Discussion in 'Boat Design' started by laukejas, Feb 6, 2015.

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### laukejasSenior Member

Hello,

I'm trying to figure out the mystery behind broadseaming of the sails. I red many articles and books about the subject, as well as dug through various forums (including this one), and I think I understand how it's done, but not how it's designed.

Now, I know that there are other sail-shaping techniques used together with broadseaming, like edge curves, darts, and so on, but for simplicity, let's neglect them this time.

Every book or article I red provides depth, width and locations of broadseams for specific sail, but doesn't explain how they are calculated.

Depth seems pretty straightforward - up to the deepest part of the sail. But nobody explains width. I had hoped that Emiliano Marino's "Sailmaker's Apprentice" will explain it, but he only gave a "1/2 inch per 3 feet of chord length" figure.

Can somebody please explain how to calculate the amount of broadseams to get a specific sail shape? Let's say I want 10% camber at 40% of chord length throughout the whole sail. How can I convert these numbers into broadseaming amount? How do I know what width should I choose for luff and leech? Do I broadseam every cloth panel, or every other one? How does cloth width influence it?

I'd be grateful if someone could explain it. I've been trying to work out a formula for several weeks now, and I'm getting more and more confused.

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### Dave GentryJunior Member

Although I've made plenty of sails and I'd like to pontificate, I'm an amateur and my answers would certainly be suspect - or likely just plain wrong.
Instead I suggest you post on the WoodenBoat forum, and hopefully Todd Bradshaw will weigh in with an answer. He's a professional small boat sailmaker, among other things, and is generous with his time when it comes to sailmaking questions.
Good luck! Making sails is fun.

Dave Gentry

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### laukejasSenior Member

Thank you. Good advice. I will post a question there too.

In the meantime, if someone knows anything, please write here!

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### Wayne GrabowSenior Member

The Sailrite company used to publish a news letter called Fabric Foils (maybe they still do). In an early edition of their newsletter, they included an article on the mysteries of broadseaming. At that time, I was beginning to use a mathematical method for designing developable surface hull shapes. After reading the article, I realized that the same principles I was using could be applied to sail design. The major difference is that fabric panels can have significantly more elasticity than plywood panels. I thought that a mathematical approach could take some of the mystery out of broadseaming. So I wrote two-page description of the idea including diagrams and sent it to Sailrite. They published the letter as an article in their July 1984 edition saying that the method appeared "quite sound and effective" although "far from complete". If you can find the article, it might give you some idea on how to actually calculate broadseams. If you cannot find the article and are interested, send me a message with an address and I'll mail you a copy of the letter. I don't have it in digital form.

Last edited: Feb 8, 2015
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### laukejasSenior Member

Interesting. I tried to find that article, but with no success. I am living in Europe, so mailing would be very expensive... Could you please take a photo or a scan of that article, please?

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### Wayne GrabowSenior Member

"Interesting. I tried to find that article, but with no success. I am living in Europe, so mailing would be very expensive... Could you please take a photo or a scan of that article, please?"

Sorry, I was in a hurry and didn't notice your location. I'll take photos and try to post it. It will take a few days.. still busy.

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### laukejasSenior Member

Thank you, I'd really appreciate it. There's no hurry, take your time

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### Wayne GrabowSenior Member

laukejas,
The sail design information I mentioned earlier has been posted on my blog: developable-surface-boat-designs.blogspot.com

Have you ever considered using a spritsail instead of a standing lugsail? Many years ago, when we lived in Panama, I built a skiff, 40" wide (about a meter) at the chine and 15 1/2 feet (4.7 m.) long. It was big enough for our family but light enough that my wife and I could cartop it on our Toyota. The rig was simple to assemble, and spars could be easily stored in the boat. Pictures of the boat are also on my blog in the article titled "panama skiff".

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### laukejasSenior Member

Thank you. I'm still working on figuring out these formulas, but it seems to be a gem.

I have considered spritsail. It is a possible arrangement, but the best spritsails I've seen were rigged boomless, and if I'd choose that, it would severely limit possible sail area (because I'd have to have to design the sail around mainsheet angle).

Your boat looks nice. How much did it weight? Are there any more photos?

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### SkyakSenior Member

You have likely read much more on the subject than I have and now poses a bunch of formulas on the 'art' of sailmaking and broadseaming. I have a good idea of the theory and physics it is based on and will try to give a short concise explanation of what they are doing, why, and why it remains an art in a world of science.

The entire objective in sailmaking is to present a curve to the wind that will result in favorable forces on the sail -a high coeficient of lift, and a small coeficient of drag -high drag in the case of downwind sails. The force of the wind reinforces and causes the sail to posses this shape. The streamwise curve is determined by the study of aerodynamics and it could take a lifetime to master even without the complexity of cloth and rig dynamics. For sailmakers purposes all they are trying to do is make a smooth acceleration curve for the air from the stagnation point to the maximum depth point so that the boundary layer thickness is minimized. At the leach the challenge is getting it flat so the leaward air stream decelerates to better match the speed of the air coming from the windward side to minimize turbulence. All turbulence takes energy and results in drag. For a foil of finite span there will always be some turbulence at the end(s) of the foil and the drag associated with it is known as "induced drag". The minimum induced drag is achieved when the span-wise loading of the sail is elliptical. Knowing this you see immediately that high aspect sails are more efficient upwind than low aspect ratio sails.

The curve that results from a distributed force perpendicular to a thin member in tension is called a catenary curve.
https://www.easycalculation.com/graphs/catenary.php

This is important for stays, the sail, anything thin with a distributed load. For example, your lug sail in the wind -a vertical section cut through it would show the sail is in a catenary curve and the depth is a function of the tension. Since we know what the force 'wants' to do to the sail we can shape it so that we get the shape we want horizontally (aerodynamic curve) from it getting what it wants vertically (the catenary curve). For example if your lug sail was ten meters tall and you were broadseaming it with one meter wide cloth (horizontal seams) you would calculate the catenary curve, divide it into the 9 seams. Each seam is the intersection of two conic sections at a slight angle (alpha) to each other. If the aerodynamic curve we want is F(x) then we would need to cut a curve in the end of the cloth E(x)=F(x) tan(alpha/2). The unsupported luff will need tension (distributed force again) so we cut a catenary curve, fold it over a bolt rope and tension that. Same for 'loose foot'. If we have a mast we fold the luff straight over the bolt rope, or in the case of the broadseamed sail above we would have to add the catenary.

To this point we have been treating the sail like a stiff film that is limited to conic shapes. All materials stretch and woven materials can rack easily. With stiff film it developes beam strength when bent that can counter the catenary. This allows the sailmaker to make the shape of the sail in say the first 10% of each end. Then the curved sail has enough section to hold up as a beam to the force of the wind through the middle. When you cut darts into paper at each end to give shape is a good example of this. Your tyvek behaves like a film or paper. All good commercial sail material behaves like film and professional sails put the shape in the foot and head.

Woven material can rack so it does not make a strong beam but this also means that it can form smooth shallow 3D shapes that are NOT conic. Because of this you can just cut the shape into the foot and head of the sail and let the wind distribute it acoss the sail. Your polytarp is woven and stretchy so this might work on it.

There are lots of other considerations that affect sail design, adjust-ability, life, stretch, turbulent air, heal angle, time correction, stability...that would cause you to 'cheat' from a theoretically correct sail. But if you are just making mud pies, you don't need to to go to chef school.

If you are tired of reading here is some sail analysis ****;

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### Wayne GrabowSenior Member

I don't remember what the Panama skiff weighed, but it was light enough that we (wife & I together) could easily cartop it and carry it from the car to the beach for launching. There were no launch ramps or trailers in Panama, so it was designed from the start to be easy to move. In addition to sails, I put a 1.5hp air-cooled outboard on it at times which was all it needed. Also had the mast break once due to strong wind, and I had to row it for a mile or more to get to the nearest dock.

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### laukejasSenior Member

I won't pretend that I understood what you wrote here, but my gut says this might be the formulas I'm after. I'll try to figure it out, given enough time. Thank you very much for this.

I see. It really looks like a nice boat. Thank you for sharing this

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### SkyakSenior Member

It's always a pleasure passing knowledge on to you. My explanation is based on physics -not art, not on anything I have read about sails. I will draw some figures to make the explanation clearer.

I should also tell you that there is nothing in my explanation that will enable you to make better sails out of lesser materials. The entire value of this explanation is to demystify sail lofting so you can 'see the forest for the trees'. Likely the best result will be for you to see there is too much to know and too little to gain by mastering it.

You will note I gave no formula for the aerodynamic profile. There is so much to know and so much has already been done I have nothing to contribute. I took a wonderful online course from MIT that I would recommend, but if you are not familiar with calculus it will be fairly incomprehensible. Pick a NACA profile, or any low Reynolds number design. The fabric you use will limit your ability to reproduce the profile. The 'art' you have read elsewhere likely recognizes this. Also, seams are difficult to do and weaker than the surrounding fabric. This is why most people working with polytarp or tyvek only put darts in the upper and lower leading edge of their sails. It's not worth weakening the sail to have even one full length seam. This is true of higher grade materials too. If you look at pictures of racing yachts you will see that the sail profiles of the Dacron sails are better than yours but still not really like theoretically correct -the carbon fiber sails ARE correct, and rather flat. And as you know, for a given pressure (wind speed) flatter means higher tension -based on the catenary curve formula.

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