I agree, Ueli. 1/7 is about 14% depth of camber, as seen on many bermudan sails in moderate going. What I take from this graph is that at 30˚ - 40˚ apparent wind, where we're most interested in getting the most drive, there's not much to choose between 10% and 14% camber, and since JR camber is not adjustable, 10% JR camber seems like a sensible maximum to aim for.

I recently came across this graph, taken originally from Marchaj, which suggests 7% camber is the "best". My instincts and experience tell me that in reality the best camber depends on the wind strength, needing to be larger in light airs, and less in strong winds, and the wind direction, needing more camber off the wind and less up wind, which of course is why bermudan sailors have all those devices to change the camber, and put up big bellied spinnakers downwind in light airs.

As usual, there is no one "answer".

https://upload.wikimedia.org/wikipedia/commons/archive/0/0b/

20110630044520%21Sail_Camber_Aerodynamic_coef.png

Alan

I’ll try to answer your numbered paragraphs:

1/ Sure enough, but only if the sail has been slackened quite brutally along the battens, like an accordion. With a normal half-slack tension, that vertical curve will not take the ‘chain camber’ or catenary curve  (..see ‘Arne’s Chain calculator’...)

2/ Right again, but my experience is that quite moderate hand-tension is enough to adjust this, and anyway, the resulting change in camber/chord ratio is within plus or minus 1% of the chord. There will be no run-away camber no matter how slack the sail is along the battens, and it is not realistic to pull the camber right out of the sail.

In practice, my sails definitely are in the low-stress family  -  only the boltropes see real loads.

As for your last paragraph,
I think you worry too much about the un-even vertical distribution of max camber. If this had led to my sails only having 4% camber on average, they would have felt rather lame, which they certainly don’t do in real life. When I replaced the hinged-batten sail on Malena with the first cambered panel sail, I may have lost a bit brute force in the new sail, but I suspect that this was because the hinged sail had ten percent camber while the new sail had only eight. The new sail was definitely as efficient to windward as the hinged sail (NL 30).

Take a look at the two photos below. Note how the first and last half-metre of the sail is quite flat, like a sail made with the shelf foot method. I noticed this and commented it on the diagram of the test panel in NL 30, p.22. My thinking is that the incoming wind sees a luff very similar to that of a hinged-batten sail or one made with shelves. The same goes near the leech.

In other words, if the wind can enter the luff and leave the leech of a sail at the same angles with all three sail types, then the three sails are likely to produce similar lift and drag as well.

My experience with different sails in my Malena in the early nineties supports this.

Finally: Over the years, I have argued quite a lot for the barrel cut way of making cambered panels. I will still do so, but only for amateurs. I begin to realise  -  finally  -  that to sell sails, it is not enough that they are good. They also have to look good  -  that is, with an appealing planform and with sails free from wrinkles. In addition, the batten ends should have tidy-looking terminations, and battens (ends), yard and mast should be painted, if not anodised.
I leave this to the pros. I mainly focus on..

ease of making, ease of handling, performance and longevity of the sail.

Arne

Arne said:- I have used barrel cut method to produce camber on a number of sails. I find that the vertical curve of the sail from batten to batten varies quite a lot by how much stretch or slack I give the sail along the battens.

I have enjoyed reading this thread, and having been for a walk to clear my head, I have the following thoughts

1/ Other things being equal, especially very little tension along the battens, the curve of the sail between the battens ought to be a catenary, meaning any shelf will be absorbed into the sag of the sailcloth between the battens, creating a D shape.

2/ Other things are not equal, and the more tension you put into the sail along the batten(s), the more the bulk of the panel will be supported at just the four corners, and the more this will allow a shelf to "inflate" to something approaching a "box" shape.

This is the same situation as the foot of a bermudan main, which can be loose footed, with all the load taken at the tack and clew. Then a shelf can be fitted to fill in the gap between the foot of the sail and the boom, which acts as an endplate, and improves the performance of the sail. Thus getting a shelf to work depends on putting tension into the sail along the batten. In a way this is contrary to the low stress principles of the junk rig, but it will result in better performance, because more of the sail panel will be close to the intended shape and camber.

It has always seemed to me to be a drawback of the cambered panel sail that it only achieves the intended shape and camber at the mid height of the panel, and the camber reduces to zero at the battens, so the average camber across the whole sail will be approximately half what was planned. That to me has always been an argument in favour of hinged battens, but I accept they require more engineering in the battens, while the cambered panel requires more engineering in the sail, but at least with hinged battens the camber is pretty much as intended right across the sail.

(...this is actually a response on a discussion starting on the Sadler 25 conversion thread, at 24th Sep. 2020. I jump to here to avoid hi-jacking that thread...)

I have used barrel cut method to produce camber on a number of sails. I find that the vertical curve of the sail from batten to batten varies quite a lot by how much stretch or slack I give the sail along the battens.
If you look at the photos of Malena and Johanna, you will see that this curve is quite round near the max-camber point (‘D-shape’). However, as one nears the luff and leech, the (vertical) curve gets flatter (enlarge the photo of Malena). This was actually described on the diagram of the test panel in JRA-NL 30. On Johanna I slackened the sail 10cm a few weeks after I rigged the sail. This gave me some extra camber (and more D-shaped vertical curve), but also resulted in a lot of wrinkles along the battens. Camber had first priority, so I kept it that way. Since I was not to sell sails, the wrinkles along the battens didn’t bother me.

By contrast, have a look at the sail of Frøken Sørensen, below. This sail  was given a round which was calculated to produce 9% camber, but this time I stretched the sail a bit (not brutally) along the battens. This happened:

• ·         The measured camber came out at only 8%.
• ·         The vertical curve ended up very close to trapeze-shaped, as if the shelf-foot method had been used..
• ·         Very few wrinkles show at the batten pockets.

I kept it that way.

On Ingeborg’s sail I did much the same, but started with a round which was calculated to produce only 8% camber. When this sail was fitted like that on Frøken Sørensen, the measured max camber came out at about 7%. I then eased the tension along the battens with about 3-4cm, and that was enough to increase the max camber to 8%. The vertical curve ended up something in between the trapeze curve and the D-curve, and without too many wrinkles  (to my eyes) in the batten pockets. I am content with that.

Actually, I produced a write-up about this back in 2009, for the Yahoo JR group.

Cheers, Arne

   
Malena           Johanna                   Frøken Sørensen

Ingeborg