Unless you're Huck Finn and willing to let the Big Muddy carry your boat with the whimsy of its current, you need a power source. There are still only two broad classes of propulsion for boats – mechanical and wind-driven. If your vessel falls into the wind-driven class you will inevitably have to deal with the process of buying sails. And unlike the tall-ship captains of yesteryear, you have many topics to consider – things like material, shape, performance, lifespan, cost, ease-of-use and technical support.
You might be surprised to know how many sailors really don't understand the underlying science of sailing. Of course, for some that blissful ignorance is not a problem. They're happy to just get out on a Sunday afternoon and enjoy the water. But for many sailors, a stiffening breeze encourages a finer hand with the sheets, a readjustment of the downhaul and a few extra glances at the telltales. Some may even decide to breakout the Genoa and see if they can add an extra knot.
If that describes you, then it's worth your time to brush up on the aerodynamics of sailing – before you make your way to the loft and get out your wallet.
POPE SAILSSailing downwind with a traditional symetrical spinnaker and pole.THE BASICS
The first scientifically accurate description of sail aerodynamics was authored in the 1970s by former Boeing engineer Arvel Gentry, first in a series of technical papers, followed by articles in "Sail" magazine. Although his work has been around for decades, it wasn't widely disseminated or popularly understood until the mid-90s, and even today you will hear dock-experts expounding on their own aerodynamic "theories."
An advantage to understanding the science behind sail performance is twofold –understanding how to trim your sails and rigging will make you a better sailor, and will also make you a better consumer when it comes time to buy new sails. Not only will you be able to ask better questions about how a particular loft can produce the proper sail for your boat, but you will also better understand the design and construction processes involved in making your sails and how they will take shape when hung on your boat.
You will also be able to provide the sail maker with more detailed information that will better enable the loft to make sails that suit your needs. For example, if you use a Cunningham instead of a simple downhaul, or you routinely adjust fore- and backstay tension while underway, your loft can provide you a sail that is better able to take advantage of those fine adjustments.
Incidentally, a better understanding of aerodynamics (perhaps better called fluid dynamics, since we're not always talking about performance of a shape in air) can make you smarter about keel and rudder performance, too. You can make better decisions about buying a boat with a fin keel, for example, if you understand how the fin was designed.
A lesson is sailing aerodynamics – much like the art of sailing itself – is beyond the scope of any one article. But if you want to know more, we've included a list of recommended reading to help you on your way (see sidebar).
SOMERSET SAILSEven modern sails are made on old loft floors.WHAT'S NEW
You probably won't be surprised to learn that ancient art of sail making has been invaded by high-technology. Two main areas have benefitted from the march of progress – available materials and design. Both contribute to better sails and both will have an impact on your buying decisions.
Sail materials have come a long way since the days of the earliest sailing boats. The biggest change, which began shortly after World War II, was a switch from organic materials to man-made fabrics. Ancient Egyptian rafts carried sails made of papyrus and later, cotton. The great ships of the 1800s wore suits of heavy canvas, strong but leaden when wet and susceptible to rot from mold and mildew. Enormous strength was required of the men who worked in the rigging of the old tall ships. A sodden topsail that had to be gathered and furled in the wind and rain of an ocean squall could test the limits of even the most capable crew. And in port, a frequent sight was a tall ship with every stitch of canvas hanging out to dry in the sun.
For the last half of the 20th century, the dominant material for sail making was Dacron polyester. Dacron was easy to manufacture, weave and sew. It was also much more durable than natural fabric, although it does stretch over time and still needs to be cleaned and kept mold-free. An even earlier man-made fiber, nylon, is still used for spinnakers.
Key characteristics for sail cloth include basic tensile strength, a characteristic called "modulus," which is the material's resistance to stretch (a good quality for upwind sails); "creep," which is the long-term stretching of a material over time and "flex loss," which is the reduction in strength resulting from folding, bending or when a sail is flogging. The last component is UV (ultraviolet) resistance, which determines the sail's ability to retain strength over long-term exposure to sun.
SOMERSET SAILSThis diagram shows how panels of spinnaker are designed.Over the last 15 years, new, man-made fabrics have dominated the high-end sail cloth market. A class of synthetic fabrics known as "aramids," short for aromatic polyamides, ushered in the "bullet-proof" fabric era, with well-known names such as Nomex and then Kevlar. Kevlar was originally developed to replace the steel belts in tires. Newer variants of these types of synthetics include HPME, Zylon and Vectran. The aramid fibers generally exhibit strength-to-weight characteristics significantly greater than steel and resist stretch and creep much better than polyester. They aren't perfect, however, and suffer from UV damage more easily than traditional polyester sails. They can also be incredibly expensive. Chemical and textile companies are constantly improving all the basic materials – including the basic polyester materials for sails – and almost all are an improvement over basic Dacron.
THE PROCESS
The latest material innovations involve sails that are laminates – layers of different fabrics or materials fused together; and sails that are "molded" over three-dimensional shapes. Laminates can take advantage of the unique characteristics of different fabrics, using one to provide a high modulus, while another layer provides better strength, for example. Often, films like Mylar are used in laminates to reduce the porosity of a sail and to provide a very smooth base.
