I went through all this while trying to salvage an old cedar log dock up at the lake. I finally ended up using blue plastic 55 gallon drums with the 2" and 3/4" NPT holes. I spent alot of time down at the hardware store with my tongue stuck between my lips trying to figure out how to make things work. What I wanted to do was get the drums under the dock at each of the corners, which meant I had to partially fill them with water, enough to position them under the dock at each corner then brace them in, then figure out a manifold for air regulators to blow out the water to regulate how much lift was needed at each corner to level out the dock...it was a royal pain in the derriere, but once I got it figured out I felt like some kind of genious. People said why not just build a new dock, first off I loved those big old growth cedar logs and nothing has the feel of one of those heavy old docks, second...I didn't want to fork out 5 grand for a replacement. I probably had 2-300 bucks in my renovation, but many hours of figuring and fussing. I actually found a guy that welds plastics locally, I had to have valves installed to allow water to drain out while air was pumped in and he welded in the fittings. I damn-near drowned myself a couple of times using a snorkle to do alot of the work, I modified the snorkle to be longer and never realized that its harder to suck air through a tube thats 6 feet long rather than 12"... I should have hired a diver, but like I said I'm cheap.
