Outboard engines

Every day more and more people are getting out onto the waters of the Great Barrier Reef in private boats to enjoy a unique lifestyle of fishing, diving and sightseeing. Many private vessels are powered by outboard engines. Unfortunately, operation of these motors causes the discharge of a number of pollutants. By keeping your outboard in good condition, and being aware of the type of outboard you use, you can make valuable contributions in limiting the impacts of outboard engines on the Great Barrier Reef.

Many small engines, such as conventional two-stroke engines used in marine outboard and personal watercraft, are high polluters relative to their engine size and usage, emitting a range of toxins into the water and the air. As substantial power is required to move small craft through water, even the better performing engines can emit far greater quantities of pollutants per hour than typical modern car engines.

A comparison of a conventional 15 horsepower carburettor two-stroke outboard and a 150 horsepower fuel-injected four-stroke outboard found that the smaller engine produced three times the emissions of the larger engine.

Responsible Reef Practices

  • Consider purchasing a new, cleaner running marine engine
  • Choose ultra low emission engines and use the OEDA VELS as a guide (see table below)
  • Ensure that your outboard motor is kept in good condition and serviced according to the manufacturer's recommendations
  • Properly match engine horsepower to the size of your vessel
  • Use the right size propeller and keep it in good condition (nicks and dents reduce performance)
  • Drive your boat conservatively. Abrupt starts, excessive speed and extended use of full throttle not only reduce fuel efficiency and increase emissions, but are also hazardous to slow moving marine animals such as dugong and turtle
  • Learn to trim your boat whilst underway
  • Reduce unnecessary engine idling
  • Reduce weight. Extra cargo in your boat reduces fuel efficiency.

Outboard motor emissions ratings

To help boat owners choose outboard engines based on comparable emissions, Outboard Engine Distributors Australia (OEDA) has implemented an emissions ratings system known as the Voluntary Emissions Labelling Scheme (VELS) for sales of new outboard engines, similar to energy and water efficiency ratings schemes for household appliances.


OEDA Australia Star Rating
Star Rating Description
OEDA Emissions Limit*
Zero Star
High emission: Will be a handful of older design two-stroke engines
> 250
One Star
Low emission: Most traditional two-stroke engines
68.4 - 250
Two Stars
Very low emission: Some two-stroke direct injection and four-stroke engines
30 - 64.8
Three Stars
Ultra low emission: Most two-stroke direct injection and four-stroke engines
5 - 30
Four Stars
Super ultra low emission: For future technologies
< 5

* The OEDA Emissions Limits rate engines based on Hydrocarbon (HC) and Oxides of Nitrogen (NOx) emissions, measured in grams per kilowatt per hour.

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