Topic: How a Sailboat Works  Hull Speed & Buoyancy
Teacher Resources:
Printable PDF Version of this Lesson Plan
Downloadable PPT of Lesson (3.9mb)
Click here to launch PowerPoint for Lesson 5
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
:

A sailboat’s hull is important for the many reasons we discussed during last class.

The one subject we didn’t cover is
Speed

Modes of Sailing

Displacement Sailing

As the sailboat moves through the water, she is constantly displacing a new patch of water

This generates waves behind the boat and will eventually cause the speed of the boat to max out

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

Planing Sailing

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

These boats can go much faster because of minimal drag

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

Forced Mode

This is not covered but worth mentioning in more advanced classes

However, this is a good intro into
hull speed

Hull Speed

Used to determine the theoretical maximum speed of a displacement sailboat

As a displacement sailboat’s hull pushes through the water, she makes waves at her bow and stern

When hull speed is reached, the boat will appear trapped between the waves she is generating

The length of the wave, and thus hull speed, is based purely on the boat’s length

Discuss how this calculation only applies to displacement hulls and not planing sailboats

How a Sailboat Floats: Buoyancy

A boat will float so long as her total volume weighs less than the density of the water it displaces

Archimedes Principle – The upward buoyant force on a sailboat is equal to the weight of the fluid that body displaces

A sailboat will float as long as the
gravitational weight
of the boat is less than the
upward buoyancy force

Boats are designed with the specific purpose of displacing enough water to float

How do you think men and women design large container ships to float?

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
Next Lesson
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
Next Lesson
Topic: How a Sailboat Works
Teacher Resources:
Printable PDF Version of this Lesson Plan
Downloadable PPT of Lesson (3.4mb)
Handout PDF for DragonFly TV Episode: “Sailboat Design by Max and Brian”
Click here to launch PowerPoint for Lesson 3
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
:

Introduction

The interaction of the wind, sails, hull, water, and keel all effect a boats propulsion

Take this time to review each of these terms

C.A. Marchaj:
“The sailing craft must be considered as a complex system consisting of two interdependent parts – aerodynamic and hydrodynamic”

Sails

Push vs. Pull: The Two Modes of Sailing

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 squarerig)

When the wind is coming abeam of or in front of the boat, the sails are able to generate
pull

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

As the high pressure air attempts to catch up, it generates lift on the sail which causes a force forward and sideways

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.

Keel

Why doesn’t the boat drift sideways?
à
the keel

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
:
DragonFly TV (PBS) Episode – “Sailboat Design by Max and Brian”
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.
Next Lesson