hull shape and heeling angles

Discussion in 'Sailboats' started by rapscallion, Mar 3, 2012.

  1. rapscallion
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    rapscallion Senior Member

    I remember seeing a webpage of a designer discussing a sportboat hull that kept a symmetrical underwater shape at several angles of heel. I believe Bieker was the designer... I can't find that page now or any discussion of the hull shape anywhere. I should have saved it when I had the chance - I was just wondering if anyone had that page saved and would be willing to share it with me.
     
  2. daiquiri
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    daiquiri Engineering and Design

    Take a rugby ball, cut it in half and you have it. ;)
    But then there is a problem of leeway angle, which would create asymmetry even on a rugby ball hull...

    I do not know the name of that designer, but you have got my attention now... Let's wait and see. :)
     
  3. Paul B

    Paul B Previous Member

    Yes, it was a Paul Bieker design. The Bieker 27 sportboat. It was designed for a friend of mine. His buisness has taken all of his time since he had the design done, so the boat has yet to be built.
     

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  4. messabout
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    messabout Senior Member

    I challenge the symetrical waterline claim for the boat pictured. Daiquiri has identified the only geometric shape that could satisfy the claim. Alas, the football shape would not perform well over any but the smallest range of velocities.

    It would be pleasing to keep the WLs and Buttocks the same at all working angles of heel. But that is not the most important of the numerous things to fret about. Distribution of section areas at various heel angles is one of the more interesting propositions. Getting the section centroids to make a straight line, when heeled, is an exercise that some designers feel important.

    A boat as simple as a sharpie can have exceptionally pretty underwater lines when heeled. Not all sharpies mind you, but those that have been thoughtfully laid out. At certain heel angles, wetted surface decreases a useful amount, But scows are the easiest of all to get a narrow, near symetrical, stright line of centroids, low wetted surface underwater shape.
     
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  5. rapscallion
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    rapscallion Senior Member

  6. Gary Baigent
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    Gary Baigent Senior Member

    Here are a couple of Jim Young canting keel, small boat designs; the chine version being very basic and his answer for a not expensive boat. The keel line is as straight as he could get it and when heeled, would act like a long keel. On both boats the canting mechanism is a faired line over the gunwhale, absolutely basic KISS.
    The third image is a canting daggerboard - just to show his innovative, yet simple ideas. From the forthcoming book on Jim Young.
     

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  7. lumberjack_jeff
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    lumberjack_jeff Sawdust sweeper

    As Daiquiri points out, it's easy if you are okay with zero form stability.

    Bolger sharpies meet the symmetry criteria at the right angle of heel.

    If the Beiker design above is symmetrical when heeled, then we're working with dissimilar definitions.
     
  8. DCockey
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    DCockey Senior Member

    Have a look at the link rapscallion posted above. It appears that as the Bieker design heels the "centerline" of the submerged portion does not remain parallel to the unheeled centerline. Think of cone floating on its side. As the cone "heels"/rotates the submerged portion retains the same shape.
     
  9. lumberjack_jeff
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    lumberjack_jeff Sawdust sweeper

    I see what you're saying, but if you put a longitudinal discontinuity (keel) on the cone, the rotated shape isn't symmetrical.

    Also, a cone has no more form stability than a cylinder.

    I'm not seeing the magic. Without ballast, I see a free-rolling float which delivers a course change with each change of heel angle.

    My sense is that you need asymmetry to get form stability. :confused:
     
  10. tspeer
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    tspeer Senior Member

    What you may be missing is the axis of symmetry is changing as the boat heels.

    Think of a catamaran with the hulls toed in so they are joined together at the bow and canted so one hull is out of the water when the other is upright. The waterlines have an axis of symmetry when the boat is floating on either hull, but the axes of symmetry are not the same. Now make the hulls thinner, and add an infinite number of them in between the original two...
     
  11. Petros
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    Petros Senior Member

    no sailboat hull is symmetrical in the water when underway, I do not think it is an issue.

    Symmetry is way overrated.
     
  12. DCockey
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    DCockey Senior Member

    "Form stability" usually means the center of buoyancy moves outboard as the hull heels. It move outboard with the cone example as long as the center axis of the cone is above the surface of the water.
     
  13. MalSmith
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    MalSmith Boat designing looney

    I think you will find that this boat has a gybing keel. You can just make out the rotated orientation of the keel on the plan view image on the website. I presume the idea is that the the keel is rotated to align with the heeled shape. In other words, when sailing heeled, with the keel gybed, the centreline of the hull is pointing below the actual heading.
     
  14. Alik
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    Alik Senior Member

    I believe symmetry of underwater part at heel is a kind of chimera. Inclinations with free flat condition will not help; say given drive force will change trim angle, and symmetry will be lost. It is just enough to provide reduced min of heeled resistance component!
     

  15. MalSmith
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    MalSmith Boat designing looney

    To illustrate my point above, I've attached the image I referred to and marked on it in red the axis of symmetry of the heeled waterline and in green the centreline of the rotated keel.

    RotatedKeel.png
     
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