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.

One of the most innovative sailmaking processes involves "molding" sails directly into their ultimate three-dimensional sailing shape. Ordinarily, sails are sewn together from panels that are specially cut with curved edges so that the sail will take on the three-dimensional shape of an airfoil. North Sails pioneered a new method: a Mylar base is carefully laid onto a solid, three-dimensional mold; then Kevlar threads are laid directly on the film. Another layer of the Mylar is applied and then the "sandwich" is then glued together. The sail will take its ultimate airfoil shape more easily, and since there are no break-points or bends in the Kevlar fibers, the sail will be both lighter and stronger than a sail sewn from conventional, two-dimensional panels.

NORTH SAILSA wind tunnel test is performed on racing sails.For cruising sails, the 3-D process, trademarked by North Sails as 3-DL (which stands for 3-D Laminate), may include either coatings or additional, more UV-resistant fabrics, to lengthen the sail's lifespan. In racing, these types of sails can often last three or more seasons, compared to barely one season for conventionally sewn racing sails. Of course, extreme racing, such as the America's Cup, may retire a set of sails after only one race.

SPECIAL SAILS

In addition to new materials and pricesses, there are also some relatively new sail types on the market, such as asymmetrical spinnakers. As the name suggests, asymmetrics fly off the centerline of the boat making, them well-suited for reaching.

Doug Pope, owner of Pope Sails and Rigging of Rockland, Maine, says most sailmakers have a number of asymmetrics for sale.

"For racing, they can dice the whole thing pretty finely, with sails for very specific apparent wind angles and speeds," Pope says. "For cruisers, we [ask] about what kind of sailing they want to do. We'll usually sell them more of a reaching asymmetric sail than a runner. They don't run downwind very well, although poling them out can help, like with a genoa."

Pope says all their sails come with a sleeve for hoisting and furling.

Asymmetrics are just the latest fad, according to Pope.

"Every year there is one particular item that, for some unknown reason, you will sell a lot of," Pope says. "One year it's travelers, some years it's roller furlers. This year was the year of the spinnakers, more than any other year."

Pope said his company did sell one or two symmetric spinnakers, "but for the most part, they've been asymmetrics."

THE BUYING PROCESS

Most sailmakers advise buying locally, particularly if you have special requirements such as a unique boat or specific cruising or racing preferences that might not be covered under standard class specifications.

If price is your only consideration, then go ahead and buy on the Internet; you'll get basic Dacron sails that will get you from point A to point B. But if you want sails that are optimized for your boat and rigging, seek a local sail loft. There are many independent lofts and most of the larger names in the business, such as North Sails, have regional lofts to handle a local customer base.

Pope has a good guide to sail-buying on his Pope Sails and Rigging website that will walk you through the process. He points out that sails should be considered durable goods – built to last more than three years and, "if cared for, can have a useful life of 9 or 10 years depending on how much they are used."

A good sail maker will ask you many questions. The basic question is what kind of sailing you do. According to Pope, "a mainsail built for trade wind sailing should be built differently than one built to cruise Penobscot Bay."

Choosing the features for your new sails is also important. For example, mainsails "have to be the most flexible sail on the boat," says Pope, because they are up in nearly all weather conditions. The number of reefs to be built into the sail can have a "significant impact on the price and versatility of this sail," Pope says.

For day sailing, he says, one reef is probably adequate, while for coastal and offshore sailing, two reefs are appropriate. Pope says that three reefs in a main are possible, but that for offshore work a tri-sail is a better alternative. He also notes that the type and orientation of main sail battens is one of the biggest questions people have about buying a new main sail. He says sails with metal slides that run on a track attached to the back of the mast do not work well with transverse battens. "These boats should stick with traditional battens unless a new track/slide system is installed," he said.

The cost of new sails has been rising each year, according to sail makers interviewed for this story. Many of the materials are impacted by the price of petroleum, so those increases can be substantial. Less expected, but perhaps even more significant, are the cost of rigging components. Pope says his loft is seeing almost weekly price increases. He attributes the rise to the demand for wire and metal in large, growing economies in other parts of the world. "In the course of a one-week," Pope said, "3/16-steel wire in a standard bale went up by $250, which will amount to 40-percent to 45-percent increases over the course of a year."

Buying a new sail or set of sails is a big investment. Taking the time to become an informed consumer can make the experience both exciting and enjoyable. The key to buying sails is to go beyond just choosing a material. Ask about items like mainsail insignias, numbers, sail ties and reef ties.

As Pope put it, "How complete is the quote?"

Tom Tripp is a freelance writer specializing in technology and marine science whose work has appeared in publications such as Northeast Boating and Chesapeake Bay Magazine. In addition to contributing features on new boats and technology, Tom writes a blog here on Mad Mariner.