About Buoyancy

If you are lost out in the wilderness, maybe in a tropical forest with a thick jungle canopy surrounding you, it is very easy to get further disoriented and walk around in circles. The best method to make a dash towards rescue and safety is to find a flowing river and follow it. Rivers will invariably lead to some kind of civilisation.

In order to cross a river you have two choices:

(1) swim, or
(2) make some kind of floatation device.

The second option is always the wiser one. Rig up some kind of floatation device like a raft. When constructing a raft you will have to ensure that it is buoyant enough to take your weight and that of your gear. If you make a raft that sinks when launched, all the effort and energy spent would have gone to waste. Essentially a sealed block of air is what gives you the best buoyancy. A simple enough formula will help you determine the size of your raft using material that will essentially stay afloat and not sink under the weight. Weight and air are fairly closely related, so try and remember the following:

1 cubic foot (1,728 cubic inches) of water = 28 kg = 28 liters.

The raft will stay afloat as long as the water displaced is equal to the weight of the passengers and the gear it has to carry. Let us work with one liter bottles of mineral water, for instance. For a person who weighs 60kg and has a 20kg pack, you will require at least three cubic feet of water, corresponding to approximately 85 empty one-liter bottles.

Now let us assume you are making a raft out of logs and not empty water bottles.

To calculate the volume of a cylindrical object in cubic feet is to double the radius, then multiply by pi and finally multiplied by the length of the item. So if we are looking at a log that is ten feet long and ten inches in diameter, the volume is:

5/12 (radius in feet) x 5/12 (radius in feet) = 0.17
0.17 x 22/7 (pi) = 0.53
0.53 x 10 (length) = 5.34 cubic feet.

This means that one such log will displace 5.34 cubic feet of water. One cubic foot of water weighs 28kg. So, the log will displace 149.5kg of water.

Now comes the problem.

You will need to know the weight per one cubic foot of the log. Different logs have different weights. Assuming that the log you are using weighs 10kg per cubic feet, the ten-foot log you are working with weighs 53.4 kg (5.34 cubic feet x 10 kg). Subtract this from the weight of the water the log will displace and you get 96.1 kg (149.5kg - 53.4kg) and this is the weight this one log will support.

Keep in mind that the log will be very low on the water and once it takes on water will increase in weight and ultimately sink. Keep a safety margin of at least 50%. So to carry about 100kg, use two logs ten feet long and ten inches in diameter, to cater for the safety margin.

After all this you will have to keep the raft stable. Make it wide. Instead of two ten feet logs, use four five feet lengths, for instance. If you can remember this simple formula for buoyancy, you might save yourself from a lot of hard work and frustration.