Just posting a conceptual design for comment.
The building blocks for my conceptual dock will be 4 x 8 sections made from 2 x 8 x 8' treated boards (edge pieces), 2 x 4 x 8' treated boards (barrel positioning and support), a sheet of 5/8" 4 x 8 green treated plywood (not shown) for deck surface (painted with non-slip paint) and various hardware pieces as needed for assembly and of course one or two 55 gal plastic drums depending on the position in the overall assembly.
If it's not obvious, two 2 x 8 x 8's form the long edges, the remaining 2 x 8 is cut into two 46 1/2" end pieces (forming a 48" width section when attached between the edge pieces). I'm expecting to use full width inside corner braces for the 2 x 8 corners to insure connection loads to adjacent sections are distributed.
Two of the 8 ft. 2 x 4's are cut to four 46 1/2" pieces and placed to hold the barrels in position exactly so the barrel will touch the decking bottom flush with the remaining surfaces. I intend to use upside down joist hangers for the 2x4's as the barrel pressure will work to push them up and out. The last 2 x 4 is cut to provide a center support for the plywood and help support the inside cross piece 2x4's. There is probably enough 2x4 material left after that cut to insert extra similar braces between the outside 2x4 barrel brace and the 2x8 end cap if needed.
Rather than using decking boards ($$$), I'm going with 4 x 8 5/8" plywood which is green treated and painted with non-slip paint as the dock surface.
This "dock section" will hold one or two barrels depending on floatation needed, and can be bolted together end to end or side to side (or both) to create whatever shape and size of dock needed. I paid $5 each for my barrels so my quick calculations show that each section will cost about $55-60 to build. One positive of this plan is that you can build small to start and add on as budget allows if you can't foot the whole thing at once.
One of the challenges of a narrow floating dock design is the inherent left-right instability problem when you have a high center of gravity, and a narrow footprint. The stability in these cases is commonly addressed (with floating docks) by building them double wide to create a wider footprint. I didn't want an 8' wide dock (pier) so I'm planning to address the stability by adding fingers off to the sides as shown. They will work like outriggers, each part helping to stabilize it's connected neighbors. The fingers can literally be placed anywhere to create berth spaces as wide and long (in 8' increments) as desired.
This image is only showing one barrel per segment in the assembled dock, which I think will be sufficient for the finger sections and possibly a few of the core pieces, but I accept I am likely to need double barrels for many if not all of the central dock sections, certainly anything on an edge.
For my implementation, loose corner augers will provide some stability, but in my case I have a boat lift for the pontoon and speed boat and one will go on each side. Barring some amazing winds, I think they should keep the dock from floating away.
So what am I missing... what have I not considered about this plan?
Opinions appreciated - Thanks, Scott
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