Draft Position
Top stripe 8.4% 45.8%
Top seam 12.2% 45.8%
Middle stripe 9.5% 46.5%
My feeling would be that the top seam has too much draft sewn in, in the sense that the stripes above and below it don’t want to “run” with that amount of hump. Something around 9% would “fit in” better. My other comment is, based on the other sails I’ve measured, the jib is certainly very full. Most of the jibs I’ve seen measure around 6% to 8% draft.
I was shooting for 7% at the lower stripe, 7.75% at the middle stripe, and 8.5% at the upper stripe. I was also looking for 8 to 10 degrees of twist. I need to make a better main so I can live with these values for now.
The problem with this jib is that I have used equally spaced panels. When ever I get around to making the next jib, I will move the top seam a little higher. This will let it influence the upper stripe more, and the middle less. I can then use jib stay sag to fine tune the middle stripe.
I think you’ve picked up on what I’ve noticed – IMHO the top seam has too much sewn-in draft, and it distorts the smooth vertical distribution of draft. The corrected draft, using the spreadsheet you provided, is top stripe 8.2%, top seam 10.7%, and middle stripe 6.1%.
We know when a sail twists off it looses draft. Let?s assume we want 10% draft all the way up the sail with 10 degrees of twist. Lets? also assume the sail is made of 4 panels, and the seams are at heights of 25%, 50%, and 75% of the luff.
If we rig up the sail with 10 degrees of twist and 10% draft at the lower seam, we might find the mid seam has 8% draft, and the upper seam has 6% draft. So in order to maintain are constant 10% draft we need to build an extra 2% into the mid seam, and an extra 4% into the upper seam.
Twist comes from moving the clew of the sail a little closer to the head. In a mainsail, easing the kicking strap or vang; in a jib with a topping lift, tightening the lift. It can’t be sewn-in directly. The function of twist is to accomodate (1) the wind gradient, and (2) the downwash at the head when the sail develops lift. For case (1), another approach to correctly aligning the sail entry to the apparent wind is to adjust the camber in the upper half of the sail. As camber increases, the entry angle of the luff increases, and this achieves the same effect as increasing the twist of a uniformly cambered sail. So an increasing entry angle can be sewn-in – sew in more draft. The down side is that this narrows the operational use of the sail to conditions which are in the middle of the wind range. Too much draft at the head is bad news both in drifting conditions and at the top of the range.
Far from me to be in concurrence with others methods on how to make sails.
I found, nevertheless, from an’Italian Club a simple tool that I decided to make and use. It is easy to use and very reproductible.
Actually I have published on www.minicoque.com the content, but I would like to publish it on RC-Sailing and have wider comments/ critics upon . How to do ? There is any suggestion ,
Thank you very much.
Claudio
PS. On request , I have put the English translation for the “Instruction for use”.
Marino:
Go to www.minicoque.com then click on “tecnique” and a new folder is open.
The top 3 actually are the "Outil à faire les pinces 1,2,3 " actually the photos + the graphic may helps understanding.
An English version for the “Instructions for use” could be published with the numerous photos on RC-Sailing if possible .
Claudio
Just did a mockup of the device with a piece of pine and balsa wood… Tried it with two sheets of paper, and it worked.
I’m not sure if the table in one of the pages tells how much draft can one induce by increasing the curve of the alum. strip… for those of us completely idiotic, does such a table exists? can you explain it for the masses (i.e. me)…
It is a lot easier than building myself a block (a process that could lead me to some serious injury!!)…
Wis,
you are a genius by having transferred my posts from Minicoque. This in fact was my desire.
Thank you very much, hoping someone will find it interesting.
Claudio
Marino
have you read the English instructions actually added ?
WIS has just recovered the full stuff here above.
I’m not sure having understood your question but the graphic on part 1 explain wich thickness is necessary to obtain a desired draft as a fonction of the panel width.
ie given: desired draft (creux de la voile) =10% Cord lenght 245 mm (panel width)
Multiplication factor (Fm%) is obtained by crossing the blue band = 0.325 %
Therefore wedge thickness (epaisseur cales) = 245 x 0.325% = 0,796 mm rounded to 0.80 mm or 8 sheet of paper of 80g/m²
that you have to place at each side of the panel between the flexible bar and the rigid plate. This tool do not exist in commerce each one can easily make one. Plexiglass bars are somewhat better less hysteresis than metal.
Claudio
Everything made sense, but the table. Your example cleared it out for me.
Will print it and make a “manual” out of it.
Bear in mind, this forum is accessed by many nationalities, and in particular, my french is non-existant (other than being able to say “Je ne parle-pas France”)
