Need help understanding my sails

[djn]

Hi All, my old boat, catalina c25, had sails that were, for the most part, one piece of dacron. On my new 1958 wooden boat, the fairly new sails are 18" strips sewned together to make up the sail. Why use one style over another? Cheers.

[Nanoose]

What JohnB said, OR, some cloths come in narrower widths - e.g. our Oceanus is (if I'm remembering right? ) 18" wide.

[djn]

Thanks all, that makes sense. I should do like the looks of my sails......they match the look of the boat. Cheers.

[Todd Bradshaw]

Seamed, narrow panels and false-seamed panels (a panel with narrow, seam-like bands across it where the fabric has been folded into a "Z" cross section and sewn down) were primarily used for increasing bias-stability in the days of softer, less-stable fabrics. With today's modern weaves, resin coatings and laminated technology sailcloth manufacturers now produce many fabrics as wide as 54" and they have enough stability to be used full width.

Narrow paneled construction is still used as it was (and for the same basic reasons) when working with some of the modern, soft, canvas-like replica fabrics like Oceanus and Clipper Canvas or even with regular Dacron when a classic look is desired.

[Ian McColgin]

But why seams rather than one big piece of Tyvek? Because sails are not flat, like sheets or window shades. They are three dimensional to give the foil shape that makes them work. When the sail is up, look at the curve. Play with it by adding luff tension (halyard or downhaul) and foot tension (outhaul). Get one of the many good books on understanding sails and enjoy a whole new world of wonderful phenomena.

[Lewisboats]

I know on the sail I recently recieved...curve was worked into the edges of the panels...putting shape into the sail at each of the seams.

[djn]

My main sail has a pocket in the bottom that has a zipper along the entire bottom from the mast to the end of the boom. I assume I unzip it if I want more billow.....for lack of a better word. The zipper has a line attached and is lead from the mast to the boom end and I can unzip or zip it from the cockpit. When would I fly it unzipped? Cheers.

[Lewisboats]

My main sail has a pocket in the bottom that has a zipper along the entire bottom from the mast to the end of the boom. I assume I unzip it if I want more billow.....for lack of a better word. The zipper has a line attached and is lead from the mast to the boom end and I can unzip or zip it from the cockpit. When would I fly it unzipped? Cheers.

I could think of a couple of good reasons to unzip your fly...

[Todd Bradshaw]

You would unzip the foot when sailing downwind, when you can use more draft in the sail and then zip it back up, flattening the sail for upwind work. Zippered foot panels were popular for a while. They work, but fell out of favor mostly due to the zippers themselves being problematic - so be as gentle as possible with the zipper as there is a limit to how much abuse thay can tolerate.

[merlinron]

i would think with the newer fabrics and thier ability to last longer and stay stiffer longer, that the manufacturers have narrowed up the stock material width because more seems will allow, or be necessary to allow better shape to be sewn into the sail.

[Todd Bradshaw]

Not really, but mostly because the typical modern cross-cut sail is generally designed and cut with computers. If you build traditionally with modern fabrics - traditionally in this case meaning using straight, parallel-sided panels and transforming the flat panels into a 3-D shape by broadseaming (varying the amount of panel-to-panel overlap in the seams) then narrower cloth widths do allow you to do more subtle shaping. Eight or ten seams with small-ish variations in their broadseams may indeed make a nicer, smoother shape in modern, stiffer cloth than the same sail with wide panels and maybe only three or four seams available for doing the shaping with bigger broadseams.

But computers shape sails differently. They don't use broadseams. They keep the seam width and panel-to-panel overlaps constant and instead, add-in the shape by cutting the edges of the panels themselves into curves. The computer's ability to accurately think ahead and predict the shape that a particular pair of mated, curved edges will make and to adjust it easily is what makes the difference. For a production loft, this also means that the sewer can concentrate on neatly sewn, constant-width seams, rather than having to think about, or even know how to shape a sail. It's already been cut-in by the compiter/plotter (which usually also draws seam overlap guidelines of the fabric) and all that is required is careful assembly.

With this kind of ability to accurately predict what curves are needed to produce a specific draft amount, draft placement, entry angle or roundness and even in-use twist, the computer can do all the shaping with wider fabrics and relatively few seams. If desired, it can even rotate the fabric panels as needed to align the cloth's maximum resistance to stretch with the predicted stresses on the sail - the radial cut.

If desired, the computer can even plot a pretty good impersonation of a traditional, narrow-paneled sail, but like the modern cuts, it will be done with shaped panels and constant seam widths, rather than broadseams. With more panels, they just make the curves in the panel edges more subtle. The final shape will be nearly identical, whether the panels themselves are narrow or full-width.

[merlinron]

intresting insight todd,

i would have thought that even with computers, more panels,ie. narower panls with more seams would make it easier to get much better shape than with wider panels and less seams......parrallel seams or broadseams, does it really make a differance in this respect(?)..... the broadseams just being a way to shape with a minimum of cutting panels to shape.(maybe just a little more strength there)

i imagine( somewhat mechanicaly) pieces of paper cut as a computer would plot them and seamed together.... several narrow flat facets producing a truer foil than just a few wider facets, even with eventual stretch/settle (the wider panels being able to streatch/give, more of course) taken into consideration for blending the shape. on the other hand.... the sail with narrower panels have more "ribs" so to speak, to hold shape(?)

[Todd Bradshaw]

I think it has to do with just how little curve it takes to create sail draft. When we add luff curve, for example, the amount added is typically as little as 2%-3% of the local chord width. The broadseaming (or panel curving on computer-plotted sails) is then used to move the excess created by the luff round into a specific position toward the middle of the sail creating draft and it doesn't take much because we there isn't that much to move. So even if we do it all with just a few seams and panels, we can still generate a shape that smooths-out nicely in use. From a production standpoint, you also have to figure that making a seam costs more than not making a seam, so you can bet that this also enters into the equation. These days, most of the sophisticated high-performance sails are radial cuts (if they aren't molded to shape like the 3-DL sails) and radials do generally use narow panels. Not only do they align the weave more precicely to the stresses, but the many panels yield a smoother shape, especially in very stable and often rather stiff fabric.

[merlinron]

i guess your last statement there sums up what i thought was pretty much status quoe for sail design with computer generated panels....... i sort of assumed that most all sails are computer generated these days and the frequency of seems are pretty much the result of the accurate shaping obtainable by computers, but as you state, more seems = more$$!.
it's always interesting to see a thread on sails specificly,and then look for your responce......thanks!