Introduction: This lesson enriches a study of turbulence, work, or power, and dramatically illustrates the energy efficiency gained by streamlining a hull moving through a fluid.
What to Expect: Begin by demonstrating the effect of hull shape on speed by towing wood blocks of a variety of shapes. Follow up the lesson with a design contest where student groups must create and shape their own wood-block hulls and then test them under controlled conditions.
- Clear Towing Tank, 3-4 m x .4 m x .4 m (a towing tank may be constructed by the class or in the school’s woodworking shop.
- Thermometer and hydrometer
- A variety of wood blocks of the same mass but different shapes
- Screw eye and length of cord
- “Smart” pulley interfaced with a display unit (smart pulley has a small generator attached; voltage produced by the pulley’s rotation is proportional to the speed of the hull being pulled)
- Force sensor or balance
- Weight set
1. Fill tank 2/3 with water. Mount a smart pulley to the end of the tank and connect it to the computer.
2. Provide the class with boats or blocks of wood of the same mass but different shapes.
3. Weigh each boat with a force sensor or balance.
4. Measure and record the temperature and density of the water.
5. Tow each boat a given distance in the tank using weights descending from the smart pulley. Draw students’ attention to the waves formed along each hull during towing. How do the waves along the hull and the wakes compare for different blocks of wood? Are all the shapes equally stable?
6. Record speed and number of weights for the different boat shapes.
7. Have each group graph the data and calculate the work done (Work = Force x Distance) and the power used (Power = Work x Time) by their boats. Compare the force vs. speed curves generated by the different hulls. What is the effect of hull shape? Of length?
8. You may wish to have students pursue the question of length by designing a series of tests. The formula for practical top speed of a displacement hull is 1.3 v lwl where lwl is the waterline length of the hull.
9. Follow-up: Divide the class into groups and give each group a block of wood to shape into a hull. This can take two to three days. When the hulls are ready, do speed trials using standard weights. Have students keep track of the effect of additional weight on each hull’s speed. Generate a curve using test results to show the effect of waterline length on speed.
Evaluation: Each group’s planning and effort will be considered; a summary or conclusion from each group will be submitted; calculations of work and power from each group will be reviewed for reasonableness. Each group should be able to state reasons for its own hull design.
Extensions and Resources:
1. See the excellent curriculum on boat design, Young America: Hands On Activities (1996, Scott, Foresman and Co.) for many ideas and related activities.
2. Read about fluid flow; a good treatment may be found in Introduction to Nautical Science by C. A. Chase. Have them find pictures of hulls moving through the water (for example, photos of navy vessels) and discuss the water flow around these vessels as seen in the pictures.
3. Introduce Stokes’ Law, which relates the speed of an object through a liquid of known viscosity to the resultant resisting force. Use liquids of different viscosities to demonstrate and quantify differences.
For very complete instructions on constructing a tow tank or wave tank, see Project Earth Science: Physical Oceanography (1995, National Science Teacher’s Association).
Towing Tank Setup
The towing tank should be filled with water to just below the hole where the string goes through the left end of the tank. This string runs out the tank and over a pulley and on it hangs a series of weights. The pulley drives a small electric generator who’s voltage output is proportional to the speed of rotation of the pulley. The generator output is attached to a voltmeter which then effectively measures the speed of advance of the model when the string is attached to the eye on the front of a model.