The physics of sailing

www.kqed.org/quest/television/the-physics-of-sailing (note)

From KQED

We have a storied, 500-year history of sailing.
But despite this rich heritage, scientists and boat designers continue to learn more each day about what makes a sail boat move
Contrary to what you might expect, the physics of sailing still present some mysteries to modern sailors.

To make a sailboat move, you need both wind and water, in addition to a sail, of course!
When a sailboat sails downwind, wind blows into and pushes against the boat's sails.
Because the wind is faster than the boat, the air pushing into the sails is decelerated.
As the sails push back against the wind, the wind pushes forward on the sails and the boat moves.

But working with just the sail and the wind in this way, the boat will move slowly and only in the direction the wind carries it.
To really make a sailboat move quickly, a sailor needs to know how to harness aerodynamic and hydrodynamic lift force.
Bernoulli's principle is a scientific principle stating that as the speed of a moving fluid or gas increases (or decreases), the pressure within the fluid decreases (or increases).
It’s the guiding principle behind the physics of lift.

By sailing closer to the wind, a boat will generate more aerodynamic lift.
To move around the sails, the wind will have to change direction.
This creates a change in wind velocity and harnesses lift force.
But instead of being fully forward of the boat, the force is now primarily sideways on the boat. This means that the sailboat will move sideways if left unchecked.

This is where the keel comes into play.
Unlike the keel in other kinds of boats, a sailboat keel is typically very large and uses the forward momentum of the boat to generate hydrodynamic lift and counter the lateral force coming from the air interacting with the sail.
Instead of lying flat or nearly flat against the boat hull, a sailboat keel drops down into the water beneath the boat like a large rudder.
When the boat moves sideways, the keel must push a lot of water sideways.
Like the interaction between the wind and the sail, the water resists the push from the keel to generate lift.
Because of the similar roles the keel and sail play in generating lift, the keel on a sailboat is sometimes regarded as a "second sail."
Between the sail and the keel, a boat generates enough lift to accelerate forward.

In fact, in this way a sailboat can even move faster than the wind!
When moving, a sailboat generates its own wind, often called apparent wind or relative wind.
This is the flow of wind acting upon the sail.
The faster a boat travels, the more of this kind of wind occurs and the more force there is acting upon the sails.
This means greater acceleration, and the boat will travel faster than the true wind speed.
Many modern, extreme design sailboats and larger skiffs are built especially with this purpose in mind -- to sail faster than the wind.

Links :

• American Institute of Physics : the physics of sailing (book)