Sail material
Hi Ron,
When i built my Bearospace Emma, I used cotton bought from Nylet which works well as far as I can tell.
Aero-Naut sailing boat kits come with something which is very light. In most of the BOMs, it's described as sail material, but in one BOM, it was called spinnaker material. I'm assuming that this means material for full-size spinnakers; I can't really see how spinnakers can work on models. Having said that, I've seen (on YouTube, IIRC) a guy and his wife in an electric dinghy going out with the model and physically making alterations to the spinnaker's setup midsail. I have seen clever ways of using them on models, but I'm pretty sure it would be too complicated for me to attempt.
Back to Aero-Naut, they sell sailcloth on their website in various colours, but I don't know if it's the same stuff. Worth a look, perhaps.
One thing to be aware of regarding this material is that it needs reinforcing around holes. I use self-adhesive reinforcement and 3mm eyelets and I've had no problems since (I had a few tear-outs prior to reinforcing the holes).
Hope this is of some use to you,
John