Posted on 22nd November

Death In Their Wake

A no go zone would 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 US 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."

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 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. "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.

The Marlborough Sounds route for New Zealand's inter-island ferries are a
highly significant area of concern in respect of the effect of boat wake on
the environment, this is one area where conservatism is the way to go, if
anything is to go at all.