The sail force

Introduction.

This first chapter of the on-line book “SAIL HANDLING AND TRIMMING” begins with some theoretical fundamentals that will help us to understand how the sail force is generated. I will try to keep this theoretical part short, simple and easily understandable, as this book is eminently practical. However, I am convinced that this basic knowledge can have great benefits for the sailor, including the following:

– Gaining speed and getting more performance out of the boat, in any weather situation.

– A well-trimmed boat is better balanced, its steering will be easier, it will require less effort and its components (rudder, autopilot, etc.) will suffer less.

– Enjoy sailing more, and also be able to participate in the “typical” conversation on deck about how the sails are trimmed; which is no small feat.

In this first chapter we will introduce the more general aspects, which we will go into more detail in later chapters.

Displacement in two fluids.

As we all know, a sailboat moves in two fluids: air and water.

– Everything above the waterline, the sails, superstructure, etc. move in the air. The forces that are produced are called aerodynamic forces, and the resultant of the forces that are generated in a sail we will call the AERODYNAMIC FORCE.

– Everything below the waterline, the hull’s hull work, the centerboard, the rudder, move in the water and the forces that are generated are hydrodynamic forces.

In this first chapter we will deal only with the forces that are generated above the waterline or aerodynamic forces, and later in a next chapter we will deal with the hydrodynamic forces, and how the balance between them is reached.

Theoretical foundations: Bernuilli’s theorem and the Venturi effect.

Bernuilli’s Theorem: The energy possessed by an ideal fluid (in our case we apply it to both air and water) in circulation remains constant, which means that:

Pressure + Velocity = Constant (see Note 1).

The fundamental idea that we obtain from Bernuilli’s theorem is that:

– If the fluid velocity increases the pressure decreases.

– If the fluid velocity decreases the pressure increases.

Note 1: Actually the formula is more complex since it has other factors, and also in the formula the velocity is squared; which will have a significant effect that we will see later (the wind force increases with the square of the wind velocity).

Venturi effect: It is useful to see graphically and in a practical way the application of Bernuilli’s theorem. As we can see in the figure above, the fluid maintains its flow rate, but if its section decreases the velocity increases (lower pressure), and if its section increases the velocity decreases (higher pressure).