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I get an error when I try to run hydrostatics. Why can't I get results?
This usually results from Orca3D not being able to find an equilibrium. Often the cause is a surface edge becoming submerged, such as the deck edge when the vessel heels. If this is the case, add a deck (or other surface) to your model to seal it. If you do not expect an open edge to become submerged, you should check your VCG to be sure that it is correct.
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Why is the displacement value too low?
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 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. |
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Why is the displacement value negative?
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 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.
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Why aren't the Cp (prismatic coefficient) and Cx (maximum section coefficient) reported?
Although Orca3D uses a mesh to compute most of the hydrostatics, certain quantities can only be computed from stations. These include the prismatic and maximum section coefficients, and of course the sectional area curve. Orca3D uses the stations that are defined in the OrcaSection command to compute these quantities. In order to get accurate values, you should be careful to use a reasonable number and distribution of stations. The ends of the hull, and any areas of distinct section change should be captured in order to get an accurate sectional area curve, and you should have stations near the station of maximum sectional area in order to get an accurate Cp and Cx (note that you don't need to find it exactly; Orca3D will interpolate, using a quadratic function over three stations, to find the maximum).
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Why is there a spike in the sectional area curve?
| • | If you have two surfaces joined in a station plane that coincides exactly with one of the station locations that you have defined, Orca3D will compute stations on both surfaces, so the sectional area there will be double what it should be. One case where this can happen is with a hull that has a planar, vertical transom, and the transom surface is modeled. Simply move the station location slightly forward or aft, so it doesn't coincide with the joint between the two surfaces. |
| • | Incorrect sectional area data can also result from including non-wetted surfaces in your selection when you compute hydrostatics. For example, if you have modeled interior surfaces, and include them in the calculations, Orca3D will include their areas in the sectional area curve (as well as their volumes, so your displacement, and all of the other hydrostatics values will be incorrect). |
| • | If you have a loose absolute tolerance setting, equal to or greater than your section spacing, Orca3D will not be able to distinguish successive stations from one another, resulting in incorrect values. |
| • | Check to be sure that you have correctly specified whether to "Mirror About the Centerplane" when setting up your hydrostatics calculation. If you have modeled the entire hull (port and starboard halves), and you check the "Mirror About the Centerplane" box, your sectional area values (and displacement) will be double the correct values. |
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