How do I anchor my floating dock?
Never underestimate the power of water. While floating docks are a good choice because they are less intrusive on shoreline ecosystems, that free-floating characteristic makes them more susceptible to wind, water and boating stresses. Wind, against the large surface area of docked boats, is a powerful sheering force. There are side loads too, sometimes more than intended, from boats docking. All the while, the dock will be twisting and undulating in wave and wake. Build the dock as short as you can manage: longer docks are stressed more and they cost more, needing repair or replacement sooner. Consider width as well, a 3′-wide dock might be a bit tippy.
Good lumber retailers, especially in cottage country, carry dock hardware. There are heavy, welded, galvanized steel hinges with removable pins, inside-corner brackets and chains designed for the job. These types of hinges allow movement so that the base plates are less likely to tear out of the dock frame. Brackets reinforce corners, which are vulnerable to twisting. Bolt the hinges and brackets through the boards, on backing plates with washers. Use galvanized bolts, plates and hinges to prevent electrolysis and rust.
As the dock swings back and forth, it places enormous loads on the shore ramp hinges. The longer the dock, the greater the leverage against the attachment to shore. There are two ways to reduce that stress–at least, partially. At the shoreline, don’t used fixed points of attachment unless you’re sure you can control movement at the outer end of the dock. Instead, consider setting the inner end of the ramp on a plate or flat support, and limit the side-to-side movement with lengths of heavy chain, attached to heavy U-bolts on each side of the ramp and on the fixed plate upon which the ramp will swivel.
At the outer end of the dock, anchors can limit side-to-side swing. Heavy, galvanized chains run to anchors set out at 45º from each corner of the dock. The chain to the “left” anchor runs from the “right” corner of the dock, from the underside, to give clearance for boats as they come and go. Eye or U-bolts and heavy, galvanized shackles connect the chains at the dock. Screw down (rather than nail) the deck planks that cover these parts so the planks can be removed if you need access later. The lake/river bottom anchors can be big eye screws, such as the power companies use to guy poles, screwed into a sandy or marl bottom; or they could be very heavy pre-cast concrete blocks, with ring bolts embedded in order to attach shackles and chains. A series of smaller weights in a cluster, still totalling enough mass so they won’t be dragged by the swaying dock, could also serve. If you can’t place anchors yourself, marine contractors can do it for you.
At the end of your boating season, plan to disconnect the dock from the shore, and tether it for winter where ice movement, whether by wind or current, is minimal. This goes back to the points about heavy hinges with removable pins and chains on shackles accessible under planks at the end of the dock. What to do with the chains to the anchors? Run a well-tied rope from them to the shore, dropping the chains to the lake or river bottom for the winter, ensuring the rope won’t be caught by moving spring ice at the shoreline.
A last point: even galvanized and heavy-duty chains and other hardware have a limited lifespan, because of the wear from movement over time. Inspect the points of contact carefully each spring.
Where I live, on the St. Lawrence River, many a marine contractor has watched gleefully as the spring parade of neglected or ice-borne docks goes past. Mine, built and handled as I’ve described above, has foiled them all.
