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Topic: How a Sailboat Works - Hull Speed & Buoyancy


Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (3.9mb)


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Primary Goal : After this lesson, students should be able to determine the proper hull designs necessary to compliment the rig they chose for the sailboat design. They will also understand the equations to make sure their boat is built within the necessary parameters to ensure seaworthiness.

Lesson Objectives :

  • Students will continue to learn the various hull designs and understand how a sailboat’s speed is affected by her hull
  • Students will learn how to calculate hull speed
  • Although most students have learned about the basics of why an object floats, this lesson should summarize buoyancy and Archimedes principle

Lesson Outline :

  1. A sailboat’s hull is important for the many reasons we discussed during last class.
    1. The one subject we didn’t cover is Speed
  2. Modes of Sailing
    1. Displacement Sailing
      1. As the sailboat moves through the water, she is constantly displacing a new patch of water
      2. This generates waves behind the boat and will eventually cause the speed of the boat to max out
      3. Because of this, when designing a displacement sailboat it is important to design a hull that easily cuts through the water as smoothly as possible
    2. Planing Sailing
      1. As the sailboat speed increases, the pressure which causes the bow wave causes the boat’s bow to completely rise up out of the water and the hull moves over the water rather than through it
      2. These boats can go much faster because of minimal drag
      3. However, these boats require a great deal of power to get on a plane and thus can be very difficult to drive for novice sailors
    3. Forced Mode
      1. This is not covered but worth mentioning in more advanced classes
      2. However, this is a good intro into hull speed
  3. Hull Speed
    1. Used to determine the theoretical maximum speed of a displacement sailboat
      1. As a displacement sailboat’s hull pushes through the water, she makes waves at her bow and stern
      2. When hull speed is reached, the boat will appear trapped between the waves she is generating
    2. The length of the wave, and thus hull speed, is based purely on the boat’s length
    3. Discuss how this calculation only applies to displacement hulls and not planing sailboats
  4. How a Sailboat Floats: Buoyancy
    1. A boat will float so long as her total volume weighs less than the density of the water it displaces
    2. Archimedes Principle – The upward buoyant force on a sailboat is equal to the weight of the fluid that body displaces
    3. A sailboat will float as long as the gravitational weight of the boat is less than the upward buoyancy force
    4. Boats are designed with the specific purpose of displacing enough water to float
      1. How do you think men and women design large container ships to float?
      2. This is a great opportunity to discuss density

Supplemental Resources :

YouTube video by LearnBiologically on Archimedes Principle: How do Ships Float?

DSN Animation Video: What is Stability?

US NAVY STEM for the Classroom: Buoyancy & Volume Lesson

Exercises/Activities :

This is a great class to conduct an experiment in buoyancy and show how different objects float. This is a good example of an experiement: Education.com Archimedes Principle

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Topic: How a Sailboat Works: Hull Type


Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (6.3mb)

Lesson 4 Handout - Displacement Worksheet (PDF)

Displacement Worksheet - Answers (PDF)


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Primary Goal : After this lesson, students should be able to determine the proper hull designs necessary to compliment the various rigs.

Lesson Objectives :

  • Students will learn the various hull designs and compare and contrast
  • Students should understand the pros and cons to earlier sailboat designs
  • After this lesson, students should be able to reference ancient designs and effectively integrate them into their own design later in the course

Lesson Outline :

  1. A sailboat’s hull is important for many reasons, including the following:
    1. Stability
    2. Safety
    3. Comfort at Sea
    4. Load Carrying Capacity
    5. Speed
  2. Sailboats can be identified by the number of her hulls
    1. Monohull – single hull
    2. Catamaran – 2 hulls
    3. Trimaran – 3 hulls
    4. Discuss how multi-hull boats are generally faster than monohull sailboats
      1. There are many reasons, but one primary reason is the reduced drag. A multihull does not need additional weight or ballast for stability since it has multiple hulls and thus a wider beam (breadth).
  3. Keel
    1. Full Keel
      1. Pros – easy to steer on a straight course through the water and not as sensitive to minor course adjustments
      2. Cons – slower to turn and increased drag due to large amount of surface area below the waterline
    2. Fin Keel
      1. Pros – turns quickly around the keel and able to adjust course faster than a full-keel
      2. Cons – smaller keel provides less resistance to forces that could cause a sailboat to go off course. Helmsman must be attentive when at the helm.
    3. Bulb Keel
      1. Provides more ballast weight by concentrating a large amount of weight
      2. This can help improve a boats stability
    4. Winged Keel
      1. Provides additional hydrodynamic stability
      2. A winged keel sailboat has the added benefit of stability while also maintaining a reasonably shallow draft capable of sailing in shallow water
  4. Hull Displacement
    1. The amount of water a sailboat shoves to the side while floating
    2. The weight of a sailboat is equal to the weight of the water it displaces
      1. Discuss the difference in weight between salt water vs. fresh water (salt water weights slightly more than fresh)
  5. Displacement – Length Ratio
    1. A measurement used to describe whether a boat is a heavy or light displacement hull
    2. This can help tell a boat’s purpose and performance
      1. Light Displacement Hull – 200 or less
      2. Medium Displacement Hull – 200-350
      3. Heavy Displacement Hull – 350 or more
    3. When calculating the D/L ratio, it is important to use the sailboat’s Load Waterline Length (LWL)
      1. This is the hull’s length where it comes out of the water at the bow and the stern
      2. This is critical, because it measures the length of the boat that is exposed to the water
    4. Racing Sailboats will generally have a much lighter D/L ratio
  6. Ballast – Displacement Ratio
    1. The weight in the keel and bottom of the boat that counter’s the sailboat’s tip or “heel”
    2. This is a good indicator of the stability of the sailboat and can help tell us the boat’s purpose (offshore cruising vs. racing)
    3. By comparing a boat’s ballast to her displacement, you can make this determination
      1. Coastal – 35% or less
      2. Average – 35% - 45%
      3. Offshore – 45% or greater
    4. These measurements do not hold true for all boats, but can be used as a general guideline

Exercises/Activities :

Provide students with a worksheet showing the different sailboats and allow the students to perform the various calculations.

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Topic: How a Sailboat Works


Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (3.4mb)


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Primary Goal : Students will learn about the different forces on a boat and how they all work together. They will understand how sail aerodynamics affects boats with different designs.

Lesson Objectives :

  • Students should understand how modern sailboats gain propulsion from the wind
  • In addition to the sails, how other forces such as wind, water, hull, and keel all interact with one another

Lesson Outline :

  1. Introduction
    1. The interaction of the wind, sails, hull, water, and keel all effect a boats propulsion
    2. Take this time to review each of these terms
    3. C.A. Marchaj: “The sailing craft must be considered as a complex system consisting of two interdependent parts – aerodynamic and hydrodynamic”
  2. Sails
    1. Push vs. Pull: The Two Modes of Sailing
      1. When the wind comes from behind the boat, the sails simply trap the wind and it pushes the sails, like a parachute. This is what most people think of when they think of sailing (mention square-rig)
      2. When the wind is coming abeam of or in front of the boat, the sails are able to generate pull
    2. Sails generate energy from the wind (flow of air) by “bending” it as it goes by. The bending of the flow of air causes a low pressure on one side of the sail and a high pressure on the other side
      1. As the high pressure air attempts to catch up, it generates lift on the sail which causes a force forward and sideways
    3. When a sailboat is moving directly downwind, it can never move faster than the wind because, at the wind speed, the sails would feel no wind. In fact, a boat going downwind can never attain the wind speed because there’s always some resistance to its motion through the water.
  3. Keel
    1. Why doesn’t the boat drift sideways? à the keel
    2. The keel exerts a sideways force on the water, which causes the boat to slightly tip, or “heel”. But by transforming the side force created by the wind in the sails into a force that counteracts, the boat is able to go forward.

Supplemental Resources :

Quest Video: The Physics of Sailing

Veritasium Videos: (1) How Does a Sailboat Actually Work? (2) How Does a Wing Actually Work?

DSN Animations: What are the Parts of a Sailboat?

Exercises/Activities :

Bring a large fan to class. Have students take turns holding their hand, palm open, and flat to the blowing air. Explain that this force is what happens when a boat is going downwind. Then have the students slightly cup their palm and slowly turn it towards the fan. Have the students note the gradual change when they rotate and cup their hand and relate that to the similar forces on a sail.

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Topic: History of Sailboats


Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (50.4mb)


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Primary Goal : Students will learn how sailboat designs have evolved and improved over hundreds of years. We want students to understand why certain designs have succeeded and why others have failed.

Lesson Objectives :

  • Sailboats have a long history which provides important insights into why boats have evolved to what they are today
  • Students should understand the pros and cons to earlier sailboat designs
  • After this lesson, students should be able to reference ancient designs and effectively integrate them into their own design later in the course

Lesson Outline :

  1. Intro
    1. Without an engine or motor (way to convert a form of energy into mechanical energy) sailing vessels relied upon wind for propulsion
    2. Sailboats are classified by:
      1. The shape of their sails and how many sails they have
      2. The location and number of masts
  2. Early History
    1. Dhow – one of the earliest sailboat designs
      1. Two or more triangular sails called “lateens”
      2. Explain the significance of a dhow’s unweighted shallow keel
      3. Generally weighted down by cargo or rocks to maintain stability
      4. Marconi
  3. Age of Sail – 16 th -19 th century period where international trade and naval warfare were both dominated by sailing ships
    1. Square Rig – aerodynamically most efficient running rig
      1. Good for sailing downwind
      2. Introduce “Sail area” – Square-rigs had an extremely large sail area that allowed it to take advantage of even light winds
    2. Ketch/Yawl
    3. Cutter – traditionally a single-masted, fore-and-aft rig, with at least two headsails
      1. Fore-and-aft rig – sail configuration is set along the line of the keel rather than perpendicular
    4. Sloop – single-masted, fore-and-aft rig, with a single headsail
      1. Bermuda rig most common
      2. Allows optimal upwind sailing and downwind sailing
        1. Introduce spinnaker and compare to wing-on-wing
    5. Catboat
  4. Modern Designs
    1. Hydrofoil – sailboat with wing-like foils mounted under the hull
      1. As the boat speeds up, the hydrofoils lift the hull up and out of the water
      2. This decreases the amount of drag, and increases the boat speed
      3. Monohull vs. Multihull

Supplemental Resources :

Sailboat History Timeline: http://www.dawn.com/news/617729/sailboat-history-timeline

Chapman’s p. 30: Diagram

Exercises/Activities :

Put a series of sailboat photos on the board and have students identify each and describe some of the characteristics.


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Topic: Introduction to Sailboats


Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (12.5mb)

Lesson 1 Handout - Sailboat Parts Identification Exercise (PDF)


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Primary Goal :

A basic introduction to the sailboat. Students should be able to identify the parts of a sailboat and familiarize themselves with some of the terminology that will be used throughout the course.

Lesson Objectives :

  • Ability to identify fundamental parts of a sailboat

  • Introduction to boat terminology used throughout the course

  • Introduction to some of the math and physics concepts that will be covered throughout the course

Lesson Outline :

  1. Establishing a Foundation: Parts of a Sailboat
    1. When looking at the many pictures of all the different types of sailboats, students may become overwhelmed
    2. It is important to emphasize that although boat designs may differ greatly, most of the fundamental elements are the same
    3. Show the slide with the 7 boats and ask the students to identify the similarities/differences
      1. Not only will this get the students engaged, but it will make them aware that they don’t know the boat terminology
    4. Learning boat terminology is like learning a new language à critical for communication

  2. Identify the Parts of a Sailboat
    1. Hull – the foundation of the boat that makes it a seagoing vessel
      1. Bow, Stern, Port, Starboard
      2. Fiberglass, wood, metal
    2. The Rig – the structures coming up from the hull that support the sails
      1. Mast, Boom, Forestay, Backstay, Shrouds
      2. Explain the importance of supporting the mast
    3. Running Rigging – all the gear and lines used to raise and trim the sails
      1. Halyards, Sheets
    4. Sails – how a boat generates speed
      1. Mainsail, Jib, Spinnaker
      2. Parts of the Sail: Luff, Leech, Foot
    5. Underwater Appendages
      1. Rudder, Keel

Supplemental Resources :

School of Sailing Glossary: http://www.schoolofsailing.net/terminology.html

US Sailing Video: Parts of a Boat : https://www.youtube.com/watch?v=vZ6O8lB0cII

Exercises/Activities :

If you have access to boat models, bring in and allow the students to take turns identifying the parts of the sailboat. It will be good to provide them with a list of all the items covered during the course.

Separate the students into groups and create a competition using both the sailboat part identification and definitions (Sailboat Jeopardy works well).


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