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  #11  
Old 05-08-2013
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Re: Why round sail edges if there going to be tied to the mast and boom

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Originally Posted by SchockT View Post
Bobmcgov,

your post implies that luff curve is something that was done in the past. It should be noted that cutting curve into the luff and leech remains an important part of modern sailmaking, even on the most advanced sail designs. It is not just the domain of "cheaper" sails.
Nono -- you misunderstood. I said that in the past, luff rounds were there primarily to create draft. In the days of manila cordage and cotton canvas. Nowadays, draft is induced at the seams (or in the case of 3DL, etc) in the panel material itself. But sails still have rounds, for lots of good reasons: to accommodate spar prebend and flex. To add area, particularly untaxed sail area. To lay against the deck (headsail) or boom (loose-footed main) & provide a seal -- end plate effect. To generate the 'knuckle' in free-flown sails like asyms & Code Zeros that give those sails their tremendous drive. And so on.

Some polytarp experimenters still use luff & foot rounds to create draft, and that's a fine approach for very small boats or lower tech materials. If you are cutting a leg-of-mutton sail out of Tyvek for your blow-moulded sit-a-top ... yeah, probably don't need to mess with broadseaming.

Matt: Sailrite also prints some inexpensive booklets on making various types of sails. The information is a bit dated, but it's a practical approach for the home sailmaker. If you really want to get precise, the Sailcut CAD/CAM program is available free on Sourceforge. It will let you design a sail based on a few critical dimensions. The software will fill in the fields to generate a middle-of-the-road thing, or you can tweak parameters like draft depth, luff round location, roach, and so on. Helps to know a bit of theory going in. We've built three sails using Sailcut. (There's a spinnaker version of Sailcut out there, too - rather harder to find,)
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Last edited by bobmcgov; 05-08-2013 at 06:51 PM.
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  #12  
Old 05-08-2013
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Re: Why round sail edges if there going to be tied to the mast and boom

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Originally Posted by bobmcgov View Post
Nono -- you misunderstood. I said that in the past, luff rounds were there primarily to create draft. In the days of manila cordage and cotton canvas. Nowadays, draft is induced at the seams (or in the case of 3DL, etc) in the panel material itself. But sails still have rounds, for lots of good reasons: to accommodate spar prebend and flex. To add area, particularly untaxed sail area. To lay against the deck (headsail) or boom (loose-footed main) & provide a seal -- end plate effect. To generate the 'knuckle' in free-flown sails like asyms & Code Zeros that give those sails their tremendous drive. And so on.

Some polytarp experimenters still use luff & foot rounds to create draft, and that's a fine approach for very small boats or lower tech materials. If you are cutting a leg-of-mutton sail out of Tyvek for your blow-moulded sit-a-top ... yeah, probably don't need to mess with broadseaming.
Good thing i got a sit in side then
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Old 05-08-2013
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Re: Why round sail edges if there going to be tied to the mast and boom

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Good thing i got a sit in side then
It is worth taking the hull form into consideration, tho -- the hull, sails, and appendages all work as a system. That will guide you in your sail design decisions. Unless you are adding amas, ballast, or radical hiking platforms for instance, you are limited in how much side force your kayak will tolerate -- which will shape how high your center of effort can be, and total upwind sail area.

Personally, I'd look into adapting windsurfing rigs for what you describe. Those folks have already worked thru many of the same design challenges you will encounter & the masts and sails are optimized for light, narrow shallow-draft vessels. I started windsurfing in the early '80s, on the old 65# BICs and Mistrals. Today's spars and sails are radically more complex than what we had.
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Old 05-08-2013
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Re: Why round sail edges if there going to be tied to the mast and boom

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Originally Posted by bobmcgov View Post
It is worth taking the hull form into consideration, tho -- the hull, sails, and appendages all work as a system. That will guide you in your sail design decisions. Unless you are adding amas, ballast, or radical hiking platforms for instance, you are limited in how much side force your kayak will tolerate -- which will shape how high your center of effort can be, and total upwind sail area.

Personally, I'd look into adapting windsurfing rigs for what you describe. Those folks have already worked thru many of the same design challenges you will encounter & the masts and sails are optimized for light, narrow shallow-draft vessels. I started windsurfing in the early '80s, on the old 65# BICs and Mistrals. Today's spars and sails are radically more complex than what we had.
I think I still have an '80s era Bic 250 sitting at my mom's house, complete with the triangular rainbow sail!
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Old 05-11-2013
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Re: Why round sail edges if there going to be tied to the mast and boom

If you think about what a sail is supposed to do - curve the wind around it - you can see why a sail needs to have smooth curves in order to perform most efficiently. A sheet of plywood CAN serve as a sail, but it is not very efficient. A curved piece of material the same size would be more efficient - making the boat move faster - than the flat board. Controlling the shape of the curve is what sailmaking is all about. Sailmakers create curves in the sail surface by broadseaming (as noted above) and by adding additional material to the luff, leech, and foot. This rounding of the edges creates a "bag" to the sail when it's hoisted on straight masts and booms. The mast, boom, and sail itself can be adjusted to create more or less curve in the sail, depending upon the wind force and direction. All this aerodynamic stuff applies to every sail, but for a kayak some things may make more of a difference than others. Going downwind, simply holding up a paddle can move a kayak along at better than a knot. You could spend $2000 to have a sailmaker design a carbon-fiber sail specifically for YOUR model of kayak and how you plan to sail it. In between are the poly-tarp, woven, or other material sails that are all possible, and which, depending upon their design, may be more or less efficient at moving the boat, and may last longer or shorter periods. You have to decide what works best for you, balancing out the material, shape, longevity and cost. Good luck!
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