Topic: How a Sailboat Works: Hull Type
Teacher Resources:
Printable PDF Version of this Lesson Plan
Downloadable PPT of Lesson (6.3mb)
Lesson 4 Handout – Displacement Worksheet (PDF)
Displacement Worksheet – Answers (PDF)
Click here to launch PowerPoint for Lesson 4
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
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 multi-hull 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 full-keel
- 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
- Full Keel
- 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 – 200-350
- 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
Exercises/Activities :
Provide students with a worksheet showing the different sailboats and allow the students to perform the various calculations.