Science of Sailing – Lesson 4

image from lesson 4

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)

Click here to launch PowerPoint for Lesson 4

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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