Design & Analysis Tools for the Marine Industry by Proteus Engineering

Hydrostatics & Stability FAQ

1. Do I need to have stations or a GF file to compute hydrostatics?
2. How can I calculate a righting arm curve?
3. I’ve installed RhinoMarine, but when I run Rhino I don’t see the RhinoMarine menu.
4. Can I run RhinoMarine with Rhino version 1 or 2?
5. With a very detailed model, computations seem to take a long time and the results don't seem correct.
6. I typed in a list of waterplanes, but only one is computed.
7. When I run Hydrostatics, I don't get Cp, Cx, or sectional area curve data.
8. I use Netscape for my browser, and the HTML file is not opening.
9. I’m trying to use HTML output, but I get the error “Could not find the style sheet”.
10. My displacement seems too low.
11. Can I change the colors in the HTML output?
12. My displacement is low or negative.
13. I hit the Calculate button, but nothing seems to be happening.
14. The Hydrostatics dialog box does not open (or takes a very long time to open) when I select Hydrostatics from the RhinoMarine menu and select the surfaces to compute the hydrostatics for.
15. Sometimes when I run Hydrostatics, the sections are not computed even though I have defined a list of section locations, so I don't get Cp, Cx, or sectional area curve data.
16. My sectional area curve has a spike in it.
17. When I look at the sectional area data from RhinoMarine, the data for some sections seems to be twice the correct value. Also, the max section coefficient is greater than 1.
18. When performing a hydrostatics calculation using displacement and CG as input (a free float calculation), why does RhinoMarine indicate that the only stable equilibrium condition is when my model is upside down (180 degrees heel) when I know that it is stable in the upright condition?

1. RhinoMarine Hydrostatics uses the analysis mesh to compute the hydrostatics, not stations, so you don’t need to have stations in the model. This makes the process of computing the hydrostatics easier, and also means that features on the model can’t be missed like they can when stations are used. For example, if you have a centerline keel that is cut up as it goes aft, and you don’t have stations defined very closely on either side of the discontinuity, a sectional integration will give incorrect answers, whereas the analysis mesh will correctly include the model’s features. [back to top]
2. You may specify a list of heel angles, separated by commas, after you have defined the initial flotation plane(s). The flotation plane(s) can be a list of waterplane heights, or can be defined by one of the other methods (e.g. displacement/CG, minimum, increment, and number of planes). [back to top]
3. For some reason, the plug-in has not been registered. You can do this manually in Rhino (it only needs to be done once). When in Rhino, type the command pluginmanager, click on Load, and browse to the directory where you installed the plug-in (by default, C:\Program Files\Proteus Engineering\RhinoMarine4). Select Rhinomarine.rhp,   click on Open, then Close the pluginmanager dialog box. [back to top]
4. RhinoMarine will only run with Rhinoceros version 3 or 4. Your version of RhinoMarine must match your version of Rhino. [back to top]
5. Only the surfaces that would potentially be wet (i.e. add to the buoyancy of the model) at the range of drafts, displacements, and heel angles that you have specified should be selected. No interior surfaces should be selected. If you are just computing upright hydrostatics, there is no need to select the surfaces that model the deck and superstructure.  [back to top]
6. It may be that you have typed in a list of waterline heights that was not recognized. The list of values should be separated by commas (e.g. 3,4,5,6). [back to top]
7. In order for the sectional area curve, Cp, and Cx to be computed, RhinoMarine must compute stations in the model (these are the only values that need stations; all other calculations are carried out on the analysis mesh). In the "Station Locations" section of the Calculate tab of the Hydrostatics dialog, click on Add to define a range of stations. Note that you must define enough stations to properly describe the hull. See also FAQ's  #16 and #17. [back to top]
8. RhinoMarine Hydrostatics first uses file associations to try to display the html file. This means that whatever program is registered on your computer to open a file with the “.htm” or “.html” file extension will be started, if you have specified that the html file ends with one of those extensions (the default file does). If you have specified a different file extension, RhinoMarine then tries to open Microsoft Internet Explorer with the file. If IE is not installed, the process will fail. The solution is to use a file extension of either “.htm” or “.html”. [back to top]
9. The style sheet converts an XML data set into an HTML file suitable for displaying in your web browser. The XML data set contains all of the raw data, and the style sheet does the formatting. So the program must be able to find it. The default style sheet is located in the same directory as the plug-in, and is called proteushydrostatics.xsl. Make sure that this file, or another style sheet, is properly specified in the Calculate tab of the dialog box. Note that you can create your own style sheet to format the data. It would be best to begin by making a copy of the default style sheet, and editing it to your tastes. [back to top]
10.  Since RhinoMarine uses the analysis mesh to describe the surface for computation, the density of the mesh must be set to a reasonable level. The easiest way to see this is to click on Adjust Meshes in the Setup tab of the Hydrostatics dialog. You will see the mesh displayed, and will be given the opportunity to increase or decrease the number of polygons. Experiment with different settings, and compare your results. You will quickly get a feel for how dense the mesh needs to be to give good results. [back to top]
11. The XSL style sheet is used to format the HTML output. The style sheet can be edited to completely change the format of the HTML file. However, if you would just like to change some colors, and are not familiar with style sheets, it is still easy to make the changes. The "Output" chapter of the on-line manual describes this simple process. Remember to save the default style sheet under a different name so it isn't lost. [back to top]
12. Surfaces have the concept of an "inside" and an "outside" that is defined by the Normal Direction. This can be displayed in Rhino by selecting "Analyze/Direction". The Normal Direction should be pointing into the water. A surface with an incorrect Normal Direction will have negative displacement. This will be obvious with a single surface model, but its effect in a multi-surface model may not be obvious, and will lead to a low (and incorrect) displacement. If you find that the Normal Direction is incorrect, use Rhino's FlipNormal function to correct it, then in the Hydrostatics dialog Setup tab select Adjust Meshes. Adjust the mesh to a reasonable level, click on Preview, then Apply, then Close. This is necessary because Rhino does not recompute the analysis mesh when you flip a normal. See the "Input Dialogs" chapter of the on-line manual for more details. [back to top]
13. The calculations are being performed, but you haven't defined where the output should go. Make sure that either "Excel Report" or "HTML Report" are checked on the Calculate tab of the dialog box, and that you have specified the appropriate file names. [back to top]
14. RhinoMarine Hydrostatics calculates hydrostatics properties of the selected surfaces/polysurfaces from the computed analysis meshes for those items. These analysis meshes are generated by Rhino using an internal meshing algorithm based on user-defined meshing parameters. After the user selects surfaces/polysurfaces for hydrostatics calculations, Rhino attempts to generate analysis meshes for those selected items which don’t already have an analysis mesh associated with them. It does this using default values for the meshing parameters. (Once the Hydrostatics dialog is open, users can adjust the meshing parameters by clicking the Adjust Meshes button in the Setup tab.) The Hydrostatics dialog does not open until the generation of analysis meshes is complete. Usually this is a fairly quick process; however, there are certain situations that can slow the mesh generation process down considerably. The most common situation is that the user has joined all or most of the surfaces in the model into a single polysurface. While this might be practical for some applications, it causes the Rhino meshing algorithm to slow down because not only do the meshes for each surface have to be computed, the algorithm must also ensure that the individual surface meshes match up precisely along adjoining edges between surfaces. RhinoMarine does not require that surfaces have precisely aligned analysis meshes between adjoining surfaces, although significant gaps and overlaps should be avoided to ensure accurate results. Therefore, if it takes a long time for the Hydrostatics dialog to open, try to explode the polysurface(s) into individual surfaces and retry the calculation. This should significantly improve the meshing speed. Another thing to consider is to avoid selecting surfaces in the model that do not impact the hydrostatics, i.e. those that are not “wet”. This both avoids unnecessary meshing and also ensures that internal surfaces that do not affect the hydrostatics properties of the model are not included. Also note that once Rhino has computed an analysis mesh for a given surface/polysurface, that mesh is saved when the Rhino model is saved. Therefore, the surface will not have to be re-meshed the next time the model is opened,  reducing the time to open the Hydrostatics dialog box. [back to top]
15. This behavior occurs in versions 2.0.7 or earlier, only when the user starts the RhinoMarine Hydrostatics plug-in (by selecting Proteus/Hydrostatics from the main menu) and then selects the surfaces to analyze in response to the command prompt. Preselecting the surfaces to be analyzed before starting the plug-in avoids this problem. This has been corrected in version 2.0.8. [back to top]
16. When RhinoMarine computes stations, it uses Rhino's section command. If you have defined a section that coincides exactly with the boundary of two surfaces (for example, where the transom meets the hull), a section is computed on both surfaces. This doubles the section area at that station, which shows up as a spike in the sectional area curve. The solution is to move the section very slightly forward or aft (even 0.1mm should do it), so that it no longer is located right at the boundary of the two surfaces.  [back to top]
17. One cause for this seemingly erratic behavior is the absolute tolerance setting in Rhino (File/Properties/Units). This is typically set to 0.01 units. But if it set to a value close to the section spacing (for example, your units are set to meters, the absolute tolerance is set to 0.1 units, and your section spacing is 0.1 meters), then RhinoMarine and Rhino cannot distinguish between the longitudinal values of two successive stations, and they will be seen as being at the same location. Therefore, the sectional area of the two stations will be added together.

    Incorrect sectional area curves can also result from the following situations:

    -specifying in the Setup tab that the model is Half, when in fact you have a full model (or vice versa);

    -including surfaces other than the wetted surface in your selection; RhinoMarine will cut curves at each station through all of the surfaces that are selected. For example, if your model includes interior surfaces, they should not be selected;

    -if your surface has a planar end (for example the aft end of a barge) and you cut a station exactly in that plane, Rhino will create two curves; one in the planar end surface, and one in the side and bottom surfaces. This will lead to a doubling of the sectional area at this location. Instead, cut a section just a very small distance from the planar end (for example, 0.001 units forward of the transom). [back to top]

18. The RhinoMarine hydrostatics component uses an iterative solver to obtain a stable equilibrium condition in which vessel displacement = vessel weight and the gravity force vector acts through the center of buoyancy.  For many marine structures there are at least two equilibrium conditions, one with the vessel floating upright and another with the vessel floating upside down (inverted equilibrium). The hydrostatics solver will report whichever one it finds first. If the model has no stable equilibrium in the upright case (such as when the VCG is very high), the solver will attempt to find the inverted equilibrium condition. Sometimes the solver will find the inverted equilibrium condition even when an upright equilibrium does exist. This can happen if the solver encounters an unstable configuration during any of its iterations. For example if the initial waterline height specified by the user results in an unstable condition, the solver will likely revert to the inverted equilibrium solution. If you are reasonably certain that your model has an upright equilibrium condition and RhinoMarine is reporting only the inverted equilibrium, try a different initial waterline height, preferably one that you know results in a stable condition and one that is reasonably close to the expected solution. [back to top]