Sailing to Windward
Question:
How can a sail boat sail upwind and what limits how close to the
wind direction it can go?
Answer:
The short answer on sailing to windward is that the sail is
shaped a bit like an airplane wing, sort of bellied out to the
side which is away from the wind direction, and this airfoil
shape creates a force in the direction of the bulge in the sail.
In the case of an airplane wing where the bulge is upward we call
the force lift. In the case of a sail where the bulge is
horizontal we might call the force pull. To keep this shape the
sail must not be pointing straight into the wind. If we let the
sail point into the wind the excess sailcloth just flaps back and
forth, not creating the airfoil shape.
What happens to create the pull is that the sail splits the air
stream passing over it into one part that travels along the curve
of the sail, around the bulge, and another part which takes a
shortcut straight across from the leading edge to the trailing
edge of the sail. Since the curved path is longer than the flat
one and the air must move faster along the downwind side of the
sail. When air is moving fast, more of its energy is tied up in
its speed and less in its pressure. The difference in pressure
between the outside of the bulge in the sail and the inside shows
up as this pull mentioned earlier.
So now we have a pull centered on the sail, pretty much
horizontal and perpendicular to the boom (that stick along the
bottom of the sail). For this pulling force to make the boat move
forward, the boom must be at an angle to the centerline of the
boat so that part of the force is directed forward. The other
part of the force will be in a direction to tend to pull the boat
sideways through the water. The fin on the bottom of the boat
offers a lot of resistance to sideways motion and very little to
forward motion so the sideways force does not have much effect on
the boat's motion.
Now we get to the second part of the question. The sail must
be kept at an angle to the wind to keep its airfoil shape. Then
the boat must be kept at an angle to the sail so that some
forward force is available. Adding these two angles together will
give you the total angle the boat must be off the wind in order
to move forward.
Those of you who are knowledgeable sailors will recognize that
I have left out a lot of stuff about relative wind, righting
moment, circulation, turbulence etc. For those of you who want to
know about these things, there is a sail theory site which might be of some interest.