oorestis said:
Does this mean you can only have one custom sail?
I don't recall, so won't answer this particular point. However, I would like to point out that you can change any of the sails in the game. This feature was added as a "do it yourself, in quick steps, without in-depth modding know-how, and share with friends, sort of way". Cool feature to use on your main boat (flagship).
With some more work you can use OpenBRF, etc, and add/change any/all sails in the game. In case you want to learn about that, you can visit the Forge subforum.
Cheers