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  #11  
Old 06-23-2013
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Re: Genoa Cut Flat for reefing????

Quote:
Originally Posted by Jeff_H View Post
I don't know of any way to calculate headstay sag. On my boat the sailmaker put a piece of tape perpendicular to the luff on my jib half way up. We then raised the jib with a light line at its head and while sailing stretched the line tight to the tack. Pictures were taken from the tack with various backstay tensions. We could then use the pictures on a monitor to reasonably measure the range of headstay sag. This was a parameter in his computer design software. My AP jibs are designed for a wide range of sag options and I use that range to allow the same sail to function across a very wide range of windspeeds.
Thanks Jeff. That's what I thought; a physical m/m is really the only way to get it right. On my old boat, getting down to the fine adjustments that would be important to someone who races, seems unnecessary. A fraction of a knot is not going to make any difference for cruising. That said, it's still important to get it close so that sails set as they should. With the weight of wire, the foil, and the sail there is a considerable amount of sag no matter how much tension may be on the headstay. I know if I don't have enough headstay tension, it's difficult to operate the furler because of the increased sag. I can almost adjust the turnbuckle based on how well the furler rolls up.
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  #12  
Old 06-23-2013
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Re: Genoa Cut Flat for reefing????

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Originally Posted by Fudzwollop View Post
Thanks heaps for your feedback guys. The boat is a 6 metre Jarcat Catamaran. Home built from ply. Not allowed to give you a link apparently as I haven't posted enough on this site. BUT if you google Jarcat Marine you'll find it

I was going to use 4oz Dacron, we don't get out a whole lot and I thought it would be easier to sew up. Also, I thought it might stretch a bit over time anyways creating draft. The alternative is to simply add some Luff curve and some Foot curve like they did in days past. I Have a copy of "Make your Own Sails", it is a bit dated but I am sure it would work fine still - like I said, I'm cruising not racing.

I am interested to understand how important adding draft is to a head-sail, and how much performance would be lost by not adding it. From what I am reading I should add a little at the very least, is this correct? And then I could add some foam etc to the middle of the luff to take up the bagging that would occur. Clearly I have little idea what I am doing here so please tell me straight if I am missing the point, I am sure I will appreciate it when I am out sailing using my DIY Genoa.

Thanks again in advance

Andrew Stokes
That's a neat little cruiser-cat! Very cool. It has a pretty good-sized genoa. Most beach cats are mainsail baiased, with tiny little jibs. It's surprising how essential those headsails are to making the boat go, tho. And without draft, a sail is not a wing. It's just a sheet of plywood. It will push the boat, but it won't pull it. Broadly speaking, you'll want around 12% (min) draft in a working headsail, measured as a function of chord length. That is, if your sail is 8' across at a certain height, it should have 11.5" of draft depth, usually located about halfway along that 8' line. Bit farther forward on a headsail, typically.

Don't count on stretch to provide draft -- Dacron sailcloth is built to limit stretch, and it tends to stretch in undesirable ways (on the bias). Luff and foot rounds work for a small sail attached to spars, but it's very hard to place the draft in the right spot that way, and again it works better with canvas or polytarp than with low-stretch materials.

Broadseaming is not hard! Really, it doesn't even need to be especially scientific. Figure out roughly where you want your draft. Call it 40% back from the luff, at each seam. Now: moving out both directions from that tick mark, go up 1/8" or so for every foot. One foot away, up 1/8". Two feet, up 1/4". and so on. You don't even have to curve both panels -- just the upper one. Connect the dots. Strike a hem allowance parallel to the top of the straight panel (I like 5/8"), and lay down some 1/2" Seamstick basting tape. Assemble sail with tickmarks aligned and curved panel exactly on your hem allowance line. Look good? No wrinkles? Sew.

FWIW, Don Casey advises 1" of negative luff round per 10' of luff, evenly distributed. I wouldn't bother with foam luff support -- your sail will probably be useable (with efficiency losses) as a sail up to 25% furled (which may = 50% area reduction). If your furler can survive the strain of sailing reefed, which is large. We sometimes deploy just a scrap of headsail on our SJ21, forereaching in big blows. We don't pretend it's aerodynamic or providing any real thrust. But it does help balance the boat & prevent rounding up while the reefed main does the work.

I'm jazzed you want to make your own sails. It's a great exercise for the cruising or daysailing person, to take an interest in what makes the boat go. But if you are gonna lay out $150 in materials, it's worth learning a bit of the craft. Resources include Sailrite & their cheap, simple pamphlets & videos;
Sailmaker's Apprentice: Emiliano Marino: 0639785801771: Amazon.com: Books Sailmaker's Apprentice: Emiliano Marino: 0639785801771: Amazon.com: Books


; and the aforementioned Sailcut, a free bit of CAD/CAM we've used to make three really gratifying sails. Your second one will be excellent.
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  #13  
Old 06-24-2013
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Re: Genoa Cut Flat for reefing????

Wow, that is awesome information bob (I assume it's Bob). Thank you for taking the time to reply with so much wonderful information. I have ordered a copy of Sailmakers Apprentice and will study it through before cutting or sewing anything. I see my flat Genoa idea isn't a good one. Draft is needed obviously and reefing needed too but how to maintain the sail shape when reefed appears to be a challenge with no perfect solution. Everything's a compromise isn't it but I am happy to sacrifice some performance for convenience, and with kids on board most times I don't mind being a bit under-powered.

Will continue to research
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  #14  
Old 06-24-2013
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Re: Genoa Cut Flat for reefing????

Quote:
Originally Posted by Fudzwollop View Post
how to maintain the sail shape when reefed appears to be a challenge with no perfect solution.
True. Many tears have been shed trying to achieve good shape in a reduced size; best method always seems to be "make the sail shorter, focused on the corners ." = Slab reefing of mainsails. And you can even tie a reef into the foot of a hanked-on genoa! Tho it's fuss on the foredeck, there's no convenient place to lash the unused sail material, and you have to move your sheets to a new clew location. Bet some kind of jibboom/Hoyt self-tacker would be stellar for genoa reefing.

Actually, the best method to maintain sail shape across wind ranges is to change the sail. So many variables -- draft depth, draft location, clew height, twist, luff sag, headstay tension, fabric weight & stretch, luff length -- are windspeed dependent, the best strategy is to have a dedicated sail for every 5kt increment. Obvious bummers are cost, stowage, and hassle of changing them out. But for the ultimate in (displacement) boat handling & efficiency, that's the way to go. Our Ballad specifies seven headsails, not including spinnaker! Four genoas, one spinnaker staysail, one inner staysail/blade jib, and one storm jib. We're chopping that down to four: drifter/code zero, #2 Genoa on a furler, #3(ish) staysail genoa, and storm jib. There will be light days the working #2 is too small as our primary; windy days we just roller-reef it to approximate #3 size & accept some loss of shape. *shrug* We like us some roller furling.

Good luck with your project & show us some photos!
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  #15  
Old 06-24-2013
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Re: Genoa Cut Flat for reefing????

My sail wardrobe will be reefable main, furling genoa and Iron Main sail. Reckon I can handle ANY wind conditions with that lot
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  #16  
Old 06-27-2013
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Re: Genoa Cut Flat for reefing????

Hey guys I do not know what m/ms means....can anyone explain please.

Cheers
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  #17  
Old 06-27-2013
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Re: Genoa Cut Flat for reefing????

Without a context its hard to answer your question but my guess is m=meter and ms= meter squared.
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  #18  
Old 06-27-2013
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Re: Genoa Cut Flat for reefing????

Quote:
Originally Posted by Fudzwollop View Post
Hey guys I do not know what m/ms means....can anyone explain please.

Cheers
From Smurph's posts? "Measure/ment", I suspect: "...a physical m/m is really the only way to get it right."

New to me.
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