For ease, I will split this step in to two sections. The first will explain the math/science that went into picking a bottle goal, and the second will quickly tell you how to figure out how many bottles to collect.Section 1
I wanted my raft to support myself, and a friend if the opportunity arises. I weight 170 lbs, and i'm factoring for a friend who weighs the same. That's 340 pounds. Add in the weight of the boat plus extra buoyancy to keep us above water, and we're at 400-500 pounds of needed buoyancy.
- The average water bottle contains .5 Liters of water.
Archimedes' principle, or the law of upthrust says:
"A body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid."
Thanks to scientific measurements, one liter of water supports one kilogram of weight.
If each water bottles supports half a kilogram, which is about one pound, than you need just as many bottles as pounds you wish to support. I used exactly 450 bottles in my raft.Section 2
First, decide how many pounds of cargo your raft should support, that number is roughly how many .5 L bottles you will need.
Easy as that!