SJR jib sheeting angle - how to measure?

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  • 15 Sep 2020 22:11
    Reply # 9238378 on 9230600

    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"

    Last modified: 16 Sep 2020 00:57 | Anonymous member
  • 15 Sep 2020 13:05
    Reply # 9236688 on 9236312
    The one question which I don't think any of us asked is:  Why did you think there wasn't enough sheeting angle on the jib panel?

    This was discussed earlier: the jiblets dont seem to start luffing before the mains.

    For some reason I also forgot how and if I drew the sheeting angle right considering the 45 deg shelf method. Now I know that I did :)

  • 15 Sep 2020 10:00
    Reply # 9236389 on 9230600

    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.

  • 15 Sep 2020 08:46
    Reply # 9236312 on 9230600

    Hi, Jami,  

    That sail looks very good to me. Looks very nicely constructed. Note that you went for the angled shelf foot method for the main panel as well. 

    No back-winding and setting very well indeed. 

    The one question which I don't think any of us asked is:  Why did you think there wasn't enough sheeting angle on the jib panel?

    Dave D.

  • 15 Sep 2020 07:08
    Reply # 9236207 on 9230600

    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.

    Last modified: 15 Sep 2020 07:14 | Anonymous member
  • 14 Sep 2020 17:19
    Reply # 9234593 on 9230600
    Anonymous member (Administrator)

    A great explanation, Graeme! Even I begin to understand it.


  • 14 Sep 2020 01:05
    Reply # 9233181 on 9230600

    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.)

    Last modified: 14 Sep 2020 08:40 | Anonymous member
  • 13 Sep 2020 23:18
    Reply # 9233031 on 9230600

    Hi Guys,

    Sorry but I've just seen this thread, and will admit that after a quick read through I'm totally confused.

    All our brains are different and we all see things from a different angle. Where I am weak in some techniques I have strengths in others, and in particular can visualise 3-D shapes and airflow. The trouble is when I write down a 'how to do it' the result is not necessarily clear for others to read, and I apologise for this lack of clarity.

    The development of the jib panels is one particular feature where I've never been fully happy with the write up, nor am I convinced that I have achieved the best and most efficient shape. It would be easy to do in tin plate, but sail cloth will always bend to balance the tensions in the cloth, and the jib panels are an extreme case of this problem.

    In my efforts to get the shape I want I've introduced the terms sheeting angle and camber shape in an effort to explain my thinking in Bermudan rig terms, and introducing the concept of the confusing 'angled shelf-foot' is my attempt to push the desired camber right up to the important luff and still not have excessive amounts of material weighing the shape down. The important thing is that it seems to work well in practice, though with the latest rigs I've drawn I have been increasing the sheet angle and camber step by step as in theory that should improve the efficiency even further. Of course there will eventually be a point where I go too far, but we haven't reached that yet. 

    The other question is the shape of the camber curve. Where low speed airfoils seem to work well with cambered forward sections and a flat run aft, the way the jib panels set is encouraging me to question the possibility of using a simple arc of a circle as a camber curve for the jib panels. It's only a thought at this point in time, but I should build a practice panel and see how it looks some day.

    Enough of my thoughts. Jami, if you are not clear about something write to me direct and even though I'm fairly busy and probably going abroad on holiday soon (Isle of Wight), I'll try to answer any questions you have. Having worked with Ian Hannay, the designer of you boat, I would like to see you get the best performance you can out of the junk rig on his hull.

    Cheers, Slieve. 

  • 13 Sep 2020 16:18
    Reply # 9232361 on 9230600
    Anonymous member (Administrator)


    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.


  • 13 Sep 2020 12:07
    Reply # 9232051 on 9230600


    this is exactly what made me think that the sheeting angle might be too small. The jiblets don’t seem to luff clearly before the mains.

    However, I just found the jiblet pattern I used. Measured from that it seems that the angle is mathematically ok.

    Last modified: 13 Sep 2020 12:07 | Anonymous member
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