A sea change is occurring in the development of outboard engines. The growth of reliable four-stroke engines is making this new option accessible to sailors where only two-stroke engines were available in the past. As is often the case, however, an era of change brings confusion, misinformation, and difficulty for the consumer in making a decision.
Let's review the two types of outboards along with their main differences and see how they stack up against one another:
Fuel Economy Two-stroke engines mix gasoline and oil, pour this mixture into the cylinder on every revolution, and burn it as rapidly as possible. At high rpms, where two-strokes like to run, fuel literally pours though them. Four-stroke engines, like your car's motor, keep the oil in the crankcase and burn only the gasoline. As a result, owners often report a doubling of range under power with the four-stroke. Score one for the new breed.
Pollution As a result of the high quantity of unburned gasoline/oil mix with two-stroke engines, they do pollute. The oil smoke is visible, has a distinct odor, and creates a sheen on the water. Most outboards blow their exhaust out of the propeller hub so as to mix it as thoroughly as possible with the water, but oil floats and eventually rises again. Oil and water do not mix well. As a consequence, some inland lakes and other enclosed bodies of water where large concentrations of two-stroke powered craft congregate have limited or banned their use. Little of this legislation and policing has to do with sailboat auxiliaries or dinghy outboards but rather with personal watercraft (PWCs), most of which currently use two-stroke engines. There's little question that the restrictions on two-strokes will eventually be widened. This will likely happen slowly, however, and will probably carry grandfather clauses for existing engines and primarily affect US freshwater areas. It is worth consideration and we'll score another point for the four-strokes.
Weight Four-stroke engines have more moving parts, thus more metal and therefore more weight. For the same horsepower, a four will weigh 10 to 15 percent more than a two-stroke. This is not important for a fixed auxiliary on a 25 footer or as a dinghy engine on a large tender hoisted in davits or by hydraulic lift. But for a cruising couple on a smaller boat, those few extra pounds can make a huge difference. Score one for the twos.
Noise Four-strokes were noticeably quieter in the days of yore. Two-strokes have been the beneficiary of a great deal of advancement in this area, however, and now the difference is slight. Still, score a half point for the four-strokes.
Maintenance and Repair Both engines use oil—two-strokes mix it with the gas, which is messy, while four-strokes must have the crankcase oil changed— also messy. The twos eat spark plugs, which are relatively inexpensive. Two strokes have been around since Ole Evinrude rowed his girl across the lake and are understood by marine mechanics even in third world countries where two-stroke parts and expertise are available to service the local fishermen. Four-stroke outboards are somewhat newer and parts and repair needs may be met with some head-scratching in the hinterlands. A point for the twos.
Conclusion If you've followed the scoring, it's a dead heat. But that belies the truth that there is only a "right" outboard for each sailor. First, if two-strokes are banned, or about to be banned, on your home waters, the question is moot. More importantly, a low-budget cruising couple in foreign waters might rightfully opt for the lighter weight, cost saving, and parts availability of a two-stroke. A big boat carrying the tender in davits and planning a voyage to inland fresh water might be wise to spring for the four-stroke.
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