My topic in 2 sheet system gradually develop into a discussion of the driving force inside the supposely "flat" junk sail. I think its better open a topic here to share our findings in the mystic draft.

I had been wondering how the straight bamboo and flat canvas on the junk sail can create the draft needed to drive the boat windward for a long time. I noticed that Hong Kong style junk sails twist when sail into wind (see attached). in 2016 I visited the Sagrada Familia by Antoni Gaudi and noted how he created a curved roof with straight beams. (see attached) On my 12 hr flight from Spain back to Beijing, I drew the 3D model of a Hong Kong style twisted junk sail (see attached) and cut horizontal section from it to find the draft. It proofed that there is a draft, but not very thick. I actually have to bend the battens a little bit to create a noticable draft.