Cross Cut vs. Radial Sail Construction
by Dan Dickison
Almost anyone who owns a sailboat and has spent any time purchasing sails understands that when it comes to sails built from panels of cloth, there are two basic types of construction: cross-cut and radial-cut. Cross-cut sails are ordinarily constructed from panels of fabric with the seams between each panel oriented in a fore and aft direction, parallel to each other and perpendicular to the leech. In most cases, the panels are rectangular or almost rectangular in shape. The fabric panels that make up radial sails, on the other hand, are usually oriented toward the corners of the sail. This means that the seams between panels aren’t parallel, but actually radiate out from the corners of the sail, resulting in panels that are almost always triangular in shape.
The panels used in each method are oriented in their respective ways primarily because of the material each is made from. By and large, cross-cut sails are built from Dacron, a trade name for polyester fibers manufactured by DuPont. Except for certain trademarked products, most woven sailcloth made from polyester is referred to generically as Dacron. And radial-cut sails are principally made of laminated sailcloth. Any fabric consisting of two or more layers of material adhered together into one is what we consider laminated sailcloth. There are essentially hundreds of such products on the market, and almost all of them contain three essential elements: 1) one or more layers of film (usually Mylar); 2) what some sailmakers call Tafetta (a woven fabric that is usually adhered on the outside of the laminate; 3) and fibers (often Kevlar, Vectran, Spectra, carbon, or polyester) that are laid into the laminate to enhance strength in a given direction.
The chief advantages of woven polyester sailcloth are that it’s extremely durable and relatively less expensive when compared to laminated sailmaking materials. Depending upon the kind of finish the Dacron gets after it is woven (in manufacturing, it is characteristically heated to shrink the fibers and tighten the weave, and then resin is applied to lock the weave together), it can end up either stiff or soft, or somewhere in between. All Dacron will stretch to some degree, but stiff or firm Dacron will stretch less than soft Dacron, and any propensity to stretch means that the sail won’t hold its designed shape quite as long. Despite the tendency to stretch, Dacron sails do tend to last a long time, and if they tear or wear through, they can be easily patched by unskilled hands with basic materials.
When Dacron sailcloth is manufactured, fibers are woven together. The longer fibers that run parallel to the length of a roll of sailcloth are called the warp fibers, and those that run perpendicular to them (or across the width of the roll) are called the fill fibers. Usually, there are more fill fibers in a given section of cloth than warp fibers, meaning that the cloth is stronger (more resistant to stretch) in the fill direction. This is the reason that almost all cross-cut sails are built with long horizontal panels so that the fill fibers will be oriented along the axis of greatest stress (vertically, in the direction of the leech).
For sailmakers, it is measurably more efficient to build cross-cut sails than to build radial-cut sails. With broad, almost rectangular panels, there is less material wasted than with the triangular-shaped panels, which is another reason why cross-cut sails tend to be less expensive. All of this means that Dacron will ordinarily end up being the choice for those clients whose chief concerns are longevity and economy, but not necessarily precise sail shape.
Because Dacron stretches, cross-cut sails require more fibers to resist stretch and hold their shape. That means that a sail built of this material ends up being heavier than an identically sized sail made from laminated sailcloth, and that’s not desirable for boat owners who are concerned with performance.
Laminated cloth, on the other hand, offers several advantages over woven Dacron. Not only is it lighter for a given amount of strength (resistance to stretch), but the panels can be cut in long, narrow triangles, oriented in line with the greater loads on a sail, and seamed in that configuration. This is possible because nearly all laminated fabrics are stronger in the warp direction than they are in the fill direction. Because of this, sail designers can more closely follow the load patterns on a given sail by carefully arranging the radial panels with one another.
So, how do you decide what type of construction is right for the new sail you have in mind? In general, if performance and precise sail shape are the overriding concerns, then it’s likely you’ll be more content with a radial-cut sail, like the tri-radial headsails and mainsails made by FX Sails. If you’re comfortable with trading off a loss in sail shape over time for greater longevity and a more economical purchase price, then it’s likely that a cross-cut sail will suit you best. Of course, these are basic guidelines, that won’t apply to everyone in the same way. It’s not uncommon for boat owners to mix and match. Whereas one owner might opt for a durable cross-cut mainsail to be paired with a tri-radial genoa in light air, another might opt for a full inventory of cross-cut sails. Remember, the type of sailing you do—inshore or offshore, racing, or cruising—the area in which you sail, and the type of boat and rig you have, are also important factors in determining what kind of sail(s) you should get for your boat.
About the Author: Dan Dickison is known throughout the sailing community for his in-depth articles on a variety of sailing topics. His resume includes stints as a staff editor at Sailing World, Editorial Director of SailNet, and Editor of Practical Sailor. In those capacities he has written principally about racing, sail handling, and maintenance. He has also written over 50 freelance articles that have appeared in major sailing publications around the world.