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Article #4: Boat motors

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Outboard motors for a boat are developed outweigh the range deficiencies shared
as a self-contained unit with engine, with electric cars. Diesel outboards are
subsidiary systems, and propeller, also available, but their weight and cost
designed to be mounted at the stern makes them impractical for most purposes.
(rear) of the craft. They are the most A few outboard motors have been produced
common method of propelling small with pump-jet propulsion replacing the
watercraft. conventional propeller. Like other types
As well as providing propulsion, of pump-jet propulsion systems, they
outboards provide steering control, as offer virtually no cavitation, making the
they are designed to pivot over their boat more manoeverable, and can operate
mountings and thus control the in very shallow water. However, the low
orientation of the propeller. The efficiency compared to propellers has
transmission leg in the water also acts seen them restricted to specialised
as a rudder even when the propeller is applications.
not providing power. History and developments
When boats are out of service or being The first practical outboard motor was
drawn through shallow waters, outboard created by Norwegian-American inventor
motors can also be tipped forward over Ole Evinrude in 1909.
their mounts to elevate the propeller and Historically, most outboards have used
transmission shaft out of the water to two-stroke cycle engines due to their
avoid accumulation of seaweed or hitting simplicity (and consequent reliability),
underwater hazards such as rocks. low cost, and high power-to-weight ratios
Types of motors - weight issues being particularly
Small outboard motors are truly important as too much weight over the
self-contained, with integral fuel tanks stern of boats tends to impede their
and controls mounted on the body of the handling. However, the high emissions,
motor and steered by a "tiller" directly and thus both environmental concerns and
connected to the motor. Such small motors the cost of technology to meet emissions
can weigh as little as 12 kilograms standards, has led to the gradual gain in
(approximately 26 pounds) and provide popularity of four-stroke cycle
sufficient power to move a small dinghy outboards, particularly for lower-end
at around 15 km/h (9 mph), far faster machines. High-end outboards have tended
than possible with oars. They are highly to remain two-stroke designs, as the fuel
portable, able to be removed by simply injection technology required to meet the
loosening their mounting clamps. Small emissions standards and fuel economy
outboards usually use a manual pull start targets costs proportionally less for
ignition system while larger models often these motors and weight issues are more
have electric start similar to a car. important.
Manufacturers have produced large Marine engines also benefit from their
outboard designs, with sufficient power proximity to an endless supply of cold
to be used on boats as long as 9 metres water by using it once for cooling and
(30 feet) or more. Manufacturers have then ejecting the now-heated water. This
also produced electric outboards. These allows marine engines to avoid the cost
are used for specialised applications, and weight of radiators and cooling fans
notably trolling for bass in the US, normally associated with land-based
where their quietness and zero emissions engines.






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