Even though we are sailors, outboard engines are often an integral part of the way we use our boats. Smaller vessels use outboard engines as their main auxiliary power and in bygone days boats up to 32 feet long were built with outboard wells. The vast majority of egg-beaters, however, are used on the transoms of dinghies.
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: Power
We quite often hear that a four-stroke outboard is 'more powerful' than a similar sized two-stroke. Obviously, a 10-hp, two-stroke has the same 10 horsepower as a 10-hp four-stroke. What the speaker may be referring to is the difference in torque. Two-stroke engines accelerate somewhat faster than four-stroke ones do, but do not take as kindly to being lugged down with a heavy load, especially at low speeds. A 25-foot keelboat fitted with a four-stroke geared down to swing a large-diameter, low-pitch prop will push through the water like it has tug-boat power. On lightweight craft, two-strokes have performed well for decades. Choosing the right engine for the job is the first step.
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. Cost
Four-stroke engines cost more, but the money difference is becoming smaller as the demand for them grows. Since many fours come with longer warranties, the extra cost might be written off as an extended guarantee. Still, it can add up to a fair amount of cash, so we'll score a half-point for the two-strokes.
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.