Navigation:  Quick Start Tutorials > Design Hydrostatics

This tutorial shows you the basics of computing hydrostatics in the "Design" condition in Orca3D. For more detailed instructions and background information  on Orca3D Hydrostatics in general, see the Hydrostatics & Stability section.

Note: Orca3D computes most of the hydrostatics parameters from the surface mesh, not in the traditional manner of integrating stations (stations are used for the sectional area curve, and the prismatic and maximum section coefficients). In general, this leads to more accurate results, and avoids the possibility of missing or mistreating features in the hull surface, such as the end of a hull skeg. The accuracy of the calculations, therefore, depends on the smoothness of the surface mesh (this is true in Rhino for other things; for example the curvature maps depend on the smoothness of the analysis mesh). To adjust the smoothness of the Orca3D mesh, use the OrcaProperties command, or the icon (), and set the values in the Orca Mesh Parameters section.

 

 

Orca3D can compute hydrostatics and stability in various combinations of waterplanes/displacement and center of gravity/heel/trim, with a range of heel angles. However, while you are fairing a hull, you are usually just interested in the hydrostatics at the "design waterline," or at a particular displacement/center of gravity, without needing to go through entering values in a dialog box each time you want to see the calculations.

To simplify the process, Orca3D has a special Hydrostatics and Stability condition called the "Design" condition. The intent is to define the Design condition once, and then as you create and modify the hull, you can compute hydrostatics and stability at that condition with a single button click. This saves having to go through the dialog to define the condition each time you wish to compute the hydrostatics. Also, the Design Condition can be used as the condition to evaluate in Planing or Displacement resistance calculations.

To define the Design condition:

1. Select Define Design Condition from the Orca3D > Stability menu, or select the Define Design Condition icon. The following dialog will appear:

2. Click on the Select Objects button, then select the surface(s) to be included in the Design Condition.

3. Select which mode you want: Weight/Center, or Flotation Plane.

4. If you wish the weight and center of gravity to be computed from the Orca Weight/Cost properties that you have assigned to objects in your model, check "Link to Orca3D Weight/Cost Items." When this box is checked, before the Design Hydrostatics are computed each time, the total weight and center of gravity will be computed from those objects in the model which have weight properties assigned to them. Note that this is only appropriate if you have modeled both sides of the model, and assigned enough weight items to account for the entire weight of the vessel. Note: you cannot use this option in conjunction with Real-Time Hydrostatics.

5. If your surface model represents just one-half of the vessel, check "Mirror About Centerplane"

6. If you want to see hydrostatic values in real-time as you edit the hull, check the "Real-Time Hydrostatics" box (cannot be used in conjunction with "Link to Orca3D Weight/Cost Items).

 

Note that if you had pre-selected one or more surfaces before starting Define Design Condition, the following will occur:

- If this is the first time that you've defined the Design Condition for this model, those surfaces will be used as the geometry for the Design Condition.

- If the Design Condition has been previously defined, the selection will be changed to the surface(s) that are already defined in the Design Condition.

To change the surface(s) that are used in the Design Condition, click on Select Objects. Everything will become unselected, and you should now select the surface(s) to be included in the Design Condition, and hit Enter.

 

Overriding the Initial Plane for Free Float Iteration

When the weight and/or center of gravity is entered, Orca3D does an iterative process to arrive at the equilibrium flotation condition. As a starting guess, a flotation plane at the mid-height of the selected geometry is used, with zero trim and heel. Sometimes this isn't a good guess, because, for example, at this point the waterplane is vastly different than it is at the true equilibrium condition. For example, if your sailboat model includes the mast, and you have included that geometry in your selection, the initial plane will be somewhere up the mast, with a very small waterplane (and resulting in a very large displacement). It's more difficult (and slower) for Orca3D's solver to converge in such a case. To avoid this, enter a different value for the Initial Plane that is closer to the final equilibrium height.

 

To compute Design hydrostatics:

1. Be sure that you have defined the Design condition, using the steps above. You only need to do that once, unless you wish to change the surface(s) to be included, or change the Design condition. Note that you should not select any geometry; the surface(s) that are used in computing the Design Condition are defined when you define the Design Condition.

2. Select Compute Design Hydrostatics from the Orca3D > Stability menu, click on the () icon on the toolbar (the icon is on a flyout toolbar with the Hydrostatics & Stability icon), or type the command OrcaPlayStabilitySimulation.

3. After a moment, the hydrostatics report will be displayed.

4. If you checked the "Real-Time Hydrostatics" box, the hydrostatics will be displayed in a dockable control, as you modify the surface with the Orca3D control points.

 

 

See also:

Hydrostatics & Stability