** Topic: How a Sailboat Works: Sails **

#### Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (5.3mb)

Lesson 6 Handout – Sail Area Exercise (PDF)

**Click here to launch PowerPoint for Lesson 6**

** 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 how to calculate Sail Area – Displacement ratio

** Lesson Outline: **

- 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

- 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

- 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 – 8-13
- Medium ratio – 14-20
- High ratio – 21-40+

** Supplemental Resources: **

Sailboat-Cruising.com: Sail Area Calculations

Cruising World: How Sailboats Measure Up

** Exercises/Activities: **

The actual calculation for Sail Area–Displacement is quite complicated. This is a great opportunity to give students practice using a scientific calculator. Also, the excel model provided can make quick calculations easier and can be used to check student’s work.