Possible reasons for this include:

  • If you have modeled only half of the hull, but not checked the “Mirror About the Centerplane” box, your values will be half of what they should be.
  • Orca3D computes most of the hydrostatic data from a surface mesh, not with the traditional approach of integrating stations. The user has control over the density of this mesh, just as you do with Rhino’s display or analysis mesh. If the mesh is too coarse, your values will be low. If they are too high, it will slow down the computations without adding appreciable accuracy. The settings may be adjusted using the OrcaProperties command. You should experiment with different settings, and see their effect on your results. As you increase the density of the mesh, you will reach a point of diminishing returns.
  • Surfaces in Rhino have the concept of an “inside” and an “outside.” The outside should be the side in contact with the water; if not, the volume of that surface will be computed to be negative. If your model consists of multiple surfaces (not joined), and some of them have the outside direction incorrect, they will deduct from the total. There are two ways to visualize the outside direction of a surface; first, you can select the Direction command from Rhino’s Analyze menu. Arrows will be drawn in the outward direction, and so should point into the water (note that for surfaces such as bow thruster tunnels, this means that the arrows will be pointing into the interior of the cylinder). If you find a surface whose direction is incorrect, use the Flip option in the Direction command to flip it to the correct direction. If you have many surfaces, this can become tedious; a more effective way to quickly see the directions of the surfaces is to Rhino’s Backface Settings. Select the Perspective viewport, and change to a shaded rendering. Right-click on the viewport title (Perspective), and select Display Options from the menu. Go to Rhino Options/Appearance/Advanced Settings/Shaded, and select Shaded. For the Backface Settings option, select “Single Color for all backfaces,” and then select a color that stands out in your model. Now, as you rotate the model, you can quickly visualize the backface (inside) of each of your surfaces. You can now use the Flip command to flip the direction of any surfaces that are incorrect.