Bottled-up Buoyancy

Introduction

When you see a submarine underwater in a movie, it probably seems as if it's moving very slowly, but actually, submarines move very fast. The fastest submarine can travel over 40 knots, which is over 75 kilometers per hour! Moving and maneuvering a large submarine underwater is a tricky task because of the forces involved: gravity, buoyancy, pressure, and friction.

First, think about the force of friction and how it is different when moving in the water. Have you ever tried running in a pool? Then you know it is much more difficult to run through water than to run through air. That is because water is more dense than air, and it is actually pushing back on you when you move. Moving though water creates a lot of friction, and submarines are designed to minimize this force by having a hydrodynamic, bullet-like shape. Submarines are also made of very strong materials, like steel, to resist the forces of high pressure the submarine encounters as it dives deeper under the water.

Submarines are large, heavy ships that are often built with steel, so how in the world do they float in the water? The answer is buoyancy: "A submarine or a ship can float because the weight of water that it displaces is equal to the weight of the ship. This displacement of water creates an upward force called the buoyant force and acts opposite to gravity, which would pull the ship down. Unlike a ship, a submarine can control its buoyancy, thus allowing it to sink and surface at will" (howstuffworks.com, 2007).

 

Figure 1. At howstuffworks.com you can play this java tutorial showing how a submarine dives and surfaces by changing how much water it stores in the ballast tank. On the left, the ballast tank is empty, and the submarine is at the surface. On the right, the ballast tank is full, and the submarine dives underwater. (Freudenrich, C. and Brain, M., 2000.; images ©How Stuff Works.)

That explains how a submarine moves up and down, but how does the submarine move forward and backward? One way for a submarine to move forward is to use a propeller, but there is a trick to this movement. When the propeller spins, it can also spin the submarine, taking away any forward movement. Without stabilizing fins, a submarine would just be a spinning tube. The stabilizing fins resist spinning by adding a force of friction, canceling out the spinning motion, which allows the propeller to push the submarine forward. The submarine can also move backward by changing the direction that the propeller spins.

In this project, you will investigate how changes in buoyancy affect whether a submarine dives or surfaces. You will build your own miniature submarine and test different levels of buoyancy with your model. You will change the level of buoyancy by adding different amounts of air and water to your model submarine.

Terms and Concepts

To do this type of experiment, you should know what the following terms mean. Have an adult help you search the Internet or take you to your local library to find out more!

• Submarine
• Friction
• Pressure
• Buoyancy
  • Positive buoyancy
  • Neutral buoyancy
  • Negative buoyancy
• Gravity
• Stabilizing fins
• Hydrodynamics
• Stability

Questions

• What components make a submarine work?
• How does buoyancy affect the direction a submarine moves?
• How do the amounts of air and water inside the test submarine affect buoyancy?

Bibliography

• This project is based upon the "Soda Bottle Submarine" project at howtoons.com:
  Bonsen, J., Dragotta, N., and Griffith, S. (2007). Soda Bottle Submarine. Howtoons.com. Retrieved December 6, 2007 from http://www.howtoons.com/?page_id=48
• Read about how submarines dive and surface at howstuffworks.com:
  Freudenrich, C. and Brain, M. (2000). How Submarines Work: Diving and Surfacing. howstuffworks.com. Retrieved December 6, 2007 from http://science.howstuffworks.com/submarine1.htm
• Find out how the scientific concepts of buoyancy were used to investigate "The Voyage of Doom" on NOVA:
  Groleau, N. (2000). Voyage of Doom: Buoyancy Brainteasers. NOVA Online, PBS and WGBH. Retrieved December 6, 2007 from http://www.pbs.org/wgbh/nova/lasalle/buoyancy.html

Materials and Equipment

• 2-liter soda bottle (1)
• Water bottle, standard size, approximately 500-700 mL (1)
• Razor blade or knife
• Drill with a 3/32-inch drill bit; be sure to confirm, but this size should create a hole that will fit the paper clip you'll be inserting
• Scissors
• Pen or needle
• Needle-nose pliers
• Large paper clips (2)
• Metric liquid measuring cup
• Permanent marker
• Chopsticks
• Stiff ruler
• Rubber bands (3)
• Waterproof sealant (such as silicone)
• Bathtub (or pool), filled with water
• Lab notebook

Experimental Procedure

1. 
   Click on the image at the right to view the Soda Bottle Submarine instructions found at http://www.howtoons.com/?page_id=48.
2. Print out the instructions.

Making the Propeller

1. To begin making the propeller, cut the bottom half off the 2-L soda bottle. Cut the bottom, as shown by the howtoons.com worksheet, to make a propeller shape that has five separate curved blades. This is difficult because the plastic is thick in certain places. Have an adult carefully use a razor blade or a knife to help you with this tricky step. See Figure 2 for a picture of the finished propeller.
   
   
   

   Figure 2. This is a propeller made out of a 2-L bottle, explained in the howtoons.com instructions.

   
   
2. Have an adult drill a hole in the small water bottle cap and drill two holes in the propeller, one in the center and another just off center. Be sure to wear safety goggles when using power tools.
3. Using scissors, cut a small circle of plastic out of the remains of the 2-L soda bottle. The circle should be about the same size and shape as the submarine (small water bottle's) bottle cap. This piece will go in between the bottle cap and the propeller and will serve as a washer-the propeller needs a slippery surface against which to spin. Using a pen or a needle, carefully make a hole in the middle of the washer, big enough for the tip of a paper clip to easily fit through. Use caution if you use a needle so you don't poke yourself.
4. Straighten one end of one of the paper clips and feed it through the hole in the top of the bottle cap, then through the washer, and finally through the center hole of the propeller. The propeller fins should be facing down, curved away from the bottle cap.
5. Bend the inside end with the needle-nose pliers, looping it through the second off-center propeller hole to secure the paper clip and two pieces together.
6. You will use the water bottle for the submarine, but first you need to add measurement markers on it to help you fill the submarine with different amounts of water during your trials. Using a metric liquid measuring cup, pour water into the bottle, 20 mL at a time, each time drawing a line with a permanent marker where the top of the water is. When you are done, you can label the lines (20 mL, 40 mL, 60 mL, ...500 mL) for easier measuring later.

Making the Submarine

1. To make the submarine, have an adult drill two small holes in the bottom of the water bottle, just like the holes in the propeller—one in the center and one slightly off-center.
2. Straighten one end of the second paper clip and use chopsticks to insert the straight end of the paper clip through the hole of the water bottle, from the mouth of the bottle into the centered hole at the end of the water bottle. This step is tricky!
3. Once you get the straight end out, use the pliers to bend the end over and hook it into the other off-center hole to secure it.
4. Use the chopsticks again to hook a rubber band onto the paper clip hook inside the water bottle submarine.
5. Without letting go of the rubber band, hook the other end of the rubber band loop through the paper clip hook inside the bottle cap propeller. If it is too difficult to pull the rubber band using chopsticks, use another paper clip to make a hook and use this to pull the rubber band to the hook in the bottle cap propeller. Make sure the rubber band is tight—the submarine's rubber band needs good tension or it won't be able to move. If the rubber band is too loose, try a smaller size. See Figure 3 for a picture of the connected assembly with a 2-L bottle submarine. Note: (a 2-L bottle is used in this figure because it is bigger, making the assembly easier to see—remember, you are using a smaller water bottle).
   
   
   

   Figure 3. This is the connected assembly. The 2-L bottle is used only as a larger visual aid—you will be using a smaller water bottle for the submarine body.

   
   
6. Connect the ruler as a stabilizing fin, using two rubber bands. Place the ruler perpendicular and so it is just centered to the submarine. Attach it with two rubber bands that form an "X" around the bottle and the ruler.
7. Using the waterproof sealant, make sure that the paper clip holes are fully sealed. Water can easily move in and out of the submarine if there are still holes, which will affect the experiment. Give the sealant enough time to dry before proceeding with the experiment—see the sealant's instructions.

Testing the Submarine's Buoyancy

1. Now you should fill your submarine with different amounts of air and water using the marks you made earlier. Starting at 0 mL, test each mark for buoyancy. You will need a data table in your lab notebook to keep track of your experiments and results. See the one below for an example.

   Amount of Water (mL)	Amount of Air (mL)	The submarine: (sinks, rises, or stays level)	Buoyancy is: (positive, negative, or neutral)
   0 mL	500 mL
   20	480 mL
   ...	...
   480 mL	20 mL
   500 mL	0 mL

2. Each time, after you fill the submarine bottle with the desired amounts of air and water, tighten the bottle cap and use the propeller to twist the rubber band around several times to build up potential energy. Why will this propeller work? Loot at it. What makes it force the submarine forward instead of backward? Which way should it spin? Be sure you count and record in your lab notebook how many times you've twisted the rubber band around and twist it the same number of times for every trial. Submerge the submarine in the bathtub or pool of water and let it go. Be sure to have an adult with you if you are testing this in a pool. What happens to the submarine? If your submarine:
   • Sinks - this result is negative buoyancy.
   • Rises - this result is positive buoyancy.
   • Stays level - this result is neutral buoyancy.
3. Keep testing, emptying, refilling, and testing your submarine until you have found a good range of buoyancy.
4. Make a graph of your data to help you analyze the results. You could make two graphs, one of Buoyancy vs. Air Volume (mL) and another of Buoyancy vs. Water Volume (mL). Do they show the same or different relationship?
5. Think about your results and make a conclusion. How did the amounts of water and air affect the buoyancy of your submarine? Why do you think buoyancy is important for the hydrodynamics and stability of submarines?