Loft problem! Whats wrong?


Trying to loft these 2 curves for a developable surface, but not luck.
The crvs are arcs and ok so I dont know what I am doing wrong?!
Some help would be appreciated
PerSailLoft.3dm(45.9 KB)

(Menno Deij - van Rijswijk) #2

Ehm, I could loft developable it without any trouble (rhino 5 sr4). Or is this not what you were looking for with the multiple surfaces?

You know that to loft correctly, that you should select the curves at the same end? Otherwise the loft goes wrong.

Lofted.3dm(62.9 KB)


Hi menno and thanks for reply
And yes I can also loft it but as you says the result is not one but several surfaces and that dont work for me.
And I thought that makeing a developable surface, the result should be one unrolleable surface, not a patchwork of a x number surfaces?!
Ofcourse within limits that is!
And I think mine is?
But any ideas to work around this.
I need one surface i can unroll from this curves


Hi Per,

Please take a look at this file: example-loft.3dm(150.8 KB)
When you loft your curves with style “Straight Sections” and unroll the surface the area is 0.45% bigger after unrolling. This is not accurate at all and unacceptable.

I recreated describing circles from your curves and lofted them to a truncated cone (Style Straight Sections) with the seam at uppermost position.
When I trim away the outside parts of the loft with the rail lines, you might notice that there’s a gap between the rails and the trimmed surface edge. Unrolling this surface is within tolerance.

Is this of any help?


Hi Gerard
I cant say?! As you can see there is a double curvature at the sides of the surface (annotation 0 & 1) and thats another thing i been working on to avoid.
I have to have straight side, going to make trimmings of the surface later on
I cant see why it has to bee soo difficult with such a simple loft??
Thanks for the try!

(David Cockey) #6

perolalars, a simple developable surface with straight edges is fundamentally not possible between the entire lengths of your starting curves. This is because of the shapes of your curves and the properties of developable surfaces. A developable surface can be unwrapped flat without any stretching or compression. Along a straight line on a developable surface the normals to the developable surface are all in the same direction.

The middle surface in menno’s file is the largest simple developable surface with straight sides possible for your curves.

The two triangular surfaces in menno’s file are also developable but they have singularities at their apexes. The curvature becomes large near the apexes and theoretically goes to infinity at the apexes. The triangular surfaces will have tangent (G1) continuity, not curvature (G2) continuity, with the middle surface.

Gerard’s surface is developable with simple curvature everywhere. There is no double curvature anywhere. The “U” isolines are straight and with the normal in the same direction. Yes, the edges are curved but that is because the edges cross the straight lines, not because there is double curvature.


Hi David
After some work I understood what was “wrong” in Gerard´s surface.
Menno´s surface is my original.
So how would you do this?
To get a surface that is possible to fabricate in steel sheet (say 3mm thick, the files uploaded are in scale 1:10)
It is possible to move one of the curves in vertical so it aligns with the other (quad to quad that is)

(David Cockey) #8

Moving one of the curves vertically won’t make what you want possible due to the shape of your curves.


  1. Use Gerard’s surface or similar and accept that the edges won’t be
    straight. If the desired straight edges are frames then the frame
    shapes would need to be modified.

  2. If the desired straight edges are frames then try to force the steel
    to fit during construction. The resulting surface probably won’t be

  3. Use Menno’s surface and accept that the surface will have some hard
    spots. Some local forming of the steel may be needed.

You might start a discussion about what to do on Metal Boatbuilding forum on BoatDesignNet There are a number of participants there who are very knowledgeable about metal fabrication.

(David Cockey) #9

A method to create a single developable surface when the ends of the curves don’t align properly is to extend the curve(s), generate a developable surface using the extended curves, and then trimming the surface to the desired size… This is what Gerard did but extending perolalar’s curves which are arcs into circles and then generating a surface from the curves.

An alternative is to extend the curve(s) a sufficient amount to create a sufficiently large developable surface, and then generate a developable surface from the curves. Attached is a file where I did this starting with perolalar’s curves from the first post. I trimmed the developed surface by creating straight lines between the ends of the curves. I then used Pull to obtain the curves on the surface which are closest to the original straight lines. The surface was trimmed using the “pulled” curves.
SailLoftDC01.3dm(200.8 KB)

(Scott Davidson) #10

in these hard cases , I like to use the DevSrf command.

It allows the editing of the rail curves to see what is developable. in this case I was able to use extend curve on the shallow curve to create a full surface. I used extend until Devsrf showed me it was complete. Then I trim the surface at the edges.

Is this an acceptable surface?

devsrf.3dm(90.9 KB)

(David Cockey) #11

I also use prefer DevSrf to Loft for creating developable surfaces, and extend the curves until the surface is large enough. Scott’s method is the same as I used in the link I posted in my previous post.

Note though that for some curves different shapes of the extensions may need to be tried. Also, for some pairs of curves a developable surface may not be possible, particularly where there is reverse curvature…


Ahh ok, thanks both Scott and David
It seams that it is close to what I want…
Have not been using devsrf so I have to get my feet wet there!
Thanks a lot!