Graeme,
Marchaj had no access to the computer power we see today. His approach to coming up with these curves, no doubt was through using a not very big low-speed wind tunnel, and finely calibrated instruments for recording forces.

The curves shown on Fig. 83, are from testing three hard aerofoils with different camber, with no mast, and with a rectangular planform with AR = 5. Marchaj himself calls this a reference only. Most (all) other tested sails would do worse than this. I chose to focus on the one in the middle, with 10% camber. Even this gave a lift/drag of only 6.4:1. This happened with a AoA of no less than 15° (L/D could been improved just a bit by reducing the AoA to 10°).
This Angle of Attack is more realistic to me, as this is similar to what I see by eyeballing my sail and wind indicator.

Paul’s calculations with AoA between 3 and 5 degrees are of little interest with soft sails in mind. These numbers fit better on aeroplane wings at cruising speed.

What I would like to see is a (computer-generated) Lift/Drag diagram with the coefficient of Lift at the Y-axis and the Coefficient of Drag along the X-axes, both to the same scale.

Right now there is too big difference between Machaj’s and Paul’s results

Arne

To both Arne and Ueli I would suggest: don’t forget, the purpose of these basic models is to find comparative data, on just an arbitrary section of a junk sail panel.

It’s a first step towards establishing some basic principles, including that huge mystery, the effect of the mast on a lug sail.

A "partial view". It is not (and it is not meant to be) the same thing as designing a sail – much less, designing a complete boat and hull package. 

By the way Arne, are you sure your diagram from Marchaj of the “more realistic sail” depicts an A.R of 3? Not that it matters much in regard to the point you are making, but for the record, it looks a lot more than that to me.

Eric wrote "... it could be possible and not too costly, to use some photogrammetry technology to model a given sail ... the surface could then be "sliced" into 2D profiles to feed the CFD". 

David can provide real life profiles for his articulated-batten wing sail, but for a cambered sail on rigid battens we can't do that, of course.

Eric's idea is good, especially if you have the skill and equipment to do it, but if a boat is tied up to a pontoon with an inflated sail, I would suggest a better way to start with, just take one "slice" - simply take a series of measurements along the centre chord of a panel, and get the profile by direct measurement. I think that would be easy enough to do. We only need one "slice" for a 2-D model to start getting good comparative data from the computer model, as Paul has demonstrated.

[Some time ago I made a series of different model sail panels from cloth, inflated them and took measurements along a vertical line between the battens, in order to look at the profile (shape of section) along that vertical line. 

It was not difficult to do, (and it also showed that the vertical section curve has a lot less to do with the way the soft cloth sail is constructed than most people would think - but that's another story  The same may well be true regarding the horizontal section curve, which Paul requires).

I am kicking myself now that I did not at the same time take measurements along the horizontal line, the central chord, but that was not what I was thinking about at the time. I threw out all the model panels a few months ago. Anyway, a real life sail could be measured up just about as easily, to obtain the profile of a single "slice", (and it would be interesting also to compare the horizontal sections of differently constructed sail panels).

(So much for rigid batten cambered sails. How you would deal with the camber that is supposed to be induced by twisting flat cut fanned sails is in the "too hard basket" for me, but I dare say it could be done somehow - maybe by using Eric's photography idea). ]

Paul,
hm, now I have slept on it and then opened C.A. Marchaj’s Sailing Theory and Practice.
His numbers regarding resulting Lift/drag ratios differs quite a lot from the numbers you have found.

Marchaj operates with alpha angles between 5 and 20 mostly, and for upwind work these are 10-15°. The resulting lift/drag will then only be 6/1 on a High-AR sail, that is, very far from the 20:1 and thereabouts as you find.

The very small alpha angle you are working with is unrealistic on a soft working sail.
Fig 83, below shows a super-good ‘sail’ and even this has a lift/drag of just 6.4:1 and with Alpha = 15°.
The next diagram shows a more realistic sail with AR =3.

Arne

(Diagrams under Arne's sketches, section 8, photos 13 and 14)

Some flow visualizations of the flat cut profile:

Port tack: 2, 4, 6, 8, 10 deg AoA

Starboard tack: -2, -4, -6, -8, -10 deg AoA

Basically, the dark blue/ violet areas are detached zones. I would say, at about 7-8° on both tacks the detachment bubble reaches the luff and the sail is fully stalling - but still drawing.

A flat cut sail profile is called a "flat plate" in aerodynamics. A lot has been investigated with this most basic profile over the past decades. The flat plate detaches almost immediately at the leech with just a tiny little bit of AoA.

(interesting enough, that the detachment bubble resembles the shape of a cambered profile at the L/Dmax point. It is then acting pretty similar to the cambered profile, with the detachment bubble "being the camber". However, the difference is the highly increased friction and the flat pressure side of the profile).

Cheers,

Paul

Paul, here are sections through the final form of Weaverbird's wingsail, which, as a seat-of-the-pants sailor, I found to be quite alpha-tolerant. Let me know if it would be helpful to have the DXF file.