"Virtual Ballast," Feature Article, July 2008

virtual ballast

A new design for ships aims to leave pests in their home waters.

by Jeffrey Winters, Associate Editor

Twenty years ago, a biologist with the Detroit Edison Co. began raising the alarm over something brought up by a diver in Lake St. Clair: an inch-long, striped mussel. It was a strange find, since the zebra mussel, as it is known, is a freshwater species native to eastern and central Europe. Before long, however, the zebra mussel became a common sight along the shores of the Great Lakes and other American waterways, clinging to just about any hard surface, including the insides of water company and power plant intake pipes. Keeping the invaders from clogging such pipes has become a multimillion-dollar maintenance issue.

Most researchers agree that zebra mussel larvae hitched a ride to the American Midwest in the ballast water of an oceangoing freighter. Ballast water has been implicated in the introduction of other invasive species, as well. Responding to this problem, engineers have developed numerous devices and procedures for eliminating future aquatic hitchhikers, from requiring ships to flush their ballast tanks offshore to installing expensive filters or sterilizing systems.

In April, engineers at the University of Michigan reported on a better way of eliminating the hazard from ballast tanks—eliminating the tanks themselves.

Back in the days of sailing, lightly loaded ships preserved their stability in the water by carrying heavy stones in their holds. As cargo ships grew larger in the modern era, ballast stones were replaced by tanks that could be filled with water for weight.

The ballast-free idea was conceived by Michael Parsons, a professor of marine engineering, in response to the problems arising from lugging ballast water across the globe. During one meeting, “Someone asked whether we really needed ballast at all,” Parsons recalled. “As the only naval architect in the room, I began listing all the reasons ballast was essential.”

It was only later when Parsons realized that the stability ballast achieves by sitting a ship lower in the water could be obtained by other means. Instead of just adding weight, you could change the ship’s buoyancy.

To reduce the transport of invasive species (above), engineers have designed a ship that can adjust its buoyancy by using flowing water.

The design worked out by Parsons and his colleagues replaces the tanks with a system of pumps and pipes below the waterline. Water would be pumped in through an opening at the bow of the ship, move slowly through a network of large conduits, and be expelled at the stern. The flowing water in essence reduces the enclosed volume of the ship, making the vessel less buoyant. What’s more, the system ends the practice of lugging water from one ocean or river to another. Because it takes just hours for water entering the bow to exit at the stern, a ship would never transport water more than a few dozen miles.

To test their idea, researchers have built a 16-foot scale model. The model was floated in a large tank at the university’s Ann Arbor campus, with water expelled through the stern to simulate the action of the ducts. (The scale of the model was too small to allow a realistic flow of water through the pipework.) Preliminary tests and computer models published in April suggest that the design works about as well as conventional ballast, and may provide an unexpected bonus: a 7 percent improvement in propulsion.

“We wanted to reduce an efficiency penalty for the system,”

Parsons said. “We didn’t anticipate an improvement.” Parsons said that the greater efficiency stemmed from the position of the pipe outlet. In standard ship designs, only the bottom half of the propeller sees much free-flowing water and therefore provides most of the propulsion. With the new design dumping a continuous flow of water out the back, the top half of the propeller can contribute a bit.

The efficiency gain may well be an artifact of the experimental setup, Parsons cautioned. A new round of testing on the model is scheduled to begin this summer. But if it’s confirmed, it could lead to big savings for bulk shippers. For a typical dry bulk carrier shuttling between European and North American ports, the fuel savings minus the extra capital costs for the ship (an estimated $540,000) would be some $2.50 per metric ton of cargo.

Even so, the new design is still more costly than the present practice of flushing the ballast tanks at sea. But in light of concerns that flushing isn’t as effective as first thought, the International Maritime Organization may call for more effective measures, Parsons said. “That’s when you could see people moving to this design,” he said.