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.*

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(**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.)