Quick Start Tutorials > Hydrostatics & Stability

This tutorial shows you the basics of computing hydrostatics and stability in any condition in Orca3D. For more detailed instructions and background information see the Hydrostatics & Stability section.

Toolbar
Menu	Orca3D > Stability > Compute Hydrostatics & Stability
Command	OrcaHydrostatics

In Orca3D, you may compute hydrostatics and stability at a range of waterplanes, or one or more combinations of displacement and center of gravity.

1. Start the command through the menu, toolbar, or command line.

2. Select the surface(s) to be included. Be careful to only select surfaces that could potentially be wet. Do not select interior surfaces, only surfaces that are part of the "displacer." Hit Enter.

3. The following dialog will appear.

Enter the Model Sinkage, Trim, and Heel, or the Weight (displacement) and a combination of LCG/Trim and TCG/Heel.

Notes on Entering Multiple Conditions

In all of the fields in the Hydrostatics & Stability dialog (except "Override Initial Plane Height"), you may enter a list of values, separated by commas or ellipses (...). For example, a list of Model Sinkages might be

1,2,3,4,5

As a shorthand, you may also enter the following to get the same list:

1,2,...,5

The spacing implied by the two numbers before the ellipses will be used until the number after the ellipses is reached or exceeded (note the commas before and after the ellipses). Multiple spacings may be entered, as in the following example for heel angles:

0,5,...,30,40,...,90,120,...,180

This is equivalent to entering:

0,5,10,15,20,25,30,40,50,60,70,80,90,120,150,180.

The matrix of flotation conditions that will be computed is created from all of the combinations of the various entries. For example, if you enter:

Model Sinkage: 1, 2, 3

Model Trim: 1.5, 2.5

Model Heel: 5, 10

12 flotation conditions will result.

If you have chosen the Weight option (rather than Model Sinkage), you may also click on the "Weight/Cost" icon, and the total weight and center of gravity will be computed from the objects in the model that have weight properties, and the values filled in to the Weight, LCG, TCG, and VCG fields (this option requires that you have Orca3D Level 2, which includes the Weight/Cost Tracking module). Note that these values represent only what is explicitly modeled (it does not double the weight when you check Mirror About Centerplane, for example), and only the objects that have Weight properties assigned to them. If you have only modeled half of the hull, you should double the Weight value, and (presumably) move the TCG to 0. Note: this is not a permanent link. You must click the icon each time you wish the values to be updated.

You can manually compute the overall weight and CG of a number of items using the Calculator icon. For example, if you know the weight and CG of the lightship, crew & effects, and fuel, you can enter each of them individually, and the sum will be entered into the appropriate fields in the Hydrostatics dialog.

4. If your model represents only half of the vessel, be sure to check the box entitled "Mirror About Centerplane."

5. If you'd like to see a planar surface that represents the waterplane in the resulting condition, check "Add Plane(s) Representing Water Surface." The centers of buoyancy and flotation will be marked as well.

6. The model can be moved so that the Z=0 plane represents the waterplane. If you'd like to transform the model, check the "Transform Model to Resultant Condition" box. Note that output results (such as VCB) are reported in the coordinate system of the original model orientation.

7. If you want to see Righting Arm data, check the "Compute Righting Arm at..." box, and enter a list of heel angles. Note that you will now also have to enter the VCG. The value of the heel angles must be between -180 to 180 degrees. For example:

0,10,...,180 will compute every 10 degrees from 0 to 180

0,-20,...,-60 will compute every 20 degrees from 0 to -60

-150,-140,...,0,5,...,60 will compute every 10 degrees from -150 to 0, then every 5 degrees from 0 to 60

8. If you want full output for each heel angle, check the "Print Full Output..." box. This may result in a very large report.

9. If you also want output written to a comma-separated-value (CSV) format (suitable for import into Excel, or parsing with another program), check "Also Write Output to CSV File," and enter the path and filename. The format of this file is self-documenting.

10. If you need to add surfaces to those you originally selected, click on "Add Objects," and select them. Hit Enter, and you will return to the dialog.

11. Click on OK. The results will be computed and displayed in a new window. Use the controls at the top of the report to navigate to the various pages in the report. Note that the report can be printed, or saved in either PDF or Excel format.

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). This can sometimes cause Orca3D's solver to have difficulty converging in such a case. To avoid this, enter a different value for the Initial Plane that is closer to the final equilibrium height.