How to increase the buoyancy of a container.

March 10, 2013 5:14 PM

Would an empty barrel stuffed with lifejackets and sealed be able to support more weight floating on the water than just the empty barrel? Ie. does a buoyant object increase overall buoyancy only if is directly in contact with the body of water?

I am trying to figure out how I can get something about the size of a keg to support my weight (~200lbs) on the water. I want to try using something like a keg for logrolling like in those lumberjack challenges. But from what I understand if the keg only holds 100lbs of liquid it will only be able to displace a 100lb person when it is empty. Would sealing additional buoyant material like life jackets inside make the barrel more buoyant?

posted by GleepGlop to sports, hobbies, & recreation (13 answers total)

Bouyant force is equal to the mass of water displaced.

So, a 55 gallon drum fully submerged experiences (55*8) 440lbs of bouyancy.

Adding things to the inside won't increase buoyancy, in fact, it will diminish it because of the increased weight.
posted by Pogo_Fuzzybutt at 5:19 PM on March 10 [4 favorites]

The lifejackets would reduce the bouyancy of the barrel. Bouyancy in this situation comes from trapped air -- whether that air is trapped in the barrel or in the foam of the life jackets. Putting life jackets in the barrel would mean less space for air, since the foam (not the air trapped in the foam, the actual material of which the foam is made) and fabric of the life jackets take up some space in the barrel and have less bouyancy than air.

For maximum bouyancy, use the empty barrel alone. The only way to increase it is to fill the barrel with a lighter-than-air gas such as helium, or to evacuate it and "fill" it with vacuum, or to fill it with hot air which is less dense than cold air. None of those options are likely to be practical of course. The only practical thing to do would be to use a larger barrel which holds more air.
posted by Scientist at 5:23 PM on March 10 [2 favorites]

Lifejackets work by increasing your volume much further than they increase your weight, thus decreasing overall density. If you are putting things inside the barrel, you are not increasing its overall volume, therefore the only way to make it support more weight would be to decrease the weight of the contents - i.e. replace the (presumably air) with helium or hydrogen.
posted by fearnothing at 5:25 PM on March 10 [1 favorite]

I want to try using something like a keg for logrolling like in those lumberjack challenges.

Lumberjack-style log rolling is feasible because logs are heavy and have rough surfaces, so they don't roll over very quickly. Even if it could support your weight, a small, light, man-made round object with smooth exterior surfaces is going to spin in the water faster than your reaction times can cope with unless you're a superhero.
posted by jon1270 at 5:33 PM on March 10

I think that the idea of putting some water in the barrel is to allow it to be partially submerged and thus make barrel-rolling more manageable. The rest of the barrel should be air, for minimum additional weight.
posted by yclipse at 5:45 PM on March 10

Yeah, you will need an actual log for this.
posted by ryanrs at 5:46 PM on March 10

Ah yclipse, you're right it would have to be partially submerged otherwise it wouldn't roll in place and just shoot out from under me. Ah well back to the drawing board...
posted by GleepGlop at 5:47 PM on March 10

Also it needs significant moment of inertia, i.e. it must be heavy and slow to spin.
posted by ryanrs at 6:03 PM on March 10

The only case in which the barrel being full of something buoyant, ex: lifejackets, would be helpful is if the barrel were to take on water. Instead of immediately being full of water, it would be full of water + lifejackets, and would therefore remain buoyant for longer.
posted by charmcityblues at 6:17 PM on March 10 [1 favorite]

You would increase buoyancy if you strapped the life jackets to the outside of the barrel.
posted by spikeleemajortomdickandharryconnickjrmints at 6:21 PM on March 10

Try sticking some paddles onto the end of the barrel so that they resist the rolling movement sufficient to allow your reaction times to catch up ?

You could set up a handicapping situation where different 'rollers' were allowed different length paddles !

Might be good if the paddles were made of something stiff but soft on the surface so that if you fell onto them they were unlikely to make a gash in your skin ?
posted by southof40 at 7:10 PM on March 10

Better would be a drum partially filled with water and internal baffles. But I still don't think a 55 gal drum will be large enough.
posted by ryanrs at 7:20 PM on March 10

I would be concerned about end over end stability with the percussive intermittent footfalls . Weld three barrels together and partially fill with water as required
posted by stuartmm at 12:18 AM on March 11