How do you get a beaver tail on the back of a NACA foil section?

Discussion in 'Sailboats' started by james3232, May 7, 2018.

  1. james3232
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    james3232 New Member

    Just messing around with the idea of designing and making a high performance keel(production taken care of). I've been wading around way out of my depth, and seem to have settled on a profile, based on advice and what looks right. I've got the profile in soldiworks, but I can't seem to find any information about to create the flat on the back, do you guys have any resources I may be able to look at?
    Regards and thanks!
     
  2. DCockey
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    DCockey Senior Member

    Are you asking how to create a flat in Solidworks, or something else?
     
  3. james3232
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    james3232 New Member

    Sorry, I should be more exact.
    I'm not sure if it's the correct name for the term to be honest, the flat at the back of the bulb so that the rear isn't just a cone. I can model it, but where to start it, and the size. I can't really see anything in Principals of yacht design, which i partly used to get somewhere near up to speed
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Here is a Dufour Yachts bulb similar to what you describe:

    Bulb flat end 3.jpg
     
  5. Doug Lord
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    Doug Lord Flight Ready

    And a few more:
    Bulb-flat end 2.jpg Bulb-from aft.jpg Bulb.jpg
     
  6. upchurchmr
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    upchurchmr Senior Member

    Why do you want such a shape?
    Just the look?
     
  7. james3232
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    james3232 New Member

    Well if every fast yacht has one, its gotta be quick...
     
  8. PAR
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    PAR Yacht Designer/Builder

    The bulb makes a big hole in the flow, so the tail is intended to help get it (the flow of water) reassembled and moving as undisturbed as practical.
     
  9. upchurchmr
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    upchurchmr Senior Member

    I don't see why it's better than just a conical tail.
    And if you want less drag, just extend the cone aft, decreasing the potential separation angle.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    This is a pretty fast bulb-on models and maybe on bigger boats:

    Bulb-spin 50 Amer 1.jpg
     
  11. jehardiman
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    jehardiman Senior Member

    It is a little less drag that a conical tail provided you meet all the criteria (i.e. Rn, speed, pitch, leeway, etc.). What you are trying to do is to reduce the lost energy in the trailing vortices by dividing a single one into two smaller ones. Waayyy to much to try and convey over the internet. I'm not sure all the techniques, tactics, and methods of multi-vortex simulation are all worked out in the general literature at the moment, so you will need to develop your own.
     
  12. rwatson
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    rwatson Senior Member

    The "flattened" bulb shape does more than vortex control, it also contributes to leeway control and performance. The heeled angle of the flat contributes to whole boat behaviour. It also contributes to reducing "unsteady motion", that messes up fluid flow.

    Check out page 651 of https://www.nap.edu/read/5870/chapter/45#651
     
  13. upchurchmr
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    upchurchmr Senior Member

    Blew me away. I'll just stop with the simple explanations. :(
     
  14. jehardiman
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    jehardiman Senior Member

    I would concur that properly sized winglets canted at the proper angle add "lift" and damping;...but a simple beaver tail...not so much.
     

  15. tspeer
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    tspeer Senior Member

    I would consider the fact that the flow does not generally meet the bulb head on, but at the leeway angle. If you took an axisymmetric bulb, oriented it into the oncoming flow, and then did the same on the opposite tack, you'd end up with something like the beavertail. The point is to have the corners of the beavertail where the flow will tend to collect after passing around the bulb.
     
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