by Reinhard Siegel
Fast powerboats can feature one or more steps in the bottom of the hull. Model hull_with_step-1.ms2 shows an example. The C-spline Lofted Surface topside_0 is the basis of the hull topside. The PolySnake n3 combines the forward portion of the chine (SubSnake n2) with the stepped aft part (B-spline Snake n0). The unwanted parts of the basis surface are cut off by the SubSurface topside, supported by the Edge Snake n4 (upper edge of topside_0) and PolySnake n3.
Bottom surface is the Ruled Surface bottom, simply spanned between PolySnake n3 (on topside_0) and the PolyCurve c0, which is a combination of the forward part of the keel (B-Spline Curve keel0, degree = 2)) and its aft part, the B-spline Curve keel1 (degree = 1).
In model hull_with_step-2.ms2 an example of a sailboat hull with a step is shown.
Basis surface of the hull is a standard C-spline Lofted Surface (hull_0), supported by 7 B-spline mcs. The raised portion aft is the Tangent Boundary Surface hull_aft_0, defined by B-spline Curves c2 (steeper raising fairbody), the transom edge c3, the Projected Snake n4 (projection of the C-spline Curve c1 onto hull_0) and the SubSnake n1, which is part of the transverse Intersection Snake n0, defining the start of the new shape aft.
The Tangent Boundary Surface hull_aft_0 has set edge continuity to “Slope” along snake n4 and snake n1 in order to make it tangent to the basis surface along these edges.
The step is shaped by the B-spline Curve curve1, which is then projected vertically onto the basis hull surface (Projection Snake n2) and onto the Tangent Boundary Surface (Projected Snake n3). The Ruled Surface step is spanned between these two snakes.
The final parts of the hull and the raised aft bottom of the sailboat hull are created by the Trimmed Surfaces hull and hull_aft.