In a world with ordinary magic, military ships should be prepared for enemy fireball-hurling wizards. Is there some special quality required of good sails that might go against fire resistance?
If there is some real world material available with 15th or 16th century technology (minus gunpowder, plus magic able of producing high temperatures, pressure or other conditions used in chemical industry but unavailable then) that would resist ordinary fireball/ big pot with hot flaming oil and still is quite good for sails, I'd be glad for hearing of it. If it is vulnerable to some other hazardous factor (cold, acid etc.), it's important. If there is nothing like this (and the answer will give some reason why existence of such material is unlikely), I can still accept it - I can always state that there is some magical material for it.