Building a Bottle Raft

I needed an after-the-exam project for my AP Physics 2 class, and we settled on a buoyancy project (Fluids is one of the topics.) of building a raft and using empty 2 L soda bottles as floats.

Design Criteria

At my suggestion, they got the soda bottles by going to a redemption center and offering 10¢ per bottle for a large bag of 2 L bottles.  (The deposit is 5¢ per bottle. The redemption center gets 7¢ per bottle, so 10¢ is enough to make it worth their while.)  The heaviest student in the class is around 100 kg, which means he weighs approximately 1000 N.  Based on the formula FB = ρVg, with the density of water ρ = 1000 kg/m3, and g = 10 m/s2, my students calculated that they would need 0.1 m3 of soda bottles, which is 100 L total volume, or fifty 2-L bottles.

Building the Raft

They actually obtained 76 2-L bottles and decided to use all of them, which means the raft could theoretically hold about 1500 N (which is about 335 lbs.).  The plywood and bottles probably accounted for 15-20 kg (150-200 N), so the expected maximum payload was about 1300 N (about 290 lbs.).

The base was a 4′ × 6′ piece of plywood, supplied by a student whose father is a contractor.  The students chose a catamaran design, and duct-taped the bottles together into two clumps.  I taught them how to use a jig saw, and they cut slots for the tape and a slot at one end to attach a rope (so we could reel them in if they got out too far and couldn’t manage to get themselves back).

Testing It Out

On the day of the event (during “long block,” so we had two hours) we brought the raft to Flax Pond.  I supplied canoe paddles and PFDs (life jackets).  Students used duct tape to attach the bottle arrays to the plywood.  I mostly kept out of their way, except for an occasional observation, such as “It looks like the bottles can shift a little side to side.”  (“Oh yeah.  I guess we need some duct tape running the other direction too.”)  After about 20 minutes, the raft was ready.

A student named Matt had made it clear throughout the design and construction process that he really wanted to try out the raft, so it was an easy consensus that he should be the first to try it, and it worked exactly as designed.  Of course, teenagers being who they are, they had to test the raft closer to its weight limit, so a couple of pairs of students went for a jaunt.  The last pair probably weighed around 260 lbs. combined.  The raft did stay afloat the entire time and the students did stay on top, though when they both leaned to the same side and paddled, one corner dunked under the water and their pants got a little wet, but otherwise they made it back without mishap.

Of course, teenagers being who they are, they all agreed that taking their phones on their homemade duct-tape-and-soda-bottle raft with almost its maximum payload on board would be a really bad idea, but most of them disregarded their own advice so they could take selfies and videos to send to their friends on SnapChat.  Luckily, they all made it back with their phones intact.  While I’m glad none of them had the aggravation or expense of dunking their phones, there is a part of me that felt that a life lesson at that moment wouldn’t have been out of place…

After we packed up the raft, we stopped at Cafe 39, a wonderful Southeast Asian cafe, for lunch on the way back, and enjoyed Banh Mi sandwiches and bubble tea before returning to school for last period.

This was their last long block physics class, and the students all thought it was their favorite long block in their four years of high school.


About Mr. Bigler

Physics teacher at Lynn English High School in Lynn, MA. Proud father of two daughters. Violist & morris dancer.

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