Paul S,
I cannot say why I find the 3-panel top section of Ingeborg to be so powerful. I can only tell from experience that these three panels have proven to drive my last four boat very well. When I sail only 3-up, I know I have no help from the four lower panels.

In Newsletter #44 there is a technical report from the first JRA-rally in Stavanger, in August 2004. Here Slieve reports from sailing my Johanna, even with only four panels set in the light winds.

In 2018 I wrote an account from a sailtrip in Ingeborg. Much of this trip we sailed only 2- or 3-up.

That write-up (rather cocky) caused quite a stir...

Have a look. http://goo.gl/nD5vHW

Arne

Two cliches are apposite here. People often say these things as an alternative to actually thinking, but these two cliches really apply:

1. "Horses for courses" ( what is most practical, or pragmatic in the situation? necessary compromises?)

2. "The proof of the pudding is in the eating" (The scientific approach. Has anyone tried it?)

Also the title of the thread: "Shape" is a nice way to describe both the camber (cross sectional shape) and the plan form (the outline shape) which are being considered together here.

It's an interesting discussion to follow.

Anonymous wrote:

Jan, that folding sailboat looks like a lot of fun. It is certainly easier to cut out a square junk sail, but I would argue that rigging one may be more difficult. I am curious as to why you used two yards. May I ask what the purpose of the second topmost one is? 

This is the yard. The lower one - it was supposed to be the batten of the upper (intended trapezoidal) panel. But it's better this way, because the yard is stronger and I could experiment more easily with the halyard attachment point. Then it stayed like that and it doesn't bother me.

Edit: In addition, any possible conversion to an upper trapezoid (or fan?) will be easier.

As I mentioned - I'm a fair weather sailor, although once I had to sail (downwind) using only two panels. Beating upwind (to winward ?) using two panels is possible, but ineffective. (Probably also because the leeboards are too short).

I suspect that the shape of the sail in my small folding boat was the result of my laziness. I didn't have to think about the shape of the top panel, and I achieved the intended effect - moving the COE closer to the bow, as is possible in the Split Junk Rig (SJR). And - in the origami SJR, the panels are very easy to cut - so I cut the bluetarp, glued it and hey - let's sail.

In any case, I sail with a certain sense of guilt about the top panel.

In moments of reflection, I sometimes wonder if Czesław Marchaj (my fellow countryman, by the way) is rolling over in his grave seeing such a rectangular sail.

Since I carefully avoid winds stronger than 18 kn - I hope I will not have the opportunity to test this sail in more difficult conditions.

Wind shear may be the explanation to why a square sail seems to be more effective in my experience. As Paul said, there is more sail area higher up in faster wind, and it may be that simple.

The Chinese Treasure ships from the 1400s are almost always shown with square sails. Images of the Keying and the Ning Po, originally named the Kin Tai Foong, seem to show square sails as well. There was a replica built of the Keying called the Keying 2 but fan top style sails were used.

Pictures of these boats is why I tried a rectangular sail.

These ships are also sometimes depicted with their sails centered on the mast, which must have made them very stable downwind. Like Lores that Graeme posted. I bet Lores was very fast and stable downwind, but without the ability to shift the sail off center on the mast, I doubt Lores made much headway against the wind.

The old Chinese boats must have had some technique for moving the center point of their sails on their mast. It could be that whatever technique they used to do so also enabled better windward capabilities, but that is pure theory. Such an ability in an already comparatively complex rig design is a head scratcher to be sure. I have seen a junk rig with the ability to move the sail slightly, but it was not perfectly centered and did it seem easy to do so.

On a side note, since my sails were fixed in their position on the mast, while sailing downwind, if I let the sail out too far, it started pushing the boat to windward and would almost dunk me in the water! I guess that's called a death roll. If the sail was able to be centered on the mast, that may not be an issue at all.

It is interesting to ponder what makes a true junk sail. From what I have read about the Keying when it visited England, reporters claimed the sails were made out of bamboo and matting! It may be possible whatever technology these old boats used is not feasible to recreate. Perhaps whatever plants they used to weave into sail cloth, or the way they wove the fibers, had some inherent aerodynamic quality.

This is an awesome website, and I have a feeling there are some very cool discoveries on the horizon.

-Colin Clayton