Saturday, July 13, 2013

The 2-liter bottle boat

Maker Camp officially kicked off on Monday, July 8 and the first project on tap was a boat built from a two-liter bottle with a paddle made of plastic lids. It completely captured Gwen's attention, and we had a lot of fun with the build!

The concept is simple: Use empty soda bottles to create the buoyant body of the boat, then add paddles powered by energy stored in the tension of twisted rubber bands to make the boat move. Throw in the use of colorful duct tape, and you pretty much have a perfect project for a summer day!

We started by attaching two bamboo skewers to the sides of our washed bottle. Later we switched to stronger dowels, because the skewers tended to bend too much under the stress of the rubber bands.

Then you measure between the dowels and create your two paddles. They need to have a length that is 1 inch wider than the distance measured and a width that is 1 inch shorter, creating a rectangle. We cut ours from the top of an old Rubbermaid-style container. Once cut, you create a slit in the middle of each and join them into a cross shape (+). We found it helped to secure the paddles in place with duct tape.

Next you attach two 1-liter bottles to either side of the larger bottle. We positioned ours just below the dowels with the backs just about even with the 2-liter. Placing them any further back interfered with the movement of our paddles. As we continued to trouble shoot the project, we found that when attaching the dowels and small bottles, it was important to pay close attention the shape of the larger bottle. If you apply tape across an inward curve, it will pull everything out of whack and the paddles will jam.

The last step is to attach the rubber bands so that they hold the paddles in place between the two dowels. We found we needed to trim our paddles a bit more, to be sure they had enough clearance to move. Gwen wanted to trim them a lot and we talked about why that may not be the best idea, since the amount of surface area directly affects the force created, thanks to the resistance of the water against that big flat surface. We also found that the length and width of the rubber bands mattered, though not nearly as much as we expected. In the end we opted for "medium" thickness bands that were just barely tight across the dowels before twisting.

To use the boat you twist the paddles, transferring the kinetic energy from your hands to the paddle to the bands and creating tension across the rubber bands. Of course you've now stored potential energy in the bands until you release then and the paddle rotates and propels your boat! Super simple and lots of fun. It was a big hit at the pool, where everyone wanted to know what Gwen had built and see it work.

One of the most fun things was discovering that the direction in which you turn the paddles matters when you release! Turn them one way, the boat moves forward. Turn the other, and back it goes. Spending some time observing both and discussing why it happens was great.

Of course the first hing Big Sister Caitie wanted to do was sink the boat and see if it works as a sub. I guess we'll try that at the pool tomorrow! We also want to experiment with the shape of the bottles to see how they affect the movement of the boat. Would a one gallon milk jug work as well? I guess we'll have to find out!

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