Boat hulls with smoothly curve stems - how to model

Discussion in 'Boat Design' started by DCockey, Feb 14, 2020.

  1. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    This is also posted on the Rhino forum Boat hulls with smoothly curve stems - how to model https://discourse.mcneel.com/t/boat-hulls-with-smoothly-curve-stems-how-to-model/96507

    Round bottom boat hulls frequently have stems which curve smoothly into the keel. A NURBS surface with four sides is not a simple fit to such a shape.
    Curved Stem Lines 1.png

    I know of three methods to model such a hull side using a single NURBS surface on each side. Each has advantages and disadvantages. A Rhino .3dm file with an example of each method is attached. The surfaces are very close to each other in shape. Note that the orange curves are isolines but do not indicate the number of control points used for each surface.
    Curved Stem 3 Method.png
    Perhaps the most obvious method is to use a three sided surface (actually a four sided surface but on side has zero length). This allows direct control over the shape of the stem. The shape near the upper corner where the isocurves converge can be difficult to adjust and maintain fairness. The surface cannot be simply extended at the upper corner. This method can be a quick way to create a surface for hydrostatic calculations and similar.
    Curved Stem A.png
    A second method is to split the stem curve to create an additional side. This also allows direct control of the shape of the stem but care is needed to ensure the stem curve has the desired continuity across the split. This method eliminates the converging isolines in the upper corner. The tradeoff is near the stem curve split the u and v isocurves become almost parallel, and are parallel at the split. This make adjustment and fairing of the surface shape in this area difficult. The surface may have problems extending at the split in the stem. This method can work well for creating a surface from a set of lines which will not be modified.
    Curved Stem B.png
    B Singularity.PNG
    The third method is to extend the surface past the stem and maintain the four sides. then trim the surface to the stem. This trimming can frequently be done with a plane on or parallel to the centerline of of the boat. This method does not provide direct control over the stem shape like the previous two methods. The stem shape is determined by the shape of the untrimmed surface. The resulting surface can be easier to adjust and fair than surfaces using the other two methods. Also while the trimmed surface usually can be extended without any problems and then trimmed as desired.
    Curved Stem C.png
    Curved Stem CUT.png
     

    Attached Files:

  2. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I generally try to put any irregular facets in a really flat area. What works for me is to have a ribbon of fairly narrow facets running along the center line, and then scab any of the three methods to that ribbon. This gives me a manageable layer to find my hydrostatic targets by tweaking the entire center line, and lets me hang rudders and skegs and keels from this. I can create a family of these ribbons with different appendages and save them as parts. However you do it, look carefully at how the actual facet is distorted from the control point grid. Changing the direction of principle curvature within a facet doesn't work well. So the split method wants its split in an area of least principle curve. Or you need to gradually warp the grid to produce smaller facets there. There is nothing really special about the sort of facet in the split type. It, like all others, will render best if there is very little torsion on opposite sides. That is what is tricky in the bow. The same thing happens at the keel joint in a sailboat, but it is easier to find a nice flat spot to land the oddball facet.

    As a practical approach, you can put the oddball facet where the hawsepipe or bow thruster goes and then convince yourself it isn't terribly important;) Sometimes, Click and Clack's advice to place a small piece of black electric tape over the offending visual is the best way to proceed. upload_2020-2-15_20-44-8.png
     
    Last edited: Feb 15, 2020
  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    philSweet - I was having trouble understanding your use of the term "facet" but then I saw that it looks like you use FreeShip. It's been a while since I tried FreeShip and my recollection is it uses a type of sub-division. I use Rhino which directly uses NURBS. It sounds like similar issues arise with both types of surface generation.

    When splitting the stem I usually place the split where the curvature of the stem is maximum. This limits the area where the u and v directions are close to parallel. But I usually only split the stem when I need a quick surface and the stem has a distinct "bend". Those type of shapes typically don't work well with the 3-sides approach.
     
  4. philSweet
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    philSweet Senior Member

    In this case, subdivision surfaces vs nurbs is a distinction without a difference. Since they are both set up to render circular cylinders correctly, they suffer from the same sorts of rendering issues. They can be set up differently, but as employed here, they ought work the same.
     
  5. DCockey
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    DCockey Senior Member

    A quick look into Freeship/Delftship showed that they are not general sub-division modelers which allow arbitrary arrangement of input points. Freeship/Delftship use rectangular arrays of points similar to NURBS. So I agree there will be the same or similar issues when modeling a bow with a smoothly curving stem in a NURBS modeler or in Freeship/Delftship. My understanding is Freeship/Delftship do use sub-division type algorithms to generate surfaces from the input points. This differs from NURBS modelers which use algebraic equations to generate surface from the input points. The end results can be equivalent.

    As an aside there are more general subdivision modelers such as Blender which allows arbitrary arrangements (other than rows and columns) of input points. These types of modelers should not have the same issues with modeling curved bows.

    What is the relationship between rendering circular cylinders and designing/modeling the bow of a boat?

    Rendering is not the primary factor which influences how I arrange surfaces in designing or modeling a boat. Other, usually more important factors can be ease of creating the initial surface(s), number of control points required (fewer are better), ability to efficiently modify and fair the surface(s), and ability to extend the surface(s).
     

  6. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Another method is to split the sheer to create four sides for the surface. This allows direct control of the shape of the stem. I do not have any experience with this method. Updated file with an example of this method is attached.
    Curved Stem D.png
     

    Attached Files:

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