I always use polycotton which I buy on ebay. I pay about £3 per metre, 45" width. Umpteen colours available, been using it for years. Nice fabric to handle and I find, ideal weight. The sails on the model barge in my avatar are polycotton.

Cheers, Nerys

I used Egyptian cotton when i did this she is a ratter 10 free sail with braine gear steering

This may sound stupid but how big is the yatch? A picture with a rule by it helps. I ask this because the boat length to some extent governs the weight of cloth used and dependent on the type of yatch it is controles if you are going to need to know about sail batons etc.
Is it just for display is it intended to sail it?
Down proof cambereick ie the material you use for the iners of pillows tends to be the go to material but the best artical with how to do it in depth is probably hear https://usvmyg.org/boats/sails/
This is the USA vintage group and it’s probably giving anything you might want to know. It’s really not that hard to do.
There are a lot of easier “cheat” ways to do things so if this lot doesn’t give you what you want get back in touch.

Hi Mark, if you can get hold of an old bed sheet, they were mostly made out of Egyptian cotton -and that's the best cotton material. Cotton poplin as it's called has a very tight weave and the weft yarns are generally easier to see -again often got in bedsheets. When you draw out the sail take into account weft and warp runs of the cotton. Suggest you look and learn from this video
All the best! Tony A..........