I just built a 16' flatbar. Basically, I took a 2"x10"x16' pine and 2 x 2"x4"x16' to brace it underneath with enough space to put my vertical 4"x4" s to lift it up out of the water. Then, I put small pieces of 2x4 in areas bw the other 2x4s to brace the 4x4s.
I don't know if this makes sense to you?
http://www.youtube.com/watch?v=g8j7duMnTuw