Exploring Design and Construction Name ______
A number of construction techniques need to be directly taught. The worksheet provides space where students record these instructions. By the end of this class, they should have made all of their decisions about the hull and many about the propellers.
Materials: Each team should be given a piece of Styrofoam that is enough to make at least two hulls, about 35 cm by 15 cm. Each team should also get a skill knife with a good blade, two alligator clips, several Popsicle sticks, and tongue depressors. They supply or are supplied their own 9-V batteries. If more than one battery is allowed, they can be clipped together to make higher voltages. The teams need to have access to glue guns, hammers, fine sand paper, voltmeters, hot air guns, and the trough to measure and race in. They also need access to ping pong balls suspended by two threads to test their propellers. Note: They should not be constructing the hull until they have completed this sheet and you have checked their design. This probably won’t occur until next class.
1) The Hull
a) You want your hull to be as smooth as possible. Describe how to do this.
Demonstrate the technique to the students and then have them write notes to remind themselves and reinforce their learning. To make a cut, select a skill knife with a long blade without any nicks in the edge. Extend the blade and make a cut using a slow continuous movement that goes partway into the Styrofoam. Repeat this several times until it cuts all the way through. Do not saw back and forth. Sharp edges can be gently sanded. Practice making smooth cuts on some scrap Styrofoam. Think of the hull as a modified rectangle. First cut out the rectangle and then cut off triangular sections at the front.
b) Draw the rear view of your boat – to exact scale – showing the position of the propeller, motor, hull, water, and trough. Give a brief explanation for your choices of each dimension.
The boat should be as long as possible – around 35 cm. This will reduce drag and friction, and travel straighter.
It should be almost as wide as the trough. This will balance easier and travel straighter.
It should be a double hull with Popsicle sticks connecting them. This reduces drag and increases balance.
The bottom should be flat for ease of construction and better balance.
The motor must be raised just enough so that the blades don’t hit the boat, water or sides.
It should have a little more than just enough Styrofoam to support a heavy motor and 3 batteries - 140%?
c) Draw a side view of your boat – at 1/3 scale – showing the position of the propeller, motor, hull, water, and trough. Give a brief explanation for your choices of each dimension not discussed in the previous diagram.
The battery should be toward the front to balance the motor which is usually toward the back.
The bottom should be flat for ease of construction and front-back balance.
The front might be slanted to help the boat plane, like water skis or a hydrofoil.
Sharp edges should be smoothed to improve C.
d) Draw a top view of your boat – at 1/3 scale – showing the position of the propeller, motor, hull, water, and trough. Give a brief explanation for your choices of each dimension not discussed in the previous diagram.
It might be pointed at the back and front so that the water can move smoothly around the shape, making as little turbulence as possible. This is the shape of a canoe, kayak, air foil, etc. It should have no sharp corners or edges. It should have just two Popsicle sticks holding the hull together, one near the front and one near the back. Note: This question has been answered assuming that the hull is made of Styrofoam. You may want to encourage or discourage the teams from trying other materials. More choices require more time.
2) The Propeller
a) Describe how to make a propeller from a Popsicle stick or tongue depressor.
Demonstrate the technique and then give them time to make notes. Place the stick in very hot water for ½ a minute. Twist the stick while a teammate dries it with a hair dryer or hot air gun.
b) The propellers have to be firmly attached to the smooth motor shaft. Describe how you attach a piece of wood dowelling to do this.
Demonstrate and then give time for writing notes. The extra material provides more grip on the axle. The axle can be forced into the wood by tapping with a hammer. When hammering, the other end of the motor needs to be protected with some appropriate small metal cylinder, i.e., a metal nut.
dowel motor protective piece hammer force
c) The propeller has to be perfectly perpendicular to the axle. Why? How will you ensure this?
You want all of the force to be forward. If propeller is not perpendicular to the axle, the propeller will waste forward force and either push the front of the boat out of the water or more dangerously, push it into the water. The dowel itself can be slightly off axis but the face of the dowel must be perpendicular to the axle. Once the dowel is on the axle, you can sand the face by attaching a battery and holding it against sand paper.
d) The propeller has to be perfectly centred on the axle. Why? How will you ensure this?
If not centred, the propeller will shake the boat and cause less speeding-up force and more slowing- down force from turbulence. The dowel itself does not have to be perfectly centred, but the propeller does. Take great care when hot gluing the propeller on the dowel.
e) What aspects can you vary when you make the propeller blade? Test these ideas before attaching the motor to the boat. Describe what you tested and what worked best.
This is an opportunity to start the open-inquiry part of this contest. Some testing of motors and propellers can and should be done before the hull is completed. This separate testing is easier and it does not require waiting for the hull to be finished. This also means that all team members can be fully occupied. The testing can be done by seeing how well the air can move a ping pong ball suspended from threads or you can have the air push down on a sensitive mass scale. Students should try the quickest tests first. Changing batteries is easier than changing motors or propellers. With each blade and battery combination, compare one, two, and three batteries. Compare Popsicle sticks and tongue depressors. Compare a medium twist and a large twist. After this, students can try more difficult blade construction – multiple blades and carving blades.