I’m watching Junkyard Wars: The Great Race and they are at the Colorado River segment. Each team is dashing about looking for oil drums to make pontoons.
How would you calculate the size or quantity of drums (55 gal presumably) to float something (like my car, which weighs about 2500lbs)?
Roughly, if a car is about 1,000 kilograms, and a drum displaces about 200 litres, and a litre weighs 1 kilogram, then five should do it, but to be conservative and to balance it out, run three together on each side.
I can say with certainty that one drum, some driftwood, and a few yards of bailing wire is more than adequate to float a small boy out with the tide into the Bay of Fundy, and back in again many hours later.
Apart from floating a car, there would be one heck of a stability problem due to a high centre of gravity, so either sling the car under the pontoons (underwater - - assuming that the river is deep enough, and that no one minds the car being submerged), or spread out the pontoons sufficiently, which of course would require more weight due to longer cross-beams.
One gets the idea that there might be a story here. 
Yup, but aside from adding that the small boy was me, that’s all there is to the story.
MrBlueSky
Seems as if my website might come in handy for this too.
http://www.1728.com/density.htm
That’s my Density, Mass and Volume Ultra Calculator.
(Accepts input in 9 different different units each).
So, we want to solve for MASS (or weight actually).
We input 55 gallons then the density (water densuty) and we learn that a 55 gallon drum will have a buoyancy of 459 pounds.
Muffin was on the right track but I thought an exact answer was needed.
Also, I think that as Muffin also stated, keeping a car balanced on top of several 55 gallon drums would be tough.