Sea Sled madness. It’s in my brain.

Discussion in 'Fiberglass and Composite Boat Building' started by DogCavalry, Nov 11, 2019.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Squirrel!

    Johnny gone...

    At high Reynolds numbers parabolic forms have slightly lower Cd that tangent ogives, and far less drag than cones. I'm not surprised at the latter, but I expected ogives to be slightly better. Intuition is only a starting point. Math and measurements are always better.
     
  2. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Lateral section is a trapezoid. Rounded lower edges, or sharp? And bottom surface parallel to water flow, or ramped such that over all projected area is < dry area at outboard leg.

    Ah, I'm over thinking this. The difference in my particular case would be too small to actually measure. But still, the visualizations are beautiful!
     
    Last edited: Jun 13, 2023
  3. DogCavalry
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    DogCavalry Senior Member

    Ramped, unequivocally. If parallel then the pressure gradient goes negative at some point causing turbulent flow from the sides of the form to the bottom. Damn, I wish i had some good cfd software, the processing capacity to use it, and the time.
     
    Last edited: Jun 13, 2023
  4. DogCavalry
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    DogCavalry Senior Member

    Single cup, too slight.
     
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  5. baeckmo
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    baeckmo Hydrodynamics

    KISS! Transverse section trapezoid, sharp corners aft, slightly rounded fw, bottom flat parallell to hull bottom, don't overthink the shape, focus on strong fitting method.

    Edit: ....and when the wake consists of a cavity (vented or vapour), the parabolic shape is the way to go.
     
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  6. DogCavalry
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    DogCavalry Senior Member

    Gotcha. Serenity is so far from optimal in every other way, it's wasted effort to perfect this component.
    Glued and tabbed. Can't screw or bolt this. Would penetrate the fuel tank.
     
    Last edited: Jun 13, 2023
  7. DogCavalry
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    DogCavalry Senior Member

    23l/hr gives 17km/hr
    46l/hr gives 34km/hr
    That's not supposed to be linear
     
  8. Geno67
    Joined: May 2023
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    Location: Old Florida

    Geno67 Clueless Member

    Better than exponential. Is linear good or bad?
     
  9. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Well, bad in this case. Can't extend my range without slowing down more.
     
  10. Geno67
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    Geno67 Clueless Member

    Well crap. Sorry.

    Did you ever fill out an application for water ferry? If you did, you need to find out what you need to do to amend or correct it to show composite instead of wood.
     
  11. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Wrong conclusion; prop efficiency varies due to nonlinear drag curve and engine sfc veries as a function of output torque and rpm. You can't tell curve shape from two points.
     
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  12. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    I will make many more points. I'm dying to see a final curve, after hull bottom is faired, spray rails on, deflector is shielding outboard leg from high speed water flow. And a very nice SS prop...
     
  13. DogCavalry
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    DogCavalry Senior Member

    Fell off a roof yesterday. Today I feel my age.
     
  14. Geno67
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    Geno67 Clueless Member

    I hope you're okay. :(
     

  15. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Very sore and bruised. That's it.
     
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