Designing a symmetrical hull under heel

Discussion in 'Sailboats' started by Justinet, Dec 22, 2018.

  1. Dolfiman
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    Dolfiman Senior Member

    About the nose-down attitude : if you check the lowest point of the station 0 (i.e. at the fore perpendicular), as long as this point stays at the level of the sea surface when the boat is heeled, we cannot called that a nose-down attitude but better a "stern-up" one, it is the natural result of both an elevation of the hull and a negative trim to maintain the equilibrium, it is especially spectacular for the most beamy hull, and in my opinion it is not a drawback at such, just a consequence of that type of design. The video below illustrates that perfectly.
    Only if the above point is going under the sea surface, then yes it is a nose-down attitude, a bad equilibrium to avoid.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    What a cool video!
     
  3. messabout
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    messabout Senior Member

    That video boggles my mind a bit. It is fun to watch but it has some unexpected evidence. The big boat is going like....really fast. Some of the windsurfers are sailing in dirty wind aft of the big boat. They are still managing to overtake the big boat. The big one has safe leeward position on some of the windsurfers but they are still out running the big guy. Are windsurfers that must faster, dirty air and all, than boats of the type we see in the video?

    It's back to the drawing board for me.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Not just windsurfers-foiling windsurfers!
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Earl,
    It is simple coupling. of heave and pitch.
    As the distance between the LCB and LCF increases heave+pitching increases. If the LCF is over the LCB, there is no coupling, it is merely heave.
     
  6. DCockey
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    DCockey Senior Member

    Back to the original question. A hull with a shape which part of a body of revolution with the axis of revolution parallel to the waterplane will keep the same waterline and will not trim when heeled. Another way to describe this type of shape is a surface with circular arc cross-sections with the center of every circular arc on a line parallel to the waterplane. The metacenter will be on the axis of revolution / circular arc center line.
     
    Last edited: Dec 24, 2018
  7. TANSL
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    TANSL Senior Member

    I do not see that clearly. Any additional explanation, please? Thank you.
     
  8. Justinet
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    Justinet Junior Member

    Believe i understand what you mean, but the issue with such a "cylindrical" shape is the poor hull stability, right ?
     
  9. DCockey
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    DCockey Senior Member

    Stability would depend on the hull dimensions and proportions, shape and CG location. Whether the resulting stability is "poor" would depend on the application/requirements. You asked about hull shapes which retain a symmetric waterline when heeled, and the "revolved" hull shape I described below is one way, and perhaps the only way, to achieve that.
     
  10. DCockey
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    DCockey Senior Member

    When a boat with a "revolved" hull as described above heels the waterline shape and underwater shape remain the same as when not heeled. This because the hull shape is part of a body of revolution.

    The center of buoyancy will be directly under the "axis of revolution". (Note the "axis of rotation" I'm referring to is from the geometry of the hull as described above.) The "axis of revolution" is on the center plane of the boat. The metacenter is the point on the center plane of the boat directly above the center of buoyancy.
     
  11. TANSL
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    TANSL Senior Member

    @DCockey and @Justinet all this seems to be correct only and exclusively if the water plane passes through the axis of revolution of the hull, a highly unlikely situation.
     
  12. Justinet
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    Justinet Junior Member

    As a matter of fact (tried to clarify my question in my second post) not looking for symmetry at any heel angle, just looking for tips or hints on how to improve the hull B WL symmetry at 15*.
    By hull stability i meant stability yielded by the hull shape not the ballast/CG. As far as i understand a perfect cylinder or "revolved" shape should have no hull shape stability (same volume at any angle), opposite e.g. of a "shoe box" type shape.
    So as with all things in hull design, it is likely a matter of compromise. I am endeavoring to improving WL symmetry at 15* without losing too much on other performance parameters.
     
  13. DCockey
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    DCockey Senior Member

    What part of my explanation do you disagree with?
     
  14. TANSL
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    TANSL Senior Member

    I am not able to understand that statement. I'm not saying it's wrong, I'm just saying I do not understand it. Perhaps a simple scheme would serve to clarify my ideas, once I know where the metacenter of which you are speaking is located. Thank you for your patience.
     

  15. DCockey
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    DCockey Senior Member

    The metacenter used for standard stability calculations. Metacentric height - Wikipedia https://en.wikipedia.org/wiki/Metacentric_height
     
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