The Rig of the Jonques de Plaisance
Suwan Macha, Jonque de Plaisance de 16,5m in Thailand
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.
The mast is unstayed, it is stepped securely on the keelson (I refer here to the 4.72M Jonquinette), the mast partner is reinforced by two boards which runs from the stem to the front of the centreboard case. The fact that the mast is unstayed means that it can absorb wind pressure better (when a mast breaks, it is often caused by the shrouds which prevent the natural movements of the mast). The absence of shrouds also means that the sail can be squared at 90º on a run or broad reach.
The mast carries only one sail which does not need to be changed (for example when the weather changes). Moreover, the masts on the JDPs are never straight – for very good reasons! The purpose of the forward rake is to facilitate tacking by making it possible to change the centre of gravity of the sail: The centre of gravity is brought forward or back by easing off or tightening the tack line – much simpler, very efficient, in fact very Chinese. It is more difficult to change the centre of gravity if the mast is straight and this is sometimes compensated by the addition of a jib, which shows a lack of understanding and is an aberration from a design point of view. This ability to change the centre of gravity of the sail can also be put to good use to make the boat more weatherly (or the opposite) if necessary.
Dimitri Le Forestier
The sails are made of Dacron 270-300g/m2, which for a traditional sailboat would be insufficient, but when reinforced with bamboo, is most adequate. They are simple and easy to make because they are cut flat.
The junk sail is a fully battened lugsail. Several vertical cloth panels are sewn together as shown on the plan and, as is often the case in traditional Chinese sail construction, the angle of the panels varies at most battens to stay parallel with the leech. The sail is reinforced on both sides with narrow bands of sailcloth (60 mm wide, with edges turned in), and the battens are bound to two rows of eyelets evenly distributed along the length of each reinforcing bands (see plan for spacings and appendix for bindings).
The sail can be set either side of the mast, but the battens should be between the mast and the sail. Because the battens greatly reduce the stress on the sail, we can use lighter fabrics than what is generally used for Western rigs.