This can be done with a revolve and a trim, but the result must be a closed brep with no trimmed surfaces.
How would you tackle this? (Obviously not like this.)
Surfacing Torpedo 001.gh (21.7 KB)
Just close the BRep once you’ve completed the trim?
If the issue is the BRep being made with a trimmed surface, why must the result not contain any trimmed surfaces?
Surfacing Torpedo 001.gh (23.6 KB)
Thanks for that. If you could post a file that would be even better ) I am not proficient in GH heiroglyphs yet 
The application that this is being imported into does not play well with trimmed surfaces.
edited my post to attach the file.
Many thanks. Yes, this is the striaght ahead way to make make this solid. Unfortunately, no trimmed surfaces allowed )
What application is it going into?
Do not use Patch, Use Loft. Try avoiding trimming, create your surfaces such that no trim exists.
IM trying to avoid a debate about why this is the requirement. I have been asked to provide the model without trimmed surfaces. The person kindly taking it to work with has made this request. If I ask why, he is faced with writing a long explanation or just deciding his time would be better spent on something else and rescind his offer.
Surfacing is tricky with three sided segments, but I face this challenge often and simply feed 3 curves into EDGE SURFACE and out pops a surface with a singularity which I ignore.
As such, this post is a proxy for this general problem regardless of the import restrictions of another app )
Well, if you want to really help you customer I highly recommend learning more about what they are trying to do so you can actually help them.
Digging up a 2 year old “proxy” does not answer the question. Where are they going with the model and what are they trying to do.
As you point out this is the standard NURBS has 4 sides issue that has been brought up so many times. It is one of the reasons robust trims exist in our NURBS. It is the reason Orca3d has tools to deals with this condition in boat hulls shapes.
Throw a singularity or two into the shape if you just need to get it into Maxsurf. Or, follow the recommendation of the MaxSurf community on that old thread. Then model the shape for real with trims in Rhino so the hull can be built or machined.
I get a bit frustrated when the answers and time of helpful experts like @Matt_Harwood get thrown to the side because someone is afraid to ask some questions.
Hmmm not sure what “afraid to ask questions” means.
Thrown to the side is a little strong perhaps and this level of pushback is exactly what I was trying to avoid and why I didnt want to discuss the specifics of this instant case.
As you point out this is the standard NURBS has 4 sides issue that has been brought up so many times
Well I dont doubt it. I started a thread out of the frustrations I had with surfacing and im still struggling. ORCA tools may be a fine product, I dont know. This is not about hulls its about modelling.
I have read the manuals, completed the tutorials, practiced and practiced, watched videos, and am still waiting for definitve answers on how to surface three sided geometry, and aparently, I am not alone.
Perhaps you would be kind enough to link to the answers to that discuss this problem in some depth. I get a little frustrated when answers to common problems like this are not rolled up into a thread that is pinned as prominetly as how to ask a question for example.
Can the application accept meshes (stl) instead?
3 sided surfaces are difficult. Mathematically compressing an edge down to zero is not a great situation for NURBS. And sometimes they cannot be avoided. So, we have a bunch of workarounds to deal with them.
- Trim them out. - https://cadcaminfo.wordpress.com/3d-modelling/surface-modelling/3sided-surfaces/
- Leave the singularity for old products, but use trims to fabricate from.
- find the best place to have a singularity so it does not effect the model. https://www.youtube.com/watch?v=ZfzYdf3zBzA
No, you are not alone. This question has been around since before Rhino and continues to be a condition everyone has to deal with. But understanding where the model is going gives you the freedom to figure out how to workaround the problem.
So back to getting on to answering your question. Where is the model going and what software are they using? Without trims they get singularities. And that can be OK in some situations. But understanding the target geometry capabilities really helps work around these problems.
Can the application accept meshes (stl) instead?
In asccii format yes. I assume that Rhino only converts the portion of the trimmed surface that is visible to a mesh during the conversion process ?
Where is the model going and what software are they using? Without trims they get singularities
Well its being modeled in Rhino, but evaluated in Maxsurf
https://www.rhino3d.com/resources/?id=86
The other problem with a trimmed surface is it cant be merged. Probably not an issue in this instant case, but Im trying to get a handle on the bigger picture here.
This model is not a car body so I assume singularities will work. I need to read those links you kindly provided. Thank you )
So I’m guessing this is for CFD analysis? The vast majority of my work is in aerospace CFD, and making models for that industry, so I have a lot of experience with these types of geometry transfers. So, if you’re just going to use something like Maxsurf for analysis (but not design, because you have Rhino for that), then far and away the better way to accomplish this is to simply convert your NURBS objects to a mesh in Rhino and run the analysis on that mesh. There’s no need to go to extraordinary lengths solving 3 sided problems and such if a mesh will suffice. Indeed the mesh generated by Rhino will reflect the geometry you create - trims and all.
Great. How do you determine what mesh density is sufficient?
Either by eye till you feel it accurately enough represents your geometry, or consult with the software mfg to see if they have any guidelines.
Yes, CFD is becoming great in Rhino:
- Orca3dCFD - https://orca3d.com/modules/orca3d-marine-cfd-simulation/
- Rhino FlowRT - https://www.simerics.com/rhino-flow-rt/
- RhinoCFD - https://www.food4rhino.com/app/rhinocfd
- Swift - https://www.food4rhino.com/app/swift
- Eddy - https://www.food4rhino.com/app/eddy
More discussion on CFD: Does anyone know how to create a Computational Fluid Dynamics CFD modeller with Rh or Gh?
Having fun with the Quadmesher in Rhino 7 WIP for a quad mesh. Still needs to some work in the front tip, but it is another way to get a quadmesh.
Be aware that STL does not have Quads, only disconnected triangular faces. So a more modern file format should be used for transfer of this to any analysis package.
True, many packages want quads, I don’t know in this specific case tho.
I was expecting a solution that extraced isocurves along the length, cut them into four quadrants and lofted along the length with a revolve at the nose and tail. Why would that not work?
It is a lot more work. Normally the UV directions is limiting and hard to make spacing exactly the same. To panel this yourself I would add PanelingTools that allow for more tools to help create quads.