FI - Sail cloth

FIREBALL INTERNATIONAL

a fireball sailor's resource

Sailcloth 101: A Primer

[Sail cloth] where was I? | go back

What is Dacron?

Dacron is Du Pont's brand name for its polyester fiber. Most of the world's polyester sailcloth is woven from Du Pont Dacron. Think Xerox, Scotch Tape and Kleenex, all brand names that have come to define a kind of product. To be politically correct and avoid litigation, we will use the term polyester rather than Dacron.

What is polyester sailcloth?

Polyester sailcloth is a fabric woven out of polyester fibers and subsequently coated with a resin. How it is woven and how it is finished influences the sailcloth's properties.

How does weaving influence the polyester sailcloth's properties?

Before getting into specifics, let's go over some terminology.

Warp is the long direction of a roll of sailcloth. Warp fibers are continuous for the length of the roll.
Fill (sometimes called weft) is across the roll, perpendicular to the warp. Fill fibers are only as long as the width of the roll.
Bias is at a 45-degree to the warp and the fill. Polyester sailcloth is almost always woven in a plain weave, where every fiber is passed over and under every other fiber in the perpendicular direction.
Crimp refers to the 'waviness' of the fibers as they go over and under the fibers in the perpendicular direction. The more crimped a fiber, the more it stretches when pulled (the 'waviness' straightens out).
In balanced plain weave sailcloth, the warp and the fill have the same amount of crimp. This leads to sailcloth that is relatively stretchy along the warp and the fill, but resists stretch on the bias through mechanical interlock between the warp and fill fibers.
In highly oriented plain weave sailcloth, crimp is limited to one direction only (usually along the warp). This leads to sailcloth that is relatively stretchy along the warp due to the crimp in the warp fibers, not stretchy at all along the fill since the fill fibers do not have to have their 'waviness' straightened out, but stretchy on the bias due to the lack of interlock between the warp and fill fibers.

The more oriented the sailcloth, the less crimp, the less fill stretch, about the same warp stretch, but the more bias stretch. If it were possible to weave polyester sailcloth without any crimp at all (it's not), it would not stretch along the warp and fill, but the bias stretch would be so great that the sailcloth would be useless. Sailcloth manufacturers play around with fiber size and crimp to provide specific properties, and sailmakers have the unenviable task of selecting the sailcloth with the desired properties. For example, a mainsail is usually very highly directionally loaded along the leech, so an oriented sailcloth should be selected to minimize stretch along the leech. However, we want the mainsail to keep its shape (camber, maximum depth, luff curve), so a balanced sailcloth should be selected to minimize bias stretch. On larger keelboats, where mainsails tend to be made out of many pieces of sailcloth, the sailmaker can use an oriented sailcloth in areas of high directional loads, and use a balanced sailcloth where bias stretch is required. On a smaller mainsail such as the Fireball's, where mixing and matching sailcloths would be expensive and impractical, the sailmaker must try to find the ultimate compromise sailcloth.

How does finishing influence the polyester sailcloth's properties?

Once the polyester fibers have been woven and the sailcloth cleaned, the sailcloth is coated or impregnated with a resin, and treated with heat and pressure. The resin (usually a urethane) bonds the fibers together to resist stretch. The heat shrinks the polyester fibers, increasing the interlock between the warp and the fill. The pressure presses the warp and fill together, further locking in the geometry of the weave. When the resin finally breaks down from use, flogging and UV exposure, the fibers start to stretch, the weave loosens up, and the sail quickly loses its shape. As with the weaving, sailcloth manufacturers are always trying to find the perfect coating that will provide the appropriate properties.

What is laminate sailcloth?

Laminate sailcloth is when layers of film, taffetas, scrims or knits are bonded (laminated) together.

What is film?

Film usually refers to polyester film, otherwise known as Mylar, brought to you once again by the good folks at Du Pont. To create Mylar film, polyester resin (almost identical to the polyester used to make Dacron) is extruded into blocks, which are subsequently drawn (stretched) into thin sheets. The drawing is done in orthogonal directions, which lines up, stretches and stiffens the polyester molecules into those two perpendicular directions, effectively creating a warp and a fill. Film might appear to be a homogeneous material, but it actually behaves like a woven fabric without any crimp in the warp and fill (less stretch than the woven sailcloth), but with some bias stretch.

What are taffetas, scrims and knits?

Taffetas are simply lightweight highly oriented single weave sailcloths. Scrims are very loosely woven open-weave fabrics. Open weave means that there are visible spaces between the fibers of the woven cloth, and the loose weaving reduces warp and fill crimp. Knits are not woven, but consist of layers of fibers tied together at their intersections. Knits have very low stretch (no crimp), and the layers of fibers can be oriented in specific directions. Knits and scrims are usually used in sailcloths weighing three ounces per sailmaker's yard or less, and taffetas are usually used in heavier sailcloths.

What are taffetas, scrims and knits made out of?

Almost anything. Originally, polyester fibers were the only thing available, and is still very cost competitive for lighter applications. As time went on, new fibers were introduced: Kevlar (those Du Pont chemists are really quite productive), Spectra, Technora, Certran, Twaron, Vectran, Dyneema, Pentex and PBO are all lighter/stronger/… than polyester, but tend to cost considerably more. If polyester film is just like polyester sailcloth, why not make sails from it? Technically it would be possible to do so, but early experiments (around 1964) quickly revealed that polyester film was very delicate and had little tear strength. To protect the film from tearing and in order to be able to stitch panels together, balanced open weave polyester sailcloth was laminated to the film (usually only on one side of the film). The film was doing the work, and the polyester sailcloth was holding the seams together. Towards the mid-eighties, with the advent of the above-mentioned wonder fibers, it was determined that it would be more efficient for the fibers to do the load-carrying work, and for the film to hold the fibers together and to control bias stretch. To protect the rough surface of the scrims and knits, they were sandwiched between two polyester films. Since the films did not have to do any of the 'heavy lifting' anymore, they could be made quite thin and be tailored to provide the appropriate amount of bias stretch.

Technically, what is the advantage of laminate sailcloth?

Polyester sailcloths are compromises. Balanced plain weave polyester sailcloth has good bias stretch characteristics, but tends to stretch in both the warp and fill. Highly oriented plain weave polyester sailcloth does not stretch much in the warp but tends to stretch in the fill and bias. The weaving process only allows for optimization of one of the three desired properties, so the sailcloth manufacturer has to decide on which one to choose. The finishing process allows for additional control over the bias stretch and some additional control over warp and fill stretch (we're up to two of the three desired properties), but that only lasts as long as the urethane resin. Laminate sailcloths can give you three for three. Warp and fill stretch can both be controlled by the low-crimp scrim (or the no-crimp knit), and the bias stretch can be controlled by proper film thickness selection. Since there is no urethane resin to deteriorate, the competitive lifespan of a laminate sailcloth mainsail is longer than that of a polyester sailcloth mainsail, and the laminate sail will keep its racing shape until flogging causes the film to delaminate from the scrim or if the sail is subjected to loads that stretch the film beyond its elastic limit.