Hydrostatics & Stability

Unlike traditional hydrostatics and stability software, Orca3D computes most of the parameters using a mesh that is generated from the surface(s), or simply a mesh model. In general, this leads to more accurate results, and doesn't rely on a station model of the hull that can easily miss features in the shape of the hull due to discontinuities, such as the end of a skeg.

Requirements

The requirements for a model are:

•	May be composed of surfaces, polysurface, meshes, or any combination of these;

•	While the model does not need to be completely sealed ("watertight"), any gaps in the model will decrease accuracy;

•	Any naked edges should not become submerged; for example, if the model does not have a deck, it will not run at heel angles at which the deck edge would become submerged;

•	The normal direction for ALL of the surfaces and mesh must point into the water. See below for information on how to check this, and change it if necessary;

•	The surfaces and meshes being analyzed should only represent the outside shell of the vessel (hull, deck, superstructure, etc.), and not the interior surfaces (bulkheads, interior furniture, etc.);

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Be aware that interior surfaces, such as a cockpit, that are intersected by the waterplane and form a well, will be treated as if the well that is formed is filled with water, up to the waterplane. See the explanation of well surfaces below. All selected geometry that is completely or partially below the waterplane, will be treated as if that portion of the geometry below the waterplane is wet.

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 study their effect on your results. As you increase the density of the mesh, you will reach a point of diminishing returns in terms of increased accuracy versus computation time.

Normal Direction

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.

Incorrect Direction on the hull surface

Direction corrected with the Flip option

If you have many surfaces, this can become tedious; a more effective way to quickly see the directions of the surfaces is to use 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. In the example below, the surface color is set to green, and the backface color is set to red.

Incorrect Direction on the transom surface

Direction corrected with the Flip option

Well Surfaces

All selected geometry that is completely or partially below the waterplane, will be treated as if that portion of the geometry below the waterplane is wet. This issue, which occurs with any hydrostatics program, will occur when the model includes surfaces that are below the waterplane, but would normally be dry. In the following barge-like example, because the interior of the barge has been modeled, there is potential for error:

In the case of WL 1, which is below the inside deck of the barge, the results will be fine. However, if hydrostatics are run at WL2, the results will be as if the interior of the barge were flooded up to WL2.

Note that this can occur not just in the upright condition, but also in a heeled condition. For example, beginning at WL1 for this barge at 0 degrees of heel would be fine; however, at some heel angle the waterplane is likely to intersect the inside deck, and cause it to be considered flooded.

In cases like this, it is best to select only the outside surfaces of the model when running hydrostatics.