What formula do I use to find the amount of cubic air necessary to keep a certain weight afloat?

Question

I'm building a boat using pontoons and am looking for the equation necessary to find how big they must be to keep the passengers dry.

Thank you.

Answer 1

Archimedes Principal states that a floating object will displace a volume of water exactly equal to it's weight.  So, the amount of water that the boat displaces will exactly equal it's weight (including all weights onboard such as fuel, water, passengers, etc).

For guidance, Fresh Water weighs 62.4 pounds/ft^3 and Salt Water weighs 64 pounds/ft^3.  In metric units, 1 cubic meter of fresh water weighs 1 metric ton or 1000 kg.  Saltwater is typically taken as a density of 1.025 so a cubic meter of salt water would weigh 1.025 metric tons.

As you work thru this remember something else also.  The center of gravity of the boat and passengers will be exactly above the center of bouyancy of the hull(s).  So, you have to do a weight calculation to see where the center of gravity is and then to a volume calculation to find the center of bouyancy in order to ensure that the vessel will float in a level condition....