**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.