Introduction: As sailboat designers know, the hull characteristics that keep a boat moving efficiently, resist handling difficulties and keep a sailboat upright, are sometimes in conflict. It can be a tricky job to reconcile them. All boats, sail or power, are compromises, but sailing craft embody more trade-offs in their design than do other types.
Most modern sailboats give nearly equal attention to stability, load-carrying ability and a speedy hull, with perhaps a slight tilt toward one factor or another, depending on the designer’s special aims. In the nearly “pure” racing sailboats, speed-producing elements are emphasized to the detriment of other factors. Cruising sailboats tend to be designed for comfort and cabin amenities instead of speed.
Sailboat hulls are designed to pursue a straight-ahead course with as little disturbance of the water as possible. At the same time the boat is moving forward, wind pressure on the sails is pushing it to one side. This lateral movement of the boat in the water is called leeway and is partially counteracted by the hull shape.
The portion of the hull shape that minimizes leeway is called either a keel or centerboard depending upon which is used in the design of the boat. This fin-shaped feature in the bottom of the hull allows forward movement, but increases the side profile of the hull thereby increasing lateral resistance.
What to Expect: Students will be making and testing their own sailboats. They may choose to use the materials that the teacher supplies, or may supply their own. The class will also make a testing tank using simple materials.
Since the tank is large and filled with water, it should be made outdoors. The sailboats will be designed to sail the length of the testing tank. You can use a fan to propel the boats, if needed. The testing tank is about eighteen feet long and about two feet wide. You may choose to have competitions for speed, sailing straight, and carrying weight.
- Sailboat Materials
- 1/2 gallon cardboard milk cartons cut in half from top to bottom so each half has a “bow” like a boat. The “bow” will need to be stapled shut.
- Straws for masts
- Modeling clay to support the masts and as needed for ballast
- Other cardboard
- Paper for sails
- Any other construction materials needed
- Uniform sized weights for carrying capacity
- Testing Tank Materials
- Cardboard boxes approximately 2′ wide by 2′ long (boxes from grocery stores are strong – get as many as needed for length of the tank)
- Roll of black plastic
- Electric or battery powered fan
Building the Testing Tank
NOTE: It is best to set up the tank outside the building, but it may be possible to have it inside. Handling the water in and out of the tank can be difficult. One class found that outside they didn’t need a fan to propel the boats. The slightest breeze was usually sufficient.
- For all but two of the boxes, cut opposite ends out of the boxes.
- For the remaining two boxes, cut only one end out. These boxes form the ends of the tank.
- Cut the sides down to about 4 inches.
- Put the boxes end to end, overlapping the ends. Support the boxes with books or large rocks where needed to withstand the pressure of the water when the tank if filled.
- Lay the black plastic over the boxes.
- Fill the tank with water.
- Ask students to suggest some considerations and goals for sailboat design (speed, stability, capacity).
- Introduce students to a variety of sailboat and sailing ship designs using books, magazines, and the internet.
- Have students use a half of a milk carton, or other materials, for the hull of the boat. The shape of the hull makes a large difference in how a boat sails. They may attach a keel and decorate the hull if desired.
- The boat’s hull must include the following compartments: dog house (where the navigation equipment is), galley (kitchen and dining area), and cabin (for captain’s quarters).
- Design a helm (steering wheel) for the ship. (Since these designs do not have rudders with which to steer the ships, the helm won’t function as a real helm does).
- Each boat must have at least three sails, which the students design and decorate.
- Attach the sails to the masts, and attach the masts to the hull, making sure they all stay upright. Attaching some string (stays) may help to hold the masts up. Students may want to design their boats so that the sails can be adjusted to control the effect the wind has on the sails and the direction the boat sails.
- Design an original flag for the boat. Attach it to one of the lines.
- Add other items to the boat as desired: lifeboats, oars, anchors, cannons, etc.
- In the testing tank, each boat will sail with a “following wind and following sea,” and must sail straight without running into the shore or sinking. Measure the distance and speed the boat sailed (speed = distance/time), and observe how straight it sailed.
- Each boat will have three trials to determine how much weight it can carry while still sailing at least six feet.
Evaluation: Students should record the speed and distance sailed before the boat hits the side of the tank. They should also record the amount of weight the boat carried. Evaluation will be a student’s written or oral report on these results and must include the student’s suggestions for improvements or modifications.
Extensions: The testing tank can be used to test boats of the same hull shape with propulsion designed by students (rubber bands, balloons, etc.).