Hello Durahambargeman.
The cloth you are asking about is an excellent quality product, it is pure Egyptian cotton and handles beautifully, several members of our Club on the Isle of Man have used it for both vintage yachts and as you need for a Thames barges. The beauty of this is it can be ironed safely and falls into place as should the real sail.
I have personally used the smaller packets only a couple of weeks ago to make lifeboat covers and intend to use it for canvas dodgers on board my model.
If you look on the modelling Timbers website the customs cutter built by Keith carries the cloth as sails in a nice brick colour.
Best of luck with your build, I don't think you will be disappointed.
Howard Q.