behind the dynamic center of gravity, the boat will likely porpoise (depending on hull design of course)

    In the case of a jet with an intake, the high pressure line, often called the stagnation line, eventually moves back toward the intake as hull speed increases. The water which is creating this high pressure line now is sucked into the intake, a low static pressure area and the lift in this area disappears.

    The bow drops aggressively. The wetted surface moves forward, the center of lift moves forward, the bow comes up and when the speed of the boat (on smooth water) reaches the speed when the decay started, it occurs again.

    This is pretty simplified as there might be some harmonic cycling after the first bow drop
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