It appears that behavior is inherent in SubD where three creases meet, two exterior and one interior,.or two interior and one exterior. Simple example attached: SubD Funny 1.3dm (1.9 MB) Added: The behavior does not always occur. See the post below.
SubD appears to have some limitations for precision modeling of surfaces.
Thank you for your input. I am testing now bevels and it is a soft crease then. The same as soft- and hard creases in good old T-Splines.
This might work and still I think that creases should also work if possible.
However, for this specific motor boat example I see that SubD is then better for the superstructure as the rough hull can perfectly be modeled with Nurbs strakes and then add the aft body propulsion details later separately.
What you’re really looking for is Edge Weighting where by selecting an edge and basically influencing how it affects the surrounding topology. Think of this as like tangency/curvature influence when modeling with NURBS.
In most other SubD/Polygonal software this is possible (Blender, Modo, Maya…etc). The Rhino team knows of this “limitation” and have said as much. If there will be the addition of this process is kind of a TBD right now. So basically it’s not a limitation of SubD modeling as much as it is just showing where things are with how it’s currently implemented in Rhino.