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Friday, 6 January 2017

Part Spun Fun With Sail Panels

Here's an update on the progress since the last post on designing the spinnaker.

Creating appropriate 2D panels

After some thinking I realised the way the 2D panels are created is important. The model I initially created in Blender was a "perfect" smooth curved surface (a nurb surface). This was based on a real sail design the I borrowed! and scaled a little to the boat.
Tiki 21 with an A2

To make 2D panels, I first needed to create a mesh representation from the perfect surface. But it occurred to me that the type of mesh was important. Each panel in the sail should be something which can be folded out flat without any stress in the panel. This is not the case if the initial mesh shown below is used.
The problem is the software simply cannot make a flat 2D panel using the mesh made of the back grid cut along the red seams. It can only approximate this. i.e. in the resulting panels the mesh edges lengths will not be the same when flattened as they were the curved model. i.e. the panel must be stretched a little to make it flat. This results in errors in the panel 2D shapes.

To solve the problem, I first converted the perfect sail into a near perfect fine mesh. Then I cut the sail into the panels. I then took each panel and deleted the internal mesh leaving only the perimeter vertexes for a panel as show below.

It should be understood that the edges of the panel (hole) are not straight, so the cut out is not flat (see below). I then created a new mesh of triangles using the perimeter vertices. This gives a mesh that can be laid out flat without any stress. i.e. it could be made from paper panels.

This might seem a bit complicated or over the top. Perhaps it is? However, I arrived here after lots of experimentation and only this way can I create 2D panels that when sewn in cloth and flown from the boat, should, in theory, adopt a shape close to the model shape!

Here's a single panel that can be flattened out, shown in its original position within the sail.

So how to mark out 3.5 meter long panels from the computer models ?

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