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Cleaner Pleasurecraft

Lower-emission boats may be coming soon to a lake near you

By Jeff J. White

Jeff J. White, shown in front of a marine engine installed in a sterndrive configuration, is a director in the Engine and Emissions Research Department within the Engine, Emissions and Vehicle Research Division. He leads SwRI's development efforts in engine and emissions research. White has led numerous projects involving a wide range of applications including light-duty vehicles, heavy-duty truck and bus engines, and various nonroad and off-highway engines. This project is focused on the development of low-emission solutions for marine applications, including sterndrive and inboard pleasurecraft.

Summer and days at the lake often go hand-in-hand. A bright sunny afternoon with a slight breeze can be the perfect recipe for boating or for exploring the lake on a jet ski. The water is clear, and the sky crisp and bright. Unfortunately, the very boats used to enjoy the great outdoors are emitting hydrocarbons, carbon monoxide and nitrogen oxides, which pollute the air.

Because boats do not emit smoke or particulate matter, the tangible evidence of polluted air is not immediately visible. We often associate the scenic landscape of a lake shoreline with families water skiing and fishing as part of enjoying the great outdoors and the fresh air. Many laws and regulations have been put into place for onroad vehicles to clean up the air, and now the focus has expanded to include nonroad sources such as boats. Southwest Research Institute studied four marine pleasurecraft to develop catalytic marine exhaust systems and to demonstrate the safety and durability of these systems in boats.

Catalytic marine exhaust systems were installed on four boats and evaluated for the reduction in hydrocarbon and nitric oxide emissions while maintaining overall performance. The engines achieved an 80 percent reduction in HC and NOx.


Environmental Protection Agency regulations to promote cleaner air began in the early 1970s, with rules written to clean up the emissions from passenger cars. While these regulations accomplished much, they were only the first step in controlling emissions. It was clear that there were other sources that also needed to be addressed such as on- highway trucks and buses. In 1987, the EPA introduced regulations to reduce emissions from these sources, addressing only on-highway vehicles in its early focus. More work remained to be done to truly clean the air.

California legislators recognized the significant emission output by these other sources, and in 1990 began developing regulations to cover off-highway and nonroad sources. These sources comprise diverse types of equipment that cannot be grouped in one or two categories. Since 1990, the state of California has required reductions in emissions from small utility engines of less than 25 horsepower; off-highway diesel equipment, excluding farm and construction of less than 175 hp; large spark-ignited engines greater than 25 hp; locomotives; off-road motorcycles and recreational equipment; and outboard motors and personal watercraft.

As the state with the nation's worst air pollution, California is authorized by the U.S. Clean Air Act to write its own emission regulations. While California has taken the lead with nonroad sources, the EPA has also adopted rules governing most of these sources at the federal level. Marine pleasurecraft is one particular nonroad source that California has in its sights.

Sterndrive/Inboard Rule

In 2001, the California Air Resources Board (CARB) adopted rules covering sterndrive and inboard (SD/I) pleasurecraft. Marine craft present special challenges. There are limits on under-deck temperatures, for example. All sections of the exhaust system must be water-cooled to prevent hot surfaces. This is done as a safety precaution to help reduce the heat generated from the exhaust pipes that are located within a confined engine compartment. Unlike passenger cars, if the engine quits running in a boat, it is not as simple as just pulling over to the side of the road and walking home. Safety is paramount with these craft, and any action taken to clean up emissions must not compromise the safety of the boat or its systems. Several early attempts to use automotive catalyst technology on boats resulted in failure. Marine exhaust systems typically mix engine-cooling water with the exhaust to cool the exhaust. The hot ceramic catalyst substrates would shatter when water splashed on them. California has acknowledged the special challenges of cleaning up emissions from marine engines. When CARB adopted its SD/I rule, it also agreed to fund a demonstration program to show that low-emission solutions could be developed for pleasurecraft, and that these solutions would be safe and durable in on-water operation.

Four boats of various drive systems were tested for this program. The engine on the left shows a catalytic marine exhaust system installed on an engine in a straight-drive configuration. The engine on the right is a sterndrive.

Demonstration Program

CARB selected SwRI to perform this program. SwRI staff members have extensive experience developing low-emission solutions for nonroad engines and equipment. The project team brought together various stakeholders and interested parties. Participants include marine engine and boat manufacturers, the National Marine Manufacturers Association (NMMA), the U.S. Coast Guard and Texas Coast Guard Auxiliary, catalyst manufacturers as represented by the Manufacturers of Emission Controls Association or MECA, CARB and the EPA.

The primary objectives of this project were to develop catalytic marine exhaust systems and to demonstrate their safety and durability in boats. Four boats were selected for exhaust system development. The boats represented various lengths with different engines and drive systems.

Pleasurecraft use several types of drive systems depending on the boat's intended application. The sterndrive design places the engine in the rear of the boat with the drive extending through the transom. The V-drive also places the engine in the rear of the boat, but with the engine facing the stern. The drive thus faces forward in the boat and goes through a gear box that reverses the drive direction (in a V), under the engine and through the transom. The straightdrive puts the engine in a pod near the middle of the boat, with a straight shaft connecting to the propeller through the hull. Each uses a different exhaust system layout, and industry recommendations stated that a catalytic exhaust system should be demonstrated on each type to fully explore the potential for water reversion. This phenomenon can cause water mixed with the exhaust to make its way upstream, back toward the exhaust manifolds. Because water reversion could cause catalyst failure, and because the different exhaust designs could have varying propensities for reversion, CARB agreed with industry's recommendation to design systems for the different drive types.

Progress to Date

SwRI engineers removed the boat engines and installed them on dynamo-meters for development of stoichiometric engine fueling calibrations. Air/fuel settings were maintained slightly rich at idle for smooth operation and at wide-open throttle for best power. They then fabricated double-walled stainless steel exhaust systems, each custom-designed for the engine compartment of the boat in which it was installed. Exhaust systems were designed to water-jacket all hot sections, including the two catalysts installed on each engine. As mentioned earlier, marine exhaust systems typically mix engine exhaust and cooling water at some point to reduce the exhaust stream's temperature. Systems were designed so this mixing process occurs downstream of the catalysts, with features to prevent water from reaching the rear catalyst face.

The exhaust systems were then mounted on the corresponding boat engines in the test cell, and the engine calibrations were refined to achieve the best emissions while maintaining good engine performance. Following final calibration, each engine and exhaust system was emission tested prior to beginning durability hour accumulation on Canyon Lake, Texas. The results reflect an 80 percent reduction in hydrocarbon and oxides of nitrogen (HC+NOx) emissions, and a 50 to 60 percent reduction in carbon monoxide emissions, compared to stock baseline emissions.

Following final calibration of the engines, each engine and exhaust system was zero-hour emission tested to measure the levels of hydrocarbons and oxides of nitrogen. Compared to the California Air Resource Board standard, the engine emission levels for all four boats showed a significant decrease in HC and NOx. The boats were also found to emit less carbon monoxide.

Current Status

The engines and SwRI-designed exhaust systems were reinstalled in the four boats, which are currently undergoing durability accumulation at Canyon Lake.

SwRI is working with the U.S. Coast Guard on durability operations. The local Coast Guard Auxiliary is providing coxswains and other trained personnel to serve as boat drivers. On-board data acquisition systems are monitoring engine and exhaust system performance throughout the durability test sequence. After the boats have accumulated 480 hours of on-water operation, they will be returned to SwRI's emissions laboratories and tested to determine performance over the durability interval sequence. Exhaust system components will be examined and photographed to assess any effects of on-water operation.

Benefits of Program

Marine engines present unique challenges with respect to the engineering of low emission solutions. While this program by no means solves every possible problem, to date it has demonstrated successful performance of these four prototype exhaust reduction systems. Lower-emission boats will benefit the public through improved air quality. They will especially make a difference on crowded lakes and rivers during holiday and vacation periods.

Comments about this article? Contact Bruce Bykowski,


The author acknowledges the team effort of the California Air Resources Board, the National Marine Manufacturers Association and the U.S. Coast Guard. He would like to also acknowledge SwRI staff members Jim Carroll, Glenn Boehle and Matt Dash for their support and expertise.

Published in the Summer 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Summer 2004 Technology Today
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