Software for generating foil shapes

I had to build a pair of rudders for my 24' catamaran; I found a few posts out there with different ways to manufacture the foils, including plotting NACA curves, making templates or guides for router, or just going the CNC way.

I was lucky to find a fellow boat builder locally with free access to a CNC machine who agreed to help. But I had to come up with a complete CAD file of the part.

I am no CAD expert, so rather than learning how to work with the different CAD tools, I found it easier to just write a "foil generator" of my own. Which is what I did. The plan worked, and I have two beautiful plywood cores now, waiting to be laminated in fiberglass.

The program that I wrote (in Python) was useful for me, so I'm posting a link here in case other builders find it useful too. It's free software, use and modify as you wish. If you have feedback or suggestions, I might be able to add features (also see questions below).

This is the link: https://github.com/zlieber/genfoil

Just click on "genfoil.py", then "Raw" and save the file.

What it does:

- Plots a curve if you want to make your own template
- Creates a 3d shape for template that you can 3d-print and use for checking your shape
- Creates a 3d shape for template that you can 3d-print and use as a router guide
- Creates a 3d shape that you can use for CNC manufacturing of your complete airfoil

As I'm not an experienced boat builder (and much less boat designer), I'm not sure what other people use, and whether this will be useful to anyone. Some questions I had:

- What are some popular planforms to use? How are they defined exactly? I configured a few for example, they are probably not very useful. I did find some articles on how to design planforms, but not much in the way of specific examples.

- Same questions for tapering - in my case I just made thickness proportional to width, so that my NACA number remains the same along the length of the rudder. Not sure if that is the right decision or not.

- My script generartes an STL "mesh" of the part, which is another way of saying it just outputs a bunch of triangles that cover the foil surface. Again, not sure if there is a better way to do this, this is just the one I found simplest.

Finally, here are some screen shots and current work in progress, as well as the STL file that I generated.

Thanks for reading, have fun and be safe on the water!