1. Lesson IV: How a Sailboat Works: Hull Type
Science of Sailing
Lesson IV: How a Sailboat Works: Hull Type
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. Hulls Identification based on # of Hulls
Which do you think are fastest? Why?
Monohull
Catamaran
Trimaran

4. Keel
Full Keel
Pros – easily tracks through the water on a straight course
Cons – slower to turn and increased drag due to surface area below the waterline
Fin Keel
Pros – faster than full-keel and easy to turn quickly
Cons – smaller keel provides less resistance to forces that could cause a sailboat to go off course  more difficult to steer

5. Keel Bulb Keel Winged Keel
Provides more ballast weight by concentrating weight at the bottom to improve stability
Winged Keel
Provides additional hydrodynamic stability and allows a shallow keel to perform more effectively

6. How much water does she displace?
Hull Displacement
Hull Displacement – the amount of water a sailboat “shoves to the side” when floating
A J/122 sailboat weighs 14,900 lb
How much water does she displace?

7. Displacement Hulls What is meant by heavy or light displacement hulls?
Displacement / Length Ratio
Use a boat’s length compared to her displacement to determine if a boat is heavy or light
D/L Ratio =
Displacement (lb) / 2240
[ (0.01 X Length (ft) ]3
Light
200 or less
Medium
Heavy
350 or more

8. Displacement Hulls
When calculating D/L Ratio, you must 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
LWL

9. Displacement – Length Ratio
Let’s use the previous example of the J/122
J/122 Specifications
LWL = 34.6 ft
Displacement = 14,900 lb
Displacement (lb) / 2240
[ (0.01 X Length (ft) ]3
14,900 lb / 2240
[ (0.01 X 34.6 ft]3
Is the J/122 Light, Medium, or Heavy?
D/L Ratio = 161

10. Displacement – Length Ratio
Racing sailboats will have a much lighter D/L Ratio

11. Ballast
Ballast – weight in the keel and bottom of the boat that counter’s heeling
Indicator of stability
This tells us the boat’s purpose
By comparing a boat’s ballast to her displacement, you can determine the best use for that boat
Coastal – less than 35%
Average – 35%-45%
Offshore – greater than 45%
Ballast
Displacement

12. Ballast / Displacement Ratio
Let’s use the previous example of the J/122
Ballast
Displacement
J/122 Specifications
Ballast = 5,600 lb
Displacement = 14,900 lb
5,600/14900 = 37.6%
Coastal – less than 35%
Average – 35%-45%
Offshore – greater than 45%