Hydrostatics & Stability

[John Meyer]

Hey guys!

I want to calculate the hydrostatic and stability of my sailing boat. The problem is that always appears the following error message:

 “One or more hydrostatics computations failed. An attempt will be made to display the hydrostatics results.”

The program then only shows the wetted area but not displacement or other hydrostatic results…

Does anybody know what the mistake is??

Thanks in advance!
JM

[Admin]

John:

I am assuming that you are running the hydrostatics using Weight as an input (as opposed to Sinkage). When Orca3D does a "free float" calculation to find the equilibrium flotation plane, it must start with a "first guess" waterplane, and then iterate from there.

The default "first guess" is the mid-height of the surfaces that you have selected for the computation. If your sailboat model includes a keel, this can lead to the first guess being somewhere in the keel. This results in the calculation of a very small waterplane, which in turn throws off the iteration process (it basically assumes that the model has to sink a lot to gain the required volume, leading to the model sinking).

To correct this, check the box that says "Override initial flotation plane...," and enter the height of a flotation plane that is close to what you expect (or is at least somewhere in the hull).

Also, note that this is discussed in the Support area of our web site, under Top Issues: http://www.orca3d.com/support/support_topissues.htm

If you have additional questions, please let us know.

Please note that the next release has greatly improved the iteration process, and will most often be able to converge without the user having to override the initial plane with a better starting point.

[John Meyer]

Thanks a lot for the fast reply!

When I am running the hydrostatics using "sinkage" the program doesn't show any error message but there is still only the wetted surface calculated. When the program shows the Waterplane and the Center of Flotation (CF), CF lies outside the volume of the canoe body! Is it important if I put the centerline of the boat on the X- or Y-Axis? And can I put the point of origin on waterline at the bow for example?

I would be very thankful for help!

JM

[Admin]

Jeff:

Yes, the model centerline must be at Y=0. From the Introduction section of the Help chapter on Hydrostatics & Stability:

Note: It is assumed that the model's centerplane is at the plane where the transverse coordinate = 0 (for example, the Y=0 plane). Incorrect results will be reported otherwise.

In the current version, X is the longitudinal coordinate, Y is transverse, and Z is vertical. The next release will let the user specify which axis corresponds to which direction.

The longitudinal origin can be anywhere you'd like it to be (bow, stern, fwd end of waterline, etc.), as long as you specify locations such as LCG from the origin.

If this doesn't solve the problem, please feel free to send your model to us at support@orca3d.com. All customer data is treated as proprietary, and deleted as soon as the support issue is resolved.

Bruce

[John Meyer]

Hello Bruce!

Thanks a lot for your help! Now it is working! If I select both "halves" of the canoe body and run the calculations then it doesn't work. Only if I select one "half" of the canoe body and activate "Mirror about Centerplane" it is working properly.

JM

[Admin]

Hi Jeff:

From your last description, I would guess that there is a problem with the normal direction on one of the halves of your hull. If you select a surface, and then go to Analyze menu and select Direction, you'll see arrows on the surface, showing the outward normal direction. These arrows should point into the water (if that surface was in the water; i.e. for a deck, they should point upwards). To flip the direction, you can either use Rhino's "Flip" command, or the Flip option in the Analyze/Direction command.

Again, quoting from the Hydrostatics chapter of the manual, under "The Model" (I'm quoting this so that you can read more, not to be obnoxious!):
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;

If you scroll down in the chapter, you'll see much more information. What happens is that when the normal direction points inwards, the volume is negative. So if one of the halves of your hull is correct, and one is incorrect, the volumes will cancel each other out.

Also, in the support section of our website, in the FAQ's, there are a couple of FAQ's under Hydrostatics and Stability that address this issue.

Let me know if you have any further questions.