Ships carry ballast water to manage their stability with different cargo configurations. When they pick up ballast water in one port and carry it to another, they also can carry along species that haven't previously been present in the destination port. These invasive species can cause a lot of problems.
Researchers at the University of Michigan are working on a "ballast-free" ship to discourage this sort of hitch-hiking ('Ballast-free Ship' Could Cut Costs While Blocking Aquatic Invaders, ScienceDaily, March 27, 2008). The system works by continually exchanging ballast water so that the water in the ship is always local:
Instead of hauling potentially contaminated water across the ocean, then dumping it in a Great Lakes port, a ballast-free ship would create a constant flow of local seawater through a network of large pipes, called trunks, that runs from the bow to the stern, below the waterline.
"In some ways, it's more like a submarine than a surface ship," Parsons said. "We're opening part of the hull to the sea, creating a very slow flow through the trunks from bow to stern.
"You're continuously sweeping water through the ship and out," he said. "So you're always filled with local sea water, not hauling water from one part of the world to the other."
And that isn't all, the analysis also indicates that ships using this system can cut their fuel expenses by something like seven percent.
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