Sail Design: Recent Advancements in an
Ancient Technology
By Thomas Turansky
Sail power was the only reliable form of long-distance transportation for mankind from
its inception roughly five thousand years ago until approximately one hundred and fifty
years ago. Beginning in the second half of the nineteenth century, advances in steam
technology began to spell the end of the age of sail and with it, likely advances in sail
design. For the next one hundred years, the principles of sail design would remain
unchanged. In the 1950s, however, new advances in synthetic fibers led to a sailcloth
revolution that forever changed the nature of sail design. This evolution in sail design
continued into the computer age with the adoption of computer-aided design software
and analytical testing of sail models. The rapid advances in sail design seen in the latter
half of the twentieth century can only cause one to look toward the future with both
optimism and amazement about what may happen in the next fifty years.
Introduction
Until about one hundred and fifty years ago, the lack of modern roads and mechanized transport
meant that mankind was almost entirely reliant upon the wind to move people and goods about
the world upon rivers, lakes, and oceans. Before the advent of reliable steam power in the mid
nineteenth century, the vast majority of the world’s commerce travelled by some form of sailing
vessel. These vessels evolved from the first reed rafts used by the Egyptians to move goods and
resources up and down the Nile more than five thousand years ago to the immensely successful
clipper ships of the late 1800s, which were able to make trips along the China-England or
Australia-England routes faster than the steam ships of the day [1]. However, by the turn of the
twentieth century, advances in steam technology had relegated sailing ships to handling a mere
30% of the world’s commerce, the least amount ever in history to this time [1], and in just
another few decades, the days of the sailing ship as a meaningful conveyor of trade would
forever be a memory.
Although the majestic ships of old would be only a memory in the years to come, sail power did
not die out as a practical form of vessel propulsion in the twentieth century. On the contrary, the
technological advances in sail power that have occurred in the twentieth century dwarf all other
advancements made in the history of mankind [2]. The rise of the pleasure yacht in the past
century has led to innumerable advances in sail design, and the intense competition between sail
makers is best seen in the America’s Cup yacht race. The America’s Cup began in 1851 with the
victory of the U.S. schooner America and has continued since then as one of the world’s oldest
active competitions [3]. By the Cup’s post-war resumption in 1958, the concept of sail design
had forever changed with the introduction of the first synthetic sail designs in the world’s
premier yacht race [2].
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The Physics of Sailing
As many laypersons know, sails serve to propel a vessel by harnessing the wind’s power;
however, few understand just how they accomplish this. Sails manage to create forward thrust
from the wind in two distinct ways: first, when sailing in the same direction as the wind, a sail
acts a scoop that traps the air flowing by; second, when sailing against the wind, a sail acts as an
airfoil that creates aerodynamic lift in the same manner as an airplane wing [2].
Sails acting in the first manner described above are aerodynamically stalled, meaning that it is
only drag that is propelling the sail forward because the wind is only acting upon one side of the
sail. In the second instance, the wind parts onto either the windward (closer to the wind) or the
leeward (farther from the wind) side of the sail, which causes a flow differential on either side
due to the sail’s curved shape and can be seen in Figure 1. As the wind flows past the windward
side, the air spreads out creating positive pressure against the sail; meanwhile, the wind flowing
past the leeward side of the sail is being squeezed together, which creates negative pressure. As
seen in Figure 2, it is the combination of these positive and negative pressures against the sail
that creates lift and propels the boat forward [4].
Figure 1: Illustration of wind flow over a sail's airfoil creating both
positive and negative pressure on the windward and leeward sides
(respectively) [4].
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Figure 2: Illustration of the combined positive and negative
forces on a sail through one cord [4].
Historical Sail Materials
The ancient Egyptians were the first people to use cloth sails to propel their boats up the Nile
River as early as 3300 B.C. Since their invention, cloth sails have been made out of many
different natural fibers such as flax, hemp, ramie, and jute, and it was flax (linen) that became the
dominant fiber used in sail making during the Age of Discovery (c. 1450 – 1600) and into the
beginning of the Age of Sail (c. 1600 – 1750) [5]. By the end of the eighteenth century, cotton
canvas had begun to replace linen sails because of its lighter weight, and, in places such as the
newly formed United States, greater availability. Cotton would remain the material of choice for
sail makers until the development of modern synthetic fibers in the mid-twentieth century [2].
The development of synthetic fibers in the early twentieth century, such as nylon and Orlon,
paved the way for new developments in sail design; however, these two early synthetics still did
not have that great of an advantage over cotton because early nylon sailcloth was incredibly
susceptible to rippling, elongation, and water absorption, while Orlon was limited to lightweight
(3.8 – 5 oz) cloth [2]. Thus, both of these new fibers presented few advantages over cotton, since
all sailors and sail makers were intimately familiar with cotton and its primary weaknesses—
water absorption, stretch, and rot—were not solved with the first round of synthetics [4].
Modern Fibers
The development of modern synthetic fibers caused a revolution in sail performance and design,
as they are much more resistant to both rot and UV damage from the sun than natural materials
like cotton and linen. In addition to their inherently greater resilience and longevity, synthetic
fibers also behave more predictably than their natural counterparts, which allows sail makers to
have much more consistent quality than in the past. The three most common synthetic fibers used
today are Dacron, Polyester/Mylar, and Kevlar/Mylar [2].
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Dacron
Dacron is one of the first and most popular synthetic fibers used in sailcloth manufacture. Dacron
is DuPont’s trade name for its polyester fiber, but since DuPont manufactures such a large
proportion of sailcloth, the term has become genericized [2].
Dacron is the most popular sailcloth in the industry due to its low stretch, high strength, and
ability to repel water [4]. These three factors are absolutely critical in sail design because a sail
needs to retain its proper shape throughout its life in order to function most effectively. Also,
Dacron’s water resistance keeps sails from getting waterlogged in foul weather, which causes the
sails to droop and thus destroys their intended shape [2].
Dacron’s major advantages over its predecessors—low stretch and high strength-to-weight
ratio—were not fully realized when the cloth debuted mostly because these principles were not
well understood at the time [2]. When Dacron was introduced in the early 1950s most sailors
cared only that it had a much greater resilience to both water and UV damage, and, as explained
below, it was not until Ted Hood revolutionized both sailcloth and sail design in the latter part of
the decade that modern sail making began [2].
Polyester/Mylar
Mylar, another DuPont trade name that has become genericized, is a film formed from melted,
and subsequently extruded, polyester resin. Then the material is mechanically stretched along in
both north-south and east-west orientations, which aligns the polyester molecules from a random
orientation to one that is bidirectional [2]. This process, as seen in Figure 3, forms a film that is
much more resistant to stretch than its parent material in the same manner that plywood is much
more resistant to shear stress than the individual plies from which it is composed.
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Figure 3: Illustration showing the molecular orientation of Mylar film [2].
This low-stretch film is then laminated to Dacron sailcloth with a solvent-based adhesive that is
highly wear resistant, thus forming a new laminate cloth that exhibits much less stretch than pure
plain woven Dacron and also has a much higher strength-to-weight ratio [2]. Since it is these two
properties—low stretch and high strength-to-weight ratio—that most affect sail performance, this
innovation in sailcloth design that occurred in the 1970s and ‘80s gives a marked improvement
on simple Dacron, as can be seen in Figures 4 and 5 below; however, this benefit comes at
increased cost due to the increased labor component in its production [2].
Kevlar/Mylar
Kevlar, yet another genericized DuPont trade name, is one of more-recently developed synthetic
fibers that has an incredible resistance to stretch (approximately eight times greater than
polyester) as seen in Figure 4. This fantastically strong and light fiber, the same used in modern
body armor, began taking the sailing world by storm in the early 1980s and has since outclassed
Mylar sails, but Kevlar’s superiority comes at a great cost since the fabric will degrade rather
quickly from sharp flexing and creasing—two very common forces on sails [2]. In addition to
this increased fragility over Mylar and Dacron, Kevlar is also an order of magnitude more
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expensive to produce. However, all of these disadvantages are outweighed by the fact that Kevlar
is also an order of magnitude better against its predecessors in both stretch-to-weight and
strength-to-weight ratios as seen in Figures 4 and 5.
Figure 4: Stretch vs. weight of Dacron,
polyester/Mylar, and Kevlar/Mylar
sailcloth [2].
Figure 5: Strength vs. weight of Dacron,
polyester/Mylar, and Kevlar/Mylar sailcloth
[2].
The revolutionary Kevlar/Mylar cloth is made by first weaving a cloth out of Kevlar and Dacron
that has the Kevlar running in one direction and the Dacron running in the other and then
bonding this new fabric to a layer of Mylar film in a process similar to that used in the
manufacture of polyester/Mylar [2]. The resulting fabric is arguably the strongest and lightest
sailcloth ever created, albeit the most expensive.
Sail Shape
As explained above, a sail is an airfoil, which means that even a small change in its shape will
drastically alter its performance. In light wind, a sail with a deep draft (see Figure 6) will
outperform a sail that is very flat, while the opposite is true in heavy wind. This means that a sail
must be designed so that it can be adjusted to suit a variety of wind conditions, which is why
sailcloth is so important, because a more stretch-resistant cloth means that there will be less
deviation from the designed sail shape.
After centuries of working with the same material (cotton), sail design had hit a plateau;
however, the introduction of new sailcloth materials in the 1950s allowed sail shape to radically
change. Two men spearheaded this sail design revolution: Ted Hood and Lowell North.
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Figure 6: Illustration of an sail’s draft and chord [2].
Ted Hood
Ted Hood began making sails in 1952 and is notable as being the man who designed and built
the sails for the America’s Cup defenders from 1958 to 1977 [2]. Hood, who began designing
sails as a young boy, realized that the key to successful sail design was in the cloth because the
way a sail stretches out ultimately defines its performance. After this realization, Hood began
weaving his own Dacron cloth, which was tighter than any previously manufactured, and he
instigated the Dacron sail revolution. For the next twenty-five years, Hood would have a virtual
monopoly on Dacron sail manufacture [2].
Hood’s approach to sail design was both scientific, through his use of tightly woven Dacron
sailcloth, and artistic in the approach he used to shape his sails [2]. As explained earlier, sail
shape is the key to sail performance since the sail acts as an airfoil against the wind, generating
lift by creating a pressure differential on either side. In order to most efficiently accomplish this,
a two dimensional medium—cloth—must be transformed into a three dimensional object: the
sail. A sail is given a three-dimensional shape through the use of many different panels all cut
with unique curves that, when sewn together, combine to form a specific airfoil as seen in Figure
7. What made Hood’s sails so good was both his revolutionary new material and the fact that he
had an artistic gift for seeing a fast sail shape. However, because Hood could not translate this
innate ability into any kind of mathematical formula, the dawn of the computer age was the end
to his reign as the world’s premier sail maker.
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Figure 7: Illustration showing how unique curves in two-dimensional fabric
form a three-dimensional airfoil when sewn together [2].
Lowell North
Lowell North, who began making sails in 1959, is remarkable in that he took everything that was
once thought indispensible about sail design and threw it out window. North studied civil
engineering while at college and then spent his early years working in the California aerospace
industry [2]. This engineering background and his undying love for science is what set North
apart from any previous sail maker.
After spending several years struggling in the sail making business, North had a breakthrough:
he stopped designing sails based upon what looks right and instead began scientifically analyzing
all different kinds of sail shapes, first through physical experiments in wind tunnels, and then
though analytical computer modeling [2]. North’s use of emerging computer technology would
revolutionize sail design forever, and would also relegate artists such as Ted Hood to the history
books. By the 1970s, North was fully designing sails with a computer, testing the designs in a
computer-simulated wind tunnel, and then cutting the panels using a computer-controlled
plotter/cutter machine [2]. North was eventually able to surpass Hood in his sail design during
the 1980 America’s Cup campaign, becoming the de facto king of sail design and innovation.
Conclusion
While sails have powered watercraft for over five thousand years, it has been in the past halfcentury that sail design has advanced from its previously stagnant condition and finally joined
the modern world. From antiquity until the 1860s, sailing ships ruled the world in both
commercial and naval sectors, reaching their zenith with the great clipper ships. Unfortunately
the field of sail design remained relatively unchanged from that zenith until the second half of
the twentieth century when revolutionary synthetic fibers and computer-aided design would
transform sail power from centuries-old technology to cutting-edge innovation. In all, sail design
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has changed more in the past fifty years than it has in the past five hundred, and it will likely
continue to advance at this rapid pace as we welcome the second decade of the twenty-first
century.
References
[1]
[2]
[3]
[4]
[5]
T. Gibbons e.d., The Encyclopedia of Ships. San Diego, Thunder Bay, 2001.
T. Whidden and M. Levitt, The Art and Science of Sails. New York, St. Martin’s, 1990.
H. L. Stone, “The America’s Cup,” The North American Review, vol. 230, no. 3, pp. 263269, Sep. 1930.
W. Ross and C. Chapman, Sail Power. New York, Knope, 1981.
Y. E. Ozveren, “Shipbuilding, 1590-1790,” Commodity Chains in the World Economy,
vol. 21, no. 1, pp. 15-86, 2000.