(...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

Arne Kverneland wrote:

Jami Jokinen wrote:

Arne,

EDIT: Is there a way to gradually increase the camber (larger in the lower panels) using the barrel cut and amateur batten pocket methods?

I see that many have introduced the practice of gradually reducing the camber in each panel as they move upwards in the sail.

In all the cambered panel sails I have designed, made and used since 1994 (six sails on six boats), I have made all the parallelogram panels with the same camber; 8 or 9% camber/chord. I have never ever thought, “Oh, I wish I had cut panel four and five flatter than the lower ones”. I only make the transitional panel flatter and then the two top panels even more so.

However, if you insist that you still want to gradually reduce camber as you move upwards, here is how I would have done it:
I would  first cut out the paper pattern for the lowest panel (with most camber and thus round) and use it for cutting out that panel. Then I would recut the pattern with the reduced round required for the next panel, and cut that panel out, etc, etc. This would be a quite quick process, as it lets you re-use the pattern 3-4 times.

Arne

[I copied this across from the Roger Taylor 3H topic, to this more appropriate topic]

When everything fails, read the instruction!

Somehow, I only opened Slieve’s spreadsheet, and missed the instruction in his Appendix 3. With the instruction in hand, the spreadsheet was easy to use, of course, even for me.

Now, I found I would compare the spreadsheet’s results on Round against the results I got when using my Chain Calculator. I used my master sails and simply picked my results from Fig. 4.5 in my Chapter 4 of TCPJR, page 7.

In my updated version of “Arne’s Chain Calculator” I suggest a quicker method of finding the needed Round, by just setting it to 0.55 times the wanted depth of camber (in mm). This hits pretty close on my sails as long as the wanted camber/chord is around 8% and the AR of the sail is between 1.85 and 2.05 (p/B between 0.209 and 0.234). With a lower or taller sail, or with a deeper or flatter camber in mind than 8%, I think the real Chain Calculator will hit closer. Still, I don’t try to say that the Chain Calculator is in the Rocket Science  League, when it comes to accuracy...

After having tried Slieve’s spreadsheet, it appears that he has used the simplified version, i.e setting Round = 0.55 Camber. Moreover, the algorithm for calculating the Round has no parameter in it for panel-height to batten-length ratio, p/B in it.  I have found when playing around with my Chain Calculator, that the need for Round goes down when the p/B ratio goes up.

Below is a filled in spreadsheet, used on two of my master sails, one with AR=1.90 and one with AR=2.20. For comparison, I have filled in the results from my chain calculator in parentheses.

Arne

David,

I think we are approaching something useful now. I looked up in my latest version of the “Arne’s Chain Calculator”. This has mainly dealt with the very large round needed along the battens to get enough bagginess. I have touched the matter of horizontal stretch in the cloth as well, but never quantified it.

Look at the very fresh photo below left. Today I have done some practical “camber measurements” on my garden fence: I fastened two screws at a horizontal distance of  4.9m (= B on Ingeborg). Then I hung a Dyneema line on the one to the left and put a 4.9m mark on it with tape in the right end. Mid between the two screws, I measured up marks, 30, 40 and 50cm down from the horizontal screw-to-screw line. These points represent the 6, 8 and 10% camber. Then I moved the tape mark in an out from the right screw, horizontally, until the line sunk to the  30, 40, and 50cm marks.

The first thing I noted was that it takes very little horizontal movement (slackening) of  the line until the bight sinks to 2, 3 and 4% “camber”. For this reason, one needs not bother with slackening the sail along the yard and upper 2-3 battens, where the sail is planned with little camber.

The result of today’s measurement was that it took...

·         5.0 cm (= 1.0% of 4.9m) slack to reach about 30cm or 6% camber,

·         9.5 cm (= 1.9% of 4.9m) slack to reach about 40cm or 8% camber and

·         15cm (= 3.1% of 4.9m) slack to reach about 50cm or 10% camber

Those 9,5cm to get 8% camber coincides well with the 8.7cm I got when measuring the length of the 8% curve in the CAD-drawing, below (there I used a number of 100mm radius circles along the curve to measure it).

Practical use of my findings.
Sooo, theoretically I should shorten the sail of Ingeborg’s dimensions 9.5cm in from the B=4.9m mark on the batten (foreward  end). That is a shortening of 1.9% of B. However, with the look of Johanna’s blue sail in mind, with her D-shaped vertical panel curves, I think I would only slacken the sail with 70% of what my measurements above suggests.

Then, to get...

·         6% camber, I would shorten the sail 0.71% of the batten length B along the batten,

·         8% camber, I would shorten the sail 1.4% of B (6.9cm in Ingeborg’s case)

·         10% camber, I would shorten the sail 2.1% of the batten length, B.

One has a choice on what to do about the excess cloth along the battens:
One may leave it as it is, where it will show as wrinkles (less than on Johanna, more like on Frøken Sørensen), or one may shorten the sail the same length by inserting 2-3 tucks along the rounded batten panel edges before assembling the panels.

I think that by using these numbers, the sail will inflate well in light winds and hit quite close to the planned camber. The vertical curve should end up closer to the box section  in Frøken Sørensen’s sail than the D-section in Johanna’s sail.

Arne

David - I've probably got the nomenclature wrong. Firstly, I was referring to panels formed from one continuous cloth. The point I was making was that, using Arne's method, when sewing the top or bottom edge, one gathers the material into multiple small tucks, to reduce the overall width of the sewn edge to the designed width of the panel, thus inducing the camber. No material is removed from the panel, so there is more of it, close to the head and foot of the panel, to create the 'box' effect. If, instead of simply oversewing the gathered material, one developed each tuck into a sewn-in pleat, - effectively creating broadseams - then the available sail area close to the head and foot would be reduced.

I was merely thinking that is how Arne achieves the box-section shown in his recent post. I quite agree that having more canvas than necessary flapping around, especially when under reduced sail offshore, is to be avoided.