As a native Michigander, I'm a sucker for news about the Great Lakes. (That's HOMES, remember? Huron, Ontario, Michigan, Erie, and Superior.) Engineers at the U of M Marine Hydronamics Laboratory have now designed a boat without a ballast tank in order to prevent the introduction of non-native species.
A ballast tank is a compartment that sits at the bottom of any large boat. When the boat doesn't have any cargo, its crew can fill the ballast tank with water to help it stay afloat. The mechanical details on how this works can be found here; but basically, the extra water lowers the boat's center of gravity and makes it more stable on the water.
Trouble is, these ballast water pools typically harbor lots of aquatic species. Researchers have identified 185 non-native species in the Great Lakes, and guess that most of them got there via cargo ship. The most famous are Zebra mussels, which are native to the Caspian Sea and were first introduced to the Great Lakes in 1988. Since then, they've disrupted ecosystems all over the U.S., out-competing local species for food and wreaking havoc in harbors, boats, and power plants.
Those U of M engineers are clever, though. They've figured out how to keep a ballast-free boat from sinking. As a press release explains:
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.
This design concept has been around since 2001, but only now have its creators built a prototype. When testing their 16-foot, $25,000 wooden scale model (shown above), the engineers found that not only does it work, but propelling it requires 7.3 percent less power than regular ships. That efficiency translates to a savings of $540,000 per ship (which is only slightly less impressive when you consider that a typical vessel costs a whopping $70 million to construct).