Sail cloth is a particular heavy grade of canvas. This does not mean the generic cotton stuff we find today. It would have been from hemp or flax linen. These fibers are far stronger than cotton and come from the vascular system of the plant that pulls water through it rather than fibers that are meant to rot away from around a seed. Comparatively cotton fibers are 2-3" long, flax 2-3 feet, and hemp can be up to 18 feet.

These long fibers, once removed from the plant, are spun into a relatively heavy weight thread for weaving. Canvas by definition is then woven with more warp (up/down) threads than weft (left/right). This actually bundles the warp threads together and creates tiny channels in the fabric that run it's length. These actually aid the water in draining, giving it a smooth path with no bumps or threads in the way. Sails would always be constructed with the grain vertical for that reason. This technique combined with the fiber content allows the fabric to swell when wet, creating water tension and draining fast enough to keep the sails from gaining too much weight.

The fabric could have come from Ireland, Germany, the Netherlands, or even the Ottoman Empire area. Those areas still produce linen and hemp fabrics today. Interestingly enough, the fabric would be much narrower than what you find in the store today, often around 3/4 yard. It helps to save yardage when angles are cut. Fabric is expensive, labor to sew up seams is cheap.

I can keep going about construction or other related things if you're interested or the sail makers don't pick up the question.