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

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

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

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?

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

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

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

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

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