Bending a boat side, taken from a cardboard model



I’m trying to model in SU a cardboard model of a trawler.

I can’t get the side to bend around the hull. It develops one weird spike, doesn’t fit the hull well, and seems just to have been projected ‘sideways’ along the green axis, while curved along the red,

I traced the hull side from the PDF of the cardboard model, and it should, when bent, lie along the hull. I copied the edge of the base of the hull to make the curved line for Shape Bender to follow

When I run shape bender, I select the side to be bent, then start Shape Bender and select the straight line giving the length and direction of the line to be bent to the curve, then the curve (a curve made up of bezier curves welded into one SU Curve).
I get this:

I don’t understand what’s happening here.

Wrong axes in my flat side component? I’ve set them with the red axis along the side, the green axis along the face, and the blue axis normal to the side of the ‘flat’ cardboard side. I hoped this would not just bend the side around a vertical axis, but along the green of the flat side.

Is there some other way? Rotate everything normal to the flat side perhaps?

Model itself is here.

After accepting the offered bend, I get this result after mirroring the side to view it better:

Or should I be using Curviloft instead, and project the texture of the side onto it?

Advice welcomed

PS. Tried it again with the hull tilted and the side vertical, but even after scaling the result to width and length, it’s a poor fit.


John it’s not altogether clear what you want to achieve. Your failure is somewhat ambiguous, it would help to see what your setup was before using shapebender.

Curviloft is generally more suited to this sort of thing. But without seeing what it is you want it is hard to be specific.
Also, it would appear to be the materials you have used that are making the file size so large for relatively simple geometry. Perhaps you would consider adding them later.


Imagine the side as a flat piece of cardboard (it is just that), which I’m trying to curve to fit the hull bulkheads.

After running Shape Bender, but before scaling the result in width and length, the side ended up much too short, and not curved deeply enough.


Trying to bend the flat surface to the frames of the boat seems like the hard way to me. And Shape Bender isn’t really appropriate in this case, anyway. Each frame has a curve and there’s the curve along the run of the hull. Shape Bender can’t do both of those curves.

I think a more appropriate way to do this is to use Curviloft as Box suggested. This will create the developed surface. If you need a flat version of it, you can flatten a copy of the hull side using Jim’s Unfold or the Flattery extension.


Thanks, both. I know Shape Bender won’t bend in two directions, but hoped it would get close enough, ignoring the slight curve in some of the bulkheads.

You’ve both said what I suspected - Curviloft is a better way to go.

I’ll try that.


John it may also be that your shape is not a “developable surface”, which means one that is flat (zero curvature) in one direction at every point. Unless a surface is developable it can not be formed by bending a non-elastic planar shape (and conversely, a non-developable surface can’t be accurately flattened into a planar shape). Curviloft does 3D interpolation without concern for developable.

As an aside, boat designs intended to be made from a small number of plywood pieces is why some of them have such a slab-sided look. Traditional boats made from a lot of narrow planks avoid this and solid wood lets the builder sand the sides to “fair” curves in all directions.


This why we are asking what you actually want to do?
Do you want to make a nice complex curve hull or do you want to make something out of card.
Normally you would use the ‘rib’ profiles right down to the keel with curviloft to create one half of the hull.
Here is a quick version with what you already have.


Because CurviLoft will make a 3d surface from any curved profiles/paths then you cannot ‘unfold’ it into a single flat sheet.
You need the various ‘profiles’ to be straight edges, that will then allow the side to be unfolded into a flat sheet, using one of several tools…


In order for a single sheet to fit the curves of the frames and the waterlines, there will need to be relief cuts made in the sheet. This would be very evident if you try this with real paper or with plywood at full size. As Steve mentioned, the fewer the planks, the more boxy the hull.


I just want the SU model to resemble the trawler, to put it into a larger model railway layout involving a small harbour, and this one ship. My friend has actually built the trawler from card, and wants to see it in the SU model of the small 4ft x 2ft layout.

In theory, the flat cardboard side can’t be bent round the complex curved bulkheads, but in practice, it has just enough flexibility to fit well enough.

I’ll use Curviloft as Box demonstrated, and that should work fine.

I might still have some difficulty projecting the image, but will see how I get on.


After much experiment, and some fiddling around with the bulkhead profiles, I have succeeded in modelling the hull sides and internal structure, using a mixture of Curviloft and Joint PushPull.

Looks like this:

Hull.skp (810.5 KB)

It’s really a bit too ‘smooth’ looking for cardboard, but for our purposes it will do well enough.

I could do it again as TIG suggests, replacing the curved bulkhead edges by straight lines, but I don’t think I need it to look too straight-edged.

Next to try to put the deck on! But not tonight,


Now how do I project the texture of the side piece onto the hull side? I tried Project Texture, but when it has applied, I can’t adjust its position. I’d like to start a new thread for this, but can’t find how, though I know I’ve done it before - it’s a right click somewhere not obvious.

AH - got it. Click on the top right ‘time since reply’ text.

Applying texture to curved surface

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