Sailboats

How Fluid Machines Work
1

Tell me about Sailboats

wind power and boat glide through the water

2

This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook

PK Bot’s answer:
I’m here to help you with fluid mechanics concepts.

Regarding sailboats, they operate based on the principles of fluid mechanics, particularly through the interaction of air (wind) and water.

When wind flows over the sails of a sailboat, it creates a pressure difference due to the shape and angle of the sails, which act similarly to airfoils. This pressure difference generates lift, propelling the boat forward.

The hull of the sailboat, which serves as a container, displaces water, creating buoyancy that keeps the boat afloat. The design of the hull is crucial for minimizing drag and maximizing stability as the boat glides through the water.

If you have specific questions about the fluid mechanics involved in sailboats or any other related topic, feel free to ask!

In crafting this response, I dedicated some time to reviewing the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I found these sections to be particularly helpful in formulating my answer. I encourage you to explore the pertinent ones directly:

Textbook sections:

  1. Chapter 4.1: Watercraft (NOT RELEVANT)
  2. Chapter 1.4: Properties of sound waves (NOT RELEVANT)
  3. Chapter 9.4: Finding Analytical Solutions for \psi and \phi Part 2 (NOT RELEVANT)
  4. Chapter 4 Fundamentals: The Wreck of the HMS Herald of Free Enterprise (NOT RELEVANT)
  5. Chapter 9 How Stuff Works: Wind turbine design step 1: rotor disk analysis (NOT RELEVANT)
  6. Chapter 2 How Stuff Works: How a propeller works (NOT RELEVANT)
  7. Chapter 7 How Stuff Works: How jetskis work (NOT RELEVANT)
  8. Chapter 9.2: Engineering Descriptions of Airfoil Forces (NOT RELEVANT)
  9. Chapter 2 How Stuff Works: How a wind turbine works (NOT RELEVANT)
  10. Chapter 4 How Stuff Works: How Submarines Work (NOT RELEVANT)

Disclaimer:

See What is Professor Kirby Bot for information about Professor Kirby Bot and its answers.

3

how does the wind flowing over the sails make the sailboat move though

4

This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook

PK Bot’s answer:
The sails are basically flexible airfoils, and they can be rotated. The boom that holds the mainsail is at 0 degrees when it is aligned from fore to aft on the boat, and at 90 degrees when the sail is rotated all the way out to starboard.

When the wind is at the sailboat’s back, the sail trim is set to almost 90 degrees, meaning the boom is rotated almost 90 degrees relative to the boat centerline. (your mainsail goes to one side and your jib goes to the other), and the sails act like parachutes–they generate force from the drag on a bluff body. The relevant aerodynamic coefficient is the drag coefficient, which could be approximated as being roughly that of a flat plate (1.28) or a hemispherical cup with the concave side facing upstream (1.38), and the force on the sail would be

F=C_D (\frac12 \rho U^2) A

where A is the area of the sail, and U is the windspeed. This configuration is sometimes called “running”.

When sailing into the wind, rather than using the sail as a “parachute” i.e. a surface that generates drag, you use the sail as a flexible airfoil that generates lift. And, because the angle at which you sail with respect to the wind varies from 0-180 degrees, the force you need to generate continuously varies from drag to mixed drag/lift to lift depending on the orientation.

Some sailors use terms “running”, “broad reach”, “beam reach”, “close reach”, “close haul”, and “in irons” to refer to wind aligned 0, 30-60, 90, 105-120, 135-150, and 180 degrees relative to the boat motion. It is also often common to describe the angle “off the wind”, which is the angle between the boat travel direction and upwind.

When sailing upwind, you usually sail in a close haul, and if you are sailing 30-45 degrees off the wind, your sail trim might be out approximately 15-20 degrees. The boat is moving, and thus the wind angle relative to the sail in the sail’s frame of reference is lower than the sail trim, maybe 7-10 degrees (this depends on both wind speed and boat speed). the forward force on the boat is proportional to C_L \sin \alpha - C_D\cos\alpha, and a good rough approximation for C_L might be 2\pi (\alpha-\alpha_0), where \alpha_0 is the angle of attack at which the lift coefficient is zero (maybe -3 degrees or the like). You can think of the sail as being an airfoil up to about 90 degrees off the wind, at which your sail trim is likely roughly 45 degrees and the angle of attack of the sail is about 20 degrees, which is about the stall point of a sail. Past that, it is hard to argue that the sail is more airfoil than parachute. The C_L of a sail maxes out around 1.3-1.8, typically at 15-20 degrees angle of attack.

A deeper sail is one that has looser fabric, and which billows out more. Deeper sails act like more cambered airfoils (more highly negative \alpha_0). This can be adjusted by the outhaul.

In crafting this response, I dedicated some time to reviewing the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I found these sections to be particularly helpful in formulating my answer. I encourage you to explore the pertinent ones directly:

Textbook sections:

  1. Chapter 2 How Stuff Works: How a propeller works (RELEVANT)
  2. Chapter 4.1: Watercraft (RELEVANT)
  3. Chapter 2 How Stuff Works: How a wind turbine works (RELEVANT)
  4. Chapter 9.1: How Airfoils Work (RELEVANT)
  5. Chapter 7 How Stuff Works: How jetskis work (RELEVANT)
  6. Chapter 9.4: Finding Analytical Solutions for \psi and \phi Part 2 (RELEVANT)
  7. Chapter 9.2: Engineering Descriptions of Airfoil Forces (RELEVANT)
  8. Chapter 9 How Stuff Works: Wind turbine design step 1: rotor disk analysis (RELEVANT)
  9. Chapter 7.1: Helicopter/Quadcopter Rotors (RELEVANT)
  10. Chapter 9 Fundamentals: Magnus effect (RELEVANT)

Disclaimer:

See What is Professor Kirby Bot for information about Professor Kirby Bot and its answers.

5

technically, PK Bot claims to describe the force “on the boat” but that is the force on the sail. The force on the boat is almost entirely in the direction of motion because the keel resists side motion. so there is another cosine or sine of the sail trim to determine the force on the boat. the \alpha PK Bot is using is the angle of attack of the sail, which is of course different from the sail trim.