Problem and question about hydrostatics

I have been having good luck with the hydrostatics command with files like play-cat2.3dm (106.6 KB)
The hydrostatics command returns what I expect when I select the hull polysurface.
But this file play-cat4-211808.3dm (59.2 KB)
where the polysurface was made the same way does not return volume displacement and I can’t figure out why. Help? water line should be 12" the same in both files.
I have another question kind of related to boat design. I’d like to find the center of gravity of the hull but this would normally involve the weights of the hull planking which would be an attribute of a solid. Is it possible to do this kind of thing in Rhino? Maybe this should be a separate thread.

Should be a separate thread.

The usual cause of hydrostatics not returning the volume, etc is naked edges below the selected waterline. Hydrostatics needs a “watertight” surface below the waterline. Naked edges are where there is an exposed edge which is not properly joined to another edge. Use ShowEdges with Naked edges selected to check for naked edges.

Your second file has a two pairs of naked edges below the waterline. Fix the naked edges and Hydrostatics should work.

Okay. I used ShowEdges and see the result. Not sure how to fix the naked edges. I thought I built this up the same way I did previous 3 designs.

Simplest way to eliminate the naked edges in your model is to rebuild the stem and bottom surfaces using Loft between the side panel surface edges.

Cause of the naked edges in your model are control points which don’t quite coincide. Do you use OSnaps when placing points?

Tip - put your curves and surface on different layers. That will make it easy to see what is a curve and what is a surface edge, and also to turn the curves or surfaces off and on.

Yes, I use OSnaps most of the time. And I’m slowly learning how to use layers. Have not used Loft. Will have to look into it. I had been building surfaces and using Join to make a polysurface. When I used What on play-cat4 it said it was a valid polysurface but didn’t give me any information about naked edges. So I didn’t know what to do. Been using Rhino for about a year. Lots left to learn.

Ugh!! when I tried using Loft to build the bottom it gave me a mess. Looks like I need to start over.

Obliviously I haven’t a clue about using Loft. It does not do what I think it should. I have better luck creating a surface from edge curves. This is yet another case of the help files not being helpful. No matter how or what order I select the curves, Loft does not give me a correct surface.

Use loft to create each surface individually using only two opposite edge curves. Start the Loft command, the select the two opposite edges near the same end. For the bottom use Straight sections and also use Do not simplify.

A common problem when using Loft is to select curves near opposite ends. Loft assumes you want the ends closest to where you selected the curves to be on the same edge of the resulting surface.

A valid polysurface can have naked edges. You need to check for naked edges separately.

Which command do you use?

Can you provide an example of the curves you have been using and what you get with Loft?

Okay. I think I got it to work.
play-cat5-success.3dm (61.8 KB)
On this file, I kept having issues with the transom and stem when using Loft and I think it was the thing about where the curves were selected. This is one of those commands that requires some “playing” with to get the hang of. What was very confusing was that sometimes it worked on the transom and sometimes not and I guess it just depended on where I happened to select the curves.
One last question. After I have made all the individual surfaces is the best way to join them to select them and use Edit->Join ? Seems to work with hydrostatics. I did check with ShowEdges afterwards and it only showed the top edges which is correct.
Thanks for all your help and patience.

Join is the command to use. You can start Join and then select two surfaces/polysurfaces with adjoining edges. Repeat as needed.

Or you can select multiple surfaces/polysurfaces and then start the Join command. Rhino will join everything which it can.