Bob,
Great project.
I just recently made up a materials list for a somewhat similar floating dock based on blue barrels, but mine was going to be half the width of yours -- 16 x 4 foot.
You changed my mind.
All great suggestions. Doubling the width and turning the drums 90 degrees should be a lot more stable than what I was planning. I too was going to use 2 x 12s. Now I think I will go with your suggestion of 2 x 10s.
Thanks,
Ken
