This thread is based on Boat hulls with smoothly curve stems - how to model I created a new thread to put the methods together in a single post.
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.
Shown below are four methods to model such a hull side using a single NURBS surface, and a method using multiple, untrimmed surfaces. 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. The examples are intended to illustrate the typology of each method, not the ultimate degree of fairness possible.
BowCurve132.3dm (15.0 MB)
Perhaps the most obvious method is to use a three sided surface (actually a four sided surface but on side has zero length); “A” in the example file. 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.
A second method is to split the stem curve to create an additional side, resulting in a four sided surface; “B” in the example file. 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.
A third method is to extend the surface past the stem and maintain the four sides. then trim the surface to the stem; “C” in the example file. 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, but with practice can be controlled with precision. The stem shape will be inherently fiar if the surface is fair. The resulting surface can be easier to adjust and fair than surfaces using the other two methods. Also the untrimmed surface usually can be extended without any problems and then trimmed as needed.
A fourth method using a single surface is to split the sheer to create four sides for the surface; “D” in the example file. This allows direct control of the shape of the stem. It creates a singularity along the sheer at the split where the u and v isocurve directions coincide. This can cause difficulties with fairing and extending the surface. I do not have any experience with this method.
Another approach uses multiple untrimmed surfaces; “E” in the example file. No singularities and the exterior boundaries are matched exactly. This example could be refined to improve the matching between surfaces.
