Design & Analysis Tools for the Marine Industry by Proteus Engineering

Easily finding incorrect surface normal direction using the Backface Color attribute

Issue: RhinoMarine performs hydrostatics calculations using the surface analysis mesh, rather than the traditional approach of stations. Surfaces in Rhino have an "inside" and "outside", and the outside must be in contact with the water, or RhinoMarine will consider the volume of that surface to be negative. Using the Backface Color attribute, it is easy to determine whether the direction is correct for all of the surfaces in the model.

Description: Surface design programs have a concept of the "inside" and "outside" of a surface. The Normal Direction, sometimes called Outward Normal, is a property that each surface possesses, pointing to the outside of the surface. In the context of hull design, the Normal Direction should be pointing into the water. If the Normal Direction is incorrect, the displacement for that surface will be negative. This will be obvious if you are working with just one surface, but if you have a multiple surface model, it may not be so obvious, because the incorrect surface will just subtract from the overall displacement, which may still result in a positive (but too low) displacement.

Without the Backface Color attribute, you can select a surface, click on Direction in the Analyze menu, and see arrows that point in the outward direction. With this approach, you must go through the entire model, surface by surface, to ensure that each surface has the correct normal direction. Using the Backface Color attribute, you can tell at a glance which surfaces need to have their Direction flipped.

This figure shows a simple sailboat model in shaded mode. From this view, there is no way to tell whether the surfaces have the correct Direction.

Using Rhino's Advanced Display Settings dialog, you can select a Backface Color, and check the option "Color Backfaces". (Note; in Rhino V3, Service Release 3c and earlier, this is in the Bonus Tools->View menu. To download the free Bonus Tools, visit http://www.rhino3d.com/3/bonus.htm.)

In the figure below, it is now immediately obvious that the Direction is incorrect on the starboard deck and one of the cockpit surfaces.

Using the Direction command in the Analyze menu, we can see the arrows pointing away from the "wet" side of the surfaces. Use the Flip option to correct the Direction.

These figures show the correct Direction on a couple of the surfaces:

Once the Directions have been corrected on the blue surfaces, the model will look like this: