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Sail Theory |
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Dynamics of Sailing
A sailing craft exists in the world of wind and water - and a sailor knows
these elements intimately. Using both wind and water, sailboats can move swiftly
and silently over the Bay. Most forms of transportation use engines to overcome
the elements, wind and gravity, while sailing works in harmony with these natural
forces.
Students will help the crew of Living Classrooms Foundation's sailing
vessels with sailing operations and should try to understand the principles
of how and why a boat sails. When sailing downwind, the wind pushes the sails
and boat along. When sailing upwind, there are a number of different factors
involved but most important is the shape of the sails. A sail is made to take
an aerodynamic shape, forming a concave surface when filled with wind. The shape
and function of a sail is similar to an airplane wing when examined in cross-section.
A sail acts like a vertical airfoil (wing) that creates a force (lift) to propel
the boat forward.
Actually, what happens is that the wind
hits and is redirected to one side or the other. If the wind goes outside the
sail's curve (leeward) it will speed up. As the wind speeds up it decreases
in pressure. Meanwhile, on the inside of the sail (windward) the air is moving
slower and creating a greater pressure on the sail. The end result of these
two complimentary airflows is a driving force on the front of the sail pulling
the boat forward and forcing the boat to the side. This side force is transformed
into forward propulsion by an appendage under the boat called a keel. The keel
keeps the boat from sliding sideways and allows forward thrust while the rudder
is the steering device, which directs the boat's course.
Tacking up Wind
While sailing it is very important to present the sails to the wind at a favorable
angle. If a boat attempted to sail directly into the wind, the sails would luff
(flap like a flag) and the boat would not more forward. Most boats can only sail
45 degrees or more off the wind; in order to sail directly into the wind, the
boat must tack (zig zag) towards it's destination.
When sailing, students will feel the normal
heeling (leaning of the boat to one side) as the wind fills the sails. Tons of
heavy lead weight called ballast are placed into the bilge (inside bottom of the
ship) to give stability and restrain excessive heeling. Living Classrooms Foundation's
sailing vessels have been designed and built with great care and are superbly
balanced.
Points of Sail
In all directions except into the wind a
boat can sail easily. The Captain and crew set (adjust) the sails according to
the angle of the wind. If a boat is heading as close to the wind as possible (45
degrees), then the sails are pulled in close to the boat (closehauled). When sailing
with the wind from the stern (rear) of the boat (running before the wind), the
sails are let out far from the boat. Often the wind is "on the beam" (halfway
between ahead and astern); at this time, logically, the sails are set halfway
between closehauled and running (this is called reaching). Therefore, a sailboat
can maneuver in any direction by proper setting of the sails.
Knowledge of the dynamics involved with sailing is truly important, yet ineffective
without smooth teamwork between Captain and crew. Cooperation and discipline contribute
significantly to the efficient sailing of a vessel. With appropriate techniques
and invaluable teamwork, a ship and her crew can meet the challenge of the sea
while experiencing the exhilaration of sailing with the wind.
