Jami wrote: ...the jiblets dont seem to start luffing before the mains.

[I wrote yesterday that my jibs luff just prior to the mains - but now not sure. I have not been sailing for about 8 months and I honestly can't remember now if this is actually correct or if it is the other way round. I think I wrote it down somewhere but I forget where ... you see the problems of advancing age!]

Does this matter anyway? I think you did also report in another post, that the boat tacks easily. I suppose in the ideal world, when turning into the wind, both parts would luff at about the same time? I can't find that earlier post where you described this "problem" but since your boat seems to sail so well and the sails look so good, I wonder if you are being too critical.

Anyway, it seems to me that the question of which part luffs first is the combined result of the cambers of both parts of the sail, as well as the foil shapes, and the sheeting angle  - not just the relative sheeting angle on its own.

Unlike the bermudan rig, the SJR has to live with fixed cambers and a fixed angle of sheeting between the jibs and mains, so when designing the sail there is always going to be an element of compromise. And as you say, these numbers are just targets anyway, most of us are not that accurate, and soft cloth is not entirely predictable in the shape it finally takes up.

What is the best all-round combination of these parameters? We don't know yet, and there seems to be no way of experimenting except trial and error when making a new sail, and keeping a record of what has been tried in the past. 

Perhaps Slieve can remind us of the figures for Poppy and Amiina Mk1. I seem to recall the progression from Poppy to Amiina Mk 1 to Amiina Mk2 has been a gradual increase in all these numbers. In 2019 Slieve wrote:"Yes, the latest thinking is 12° sheeting angle and 10% camber for the jibs, and 7% camber for the mains. As to whether it is better than the earlier figures we cannot be sure, but in theory it should be better and there are no stalling problems".  And in another email: "Things have moved on since the original draft notes were written. Amiina's latest rig uses 12° sheeting angle and 10% camber, and sets well and pulls well". 

In one of the other threads Jamie wrote that his targets when making his sail were 8% camber for mains, 11% camber and 12 degrees sheeting angle for jibs.

I did similar on Serendipity. 8% and 10% and 12 degrees sheeting angle.

Dave D. and James G. what did you guys use?

Slieve wrote: "If building another rig for myself I would certainly be looking at even increasing these numbers, but for the moment these numbers are known to work well". 

When new sails are made in the future it will be good if we can be kept posted on the latest numbers, as I am sure there is more that can be learned. 

PS here is another thought. When discussing the cambers on "normal" junk rigs, one assumes were are referring to measured camber - in the manner described by Arne which involves using a measuring stick and measuring the actual camber of an inflated sail. In these SJR discussions, however, we tend to be referring to theoretical camber - that is the result of these geometric diagrams used in lofting the sail. But an inflated SJR sail probably has more camber in it than the theoretical number, as it is made from soft cloth not tin plate. I suppose we should all measure the actual cambers when having these discussions, so as to be "comparing apples with apples"

I agree with David, that sail looks really good.

(Must have been quite a lot of work making all those hinges though.)

I'm most impressed, well done Jami.

Thank you all,

with all this info (and especially the "sheeting angle for dummies" by Graeme) I managed to find out that the sheeting angle of my jiblets is something around 11,5 degrees. I think this is close enough, when 12 deg was the target.

Also, email sent to Slieve. Thanks for the offer.

And to anyone interested, a short clip of the reefed sail, jiblets visible. The sheeting angle is not easy to see from this angle, though.

I have no trouble with the maths but I do have trouble visualising 3 dimensions, so I made a plywood model.

(Actually, since most of the panels are identical on the delightfully simple Amiina sail, and my sail was small, I went further and used the plywood model as a tailor's dummy for the jibs, and made another one for the mains, but that's another story and not really necessary.)

I think Len has over-thought the problem of jib sheeting angle, and made it a bit confusing. However, as Slieve says, we all see things differently.

Here is the 3-dimensional model, with a sheeting angle and camber marked on it.

Now, if this were a 90 degree shelf, that is all there would be to it.

However, there is a very slight complication here due to the fact that the shelf is angled at 45 degrees. This means that in the vertical plane, the actual camber and sheeting angle become less.

To overcome that, and get the sheeting angle and camber that you actually want, all the measurements need to be scaled up (multiplied) by 1.4142

(Not added, as jamie wrote, but multiplied).

This scale factor depends on the angle of the shelf and it is simple trigonometry, but all that is necessary to know is that for 45 degree shelf, the scale factor is 1.4142 and all the measurements need to be multiplied by 1.4142 in order to achieve the actual numbers we want in the plane which is at right angles to the sail.

This diagram might help. 

So here is what you have to do:

(1) Draw a diagram on a full size sheet of paper, with the sheeting angle you want and the camber you want.

(2) Then you have to scale all the measurements up by multiplying all the heights by 1.4142 and draw a new curve. (shown here in red).

That red curve will be the correct curve when the shelf is angled at 45 degrees.

That's really all there is to it.

If you follow Slieve's detailed instructions unquestioningly, this is the result you will achieve.

Now for jami - if you want to know the sheeting angle, you need to measure up your jib and reverse the procedure.

Jami, I think you can take one of your jib panels off and take some simple measurements.

If you want to calculate your actual sheeting angle (as defined by Slieve) measure the length of the leech of a shelf (L) and measure the length of the foot of the shelf along a batten (B). 

Now reverse the scale factor by dividing L by 1.4142 to get C.

C divided by B is the tangent of the true sheeting angle.

By using inverse tangent tables you can find the angle. 

Here is a worked example. Suppose the length of the foot of your jib shelf when measured along the batten is 1.2 m or 1200 mm.  B=1200

Suppose the leech of one of your jib shelfs measures 330 mm.   L=330

Reduce this by dividing by 1.4142 to get C   C= 330/1.4142 = 233

then the Tangent of your sheeting angle= C/B = 233/1200 = .1942

Ask Google what is the inverse tangent of .1942 in degrees (you need to specify degrees, or google might give you the answer in  radians and we won't go there,)

The inverse tangent of .1942 is 10.99 degrees, so we can say the sheeting angle in this case is near enough to 11 degrees.

If you think your sheeting angle is too small, I suppose as you say, in your case, you might be able to make new "hinges". I am not sure if your centre panel would need to be re-cut (because I can't visualise it) but I expect you would get away with just altering the hinges  if the change is not too great.

...........................................................................................................

(PS for anyone making a SJR sail for the first time: you still have to loft the centre panel, but the same principles apply.

The fun part about my plywood model is that I did not need to loft the centre panel.

I just set the two shelfs up at 45 degrees then planked the middle and trimmed the "chines" like making a flat bottom punt.

I don't think you have to make a full size plywood dummy like I did (Slieve says its a waste of time, and I agree it is not so practical for a large sail), but I do recommend anyone to at least make a paper or cardboard scale model before lofting and cutting cloth. Everything will then become clear.)

(pps I used 12 degrees for the sheeting angle of the jibs. (That is, the actual sheeting angle as in diagram (1) above). This was the sheeting angle used on Amiina's Mk 2 sail and I don't know if anyone has tried greater than that yet. I find my jibs luff just slightly prior to the mains so I feel this is probably close to the ideal but someone will try more, eventually, and then we will know. For cambers, I used 8% for the mains and 10% for the jibs and that looks about right to me, until someone tries more. I thank Slieve for giving me these parameters.

I left the top panel unsplit and will do the same on my next sail. K.I.S.S.  I gave the top panel 6% camber. This sail is built for sailing in sheltered water and it works well.

Here is a warning for first-time SJR makers - be sure to mark your cloth when making the panels, for top and bottom.  Because of the very small angle of rise on the Amiina Mk 2 sail, a jib panel looks the same upside down as right way up, and if you don't mark them you will forget which way is up. But if you sew a panel upside down, when you hoist the sail you will find the sail will never set properly and the reason for it is not obvious, so you will get crazy trying to make it set better. So mark your panels for right way up at the time you cut the cloth. That is a lesson I learned the hard way.)

Jami,

I’s easy to get impressed by maths, but in this case, with more or less stretchy cloth and more or less accurate sewing, plus that mast, etc. my guess is that it takes some testing and adjustments to hit the right spot.

Arne