1. Science of Sailing Lesson VI: Sails Science of Sailing
Preserving America’s Sailing Legacy * Engaging Sailing’s Next Generation

2. Introduction
We have already learned how a sailboat uses her sails for propulsion
The difference in pressure between the two sides generates lift (like an airplane wing)
Low Pressure
Wind
High Pressure

3. Introduction
A sports car enthusiast searches for a car with a high horsepower / weight ratio…
For a sailboat, we use a similar type of measurement to determine the potential speed (or acceleration) of the boat
What variables do you think we use for a sailboat?
horsepower
weight
Faster Car

4. Sail Area
A sailboat’s propulsion comes from the wind on the sails and is proportional to her…
Sail Area
Jib Sail Area
150 sq ft
Measured by calculating the area of each of the sails and then simply adding those numbers together
Measured in square feet
Mainsail Area
188 sq ft
J/80 Total Sail Area
338 square feet

5. Height (Mast) x Base (Boom)
Area of Mainsail
Although most sails are not a perfect right triangle, we will calculate the area of the mainsail by measuring…
Height (Mast) x Base (Boom) 2
Actual sail measurements are slightly more complicated as most sails have a natural curvature shape
Mast
Boom

6. Height (Luff) x Base (Foot)
Area of Headsail
Although most sails are not a perfect right triangle, we will calculate the area of the headsail by measuring…
Height (Luff) x Base (Foot) 2
A common measurement for the headsail area is the “foretriangle”
Luff
Foot

7. Sail Area – Displacement Ratio
In order to compare boats, we calculate the…
Sail Area / Displacement Ratio
This shows how much power per lb of displacement
Under this calculation, we assume that displacement is the sole limit of a boat’s speed
While not completely accurate, as we have seen with the different hull designs, this ratio provides a good way to compare boats
Why do you have to divide the boat’s displacement by 64?
Weight of Seawater = 64 lb / cubic ft

8. Sail Area-Displacement Ratio
The higher the ratio indicates a high performance boat designed for racing, but generally more difficult to handle
Low
Medium
High
8 - 13
21 – 40+

9. Sail Area-Displacement Ratio
Let’s do an example using the Cal 25 Sailboat
Sail Area (sq ft)
[Displacement (lb) / 64 ]2/3
Sail Area = 315 sq ft
Displacement = 4500 lb
315 sq ft
[4500 lb / 64 ]2/3
SA/D Ratio = 18.5