Board revved up over motors

Study: Higher horsepower may hurt river less

Posted: Tuesday, March 28, 2006


  An angler waits by his boat after a day of fishing on the upper Kenai River on Sunday. A proposal to raise horsepower limits of boats on the river from 35 to 50 is before the Alaska Department of Natural Resources. Photo by Joseph Robertia

An angler waits by his boat after a day of fishing on the upper Kenai River on Sunday. A proposal to raise horsepower limits of boats on the river from 35 to 50 is before the Alaska Department of Natural Resources.

Photo by Joseph Robertia

A proposal to raise outboard horsepower limits on the Kenai River from 35 to 50 is gaining momentum. Proponents of 50 hp motors on the river say they won’t rock the boat — or at least not as much as the current 35 hp motors.

“This will actually cause less boat wake,” said Ken Lancaster, president Kenai River Special Management Area Advisory Board.

Earlier this month the KRSMA board voted in favor of recommending the Alaska Department of Natural Resources change boat regulations to allow boats with 50 hp motors on the river instead of just 35 hp, an amendment proponents say will lead to smaller wakes, less pollution and greater safety on the river.

The proposal is based in part on results from a two-phase study being conducted by the U.S. Army Corps of Engineers.

The first phase of the study, completed in 2000, found that rather than increasing a boat’s wake, a higher-powered motor could actually reduce it by shortening the amount of time it takes a boat to get on-plane, the point at which the boat travels over the water instead of pushing through it, resulting in less wake.

With a 35 hp limit some boats, such as a 21-foot boat loaded with clients and gear, may not get on-plane at all, Lancaster said.

“You get a boat that plows rather than planes,” he said. “We’re not trying to make a speedway or anything like that, we’re just trying to make a safer river.”

Boats that plow through the water can create larger wakes and, consequently, put neighboring boats at risk of taking on water.

“That’s really dangerous,” said Joe Connors, a member of the KRSMA board. “You can get swamped really easy.”

Limiting the wakes produced by boats is critical to protecting Kenai River shorelines.

The energy of a boat’s wake is determined by its height, speed and frequency, and the greater the energy of a wake, the greater its potential to erode shorelines.

Factors limiting or increasing boat wakes include more than just horsepower, said Jack Sinclair, Kenai area superintendent for Alaska State Parks.

KRSMA board’s recommendation to DNR, which also proposes a 21-foot boat length limit, addresses only two of them, he said.

“There are some limitations in the proposal before us,” he said.

The completed first phase of the U.S. Corps of Engineers study cites neither horsepower nor length as a leading factor in determining wake sizes.

In a summery of boat-wake measurements made on Johnson Lake and the Kenai River study results suggest the shape of a boat’s hull rather than the boat’s length or motor is the leading factor in determining wake energy.

“While other factors cannot be ruled out, hull type is the most apparent difference between two boat groups that exhibit differences in wave characteristics,” the study says.

The study found that v-hulled boats create average wave heights ranging from 29 to 60 percent greater than flat-bottomed boats when driven at varying speeds with 35 hp motors.

Sinclair said KRSMA board’s recommendations are just a first step in addressing a package of issues that are well overdue for review.

“This discussion ... hasn’t happened in 20 years,” he said.

The horsepower of boat motors on the Kenai River was last addressed in 1986, when it was reduced to 35.

Study results found that horsepower did make a difference in wave characteristics.

Boats tested in the study created waves up to 20 percent higher using 35 hp motors at maximum power than using 40 or 50 hp motors at maximum power. Some boats, however, showed no significant difference in wave heights between the two motors.

Sinclair said other factors contributing to boat wake-induced bank erosion include the boat’s proximity to the shoreline and its weight.

The lighter a boat and the weight it is carrying the less likely it is to create large wakes. Wakes created by boats traveling down the center of the river are less likely to cause bank erosion than wakes created by boats traveling near shore.

Proponents of 50 hp limit, however, say the board’s recommended horsepower limit has the added benefit of reducing pollution.

Connors said many of boats on the Kenai River use 50 hp motors detuned to 35 hp to comply with regulations and use fuel less efficiently than motors that are not detuned.

According to a paper written by Connors and Jeff King, also a member of the KRSMA board, local motor dealers who attending a KRSMA board workshop in December said that in older carburated boat motors detuning can increase the amount of unburned fuel released.

Sinclair said that the board’s motion is only advisory and it is likely to evolve before any changes occur. The board will probably receive a response from the Division of Parks and Recreation and DNR before the board’s April 13 meeting, but will not formally be taken up until next fall, he said.

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