The mulitpurpose attenuator sounds like a good deal, especially because, as some marina owners contend, the purchase of such a system returns the original investment in fewer than 10 years. But as promising as this sounds, multipurpose attenuators aren't for every marina. Those tempting financial return can crumble in the wake of maintenance, insurance, and replacement costs.

Even stand-alone wave attenuators are not suitable for every marina. Though sometimes referred to as a "breakwater," the function of an attenuator is different than that of a traditional rubble-mounted system. While a rubble-mound breakwater absorbs the entire column, an attenuator interrupts a wave's elliptical motion (which extends to a depth of one-half its length). Therefore, the greater the mass and depth of the attenuator, the greater its dampering effect. However, even the best attenuators are incapable of handling wave heights of more than five feet, and wave periods of more than four seconds. But where conditions allow, floating attenuators provide an effective and economical alternative to a traditional breakwater, minus the hassle of environmental impact and permitting red tape.

A $100,000 bonus
And, if perchance the attenuator can do double, or even triple, duty, all the better, right? Definitely, says Bob Rashleger, director of engineering and construction with Skipper Marine Development. "By installing a ShoreMaster attenuator dock with attached finger piers for the Harbor Club Marina in Sturgeon Bay, Wisconsin, the marina gained 20 slips," he says. "The resulting rental revenue- $100,000 annually- made the project financially feasible."

Sheltered on two sides, the marina is battered by boat wakes and wind waves from three miles of open fetch stretching into Lake Michigan. In winter, passing ice breakers in the adjacent shipping channel ram frozen sheets toward the attenuator dock. During the on-site engineering phase of the project, a ShoreMaster floating wave attenuator was deemed the ideal combination attenuator/dock for the location. Lake Michigan's abuse wreaked havoc on earlier wave attenuation systems, but according to Rashleger, "this post-tensioned system makes it possible for the attenuated dock to survive these tough conditions."

According to Rashleger, Harbor Club's attenuator minimizes stress damage by allowing for slight movement between the sections. Dock units are connected by hermetically sealed cables within tubes molded inside the concrete. The cables, not the concrete, absorb the attenuator's movement.

Generally, multipurpose attenuators such as the one at the Harbor Club Marina are practical only in relatively protected sites with less than one mile of open fetch and with predictable wav patterns of short periods and small heights (under two and half feet). But each site is unique, and this decision requires a careful wind and wave analysis by an experienced engineering team.

The drawbacks
At first glance, Harbor Club's three miles of open fetch would have seemed to rule out the option of attaching fingers to the attenuated dock. Nonetheless, the distinctive characteristics of the marina's site - along with the right product - proved this option not only feasible, but successful, as well.

However, Harbor Club's new mooring space is not without drawbacks. The attenuator pier cannot prevent waves from washing over the dock during rough weather. "It's not the best situation for pedestrians," Rashleger acknowledges, "but usually, people aren't out there during storms."

On the other hand, down in Lake Ozark, Missouri, the Regatta Bay Condominium Association opted to keep its attenuator separate form the dock system. Manager Ann Glynn described the attenuated dock as "not desirable for pedestrian traffic."

At Paradise Cove Marina on Lake Travis near Austin, Texas, the "worst" can strike on even the mildest days. "Wind and heavy boat traffic do less damage than a singe Carver (yacht) on a calm day when its three-foot wake marina barrels through the marina," says marina co-owner Ron Doll. "It tears everything up." A floating dock would "serpentine and roll at the same time," says Doll, rendering the dock dangerous place for pedestrians or moored boats.

Paradise Cove is responding to the problems created by boat wakes with an especially large and heavy attenuator in addition to an experimental displacement wave wall. Like Regatta Bay, the marina will not construct additional slips.

A separate attenuator is a must for Paradise Cove's seemingly sheltered location, says ShoreMaster sales representative Roger Squires, "because in their particular situation, it's simply safer for the people and for the boats to keep the attenuator detached from the dock system."

Squires stresses the importance of ensuring that marina operators gain sufficient understanding of their particular sites so they can select the right system.

He recommends seeking more than one professional opinion and contracting the work to a company with a proven track record in designing and constructing attenuated systems.

The right system may prove to be a combined dock/wave attenuator. But in a situation with the potential for high waves, all that can be expected of an attenuator is that it endure a day-in, day-out beating.

Anna Ossanaa is a freelance writer with a degree from Iowa State University of Science and Technology.