Lesson 3

Super Sails! - Lesson 4

Topic: How a Sailboat Works: Hull Type


Teacher Resources:

Printable PDF of Lesson Plan for Part 1 - Hulls

Printable PDF of Lesson Plan for Part 2 - Sails

Downloadable PPT of Lesson 4 - Hulls (6.4MB)

Downloadable PPT of Lesson 4 - Sails (5.3MB)

Lesson 4 Handout - Displacement Worksheet (PDF)

Displacement Worksheet - Answers (PDF)


Click here to launch PowerPoint for Lesson 4 Part 2 - Sails

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
  • Students will about what what goes into sail design

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

Lesson Outline - Part 2 - Sails :

Primary Goal: After this lesson, students should be able to determine the final piece of their sailboat design, the sails. After reviewing how sails generate speed for a sailboat, they will learn how to generate speed for their boat while also taking into account the many other factors affected by a boat’s sail area.

Lesson Objectives:

  • Students will review how sails are able to generate lift, and thus speed
  • Students will learn the importance of and how to calculate Sail Area
  • Students will learn about the Sail Area – Displacement ratio

Click here to launch PowerPoint for Lesson 4 Part 2 - Sails

I. Intro

  1. A sailboat uses her sails for propulsion by generating lift (upwind) or by blocking the wind and being pushed (downwind)
  2. Just like a sports car is interested in a high horsepower – weight ratio, sailboats use a similar type of measurement to determine the potential speed, or acceleration, of the sailboat

II. Sail Area

  1. A sailboat’s propulsion comes from the wind on her sails and is proportional to the area of all the sailboat’s sails
  2. This is measured by calculating the area of a each of the sails and then simply adding those numbers together:
    1. The measurement of sail area is calculated using square feet
    2. For more advanced courses you can discuss the measurements of E and P for the mainsail and I and J for the headsail
  3. It is also worth noting that actual sail measurements are more complicated because of the curvature shape of the sails

III. Sail Area / Displacement Ratio

  1. In order to compare sailboats with one another, we use the Sail Area – Displacement ratio
  2. This shows how much power the sails generate compared to each pound of displacement
  3. Under this calculation, we are assuming that displacement is the sole limit of a boat’s speed due to the reasons discussed in previous lessons
    1. The calculation also involves dividing the displacement by 64. This is done because the weight of seawater is 64 lb/ cubic foot
    2. The higher the ratio indicates a high performance sailboat usually designed for racing
  4. Because of the large sail area, these boats are sometimes more difficult to handle and can become easily overpowered in high winds
    1. Low ratio - 8-13
    2. Medium ratio - 14-20
    3. High ratio - 21-40+

Exercises / Activities:

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

Additional Resources:

National Geographic / Volvo Ocean Race Wave Simulators

Experimental Sail Design Images - Bing

The Maltese Falcon Yacht - Bing

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