America's Cup sail design
 

....
All seamless string sails do have seams. Molded sail technology relies on
flat mylar in a roll form, they broad seam the mylar for the initial shape.
Ain't marketing a great?

Yeah, I know a sail maker who worked with a major on the last cup and
he claimed that they routinely re-cut "molded" sails but their license
with North required that they cover any seams with draft stripes. I
guess the marketing boys didn't want to confuse the general public...

-- Tom.