Calculating Air Volume for Boat Stability in Water

[champ2823]

Being that I have little experience in physics, but have grown a fascination for it, I am hoping you guys can help with a problem. A friend of mine wants to make a few pontoons to make his boat more stable. It's just a smaller 15' aluminum boat that he uses for hooping (catching lobsters).
He wants to know how much air volume he needs to float about 300 pounds. How much weight a specific volume of air will hold in water. What exactly would the equation be to figure that out? I'm guessing it's so simple that I've overanalyzed it. I'm hoping that I've made myself clear enoug.

 

[Tide]

1 cc of air will displace about 1 gram of water. That works out to about 62.4 pounds of water displaced by 1 cubic foot of air. The weight of the air is negligible. I think that's all you need.

 

[Kazza_765]

All you need is Archimedes principle. That is, the upwards thrust on a submerged object is equal to the weight of water that is displaced. So to provide 300 pounds of floatation you need to displace 300pounds of water (I'm not sure how much that is, I use metric). If you use an air filled device to displace that water, then the weight of the air is negligible. But if you use something heavier (say wood for example), then you need to add the weight of the wood to the weight of the boat.

 

[russ_watters]

I assume that means the boat will hold 300lb of cargo, so remember to factor in the weight of the boat when doing the calculations. And oversize the pontoons by 25%-50%

 

[champ2823]

Ok thanks guys, I was looking at Archimedes principle and figured it had to do with that.