University Park, Pa. A "No Wake Zone" may be better than a speed limit to prevent the pollution and water quality problems that can occur when pleasure boats stir up a lake bottom, a Penn State study has shown.

Dr. David Hill, assistant professor of civil and environmental engineering, says, "One might think that putting in a sign in shallow water that says, '8 mph would be a good way to prevent turbulence from prop wash that can stir up shallow lake bottoms. However, our study shows that imposing a uniform speed limit can lead to significantly different impacts for boats of different size.

"In addition, we found that at between 6 and 8 mph, in waters shallower than 6 to 8 feet, there is maximum potential for prop wash to stir up lake sediments. So, an 8 mph speed limit could aggravate rather than reduce turbulence problems."

Hill will present his results, Friday, Nov. 9, at the North American Lake Management Society Symposium in Madison, Wis., in a paper, "The Hydrodynamic Impacts of Recreational Watercraft on Shallow Lakes." Michele Beachler, a master of science candidate in civil and environmental engineering, is co-author. The study is part of her masters thesis.

Hill and Beachler conducted their study at two lakes in northern Wisconsin used by recreational boaters including, water skiers, fishermen and personal watercraft fans. The two Penn State engineers instrumented the lake bottoms at depths from 3 to 7 feet with an acoustic Doppler velocimenter to measure the water velocity induced by passing boats and an optical backscatter sensor to measure the turbidity of the water. Then they passed different watercraft, including inboard and outboard boats, at different speeds and different depths over the instrumented lakebeds.

"We did not see much impact from personal watercraft in water depths greater than 3 to 4 feet," Hill says. "There was not a big difference between inboard and outboard boats, either."

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