Although flight is a recent human achievement, the wind was harnessed for
transportation long ago. The exact date is uncertain, and there is no
historic person to credit, but an engraving on an Egyptian pot more than
5000 years ago clearly illustrates a major invention: the sail.
Navigation of the water has four basic components that were more or less
developed one after another: floating, rowing, sailing, and motoring.
The first travel on water was simply by floating, and in different parts
of the world today we see continued use of ancient methods. The Tamil
people of Sri Lanka simply place a log under an arm to achieve flotation;
in New Zealand the Maoris lash bundles of reeds together to form a raft;
the Sindhi of Pakistan bob inside pots; while some Iraqis use goatskins
filled with air.
At one time, standard gear of a Roman soldier included an inflatable skin
for river crossings. These various flotation devices were gradually combined
and refined to accommodate groups rather than individuals. Reed platforms,
buoyed by inflated skins, were built large enough to transport elephants into
battle. Entire villages were carried to a new destination upon a single
Once afloat, humans developed means to use or resist the currents of the
water. Poles were utilized to push against the river bottom. In deeper
streams, paddling by hand gave way to wooden paddles, then oars that were
pulled in unison. But rowing was certainly more pleasant with the
current than against it. In fact, many early water travelers preferred a
one way voyage - the ease of return by land was worth the extra time.
The invention of the sail, and its many refinements, is another great
example of human observation and employment of natural phenomena. By
harnessing the power of the wind, people were able to travel with greater
agility and for longer distances than ever before. It is also
interesting to note how the materials and designs of sailing ships varied
according to the characteristics of the local body of water, wind
patterns, and surrounding environment. Archaeologists have discovered
much evidence around the Mediterranean sea, including the Nile, Tigris
and Euphrates rivers, that documents the development of the sail from
both native experience and cultural encounters.
The first sail unfurled on the Nile at least as early as 3100 B.C. From
the drawing that survives from that date, the sail appears to be no more
than a square patch of material fastened to a stick near the front of the
boat. The artistic rendition may have been oversimplified, but it's
plausible to imagine the first sail as indeed being rather primitive.
The importance of this contraption was immediately recognized, however,
and modifications continued throughout the Egyptian dynasties. By 2400
B.C the sail had become oblong and rose on a tall mast, allowing it to
catch the breezes that flow between the many cliffs along the Nile.
Everywhere else in the Mediterranean the sail was low and square, and
eventually due to ease of management the Egyptian design conformed.
Rigging, the means of support and control, consisted of several lines
that were either fixed in place to strengthen the mast from which the
sail was hung, or movable so that the sail could be raised, lowered or
Although refinements continued, the shape of a sail remained the same for
nearly 4000 years. Its biggest aerodynamic drawback was that, although
it could be angled to achieve maximum thrust, the square sail received
air only from the rear - the boat was pushed by the wind. Sailing was
revolutionized in the 9th century A.D. by the lateen or triangular sail,
probably invented by Arab seamen. Hung fore and aft (front and back) of
the mast and easily shifted, the lateen sail received wind on either side
- in effect, the boat was pulled as well as pushed. The inventors of the
triangular sail recognized that their design greatly improved boat speed
and responsiveness, but they did not understand the aerodynamic
principles which were being applied. We will investigate these
principles now, and in the process learn more about how airplanes as well
as boats manage to sail the wind.
One would think that a boat could only move in the direction that the
wind was blowing - that is, downwind. But a triangular sail allows a
boat to move toward the wind (windward). To understand how this movement
is accomplished, we first need to identify some of the parts of a sail.
The leading edge of a sail is called the luff; it's positioned at the
front, or fore, of the boat. The trailing edge at the back, or aft, is
called the leech. An imaginary horizontal line from luff to leech is
called the chord. The amount of curvature in a sail is called the draft,
and the perpendicular measurement from the chord to the point of maximum
draft is called chord depth. The side of the sail that the air fills to
create a concave curve is called the windward side. The side that is
blown outward to create a convex shape is called the leeward side. We'll
return to these terms as we proceed.
A boat is moved in a windward direction by using forces that are created
on each side of the sail. This total force is a combination of a
positive (pushing) force on the windward side and a negative (pulling)
force on the leeward side, both acting in the same direction. Though you
wouldn't think so, the pulling force is actually the stronger of the two.
In 1738 the scientist Daniel Bernoulli discovered that an increase in air
flow velocity in relation to the surrounding free air stream causes a
decrease in pressure where the faster flow occurs. This is what happens
on the leeward side of the sail - the air speeds up and creates a low
pressure area behind the sail. Why does the air speed up? Air, like
water, is a fluid. When the wind meets and is divided by the sail, some
of it sticks to the convex (leeward) side and hitches a ride. In order
for the "unstuck" air just above it to move past the sail, it has to bend
outward toward the flow of air unaffected by the sail. But this free air
stream tends to maintain its straight flow and acts as a kind of barrier.
The combination of the free air stream and the curve of the sail creates
a narrow channel through which the initial volume of air has to travel.
Since it can't compress itself, this air has to speed up to squeeze
through the channel. This is why the velocity of flow increases on the
convex side of the sail.
Once this happens, Bernoulli's theory takes effect. The increased air
flow in the narrow channel is faster than the surrounding air, and the
pressure decreases in this faster flowing area. This creates a chain
reaction. As new air approaches the leading edge of the sail and splits,
more of it flows to the leeward side - air flow is attracted to low
pressure areas and repulsed by high pressure areas. Now an even larger
mass of air must travel faster to squeeze through the channel caused by
the convex sail and the free air flow, causing an even lower air
pressure. This continues to build until the maximum speed is achieved
for the existing wind condition, and a maximum low pressure area is
created on the lee side. Note that the air flow increases only until it
reaches the deepest point of the curved shape (the chord depth). Up to
this point the air is converging and speeding up. Beyond this point the
air diverges and slows down until is again the speed of the surrounding
In the meantime, just the opposite is happening on the windward side of
the sail. As more air travels to the leeward side there is less air on
the windward side to travel through the expanded space between the
concave side of the sail and the free air stream. As this air flow
spreads out it slows down to a speed less than the surrounding air,
creating an increase in pressure.
Now that we know about these potential forces, how do we actually develop
them in order to move our boat? We need to create an ideal relationship
between the sail and the wind that will allow the wind to both speed up
and flow along the convex curve of the sail. One part of this
relationship between sail and wind is called the angle of attack.
Picture a sail pointing straight at the wind. The air will split evenly
to each side - the sail sags instead of filling to a curved shape, the
air does not speed up to form a low pressure area on the lee side, and
there is no movement of the boat. But if the sail is angled to the wind
to just the right degree, the sail suddenly fills and the aerodynamic
The angle of attack must be very precise. If the angle remains too close
to the wind the front of the sail "luffs" or flaps. If it's angled too
wide the flow lines along the curve of the sail detach and rejoin the
surrounding air. This separation creates a "stall zone" of whirling air
that causes a decrease in speed and an increase in pressure. Because a
sail's curvature will always cause the aft end of the sail to be at a
greater angle to the wind than the leading edge, the air at the leech is
unable to follow the curve and returns its direction to that of the
surrounding free air. Ideally, separation shouldn't start until the
airflow reaches the leech. But as a sail's angle of attack widens, this
point of separation gradually moves forward and leaves everything behind
it a stall zone.
You can see that, along with having the correct angle of attack to allow
air to pass smoothly onto it, the other important factor in the wind to
sail relationship is that the sail must have the correct curvature so the
air attaches all the way aft. If the curve is too slight the air flow
will not bend out, and there will be no squeezing effect that increases
the speed. If the curve is too deep the flow cannot remain attached.
Therefore, separation can occur from too much curvature as well as from
too wide an angle of attack.
So now we know how pressures on the sail are developed in theory and in
practice. But how do these pressures move a boat forward? Let's take a
closer look, which we can do because air pressure can be measured rather
At sea level air pressure is 2,116 pounds per square foot. When the air
flow on the leeward side of the sail is increased, you recall that air
pressure decreases. Suppose it decreases by 4 pounds per square foot.
Likewise, air pressure on the windward side increases - let's say by 2
pounds per square foot (remember, the pulling pressure is stronger than
the pushing pressure). And even though the leeward pressure is negative
and the windward is positive, they both work in the same direction. So
now we have a total of 6 pounds per square foot. Multiply
that by a 500 square foot sail and we've created a total force of 3000
pounds on the sail.
Each point of the sail has different pressures working on it. The
strongest force is at chord depth, where the curve of the sail is the
deepest. This is where air flows fastest and pressure drops most. Force
weakens as it moves to the rear and separates. The direction of these
forces changes also. At every point in the sail the force is
perpendicular to the sail's surface. The strong forces in the forward
part of the sail are also in the most forward direction. In the middle
of the sail the force changes to a sideways, or heeling, direction. In
the rear part of the sail the force grows still weaker as wind speed
decreases, and causes backward or drag direction.
Each force on a sail can be calculated to determine the relative strength
of its forward, heel and drag components on either side. Since the
forward forces are also the strongest, the total force acting upon the
sail is in a slightly forward, but mostly sideways, direction.
Increasing the power of a sail to gain more forward drive also results in
a much greater increase in the heeling force. So how does one move
forward into the wind when the greatest force is to the side? This
involves the angle of attack of the sail to the wind, and the resistance
of the boat to the other fluid involved here: water.
The direction of the total force is nearly perpendicular to the sail's
chord. When a sail's chord is parallel to the boat's centerline, the
main force is almost completely to the side. But if the sail is angled a
bit so the sail force is in a slightly more forward direction, the boat
itself moves forward at once. Why? The centerline, or keel, of the boat
acts against the water in a manner similar to that of the sail against
the wind. The keel produces a force that opposes the heeling force of
the sail - it keeps the boat from simply going in the direction of the
sail force. And although total sail force is always to the side when
sailing into the wind, a proper angle of attack will move the boat
Suppose you hold a mop perpendicular to the floor and push down - there
is no movement. But if you angle the mop just a little and push again
with the same amount of force, the mop slides easily across the floor.
This is what occurs when the sail's angle of attack is altered. The
farther the sail is angled from the centerline of the hull, the more the
force points forward rather than to the side. Combine that slight
adjustment in forward force with the opposition of water to air, and we
have a boat shooting windward because it is now the course of least
resistance. Add to this irony the fact that a boat moving against the
wind can go faster than if it were to turn and travel with the wind.
Such is the power of aerodynamics: definitely not magic, and hopefully
not quite such a mystery.
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