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  1. #1
    jdog800's Avatar
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    Center of gravity

    Does anyone know where the center of gravity is on the gpr hull without a motor installed?


  2. #2
    Moderator OsideBill's Avatar
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    Post this in the Conversion Forum I believe Hydrotoy's knows the anwser.

  3. #3
    jdog800's Avatar
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    Thx Hydrotoys for info on weights.

    Still not quite the answer I was looking for.
    I asked the same question over on the seadoo side with little help so far but im trying to get a better overall understanding of the physics going on with the loading and unloading of the watercraft pump in its relation to hull angle.
    How much weight is being kept infront of the intake? I have been thinking that the reason that the intake is slighly behind center mass is to keep it completely in the water at all times....yet at speeds, when the boat plains out, the weight mass is lifted out of the water by the hull. By shifting our weight to the back of the ski(superman), we are changing the center of gravity back closer to the intake grate. Two opposing weights (motor and rider)riding a wave of water, balanceing the boat so that the ride angle keeps the pump at a perfect angle of effecience(straight).
    This is why more angle of the ride plate allows for more rocking back of the boat.
    In theory the longer the boat(what the water see's) after the center mass, the flater it will ride in the water. The shorter the boat,after center mass, the more angle the boat will take.
    Now haveing said all that, my point was to say that i think the correct angle for the boat to be in is where the pump is basicly parrallel to the waters surface just below the water line. Making the axil flow pump its most effecient.

  4. #4

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    Quote Originally Posted by jdog800 View Post
    Thx Hydrotoys for info on weights.

    Still not quite the answer I was looking for.
    I asked the same question over on the seadoo side with little help so far but im trying to get a better overall understanding of the physics going on with the loading and unloading of the watercraft pump in its relation to hull angle.
    How much weight is being kept infront of the intake? I have been thinking that the reason that the intake is slighly behind center mass is to keep it completely in the water at all times....yet at speeds, when the boat plains out, the weight mass is lifted out of the water by the hull. By shifting our weight to the back of the ski(superman), we are changing the center of gravity back closer to the intake grate. Two opposing weights (motor and rider)riding a wave of water, balanceing the boat so that the ride angle keeps the pump at a perfect angle of effecience(straight).
    This is why more angle of the ride plate allows for more rocking back of the boat.
    In theory the longer the boat(what the water see's) after the center mass, the flater it will ride in the water. The shorter the boat,after center mass, the more angle the boat will take.
    Now haveing said all that, my point was to say that i think the correct angle for the boat to be in is where the pump is basicly parrallel to the waters surface just below the water line. Making the axil flow pump its most effecient.
    I've talked to an aerodynamic engineer about this very thing, and a lot of things that were discussed applied to hydrodynamics as well. One of the things that he explained was that jets and (boats at times) are designed from the center out. The center of gravity should be in the intersection of the x/y/z axis. The majority of the weight should be positioned in a place that when at speed the weight helps to keep the boat balanced, not in just a postion to maximize speed. Think about the weight of the engine and the body position of a racer at speed. They are NORMALLY positioned in the center of the hull right in front of the intake. It also must take into consideration that a PWC has an intake with an impeller, not a propeller, and so when riding must be at a much flatter angel of attack to keep the intake in the water.

    I've looked at our beloved GPR hulls with a lazer. It is unbelieveable what you can see. You can actually see the hull changeing shape as the lazer simulates the hull rising out of the water gaining speed, and all the while the working parts of the hull are still in the water. It is like you say, as the boat gets faster the relationship or ratio between the hull length and its width begin to change. Even though the hull is getting shorter while going faster, it proportionally is getting much thinner in relationship to its width the faster it goes. It seems that our hulls were designed to operate much faster AND safer at a 1-2 degree angle of attack.

    About us needing a longer boat the faster we go.... according to the lazer, the ride plate extends the length of the hull, but not in the place where you would think. The flat part gives you lift, and that is well documented. According to the lazer, it is the angled sides and that part of the plate that houses the speedo that EFFECTIVELY increases boat length. That part of the plate is less than 2 degrees. This part of the plate seems to stabilize the hull by making it longer. According to the lazer, the flat part of the plate is not subjected to high pressure water when the hull is riding at 1-2 degrees like the rest of the working parts of the hull. Parts of the angled sides (dead rise angle) and the speedo area are. It is the high pressure water that is literally pushing these parts of the plate to make the stern ride in a flatter angle of attack as well as give the stern lift.

    One other thing that really blows me away is that at a very flat angle of attack, the EFFECTIVE part of the intake, that is the part of the intake taking water directly into the gullet seems to shrink. A smaller intake at higher speed is exactly what is needed. So I think you are right, the flatter the boat rides, the more efficient the pump becomes. It is just that unless you are at very high speeds, a flat riding hull creates so much drag that it robs you of speed in spite of pump efficiency. The hull really looks like it begins to get efficient at 74-78 mph. Until you get there, riding at a higher angle of attack nets better speeds at the expense of sacrificeing stability.

    I hope that Santa sends me a cam corder that I can use to record and down load a lazers view of our hulls for all the forum member to see. I promise you it will spark a lot of good discussion as well as debate. It is the most interesting thing I have seen (concerning PWC's) in a long time.

    JMO

  5. #5
    Hydrotoys's Avatar
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    One other thing that really blows me away is that at a very flat angle of attack, the EFFECTIVE part of the intake, that is the part of the intake taking water directly into the gullet seems to shrink. A smaller intake at higher speed is exactly what is needed. So I think you are right, the flatter the boat rides, the more efficient the pump becomes. It is just that unless you are at very high speeds, a flat riding hull creates so much drag that it robs you of speed in spite of pump efficiency. The hull really looks like it begins to get efficient at 74-78 mph. Until you get there, riding at a higher angle of attack nets better speeds at the expense of sacrificeing stability.
    True of all jet boats. In the Jet boat world they call this the intake "window." The window is increased with greater angle of attack, which leads to an overfilling of the pump... along with a multi-angle prop, which in essence is wedge shaped leads to the attempted compression of water in the pump.

  6. #6

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    drill a hole.

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