I am building a 12x12 dock with a 12ft landing 5ft wide, I would like to hing the landing so can have a floating platform to take out to middle of pond when we are swimming. how many 50 gallon drums do you think I will need to float platform and float landing. note plat form will have 2 4x4 post on land with 1 bolt in each side to allow up and down movement with diffrent water levels, so only one end will have to float at the attched floating platform