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twhite View Drop Down
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    Posted: 29 February 2012 at 12:34am
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Hi folks,


I'm rather new to making sails, I hope you don't mind me asking a few questions. Mostly about the basics.


A couple of years ago Shannon and I bought a Dufour 27. Neither of us had any experience with sailboats, but it seemed like a very good quality boat, and we got it for a really good price. It came with quite a pile of parts, and what appeared to be a roll of sail cloth. When we got around to looking at it, we found that someone had started to make a mainsail. It turned out to be a 8 ¼ ounce material (American measure). It's pretty stiff stuff, although not overly so. All of the panels were cut out already, a bit larger than necessary. the luff was marked, and one of the battens .But nothing else. It's laid funny also, the panels go horizontal, so they are perpendicular to the luff. It's my understanding that they should be perpendicular to the leech, so the leech doesn't stretch.


I ran across a program called sail cut on the Internet. It seems to be a pretty sophisticated program, it will allow me to cut any width of material, to any shape whenever. It will lay out the individual panels for me, although I need to do a little more work to get it to run. It seems to be written mostly for racers, and used mostly by them. They used to have a forum, but then they got too many ads for Viagra, and closed it down. Anyway, there's little there to suggest any decent starting points.


So I got several books on sails and sail making, even plowing through Tony Marchaj's book: sails and sail performance. But when it comes down to designing the sail, I'm as much in the dark as when I started. What I need are some basic starting assumptions, such as what a sailmaker would use.


So here are the basic facts:


The boat is a Dufour 27, it's a medium displacement fin keel cruiser/racer. We're using it for cruising. We live in the middle of the Oregon Coast, about 600 miles north of San Francisco. The coastline here tends north and south, as do the winds. The wind tends to be fairly strong, 20 or 30 kn regularly in the summer. So are either going with the wind or beating against it.


The sail dimensions are these:


Luff: 8800 mm


Foot: 2800 mm


Leech: 9200 mm


Surface area: 14.4 m²




Basically, what I need to know is the amount of camber to start with, and position thereof. I've read that 10° is appropriate in an intermediate wind, and that there is no advantage in having less than 7° no matter how high the wind pipes up. So I'm guessing, that I would start with 12°, or possibly 15°.


As far as position, I've heard several figures, from 33% to 50%. I tend to doubt the 50% figure as being more appropriate for light air.


I also understand that the draft needs to decrease the further up you go on the sail. But here I truly don't have a clue. Should it be 2% less in the middle, and 4% less at the top? Or should the difference be more dramatic.?


In the program it asks for a twist angle. Which appears to be the angle that the sail actually twists. But we can't tell from our old sail, it's so stretched out of shape that if we sheet it down tight the leech hooks around to windward. Any suggestions here?


If there is anything else I've missed, would appreciate you letting me know about it.


Thank you for your help, Tom


Tom Whitehead

Shannon Lenz

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MattGrant View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattGrant Quote  Post ReplyReply Direct Link To This Post Posted: 29 February 2012 at 8:01am
Hi Tom and Shannon,
Yes for a vertical cut sail the panels should be parallel to the leech. If the sail were cut as a standard horizontal cut, the seam would be perpendicular to the leech.
 
I am not familiar with the Sail Cut freeware. We use a program here at Sailrite called "Prosail".
 
I suggest that you purchase Jim Grant's book "The Mainsail Manual". It will give you the basics on traditional lofting of a sail. His book will explain how to convert desired camber into draft and position. Ultimately you end up with broadseaming figures which is how the shape is actually built into the sail and positioned.
 
Thanks, Matt
Sailrite
 
 
 
Best Regards,
Matt Grant
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Post Options Post Options   Thanks (0) Thanks(0)   Quote twhite Quote  Post ReplyReply Direct Link To This Post Posted: 29 February 2012 at 10:43pm
Hi Matt,

Thanks, we bought all five of Jim Grant's books, and I did read through the mainsail book.  I do remember it having something in there about the broad seaming process. That's not the problem, however.  What we really need is a good first approximation on what our camber and position should be. What we're looking at,is building an all-purpose sail.

So, if you were to design a sail for our boat, and our section of the Coast, what percentage of draft would you use?  And where would you place it? And how would it change as it goes up the mast?

Best wishes, Tom
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattGrant Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2012 at 8:22am
Hi Tom,
The Mainsail Manual does not get into detailed variations of draft depth or maximum position aft. It assumes an all-purpose design and shape. This means draft position roughly 45 percent back (along the lowest seam) moving forward toward the top of the sail. And draft of about 1 foot depth to 10 feet of chord. This is 10% camber. Modern sail design packages would call for roughly 14% toward the top all the way down to 3% at the bottom. I guess what I am saying is the process is a little different for computer design and traditional sail lofting. Follow Jim's book and you will be happy with the sail shape. I hope this helps :).
Thanks, Matt
 
Best Regards,
Matt Grant
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Post Options Post Options   Thanks (0) Thanks(0)   Quote twhite Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2012 at 12:45pm
Hi Matt,

Thanks,I hadn't thought about having more camber at the top of the sail.  It makes sense though.  And moving the camber forward as you go up, makes sense as well.  So, I'll work up a design both on the computer, and manually as well, just to doublecheck.  Shannon has already talked to you guys about getting the rest of the hardware, will call you when we're ready for it.

About having a larger camber towards the top of the sail?  On our old sail, the forward end of the batten forces the sail to assume a V shape. It does that for both of the top two battens.  Not very aerodynamic, to say the least.  Should I use tapered battens, at least for the top two?

Best wishes, Tom
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattGrant Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2012 at 1:24pm
Yes Tom, it sound like the top two battens are too stiff. Or you might need the tapered battens. The hard angle at that region of the sail will not help with performance.
Best Regards,
Matt Grant
Sailrite
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