2. Introduction
When you see a sailboat out on the water, you know the wind is causing the boat to move, but how?
The interaction between many variables:
Wind
Sails
Hull
Water
Keel

3. Introduction
“The sailing craft must be considered as a complex system consisting of two interdependent parts – aerodynamic and hydrodynamic” - C.A. Marchaj

4. Sails
Push vs. Pull
Push
When the wind comes from behind the boat, the sails simply trap the wind and it pushes the sails
Sailing downwind
Wind

5. What causes the sails to pull the sailboat?
Push vs. Pull
Pull
When the wind is coming abeam (from the side) or in front of the boat, the sails are able to generate pull
Wind
What causes the sails to pull the sailboat?

6. Sails Think of wind as simply…the flow of air across the sails
The difference in pressure between the two sides generates lift (like an airplane wing)
Low Pressure
Wind
High Pressure

7. Sails
When the difference in pressure creates lift on the sail, the resulting force vector is perpendicular to the boat
Wind
If the force vector is sideways, why doesn’t the sailboat simply move sideways?
Force Vector

8. Keel The keel keeps the boat from drifting sideways
Exerts a sideways force on the water, causing the boat to slightly tip, or “heel”
By transforming the side force created by the wind in the sails into a force that counteracts, the sailboat is able to go forward
Sail Force Vector
Keel Force Vector

9. Where are the high and low pressure areas when you turn the rudder?
Keel and Rudder
The rudder can also act as a counteracting force
Where are the high and low pressure areas when you turn the rudder?
Sails
Keel
Rudder

10. Keel and Sails Combined
The keel and sails produce sideways lift forces
The sum of those two force vectors determines the boat’s forward direction
Keel Force Vector
Why is the forward direction so much smaller than the two force vectors?
Sail Force Vector
Keel Force Vector
Sail Force Vector