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Sunday, 4 December 2016

Spinning the Spinnaker

I wonder if the meaning of the expression "it's taking shape" comes from sail making?

After a number of dead ends, I think I am making way. I now have a 3D model of an A2 spinnaker sized to the model of my Tiki 21. I can mark out the seams and make a 2D surface development of the panels for cutting. I've tested this by cutting out paper panels to make a model.

Here are a few notes for those that are interested on the journey and lessons learnt.

Sail Design

The first thing I need is a good sail shape. Then I need to realise the design in cloth. Both tasks are difficult. I need to know what shape is right, and then how to make this from flat cloth.

I don't know what shape is exactly right. Clearly I can look at lots of pictures and guess, but I don't want to spend too much time and money on a guess. My approach is to copy an existing shape for the design and use a computer to build a 3D model and develop the necessary 2D panel shapes from the model.

There's plenty of detail on all this in Brian Hancock's excellent blog and website.

Sail Shape

To obtain a good shape, I've been a bit naughty. If you search the web you can find 3D visualisations of sail designs. You can't download these, you can just "look". But that's enough for me. If I look from three orthogonal axes and take screenshots, I can reconstruct a 3D model. I did this in Blender 3D.

Orthogonal Screenshots as a template design
Tiki 21 with an A2  Tiki 21 with an A2

The sail design shown has the following dimensions: Luff: 6.2m, Leach 5.8m, Foot 4.6m Total area - 21.5 sq m. 

With my sail shape defined, I need the panel layouts.


1. Sailcut CAD

Sailcut CAD  is a program with a long history and allows a sail design to be specified in terms of a set of parameters, e.g. foot curve, luff curve, twist, mid girth etc. It takes the design and creates 2D panels with seams. These are cut from cloth and the sail assembled. Sailcut allows head and fore sail designs in various panel layouts, including radial designs.

I took my desired shape and made a set of measurements according to the parameters required in Sailcut. All looked good. I exported the 3D design from Sailcut to Autocad DXF and imported into Blender 3D. In this way I can compare my desired design with the generated design from Sailcut.

After a number of iterations, I found that Sailcut simply will not give me the shape I want. It is expecting the luff to be bent on to a forestay with a small curve in it due to the wind load. It doesn't expect a flying sail, fixed at the head and tack. No matter how hard I try I cannot make it work. Time to rethink.

2. Blender 3D

Blender 3D is an amazing tool. It's also free! It's primary purpose is to develop 3D games. It supports 3D modelling, photo realistic rendering, animation, physics models and plenty more. The image below was create using blender.

It is so good that you can use it for many 3D CAD applications. Since I have a 3D model of my desired shape, can I use Blender to create the 2D panels? If I can, I don't need Sailcut.

A bit of searching on the internet and I found an add-on for Blender called Sailflow. This add-on allows sail shape development using parameters a bit like Sailcut, and panel development. But I don't need the shape development, as I already have a design. I tried this add-on and although it looks promising, the panel development doesn't quite work. I created 2D panels, but it makes a mess of some panels. It's just not working for me.

3. Dynamics of Cloth

It is clear that a smooth 3D surfaces can be approximated with a mesh. The finer the mesh, the more accurate. If the mesh is used to define flat panels, cut out in cloth and assembled, the stretch in the cloth means that the sail, when flown, will take up the smooth shape. To make this work exactly you need the panels to be the right size according to the expected wind strength and cloth stretch characteristics. Working that out is too difficult. After all, it's a Tiki 21 and not an America's Cup trimaran.

If I assume that for the panel sizes I want, the stretch etc. will work out, then I can disregard the dynamics and just flatten out my 3D panels into 2 dimensions. It turns out that Blender has the ability to do this using a feature called UV unwarp. I tried this out and compared the output with a Sailflow panel that did work. They are the same. So I can ignore the add-on scripts and just use Blender's built in tools, but there are 25 panels and it will take time to do. Surely there's a short cut?

4. Paper Model

More searching the internet. This time for a Blender add-on for surface development. I found Paper Model. This add-on tool allows a mesh to be cut with seams and the panels exported to an SVG file.

It's purpose is to make a layout for assembly into a paper model. It is very close to a sail panel layout, just a lot smaller. I tried it and here's the result


I'm making steady progress. It's fun and I'm learning a lot. The following considerations are still to come:
  • How smooth a model (how fine a mesh) should I use?
  • Double check the tack sheeting angle alignment with sail's radial panels.
  • What seam layout do I need?
  • What detail do I need at the seam junctions between 3 or 4 panels?
  • What is the best order of assembly?
  • How to best mark out the panels on materials?
  • Do I need to make a test sail to gain experience?

Watch this space for more ramblings.

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