Wood floats and epoxy/glass is neutrally buoyant so it's not going directly to the bottom unless you're haulling rocks.

Since people also mostly float, you only need enough volume to make up for the non-floating gear that will not have tumbled into the river.

Each cubic foot of tank space will make up for 60 lbs or so of gear.

Have fun,

Laszlo

The key is to get enough flotation so you can stand in the boat and have some freeboard so you can bail it out.  Just staying above the waves doesn't help much if you are away from shore, wor worse close to a rocky shore.  Laslo has the right number, about 60 lbs/cubic ft, so you can figure about how much for yourself and some gear.  If you are sailing you also need to account for the mast, sails, etc. 

The British seem to take flotation requirements pretty seriously. For small boats they require buoyancy equal to 2.5 times the weight of the boat and equipment without the crew. So if your boat weighs 150 lbs. and you generally have 100 lbs of equipment aboard you would need 250 lbs. of buoyancy or a little over 4 cu ft. If the boat is wood it probably has buoyancy of about 1/2 its weight or 75 lbs. So about 3 cu. ft. of buoyancy should do.

Thanks, Guys, I can work with this and will sort it out.  Will do the math and make bow & stern air tanks to suit.  May have to rethink that big-block Chevy inboard I was thinking of...