Draft when heeling a sailboat

Discussion in 'Stability' started by jessepietila, Oct 24, 2016.

  1. Petros
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    Petros Senior Member

    the first water to enter the hull when heeled is always from the waves. you will not have smooth water when there is enough wind to heel the hull over that far.

    it usually comes over the gunnels at the maximum beam point, though with an unconventional hull design that might be different. I am having a hard time wrapping my head around the issue you are trying to solve. There are so many forces involved, that have to be balanced, for you to be able to heel the hull over in the first place. you can not really look at just a hull by itself becuase so many things change when you add a keel, rudder, rig and sails, mass of the crew, waves, wind on both the hull and rigging...

    Good luck.
     
  2. SukiSolo
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    SukiSolo Senior Member

    Does it? It will if you include keel/centreboard, but not necessarily the hull form!.

    One thing worth bearing in mind is the structural depth of an open transom. At a certain point you will need a tie bar to stop the hull sides spreading and cracking the hull. Depends too on rudder hanging arrangements. Also worth considering are the loads when boarding or getting out of the hull when a little aft of normal position ie scooping in water. AFAIK for most purposes you can treat the rig/sail weight as neutral vertically on small craft, large ones are a bit different. Not forgetting that the hull may generate vertical lift with increased speed that counters any down force from the rig. At low speed it's not worth worrying about.
     
  3. mydauphin
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    mydauphin Senior Member

    Does this matter, doesn't the keel and boat have to be able to handle being lifted out of the water completely. My only concern is hitting the hard bottom.
     
  4. jessepietila
    Joined: Oct 2016
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    jessepietila Junior Member

    Hi,

    mind the raw sketch, I am mainly concentrating on hull at the moment.

    Displacement in 3D-model itself is close enough ok at the moment.
    Double chine hull to be made out from glassifiber reinforced 9mm veneer.


    L 8m
    Beam 2,2m
    Draft 1,X0

    Weight 1100kg
    Weel 600 Kg
    weight ratio 55%


    Having no touch to MAXsurf or any analytical hydrodyna-sftwr it would be cool to share some thoughts !


    Jesse
     

    Attached Files:

    Last edited: Oct 26, 2016
  5. TANSL
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    TANSL Senior Member

    I've tried to work with the model, but there is a hidden element that prevents me to manipulate surfaces. When exporting to Maxsurf appear every surface that you have defined, but untrimmed, and I find impossible to trim them. See attached image.
    As it is a simple model, if you provide me several cross sections, plus the longitudinal profile, I could create a valid model for Maxsurf.
     

    Attached Files:

  6. jessepietila
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    jessepietila Junior Member

    Ok, I see,

    please have a look on following files,

    Sections / stations are created in 500mm interval starting from vertical transom,


    Jesse
     

    Attached Files:

  7. DCockey
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    DCockey Senior Member

    Rhino includes the Hydrostatics command which has an input for the waterline height, and which calculates:
    • Volume Displacement
    • Center of Buoyancy
    • Wetted Surface Area
    • Waterline Length
    • Maximum Waterline Beam
    • Water Plane Area
    • Center of Floatation
    The hull surface below the waterline cannot have any naked edges. http://docs.mcneel.com/rhino/5/help/en-us/commands/hydrostatics.htm

    For heeled calculations copy the hull surface and use rotate to heel it to the desired angle. Run Hydrostatics and check the calculated displacement. If it is less than the target displacement change the waterline height to a higher number and vice-versa. Iterate until the calculated displacement is close enough to the target displacement.

    Why is the model 85,000 mm from the origin?

    Why is the model split at the waterline?
     
  8. TANSL
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    TANSL Senior Member

    Keep in mind that the trim of the boat changes as increases heeling.
     
  9. DCockey
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    DCockey Senior Member

    Also keep in mind that the CG location for a small, open sailboat such as the design under consideration is strongly affected by the occupant location, and the occupants may move as the boat heels.

    Trim can be varied for hydrostatics calculations in Rhino by rotating the model. Vary trim until the Center of Buoyancy location is as needed.

    As an earlier post said water is most likely to get into the boat over a low transom due to waves. Also when the boat is sailing in conditions where heeling is significant the speed will be high enough that the height of water on the transom will be lower than the static height.

    A reasonable approach to selecting the transom height would be to look at other similar designs which are successful. One part of design expertise is to know when to use other successful designs as a baseline.
     

  10. jessepietila
    Joined: Oct 2016
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    jessepietila Junior Member

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