Boat Motors and Water Quality

As Minnesotans, we love cruising along the lakes in our watercraft. Whether we are heading to our favorite fishing spot, waterskiing, or taking a sunset cruise, not much can top the feeling of taking in the fresh lake air.

Yet as the number of motorized boats and size of motors on Minnesota’s lakes continues to increase, questions arise about the potential effects these boats have on the lake environment. In the last 20 years, there has been a 36% increase in the number of registered boats of all types in Minnesota. Moreover, the number of motorboats between 16 ft and 26 ft in length increased 118%, while the number of motorboats less than 16 feet decreased by 27%. Average horsepower went from 46.1 to 74.5 from 1987-2001 in a DNR survey of west boatmixingcentral Minnesota. Our boats are becoming larger and faster, which increases the potential to effect water quality.

So how do boats affect water quality? The Wisconsin DNR did a study on the effects of motorized watercraft on aquatic ecosystems. Boats can affect water quality in a few different aspects. First, they can add metals and chemicals to the water column. A certain amount of the fuel that enters into a motor is discharged unburned and ends up in the water. Two stroke motors can emit 25-30% of their unburned gas and oil mixture into the water. In contrast, four-stroke motors emit 97% less air and water pollution than old two-stroke motors. This pollution can affect the pH and dissolved oxygen in the lake, which can influence the type and abundance of fish and wildlife.

Another main impact by motors is churning up the lake bottom in shallow areas. This action stirs up the lake sediment, re-suspending nutrients (phosphorus) that are at the lake’s bottom. When these nutrients reach the surface of the water where the algae are, they can feed algae and cause and algal bloom. This stirring can also decrease the water clarity because of additional particles suspended in the water column.

So what can you do to protect your lake? 1) Establish no-wake zones in shallow areas with waterfowl nesting and bulrush stands. “Slow no wake” means operation of a watercraft at the slowest possible speed necessary to maintain steerage, but in no case greater than 5 miles per hour. 2) Educate lake users to avoid sensitive areas and drive slowly through shallow areas. 3) Upgrade your boat motor from an old two-stroke engine to a four-stroke engine. Four-stroke engines use fuel more efficiently, produce cleaner exhaust, and run more quietly than traditional two-stroke engines.

To reduce your impact while boating, there are other easy changes in behavior we can all practice. 1) Keep your boat property trimmed – an engine in the water makes much less noise and creates less wake; 2) keep your engine well-tuned so that it runs more efficiently, pollutes less and is quieter; 3) be respectful to wildlife and loons, keeping a distance of at least 200 feet away at all times; 4) consider the size of your boat and motor when choosing a lake for recreation – smaller lakes are not appropriate for large boats or engines; 5) remember that swimmers, canoeists, kayakers, sailboats and other non-motorized users always have the right-of-way.

To read more about the Wisconsin DNR boating impact study visit:


  1. tom weber says:

    This message needs to be passed on to our younger generation in hopes of influencing a change in the actions taught by example by the wise older adults who have not embraced the impact all of us as users make upon our waters. Bigger, faster does not make us “rulers” in fact we have the potential to destroy when we are not educated or want to be educated as to what it takes to respect each other or our environment. Pirz Lake ” oldtimers ” have taught their children that their Grandfathers owned the lake and ALWAYS went as fast as a 3hp motor would go and NOTNING bad ever happened to the water. Todays HP is overkill fo a 67 acre lake but ” let’s kill the messenger ” after all it is OUR RIGHT as it has always been on public water. Today Pirz Lake is in the second full growing season of newly introduced Curly Leaf Pondweed and the boaters are simply running through the dense vegetation until chopped and left to spread throughout the perimeter of the lake. Maybe it is too late for Pirz Lake but we can become an example of WHAT NOT TO DO ! tommy weber