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Outboard Motor

Updated on November 6, 2010

An outboard motor is a detachable, externally mounted propulsion system for small boats. An outboard motor consists basically of an engine or motor connected to a propeller by means of a drive shaft and gears. By far the most common outboard power plant is the two-stroke-cycle gasoline engine. However, rotary Wankel engines are sometimes used in racing outboards, and electric outboard motors are sometimes used on small fishing boats.

Size Range

The two-stroke gasoline outboard has one to six cylinders and is built in sizes ranging from 2 to 150 horsepower. The smaller sizes (some as light as 11 pounds) provide easy portability and adaptability to a variety of small craft. The larger sizes weigh 250 pounds or more. It is important to match the size of the outboard to the size of the boat. In general, the outboard should be strong enough to plane a fully loaded boat at cruising speed.



The outboard is usually mounted on the transom of the boat by means of adjustable bolt-on brackets or clamp screws. The outboard pivots from side to side for steering. In the smaller outboards, the boat is steered by moving a handle attached near the mounting, thereby pivoting the outboard and shifting the direction of the propeller's thrust. In the larger outboards a steering wheel, attached to the outboard by cables, pivots the outboard.

The outboard also can be moved through a vertical arc. A small change of angle from the vertical position of the outboard is called trim. A large change from the vertical position, raising the propeller out of the water, as for mooring, is called tilt. Smaller outboards have manual trim and tilt. Some of the larger outboards have power-driven trim and tilt.

Outboard Motor Construction

In addition to its gasoline or electric motor, an outboard motor includes a vertical drive shaft; forward, neutral, and reverse gearing; a propeller shaft; and a propeller. In addition, the outboard has a small water pump to circulate water throughout the engine cooling system and carry heat away from the engine to an exhaust port.


The propeller is made of aluminum, bronze, stainless steel, or plastic and has two or three blades. It is mounted on a shock-absorbing rubber hub and connected to the propeller shaft by means of a shear pin. If the propeller strikes an underwater obstacle, the pin shears, often preventing damage to the propeller and other parts of the outboard. A spare shear pin should always be carried.

The size of a propeller is specified in terms of diameter and pitch. The diameter is the distance across the center of the circle made by the revolving blade tips. The pitch is the angle of the blades. The correct propeller size and pitch must be used to obtain maximum performance from an outboard motor.

Fuel Mixture

For the smallest engines, gasoline is stored in a small tank attached to the engine. For larger engines, the fuel is stored in a separate tank that is connected to the fuel pump by flexible tubing. The engine is lubricated by a mixture of gasoline and oil in the ratio of 50 to 1. Gasoline, oil, and air are passed from the carburetor through the crankcase and then to the cylinders (where the gasoline and air provide a combustible vapor) by way of intake ports. The oil does not vaporize. Instead, it forms fine droplets that lubricate the cylinder walls, crankshaft, and connecting-rod bearings, and other moving parts.


The effects of continuous low-level water pollution by gasoline outboard motors generally were overlooked until the late 1960's and early 1970's, when the Environmental Protection Agency and other organizations conducted tests in lakes in Florida and Michigan.

In a 1970 report in National Fisherman, it was estimated that roughly 10% of the fuel-oil mixture used in outboard motors was discharged into the aquatic environment rather than being burned in the engine. Beginning with the 1972-model outboards, overboard loss of fuel and oil was significantly reduced by the development of drainless outboard engines in all sizes from 2 to 150 horsepower. In a drainless outboard, un-burned fuel-oil condensate is recycled to burn in the engine instead of being exhausted into the water.

History of the Outboard Motor

Cameron B. Waterman patented the first commercially successful outboard motor in 1905. He sold about 30,000 motors by 1915, but their heavy cast-iron construction made them vulnerable to competition from lighter motors.

Another successful outboard motor pioneer was Ole Evinrude, who was granted a patent for an outboard in 1910 and began production the next year. He was spurred to his invention because he needed to row across a lake to get ice cream for his fiancee. Evinrude's outboard, partly made of lightweight aluminum and soundly designed, was a forerunner of present-day outboard motors.

In 1921 the Johnson brothers brought about a substantial reduction in the weight of out-boards with the first outboard made primarily of aluminum alloy. Later improvements included a full gearshift, a V-4 outboard engine, and electric starting.


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