plastic Raft

It's summer. I'm bored. Let's make something out of trash! Actually, I'm making this raft out of 1.25 inch pvc pipe, some metal mesh, my FVS graduation sign, and a lot of empty plastic containers, all attached with old screws I found in our many messy drawers. I used the physics Iearned in school to calculate how many gallons I would need to stay afloat on my raft, and that number came out to be around 45 gallons. Ideally I could use 45 milk jugs, but I'm making do with other sizes and shapes, too.

The frame, with Pigeon the cat

Adding the seat: thanks FVS!

The first load of containers

Attaching the first container

A donation from a neighbor

Attaching containers to the frame

Collecting more containers

Filling in the center

Side view

Nearing the end of the build

Paddle made from pvc and plastic sheet, with bottle cap washers

The Calculations:

Buoyancy Force (Fb) =

Submerged Volume (Vs) x Density of fluid (D) x gravity (g)

The density of water is about 997 kg/m^3

g = 9.8 m/s^2

Knowing that the buoyancy force balances out the force of gravity on my body (Fg), I found that Fb must be equal to or greater than 715.5 Newtons (N).

715.5 N = Vs x 997 kg/m^3 x 9.8 m/s^2

Vs = 0.0732 m^3

That doesn't seem like much, since the answer is in cubic meters. But a conversion to gallons shows that my raft needs around 20 gallons of completely submerged volume to hold just my mass. I doubled that number for how many gallons I needed to collect, because I wanted all the containers to be only halfway submerged, so I wouldn't get wet. This calculation doesn't take into account the mass of the raft, though the result doesn't change much if that mass is included.

Testing the raft:

Packing up the car

Attaching the outriggers

The final design

Plastic Raft

I did the math, I built the raft, the only thing left to do was to try it. I had to put faith in my math and my design, and trust that I had built a capable vessel. I didn't know if I was going to sink or float, but I wasn't too keen on getting wet. I was delighted when I stayed stable and above water. My raft project was a success!

My brother and my parents joined me to watch the maiden voyage of the "Drowning Dane" and once they saw how well the raft worked, of course they wanted to try. My math was based on my mass, so I knew that my mom, who weighs less, would be able to float. But my dad and brother weigh more than I do. They still tried it.

My mom went after me, with no mishaps. She even felt comfortable enough to try standing on it, which didn't work very well.

Next was my brother. He only got a little bit wet, partly because the raft is a few containers shy of being built for his mass and partly because some of the containers had begun to fall off.

Finally, my dad attempted to float on the raft. This sunk it, almost immediately. By this point, the raft was not holding together well, and some containers had filled partly with water. Though I overestimated the number of gallons needed, the raft couldn't take the greater weight, the loss of several containers, and the extra weight from water-logged containers. When my dad tried to get on, the edge of the raft tipped into the water and the open containers filled up, sinking the raft.

In the end, the math worked out. I proved that open containers on a balanced raft won't fill with water as long as the weight limit isn't exceeded. Screws don't work too well to attach plastic to plastic (the screws rip out), but overall the design worked pretty well. It was easy to balance just by shifting my body position. My little paddle really didn't work, mostly because the plastic was too flimsy, but also because it was too small.

What happened to the raft when I sat on it?

When I put pressure on the containers, those at the end were pushed out, away from the water (purple arrows). When I calculated the minimum submerged volume, I assumed all containers could be made to float halfway in the water. But because they were pushed away, the containers at the edges were barely submerged at all (yellow highlights show submerged portions).

The outriggers did nothing, so much that we took them off so I could paddle more easily. There wasn't enough weight on the jugs at the ends of the rods, and the rods were not stiff enough. They were barely submerged at all.

Ideally, every container would be halfway submerged when my full weight was applied to the middle of the raft (yellow highlights show submerged portions). Everything on the raft is either submerged or is dead weight. If a container is mostly out of the water, then it is doing more harm (adding weight) than good (displacing the water to cause buoyancy). My raft was working at a less than optimal level of ability.

The simplest way to fix this issue would be to wrap a rope around the edge of the raft, pulling all the edge containers towards those in the middle. Not only would this keep them from being pushed out, it would also keep the containers in the center--which were taking the most pressure--more secure.

This is one of my strangest projects so far. What's funny is how technical this was, with all the calculations I did, and the result was essentially a pile of trash. This entire project took me a month to build, not because I didn't know how to build it, but because it took a while to collect 47 gallons of volume. The finished raft actually has around 47 different containers (I may have forgotten one or two in my notes), even though they were all different sizes. If I had just relied in my family's consumption of milk and laundry detergent, this project probably would have taken me a year. Thanks to all the neighbors and friends who helped me complete this project!

Summer 2020

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