SJR jib sheeting angle - how to measure?

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  • 13 Sep 2020 09:24
    Reply # 9231879 on 9230600
    Anonymous member (Administrator)

    Jami,
    could I ask which part of the sail luffs first when you gradually head up into the wind? My guess is that one would want the jiblets and mainlets to luff at the same time, or the jiblets a moment before the mainlets (..ask Slieve...).

    This may be a more practical approach to getting the jiblets’ sheeting right than diving into the maths. If you then make adjustments to just the middle jiblet until you are happy with it, you can finally copy that setting onto the other jiblets.

    Just an unqualified armchair idea...
    Arne


  • 13 Sep 2020 07:34
    Reply # 9231784 on 9230600

    Thank you for this. I still don't understand why one would need to subtract anything from the jiblet panel... but it looks like I haven't been totally lost after all. 

    No matter how much I try to remember, I can't say if I added the 1.414 to the sheeting angle triangle or not. I may have to measure this.

    Should the sheeting angle be too small, I have a moderately easy way to increase it, thanks to the separate/hinged panel structure of the sail: I can add a Roger Taylor -kind of thinking and make increasingly longer hinges to the jiblets.

    Last modified: 13 Sep 2020 07:35 | Anonymous member
  • 12 Sep 2020 22:19
    Reply # 9231127 on 9230600
    Anonymous wrote:

    I might be too blind to find or too stupid to get it, but I don’t seem to understand how to measure the jib sheeting angle on the jibs of a split sail. By measuring I mean while making the jibs (or the jib lenses to be accurate).

    Let’s say the chord length is X and the desired sheeting angle is 12 degrees.

    Does one

    - Count trigonometrically the distance needed to achieve 12 deg angle to line X or

    - Count the above and multiply it by 1.414 (as in 45 deg sehlf foot method)

    ...and add this  measurement to the leech of the jib lenses

    Or is the method something totally  different?

    My mind has trouble with only words to describe this so I have added some pictures... shelf-start and shelf-end. Basically, one would start with the pattern for their 45degree shelf (the first picture) and then add a triangle (the second picture) where the angle at corner A is 12 degrees. Arriving at this angle and shape and of course the distance at B-C can take whatever path is easiest for you. A long yard/meter-stick (or other straight something or other) and a protractor will do for example. Just lay them out on your pattern and don't worry about what the distance at B-C is so long as the angle is right. Or use a calculator such as https://www.calculator.net/triangle-calculator.html. which tells us that if the chord is 1.0 then B-C is 0.20906. So chord times 0.20906 would be the length of B-C to use.

    But... your shelf is at 45 degrees and so this 12 degrees added to the shelf will in fact end up adding less than 12 degrees. So one could take the 0.20905 and multiply that by the 1.414 you mention to fix this.

    Now that you have done this. The wine glass shaped jiblet is going to be too wide and will need to be decreased by the width/1.414 of the extra triangle at each point along the chord.

    Ok, so far all of this assumes that the vertical shape of all these panels will be perfectly straight. That is that the shelf will be perfectly flat and that the jiblet itself while forming a nice curved foil and will be nice and straight up and down. None of this is true of course. The shelf and triangle will not be a 45degree straight line but have a curve where the part nearest the batten will be almost horizontal and the point where the shelf touches the jib will start to approach vertical. That is looking at the Luff of the sail you will find that the jib, shelves and extra triangles will form somewhat of a concatenary curve when viewed from behind. I do not know if, having done the calculations as above, the average sheeting angle between the top and bottom of the jiblet will still be 12 degrees.

    On the other hand, various people have built these sails and written down what measurements have worked best (or at least should be able to tell you). I would think the place to start is with https://junkrigassociation.org/Resources/Documents/Slieve's%20Files/C%20and%20SJ%20P56-66%2012-03-17c.pdf In this case, 8 degrees is added to the shelf. Note that the protractor method seems to have been used to do this, no calculating. The fact that the shelf is 45 degrees also seems to have been ignored which is probably ok because, as mentioned above, that part of the shelf is close to horizontal.

    So the real answer is that we can only do the best we can without some better modeling than we have now. In theory all of these things can be taken into account. We got to the moon using sliderules after all. However, so far everyone seems to have assumed flat panels and pretty much gotten away with it... and because they have started there, any experimental improvement is based on the same method of calculations. That is, the jib sheet angle is the calculated angle that seems to have worked best at whatever the real world angle (or average) turns out to be.

    2 files
  • 12 Sep 2020 21:46
    Reply # 9231052 on 9230600

    Jami, 

    Hopefully much easier to understand.


    On page 58 of Slieve's notes, which contains the drawings for construction of the panels, the bottom drawing is a plan view of the finished sail.


    The distance you are looking to measure is from  A6 to X6, giving the distance the finished leech SHOULD be from the batten line. ( The horizontal line at the very bottom of the bottom drawing).


  • 12 Sep 2020 21:31
    Reply # 9231032 on 9230600

    In trigonometry, the Tan of 10 degrees is 0.1763 or thereabouts, according to my phone calculator anyway.

    I'm no mathematician and going on secondary school maths, using Tan gives the length of the opposite side of the triangle you are measuring, with the adjacent being the chord length, which you know and the angle of the corner you are measuring from.


    I'm talking about the measurement of the finished sail.

    So, hanging a finished panel horizontally between two battens and placing a straight edge across the battens at the leech, the measurement you want is how far down the leech of the centre panel, is below that straight edge.

    The triangle ( measured from the luff) you are measuring is Hypotenuse ( measured from luff to leech of sail hanging down). Adjacent ( the length of the batten) and the Opposite ( the measurement you are looking for).

    I'm on the phone on which the typing is acting up, so sorry about the delay.


    I'm not sure that the various procedures in drawing the templates can be used to determine what the leech distance should be.


    Rather, you know the length of the batten, the sheeting angle you want and the formula above should give you the distance the leech of the fully inflated sail should be away from the batten.  Allowing for the fact that the sail isn't rigid

    I've probably made it totally non-understandable.

    Not sure the above is any help at all. Sorry.

  • 12 Sep 2020 20:46
    Reply # 9230959 on 9230600

    Yes, I read the whole package - without it I would never have been able to construct a split sail. 

    It just suddendly hit me that my sheeting angle might be too small.

    Where did you get the number 0.1763 to your formula from?

    Last modified: 12 Sep 2020 20:47 | Anonymous member
  • 12 Sep 2020 20:42
    Reply # 9230958 on 9230600

    Have you read Slieve's notes in the public area, which are what I used when I made mine?

    I have a photo on my profile which helps show the difference between the camber of the sail versus the sheeting angle, which may be of some help.

    Slieve's notes are good.

    It's a while since I measured mine, but if the chord length is 100cm and the sheeting angle is eg, 10 degrees, I used the following formula to see what the distance at the leech should be on the finished sail.

    EG: TAN(10)= 0.1763 x 100cm = 17.63cm.

    That 17.63cm would be the distance that the leech of the centre panel is, when the constructed sail is fully inflated by the wind, from a line held taut across two battens at the rear.

    Unbelievably, my actual measurements worked out pretty much  where they should have been according to the maths.

    Hard to measure exactly because the sail is obviously floppy and doesn't hold the exact shape a more rigid material would.


    Hopefully someone else can explain a lot better than the above.




  • 12 Sep 2020 17:35
    Message # 9230600

    I might be too blind to find or too stupid to get it, but I don’t seem to understand how to measure the jib sheeting angle on the jibs of a split sail. By measuring I mean while making the jibs (or the jib lenses to be accurate).

    Let’s say the chord length is X and the desired sheeting angle is 12 degrees.

    Does one

    - Count trigonometrically the distance needed to achieve 12 deg angle to line X or

    - Count the above and multiply it by 1.414 (as in 45 deg sehlf foot method)

    ...and add this  measurement to the leech of the jib lenses

    Or is the method something totally  different?

    Last modified: 12 Sep 2020 20:12 | Anonymous member
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