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
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; http://www.amazon.com/Sailmakers-Apprentice-Emiliano-Marino/dp/0071376429; 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.