Sea Rayder F16 tries to go left while on plane

Discussion in 'Jet Drives' started by KStro, Jun 4, 2021.

  1. Lordneeko
    Joined: Aug 2021
    Posts: 3
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    Location: Georgia

    Lordneeko New Member

    See that still makes me feel like some how the weight on one side is pulling the boat to the other side. With the right side of the boat pushing down, the force of the water on the front of the boat would be giving a slight push up and to the left, right, like if you were to somehow deploy the right flap on an airplane? I wonder if how light and short this particular boat is, plus the heavy 175hp V6 in the back, contributes to that being a normal handling problem?
     
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Ok this is what goes on. I made a couple of sketches in another context, but to show the same issues, hope you can live with the recycling..... The first is demonstrating the real flow into a hull opening, and the importance of rounding of edges. The inlet is what is called a hydraulic sink; it has to present to the flow a lower pressure than the surrounding, alas the water "seeks" its way along paths even from the sides. The heavier load, the more fluid is entering diagonally across the inlet sides.

    As you can see, your Sportjet inlet has a really bad shape for this flow condition, its sides are square, their longitudinal shape is wrong, and the grating bars have square corners. On top of that, there is a possibility that the sealing "grommet" in front of the inlet might develop a leak, allowing air to mix into the flow path from above (I previously said you should check its condition, have you done that?). It may work when new and shining, and in a boat without any kind of add-ons, but not with scarred surfaces.

    Next image shows a cut through the center of the inlet duct, demonstrating the ideal inflow velocity profile, that is distributing the flow evenly over the entire impeller inlet area.

    The third picture shows the result of the common disturbances occurring when the jet is operated outside its design load and speed range. Note that all non-symmetrical inflow will cause an asymmetrical outflow as well. This means that in one flow zone the flow over impeller blades and stator vanes is stalling (ie not following the foil profile), and in another zone it goes straigh through, without any energy addition, ie there is no work done. This is where you get your unwanted steering effect (and, of course a consequent loss of performance).

    And the fourth picture shows why there is a difference in cavitation noise depending on which way you turn. The Mercury jets are clockwise rotating (as seen from behind), and when you make a turn to port, the aft part of the hull is sliding to starboard, which induces a slight crossflow into the inlet. This collects in the impeller end of the duct into an inlet swirl. When this swirl rotates the same way as the impeller ("positive prerotation"), cavitation is suppressed to some extent (and vice versa with negative prerotation of course). When there is no cavitation, the negative prerotation results in an increased impeller load, that in severe cases may bog down the engine. Note that most other jet units operate counter-clockwise.

    The prerotation problem is often seen where inlet grating fingers are missing or to short/low in the fluid direction; they have the function of flow straighteners in oblique flows.

    So, what medicine is available then? First, check air tightness of course. Then check if another version of grating (from another Sportjet) might fit, there seems to be quite a number of improved versions. There may also be after-market models available. And, of course if you are handy with metal-working, a reshaping of the present inlet is possible, but I don't know how much material there is in the grating frame. But any serious rounding of the original would be an improvement. If that's your route, I might add a few words later, Good Luck!!

    DPjet 001.jpg JetInlet1.jpg JetInlet2.jpg JetinletSwirl 001.jpg
     
    ziper1221 and DogCavalry like this.

  3. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry Senior Member

    Fantastic, @baeckmo! Your posts are a master class in theory and practice.
     
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