Floating Docks - 

part 3 of 3

Freeboard: how low can you go?

The distance from the top of a floating dock to the water, or freeboard, is largely determined by the size and shape – and, therefore, the buoyancy – of the flotation, versus the weight of the dock’s framing materials. Freeboard can range from 25–60 cm, but many dock builders aim for somewhere around 40–45 cm. There’s no perfect figure. What really counts is that freeboard matches your needs. If you’re on a quiet lake and only need to moor a canoe and a small fishing boat, a dock with low freeboard might work well. Larger vessels and bigger waves require more freeboard to keep boats beside the dock, not sitting on top of it.

Floater turnout

The exact amount of flotation a dock requires can be a bit of a guessing game, because it depends entirely on the building materials used (cedar being lighter than plastic composites, for example), required freeboard, and the expected loads the dock will support. Professional dock builders who work with one particular type of flotation, whether it’s EPS billets or plastic float drums, will have learned through experience how much flotation they need for a given dock design. Tom Hopper, of Polywest, manufacturer of Seaco Marine floats, uses a set ratio when he’s building a wood-framed dock with Seaco floats: “Generally, two pounds of flotation to one pound of dock works as a rule of thumb to give 12 to 14 inches of freeboard.”

Once you change floats or materials, any neat ratio goes out the window. For this reason, if you are building your own dock, it’s best to go with a proven dock design or let the float manufacturer help you calculate the required flotation given your chosen materials and dock configuration.

Expanded polystyrene (EPS) billet Closed-cell foam float available at cottage-country building centres. (Avoid open-cell EPS, which readily absorbs water.) Note that extruded EPS billets (such as Styrofoam brand) are much stronger than moulded EPS billets. Relatively inexpensive, but must be protected from physical damage by the dock frame.

Polyethylene float (bolt-on) Plastic floats, either hollow or filled with open- or closed-cell EPS. Air-filled floats will sink if punctured; EPS-filled floats will not. They are attached to bottom of dock and do not require protective subframe, unless winter haul-out requires dragging over rocks.

Polyethylene float (build-on) EPS-filled plastic floats, in rectangular or square sections. Built-in channels accept standard lumber; float becomes part of subframe and creates an integral fender. More pricey than bolt-on floats, but require less framing.

Welded steel pontoon Spiral-welded steel tube capped at each end. Extremely heavy, stable, and damage-resistant. Can be repaired if punctured and can overwinter in harsh conditions. Very expensive.

The modular

Over the years, crafty cottagers faced with taking docks in and out have fashioned some clever ways to lighten the load, usually involving removable sections of decking. A good start, but when you’re limited to lumberyard materials that are screwed or nailed together, there are only so many pieces you can pull apart and put back together again. Builders of modular floating docks have solved this dilemma by using newer materials like extruded aluminum and self-contained float drums and putting them together in a way that allows the entire dock to be disassembled without destroying it in the process. The theory: Many pieces make light work.

Dan Doig, president of Offshore Performance Marine Inc., developed one such product, called Dock in a Box, that delivers entire docks to your door – in pieces. “The philosophy was to try to make everything manageable between two people so you wouldn’t need to have a crew,” he explains. Rather than relying on wood framing, Dock in a Box uses an aluminum subframe that boltsto square polyethylene floats filled with expanded polystyrene. Composite lumber decking, made from wood fibres and polyethylene, is assembled into panels, which slide into the grooved subframe and are held in place by a removable end cap. To remove the dock come fall, simply take off the end cap, slide out the decking, unbolt the floats, and carry the parts away.

According to Doig, composite wood (light-coloured to stay cool in the sun) offers two advantages: Heavier than real wood, it helps compensate for the lightweight aluminum skeleton, and it requires absolutely no maintenance. Like other modular docks, the sections can be configured into letter shapes or large docks. Dock in a Box sections are available in 4', 6', and 8' widths, from 10'–24' long, at about $26 per sq. ft. of dock (ramp and anchors extra).

The all-in-one

Most floating docks have three main components: the flotation, the frame, and the decking. As plastic moulding techniques developed, however, a new creature – in which the flotation was itself both deck and frame – was born. Early versions, using small building blocks that linked together with flexible corner connections, like ice cubes in a tray, were less-than-stellar performers, writhing and undulating as wakes and waves rolled underneath. Today, the all-in-one concept has improved, with several manufacturers offering much larger sections linked stiffly to form a stable dock platform.

It seems unusual that a light dock with a lot of buoyancy could also be stable, but Don Dailey, co-owner of EZ Dock Ontario, explains that the floats are “barge-bottomed,” with the entire dock in contact with the water, not just the floats around the perimeter. Also, rather than simply being flat, the bottom of each float has a number of chambers acting like suction cups on the water. When waves strike, the dock tends to rise and fall gently as a whole, rather than reacting directly to the crest of the wave and rippling along with the water. Accustomed to dealing with skepticism, Dailey points out that EZ Dock installations can be made to comply with the provisions of the Americans with Disabilities Act. This legislation, he explains, includes a requirement that horizontal surfaces are “extremely stable so that someone in a wheelchair, on crutches, or with balance difficulties has to be able to walk on this dock without any untoward occurrence.”

EZ Dock sections, which cost about $325 per square metre, come in several modular sizes, up to two by three metres. The sections connect with couplers that can flex a bit. “It’s almost a Lego-type situation,” says Dailey. “You can disconnect and relocate pieces to create a slip or make a T- or an L-shape. You can change the shape from year to year.”

Hardware

Hardware for marine use should be stainless steel or hot-dipped galvanized to resist corrosion, especially if in contact with ACQ pressure-treated lumber (See “The Latest Treatments,” p. 128). Buy the heaviest hardware you can afford, with holes that line up, so two pieces can be through-bolted. If you take your dock apart for winter, choose hinges and connectors that disconnect easily.

Hinge: Use to attach dock to ramp and ramp to shore, and to connect dock sections, especially long ones.

Dock connector: Some systems use specialized connectors to attach sections together to form a rigid, unified whole.

Corner bracket:  Use to reinforce corners and as a backer plate for attaching hinges and connector plates (hence the importance of matching hole patterns).

Resources

cottagelife.com's Product & Services Directory - Docks & Accessories category

The Dock Manual, by Max Burns, published by Storey Books. 

Float manufacturers often supply plans for their own products. 

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Former editor of Cottage Life, David Zimmer has a cottage at Byng Inlet on Georgian Bay, where he will never ever (ever!) build a crib dock again.

Floating dock photo courtesy of Dockinabox

Published in the June 2005 issue of Cottage Life magazine.

Copyright © 2005 by Cottage Life. All rights reserved. Reproduction of any article, photograph, or artwork, for other than personal use, in whole or in part, without the written permission of the publisher is strictly forbidden.