1. Lesson V: How a Sailboat Works Hull Speed and Buoyancy
Super Sails!
Lesson V: How a Sailboat Works Hull Speed and Buoyancy
Preserving America’s Sailing Legacy * Engaging Sailing’s Next Generation

2. Introduction A sailboat’s hull is important for many reasons:
Stability
Safety
Comfort at Sea
Load Carrying Capacity
Speed

3. Displacement vs. Planing
Hull Speed
Displacement Sailboats
As they move through the water, they are constantly displacing a new patch of water
This generates waves behind the boat
Hull designed to cut through the water
Planing Sailboats
Able to sail on top of the water
They can go very fast with minimal drag, but require a great deal of power to get “on plane”
Displacement vs. Planing

4. Only for Displacement Hulls!
Hull Speed
How do you determine a boat’s maximum speed?
As a displacement boat’s hull pushes through the water, she makes waves until she’s sitting between those waves
This maximum speed is called “hull speed”
The length of the wave is based on the boat’s LWL
Hull Speed =
LWL = the length of the waterline in feet
Hull Speed = speed of the sailboat in knots
Only for Displacement Hulls!
Why??

5. J/122 LWL = 34.6 ft 7.9 knots Bermuda 40 LWL = 29.8 ft 7.3 knots
Hull Speed
What is the hull speed of a J/122?
What about a Bermuda 40?
J/122
LWL = 34.6 ft
7.9 knots
Bermuda 40
LWL = 29.8 ft
7.3 knots

6. Laser LWL = 12.5 ft 4.7 knots Hull Speed
What is the hull speed of a Laser?
But Lasers have been recorded at speeds of as much as 16 knots! How is this possible??
When a sailboat planes, it is able to sail much faster than her hull speed
Laser
LWL = 12.5 ft
4.7 knots

7. Buoyancy How a Sailboat Floats
When a 2,000 ton sailboat is launched into the water, what is keeping it from sinking?
A boat will float so long as its total volume weighs less than the density of the water it displaces
Review: How much water does a 2,000 ton boat displace?
Buoyancy
– video on buoyancy

8. Weight Buoyancy Buoyancy Archimedes Principle
The upward buoyant force [on a sailboat] is equal to the weight of the fluid that body displaces
A sailboat will float so long as the gravitational weight of the boat is less than the upward buoyancy force
Boats are designed to displace enough water to float
Weight
Buoyancy

9. So how is the weight of a massive tanker not enough to make it sink?
Buoyancy
A large boat floats because…even though it weighs a lot, it displaces a greater amount of water that weighs even more
Wide hull
Not solid (full of air)
So how is the weight of a massive tanker not enough to make it sink?