Outboard motors consist of the engine, motor leg and propeller in one self contained unit that mounts on the transom of the boat. You can check out details at http://en.wikipedia.org/wiki/Outboard_motor.
Early outboard motors were of two stroke carbureted design. Two stroke engines burn oil in the fuel mixture and fire on every stroke. This type of engine is typically relatively noisy and smoky, however, it is lighter and the low end torque is extremely good due to the design and power to weight ratio.
For many years, engine manufacturers have invested heavily to bring outboards into EPA compliance, concentrating on improved fuel economy and reduced emissions. Electronic fuel injection (EFI) systems replaced carburetors and provide slightly better fuel economy.
Direct fuel injection (DFI) systems are similar to EFI, except that they inject the fuel mixture directly into each cylinder. While their technologies are different, the three most popular DFI systems are the Mercury Optimax, Yamaha HPDI and Evinrude E-Tec.
Four stroke outboard motors utilize technologies similar to automotive engines. They do not burn oil in the mixture as they get lubricated through oil in the crankcase. Four stroke engines fire every other stroke. These engines are quieter, smoother and have typically better fuel economy and lower emissions. However, due to the heavy investment in these new technologies, they are the most expensive.
Price Considerations
Due to increased requirements by the EPA, fewer carbureted two stokes are available on the market today. Where available, these engines are very inexpensive when compared to DFI or four strokes. DFI engines are more expensive than traditional two strokes and Four strokes are the most expensive. The investment in R&D by all manufacturers to bring these technologies into compliance is very costly. One must consider whether the fuel savings outweigh the initial cost to purchase and therefore, what is the breakeven period to realize true dollar savings. Try to find performance data that indicates the fuel economy of the boat you are considering. It is best to use "Cruise" data rather than "Wide Open Throttle" data. Based on the price of fuel, you can calculate the cost per hour to operate each engine. Comparing this rate to the difference in price at the time of purchase, you can determine how many run hours it will take to offset the initial investment. In many cases, particularly with DFI technologies, you will find that it could take several years to recoup the additional cost of the four stroke.
Power Considerations
Low end torque is particularly important for towing skis, wakeboards and other towables. Inherent in their design, two strokes have much better power, or low end torque, than four strokes. Given this factor, combined with the added weight of a four stroke, you can generally get the same performance with less horsepower in a two stroke design. Technologies such as Mercury's Verado, utilize superchargers for added low end torque, but at added initial expense.
Noise and Vibration Considerations
At idle and low speeds, four stroke technologies win this consideration hands down. However, at cruise speeds, the noise levels are more similar, though four strokes still have a slight edge. If this is important, check out the difference in noise levels at cruise speed. They may not be as different as you think.
Maintenance Considerations
It is typically cheaper to perform annual maintenance on two strokes as compared to four strokes. This is because the oil and oil filter requires periodic changing. However, two strokes burn oil as the engine runs and this added expense typically costs more.
The Green Choice
When it comes to emissions, four stroke technologies typically exceed EPA regulations by a great margin. However, high pressure direct fuel injection technologies such as the Mercury Optimax and Yamaha HPDI make two stroke outboards very economically friendly.