Pleasure Boat Wakes Stir Up Trouble

UNIVERSITY PARK, Pennsylvania, November 16, 2001 (ENS) - 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. Even at low speeds, the boats can stir up sediment and block sunlight from reaching underwater plants, the researchers learned.

"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," said Dr. David Hill, assistant professor of civil and environmental engineering. "However, our study shows that imposing a uniform speed limit can lead to significantly different impacts for boats of different size."


Small watercraft may need stricter speed limits (Photo courtesy U.S. Environmental Protection Agency)
Previous studies have shown that stirring up the sediments on a lake bottom can cause less light to get to aquatic plants growing there. Water clarity also affects water temperature as well as quality, and has impacts on human lake users as well as wildlife.

Stirring sediments can lead to increased levels of nutrients and contaminants in the water, and allow them to be transported to other regions of a lake.

"We found that at between six and eight miles per hour (mph), in waters shallower than six to eight feet, there is maximum potential for prop wash to stir up lake sediments. So, an eight mph speed limit could aggravate rather than reduce turbulence problems."

Hill presented his results, "The Hydrodynamic Impacts of Recreational Watercraft on Shallow Lakes," last Friday at the North American Lake Management Society Symposium in Madison, Wisconsin. Michele Beachler, a master of science candidate in civil and environmental engineering, coauthored the study as part of her master's 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 examined the lake bottoms at depths from three to seven 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.


The wakes from small boats can stir up sediments from lake and river bottoms (Photo courtesy General Electric)
"We did not see much impact from personal watercraft in water depths greater than three to four feet," Hill said. "There was not a big difference between inboard and outboard boats, either."

With water skiing boats, including a 16 foot, 150 horsepower outboard and a 19 foot, 275 horsepower inboard, the Penn State engineers found that at very low speeds, as well as at very high speeds, there was little impact. However, at speeds near six to eight mph, where the boat was "near plane" or close to skimming the water, there was maximum potential to stir up the lake bottom.

Using the data from the study, Hill and Beachler have developed a computer program that can predict the water velocity at the lake bottom at different boat speeds and water depths. They hope to produce guidelines that can be used by lake managers to decide what speeds can be allowed in shallow parts of a lake.

Hill points out that the study also holds important implications for commercial boats, such as ferries. He notes that ferries often leave their propellers turning while docked, which could cause turbulence that could stir up a lakebed.