Hydrostatics differences between rhino and Hydromax

[nemo]

Hi
I'm designing a Mini650, now I'm into the stability checks.. I checked the displacement with Rhino and it says it's 930 Kg, while Hydromax (after importing the rhino file as .igs) says it's 830kg!
Since it's a big difference for such a small boat, which method should I trust?
Stefano

[Tim B]

Very strange, check that the water-line is the same. I trust rhino as it agrees with my code for a given hull-shape and my code is derived from first principles. I doubt it is any problem with the iges format, but check a few offsets to be sure. Just repeat the process very carefully. you may have taken the keel into account in one and not the other. The volume is usually defined as the volume within the canoe hull. so check that the method is the same in each case. If you want me to have a look at, drop me an e-mail at Tim@MarineDesign.tk and I'll send you an adress which will handle attachments.

Hope this helps,

Tim B.

[nemo]

Great, now the displacements match!
I found that Maxsurf doesn't like so much Rhino trimmed surfaces.. I had to rebuild the hull with untrimmed surfaces and then export again in .igs format.
Now I'd say the final displacement of my mini is 965 kg, which includes keel and bulb.
I took a quick look at your website, but I still didn't have enough time to read more about that spanish code, sounds interesting and free.
I think it shouldn't be so difficult to write a hydrostatics code that could work with rhino, maths behind those softwares is kind of easy, I guess the most diffucult thing is reading offsets from the drawing.

[Dutch Peter]

Can the fault be the importing of the file or the difference in "moulded" or "outside skin", if you know what I mean.

[Dutch Peter]

You beat me to it Nemo!!

[Raggi_Thor]

Hydrostatics in Rhino: Have you tried RhinoMarine?

[Tim B]

Actually, taking the surface data out of rhino is easy if you don't mind the slight appoximation of using an unstructured surface mesh. This is how you do it... first of all select the surfaces you want to export, then select EXPORT from the file menu. choose to save as raw triangles *.raw ; then play with the controls until you are happy with the mesh. The defaults are a pretty good starting place.

This will give you a text file with a format as follows:

<object name>
x1 y1 z1 x2 y2 z2 x3 y3 z3
....

so all you need to do is integrate the area between that triangle and the waterline. Do this for each triangle, then you have a displacement volume. Obviously you can extend this method.

Cheers,

Tim B.

[terhohalme]

I have used Phaser (now Rhinomarine) and Rhino for several years succesfully. It is a must!

[MikeJohns]

Originally Posted by terhohalme

I have used Phaser (now Rhinomarine) and Rhino for several years succesfully. It is a must!

I woult agree most strongly.

If you are involved in marine design and using Rhino then Rhino Marine is an excellent well priced utility.

[Navaldesign]

Usually Hydromax results are correct. However, check that sections are correctly formed and, if you use trimmed surfaces, make sure you have enabled the relative button.

[Andrew Mason]

I found that Maxsurf doesn't like so much Rhino trimmed surfaces.. I had to rebuild the hull with untrimmed surfaces and then export again in .igs format.

Maxsurf doesn't read Rhino trimmed surfaces. It will read them as untrimmed surfaces and you will need to retrim in Maxsurf.

Hydromax requires that surfaces form an unambiguous external shape and often the lack of precision of the edge matching of Rhino surfaces (gaps, overlaps) prevents it from forming correct sections through the hull.

I think it shouldn't be so difficult to write a hydrostatics code that could work with rhino, maths behind those softwares is kind of easy, I guess the most diffucult thing is reading offsets from the drawing.

ROTFL!!!

[MikeJohns]

Originally Posted by Andrew Mason

Hydromax requires that surfaces form an unambiguous external shape and often the lack of precision of the edge matching of Rhino surfaces (gaps, overlaps) prevents it from forming correct sections through the hull.

Andrew
Maxsurf is a wonderful program for sure, but if you are getting gaps and overlaps in Rhino then you haven't created your surfaces properly! Rhino marine won't function with a hull so defined either.

The big problem is Maxsurf not accepting trimmed surfaces, designers who use Rhino haveg to try and re-define their hull in rhino without any surface trimming. That is when we end up with gaps and overlaps.

[Andrew Mason]

Mike

Designers do not need to define their hulls in Rhino without surface trimming. Maxsurf reads all of the surfaces from Rhino correctly, it simply discards the trimming information.

As a result the designer will need to specify the surface trimming again in Maxsurf, however if the trimming has originally been defined with surface-surface intersections this takes very little time.

[Tim B]

Then by definition, Andrew, that is incorrect.

A properly imported trimmed surface should be the same shape as the origenal trimmed surface.

Tim B.

[Navaldesign]

Hi Tim B

as Andrew has already mentioned in a previous note, dynamic changes in the trimmed surfaces within Maxsurf require information that the export facility of Rhino or other similar programs simply can't supply. This means that even if Max imported the trimming information and could actually represent on screen
the write contour, as soon as you changed something on your surface, the trimming information, not being a dynamic one, would be wrong. Actually as you now trimming a surface only means "hide" a portion of the surface, without changing the surface itself. So the solution is simple: trim surfaces with surfaces, and, once in Max, retrim and make the trimming surfs invisible.
Even this way, however, the link between trimmed and trimming surfaces in MAax, sometimes gets lost.

Navaldesign
www.navaldesign.it