In order to compute the hydrostatics properties of your hull model, Orca3D performs a numerical integration on the surfaces that you select for the hydrostatics analysis. This numerical integration is affected by the orientation of the surface, where in this context “orientation” refers to the direction that the surface normals are pointing. By convention, Orca3D assumes the surface normals point into the water (i.e., the normals are on the wet side of the surface). One might be tempted to say that this is the equivalent of assuming the normals point “outward” from the hull surfaces; however, this is not always true. Consider the case of a bow thruster opening in the hull. Here, pointing into the water will have the normals pointing inward. Surfaces whose normals are outward create positive displacement; when surface normals are inward, the displacement of that surface is negative. For the bow thruster example, it’s correct for the buoyant volume of the surface to be negative.
You can use the Rhino Dir command to determine and/or change the normal direction of your hull surfaces. However, an easier way to visualize the surface normals without having to remember to use the Dir command is to use the Rhino backface coloring option. To do this, run the Rhino Options command. In Rhino Options go to Appearance > Advanced Settings > Shaded. In the Shading settings set Backface settings to Single Color for all backfaces and set the Single backface color to a color that you don’t typically use for you surfaces. Now when you turn on shading mode in a Rhino viewport, you can easily see the surface orientation because the backfaces (the side opposite that to which the normal is pointing) are shaded in your chosen backface color. If any of your surfaces have incorrect normal directions, you can use the Rhino Flip command to correct them.