** 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 : **

- A sailboat’s hull is important for many reasons, including the following:
- Stability
- Safety
- Comfort at Sea
- Load Carrying Capacity
- Speed

- Sailboats can be identified by the number of her hulls
- Monohull – single hull
- Catamaran – 2 hulls
- Trimaran – 3 hulls
- Discuss how multi-hull boats are generally faster than monohull sailboats
- 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).

- Keel
- Full Keel
- Pros – easy to steer on a straight course through the water and not as sensitive to minor course adjustments
- Cons – slower to turn and increased drag due to large amount of surface area below the waterline

- Fin Keel
- Pros – turns quickly around the keel and able to adjust course faster than a full-keel
- 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.

- Bulb Keel
- Provides more ballast weight by concentrating a large amount of weight
- This can help improve a boats stability

- Winged Keel
- Provides additional hydrodynamic stability
- A winged keel sailboat has the added benefit of stability while also maintaining a reasonably shallow draft capable of sailing in shallow water

- Full Keel
- Hull Displacement
- The amount of water a sailboat shoves to the side while floating
- The weight of a sailboat is equal to the weight of the water it displaces
- Discuss the difference in weight between salt water vs. fresh water (salt water weights slightly more than fresh)

- Displacement – Length Ratio
- A measurement used to describe whether a boat is a heavy or light displacement hull
- This can help tell a boat’s purpose and performance
- Light Displacement Hull – 200 or less
- Medium Displacement Hull – 200-350
- Heavy Displacement Hull – 350 or more

- When calculating the D/L ratio, it is important to use the sailboat’s Load Waterline Length (LWL)
- This is the hull’s length where it comes out of the water at the bow and the stern
- This is critical, because it measures the length of the boat that is exposed to the water

- Racing Sailboats will generally have a much lighter D/L ratio

- Ballast – Displacement Ratio
- The weight in the keel and bottom of the boat that counter’s the sailboat’s tip or “heel”
- This is a good indicator of the stability of the sailboat and can help tell us the boat’s purpose (offshore cruising vs. racing)
- By comparing a boat’s ballast to her displacement, you can make this determination
- Coastal – 35% or less
- Average – 35% – 45%
- Offshore – 45% or greater

- 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

- A sailboat uses her sails for propulsion by generating lift (upwind) or by blocking the wind and being pushed (downwind)
- 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

- A sailboat’s propulsion comes from the wind on her sails and is proportional to the area of all the sailboat’s sails
- This is measured by calculating the area of a each of the sails and then simply adding those numbers together:
- The measurement of sail area is calculated using square feet
- For more advanced courses you can discuss the measurements of E and P for the mainsail and I and J for the headsail

- 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

- In order to compare sailboats with one another, we use the Sail Area – Displacement ratio
- This shows how much power the sails generate compared to each pound of displacement

- 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
- The calculation also involves dividing the displacement by 64. This is done because the weight of seawater is 64 lb/ cubic foot

- The higher the ratio indicates a high performance sailboat usually designed for racing

- The calculation also involves dividing the displacement by 64. This is done because the weight of seawater is 64 lb/ cubic foot
- Because of the large sail area, these boats are sometimes more difficult to handle and can become easily overpowered in high winds

- Low ratio – 8-13
- Medium ratio – 14-20
- 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