ANSWER: As often is the case, the answer is simple: in ancient times, the Chinese who were nevertheless great inventors, did not have fabrics strong enough to make sails. At other times, the use of fabrics was forbidden by local warlords (perhaps to discourage their citizens from finding out if the grass was greener elsewhere).
There wasn’t much left in the back yard except for bamboo which, if intelligently set in the sail, reinforce it dramatically. Faced with this arrangement, the Chinese easily figured out how to join these ‘booms’ with sheet spans and main sheets, in order to control the angle of the fall and to reef the sail without having to even change course!
But that’s not the end of the story and here is an example of pure Chinese finesse. You may be wondering: “why are the masts leaning the wrong way?”
ANSWER: A sail must not be in the way of tacking. Much later, we invented the jib … The Chinese came up with something better: All that is needed is to place the foremast well ahead and to give it a forward rake.
A sailing junk equipped with straight masts may experience difficulty when tacking: If a jib has been added to a junk rig, you now know why … Not aesthetically pleasing.
If you let the sail find its own centre of gravity (Fig. 1), you have a sail in its natural state.
Tacking is practically automatic on a junk rig: The forward rake of the mast makes it possible to change the centre of gravity of the sail, by pulling in the tack line:
The sail is pulled back (Fig. 2) and the boat starts tacking. As soon as the path of the wind is crossed, you can release the tack line and the sail’s weight makes it return to its natural state. The boat can continue on its new course without additional manoeuver.
It is worth noting that the bamboo battens provide a most effective reinforcement. This in turn allows us to use lighter fabrics than would be required for western designs. In fact, the sail area is set at its maximum for light weather since reefing by one or more battens is simple and quickly achieved.
Another interesting point is that if the sail suffers minor tears or holes, this will not affect its effectiveness.