I glad you found it.
As to the negative volume, that is completely valid from a NURBS surface modeling point of view. Certainly it is not useful for your hull modeling application.
By way of example, here are three rail sweeps using identical circle rails and a line profile.
I’ve set the backface color in the display mode to be magenta.
The one of the left has a negative volume, the one in the center has a positive volume, and the one on the right has a volume of zero.
All three are valid surfaces from a NURBS surface modeling point of view:
Clearly, only the center example is of use for modeling real world objects,