I was wondering about dock flotation with plastic barrels on my way to work this morning. What load rating can you get from 30 and 55 G plastic barrels that are all sealed up?
Flotation of a poly drum is around 8 pounds per gallon, minus the weight of the empty drum.
Since you don't want the drum to be totally submerged, divide the capacity in half. That'll give you about 200 pounds per 55gal drum, with plenty of room for error.
The deck structure is plenty heavy (it can't help but be) enough for stability, assuming it's long and wide enough. Taking away flotation capacity doesn't make a lot of sense to get stability. Just use heavier lumber or add stuff to your deck and I guarantee it will be very heavy. My 8x16' deck weighs ~1000lbs not counting the 20 drums. It could float my Tacoma pickup pretty easily if I ever get the urge.
