Choosing the right flotation: The case for encasement

Choosing the Right Flotation: The Case for Encasement

Marina Dock Age May/June 2003
written by Douglas E. Pluth, P.E.

Just as any building is only as sturdy as its foundation, a marina's dock system is only as stable as the floatation that supports it. In waterfront installation, floatation can account for a significant portion - as much as 10 to 18 percent - of construction costs, so it's a common target for cost-cutting efforts. And because floats remain mostly out of sight after installation, they are even more likely to be under-budgeted.

While most forms of flotation are essentially similar, their safety and durability may vary widely. So, while it's tempting to go after the lowest price, it's important to realize the significant differences in quality that exist. By selecting the proper flotation, you will ensure adequate support for your docks, and reduce the chances of encountering structural problems for years to come.

Although many types of floatation are available today, expanded polystyrene foam (EPS) is still the most common. EPS is easily identifiable by its white billets and is familiar to most people as the material used in inexpensive beer coolers.

In most basic form, EPS flotation consists of large blocks of exposed foam directly fastened beneath a docks substructure. Used this way, the foam is left in direct contact with the water. These floats will do the job- for awhile. Many marina owners, contractors, and engineers now avoid using EPS in this manner, for a variety of reasons. In fact, the drawbacks to exposed foam installations have actually led some government agencies to ban them in certain parts of the country.

Problems with exposed foam

The most obvious problem with exposed foam flotation is that it erodes easily. Typically, EPS blocks are fastened to a dock with metal bands or long thru-bolts. As the polystyrene ages, these fasteners may become loose or rust out altogether - usually in locations where inspection and repair are difficult. When this happens, the floats are likely to crumble or break apart, littering the waterways and beaches with foam billets. Exposed foam blocks will also be eroded by wave action, ice, boat fenders, and anything else that might rub against them. Even aquatic animals can undermine exposed EPS floats: Otters and muskrats have been known to burrow in them, making large holes in places that can't be seen.

Another common problem with exposed EPS is its permeability. In a marine environment, this means that even high-quality exposed EPS, when left unprotected, will absorb considerable amounts of water over time, resulting in a reduced carrying capacity. While new foam may provide a carrying capacity of 10 pounds per square foot, exposed EPS that has absorbed water may support only 8 pounds per square foot - a 20 percent reduction.

Water absorption isn't the most threatening problem, though; a greater danger can arise when oil and fuel in the water are absorbed. Even trace amounts, over a long period of time, will accumulate and react with EPS to create a thick, flammable sludge. This leaves your docks, and your entire marina, exposed to a hidden fire hazard.

When exposed, even high quality EPS will absorb considerable amounts of water over time, resulting in reduced carrying capacities.

Encased EPS floats create an airtight container that protects the polystyrene core from erosion and absorption.

Cover your bases, and your floats

With these considerations in mind, the initial savings promised by exposed EPS hardly adds up to a bargain. So what is the alternative? Today's trend is to encase EPS flotation units. While a polystyrene foam core still accounts for the dock's floating foundation, a polyethylene encasement protects the float from damage and assures an extended lifetime.

Polyethylene-encased floats come in two basic types, and the main difference between them lies in the manufacturing process. Some manufacturers use a thermoforming process that begins with a foam core and, working from the inside out, covers this core with a plastic case. Because thermoforming starts with a flat sheet of plastic that is pushed or stretched into the box-like shape of the float, the edges and corners of the encasement end up thinner and weaker than the flat portions. This process also requires the final seams to be welded shut, resulting in a weaker joint that is likely to split.

Other manufacturers use a rotomolding technique, which forms a six-sided, one-piece, seamless shell. Rotomolding uses heated, rotating molds to forma polyethylene circulating inside the shells tends to accumulate in the corners of the molds, creating thicker, stronger shells at these critical stress points.

After a rotomolded shell is formed, small beads of EPS foam are injected and expanded with steam inside the shell. This process creates an airtight container. While both manufacturing techniques shielded foam floats from the marine environment, thicker, rotomolded, airtight encasement offers the most protection. The most common wall thickness is .15 inches, with thicker walls available. Thinner walls are not recommended.

Attaching your floats

When choosing an encased float, many buyers overlook another important detail: its attachment to the dock or structure. The main consideration here is whether the attachment method undermines the encasement itself. In floats that are not rotomolded, the attachment system often requires drilling bolt water to enter the float. Rotomolded shells, on the other hand, often have slots or holes molded into the side flanges, providing a place for fastening, without compromising the container's airtight integrity.

Exposure to petroleum pollutants, wave action, abrasion, and harsh ice conditions may be unavoidable for many marinas, but their adverse affects can be minimized by simply choosing the right floatation. The choices are fairly simple, and it only takes moments to determine if your dock's foundation will tolerate what a marine environment can dish out- moments that will make the difference between a dock that lasts, and one that falls apart.

Douglas E. Pluth is an engineer at Galva-Foam, and a licensed P.E. in several states.