Jet Boat Brakes

Discussion in 'Hydrodynamics and Aerodynamics' started by Phancy, Sep 22, 2013.

  1. Phancy
    Joined: Sep 2013
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    Phancy Junior Member

    I used to own a PWC and a small Bayliner.

    When you drop the reverse gate down and apply throttle to stop your jet boat.

    What causes the boat to slow down?

    What causes the bow to dip down?
  2. IMP-ish
    Joined: Jan 2011
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    IMP-ish powerboater

    The reverse gate over the jet nozzle redirects the flow forward and down. (down angle to clear transom.) The boat slows down quick or goes in reverse because the jet force is now directed forward instead of backward. The bow dips down because the stern is pushed up.
  3. Phancy
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    Location: Lewiston, ID

    Phancy Junior Member

    I am a 70 year old retired mechanic, so if I make typos please forgive me.

    I think the above statement is wrong; does anyone want to try to correct my thought process?

    I think it is the 'Murphy' effect, for a name to call it.

    I will answer emails if there are not too many.

    This thread is going to be fun.
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Huh ? How about you tell the world about this " Murphy" effect ? The answer you were given seems to cover it pretty well.
  5. Phancy
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    Phancy Junior Member

    The suction is the culprit

    Say your pump is removing water from under the boat (AKA lake, ocean whatever) at 2000 GPM.

    What replaces the water removed from the lake? The boat of course replaces the water.

    When you are sitting still at full throttle, tied to dock, flow has a chance to set up into the inlet of the pump.

    When you are going along at 35 MPH, this flow does not have a chance to set up.

    Since water can’t move instantly, but the pressure wave can move at the speed of sound in water (about 4400 fps)

    This suction affects the whole under side of the boat equally tapering off to the water line, the bow wants to plow into the water.

    The stern can’t go into the water because it is a big flat area, leaving only the moving pointy end to replace the water.

    And so the breaking action is the amount of water pushed aside buy the boat.

    If you were to replace the reversing gate with a ‘T’ feeding the water into the base of the ‘T’ equalizing all thrust of the pump it would still brake to a stop, just would not be able to go into reverse.

    If the description that I received were true, your acceleration in reverse would be at the same rate as your braking and also the stern would lift up in reverse it never did for me.


    If your pump is taking in water at the rate of 2000 GPM, the boat is moving forward at 35 MPH not all the water that the boat is displacing is going into the pump.

    Some is pushed to the sides, you know, making a wake.

    Say it is 2000 GPM being pushed aside for a total of 4000 GPM being moved by the boat and motor combination, this is only a guess and it will vary some with the boat hull and speed it could be a lot more.

    We then add a shoe to the underside, this pressurizes the inlet of the pump and the suction moves to the back side of the shoe, again affecting the whole underside of the boat.

    Add a Murphy to the underside of the boat and things change.

    From the front it looks like a shoe or scoop, but from the side it is NOT tapered into the hull, it is a box to replace the water that is deflected up into the pump.

    This box goes all the way to the stern (get ride of the ride plate it slows down airflow) allowing the air behind the boat to replace the water entering the pump.

    The boat will ride a lot higher in the water on plane, you will go faster and with more efficiency, but in a turn it might get a little squirrelly.

    The closer your inlet is to the back of the boat, the less directional stability you will have.

    The frontal size of the Murphy can be approximated by your pump volume.

    2000 GPM is 46200 cubic inch, with an inlet of 10 inches wide at 35 MPH or 3080 FPM

    With simple math your inlet should drop about 1.5 inches below your hull as a starting point.

    Your boat at plane will actually have less in the water than the same boat without the Murphy.

    And as a side note: if a little air enters the pump it will not hurt performance, but could cause over revving. The reason you get a performance boost is the water in the straight section of the nozzle will continue to accelerate.

    So something to limit revolutions in rough water would be nice.

    I built a Murphy when I was still boating on Lake Powell, it works.

    An increase in speed and efficiency is always a good thing.

    And no part of my name is Murphy.
  6. Phancy
    Joined: Sep 2013
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    Phancy Junior Member

    Simple Test

    I have had a nice night of rest; 70 year old fossils need all the beauty rest they can get.

    Maybe this will help.

    You don’t have to modify your watercraft to see how the principal works,

    Fasten a rope to a 4’x8’ piece of plywood about 2 inches from edge centered on small side of the plywood, leaving enough tail on rope for a handle.

    Put some weight on top side of the board, to ride on it, one of your offspring will do, weight should be toward the back or it will submarine.

    Tow this contraption across the water at say 15 mph, offspring should enjoy.

    A. Note how much effort it takes to move it along the water and how much water is being displaced.

    Next fasten a 6”x2”1” to the underside center of the board, looking from the front it will be 6” wide and 1” deep.

    B. Test again noting as above.

    Now cut a hole in front of board 6” by about 3” to allow the water that is dammed up in front of the scoop to escape up through the board

    C. Test again noting as above.

    Cut a 1”x6” plank the same to fill the area behind the already attached piece of wood extending all the way to the back edge of the plywood.

    D, Then the final test.

    Compare your notes.
    Just to give it some scale ‘A’ is 0%
    ‘C’ will have the most drag and be 100% for the top of the scale, also largest wake.
    ‘B’ will be in the 50% area.
    And ‘D’ will be around 25% or less.
    All numbers are approximate, but should give you an idea of what is happening under the board.

  7. Phancy
    Joined: Sep 2013
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    Location: Lewiston, ID

    Phancy Junior Member

    24 hours

    It has been over 24 hours since anyone commented on this link.

    I can think of several reasons for this lack of response:

    A. I am too dumb to understand anything you might try to explain to me.
    B. You can’t figure out what I am talking about.
    C. I forgot my bath last night, old people smell funny.
    D. All of you are stunned to find out the world is NOT flat.
    E. All of the above.

    Some kind of comments would be nice.
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