How thin or thick can the sailcloth be?
There is no lower limit. However thin the cloth, we can print it. The awkward direction is the other one: once cloth is thick enough to stop light passing through, the graphic has to be printed on both sides to read correctly from both — and that is its own piece of work.
Thin cloth is the easy case
A light spinnaker is translucent. Print one side and the graphic reads from both, because the sail is lit from behind as often as from in front. This is why the lightest racing cloth — the material everyone assumes must be the hardest — is in practice the most forgiving.
It is also why weight and thinness are not the constraint people expect them to be. The limit on a light sail is what the print weighs, not whether the cloth can take it.
Thick cloth is where it gets interesting
As cloth gets heavier and more opaque, a single-sided print becomes a graphic that only exists on one side. The answer is to print both sides, in register, so the sail carries the same image port and starboard.
Getting two sides to line up on a large, flexible, three-dimensional surface is not trivial, and it is something we have spent real time on.
Applied graphics and the same problem
The same issue appears with applied graphics. Mount a graphic on both sides of an opaque sail and the reverse image ghosts through behind the one you are looking at. SAIL-STICKER carries a light blocker embedded in the material, so the two sides stay independent and each reads clean.
Related answers.
Every answer on this site is written from real sails, real cloth and real production — not from a template.