Hi JTB, in this case it looks like your resulting surface has some corners that fold back over themselves. Like for example in this spot here:



That's actually a corner point of a 4-sided surface at that spot, but the control points near that corner are rather jumbled around - this type of situation will tend to mess up the calculation of an offset surface (which is part of the shell operation), because it causes the surface normals to wildly flip around at that fold-over spot.

Surfaces need to have somewhat more of a regular grid layout with a more distinct corner to be able to be offset.

Also, normally you want to try and have your input curves to the Network surface be touching each other. I did some work so that you could get some results without requiring exact touching, but in this case your curves are really quite far away from each other, that will tend to make less predictable results.

You can get better results for this type of a shape if you try to make a slightly larger, more rectangular shaped layout of your curves to start with, and then trim away the excess outer portion of that rectangular patch to get your smooth exterior outline.

- Michael