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 multihull 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 fullkeel

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

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 – 200350

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

Because of the large sail area, these boats are sometimes more difficult to handle and can become easily overpowered in high winds

Low ratio  813

Medium ratio  1420

High ratio  2140+
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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