An outboard motor is a propulsion system for a boat developed as a self-contained unit with engine, subsidiary systems, and propeller, designed to be mounted at the stern (rear) of the craft. They are the most common method of propelling small powered watercraft.
As well as providing propulsion, outboards provide steering control, as they are designed to pivot over their mountings and thus control the orientation of the propeller. The transmission leg in the water also acts as a rudder even when the propeller is not providing power.
Small outboard motors are truly self-contained, with integral fuel tanks and controls mounted on the body of the motor and steered by a "tiller" directly connected to the motor. Such small motors can weigh as little as 12 kilograms (approximately 26 pounds) and provide sufficient power to move a small dinghy at around 15 km/h (9 mph), far faster than possible with oars. They are highly portable, able to be removed by simply loosening their mounting clamps.
Over time, manufacturers have produced larger and larger outboard designs, with sufficient power to be used on boats as long as 9 metres (30 feet) or more.
Historically, most outboards have used two-stroke cycle engines due to their simplicity (and consequent reliability), low cost, and high power-to-weight ratios - weight issues being particularly important as too much weight over the stern of boats tends to impede their handling. However, the high emissions, and thus both environmental concerns and the cost of technology to meet emissions standards, has led to the gradual gain in popularity of four-stroke cycle outboards, particularly for lower-end machines. High-end outboards have tended to remain two-stroke designs, as the fuel injection technology required to meet the emissions standards and fuel economy targets costs proportionally less for these motors and weight issues are more important.
Electric outboards are used for specialised applications, notably trolling for bass in the US, where their quietness and zero emissions (or, more accurately, displaced emissions) outweigh the range deficiencies shared with electric cars. Diesel outboards are also available, but their weight and cost makes them inferior for most purposes.
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