Outboard motor

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Basic parts of an outboard motor Outboard diagram-01.svg
Basic parts of an outboard motor

An outboard motor is a propulsion system for boats, consisting of a self-contained unit that includes engine, gearbox and propeller or jet drive, designed to be affixed to the outside of the transom. They are the most common motorized method of propelling small watercraft. As well as providing propulsion, outboards provide steering control, as they are designed to pivot over their mountings and thus control the direction of thrust. The skeg also acts as a rudder when the engine is not running. Unlike inboard motors, outboard motors can be easily removed for storage or repairs.

Marine propulsion system for generating thrust for ships and boats on water

Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electric motor or engine turning a propeller, or less frequently, in pump-jets, an impeller. Marine engineering is the discipline concerned with the engineering design process of marine propulsion systems.

Boat vessel for transport by water

A boat is a watercraft of a large range of type and size. Ships are generally distinguished from boats based on their larger size, shape, and cargo or passenger capacity, and their ability to carry boats.

Propeller Device that transmits rotational power into linear thrust on a fluid

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blades, and a fluid is accelerated by the pressure difference. Propeller dynamics, like those of aircraft wings, can be modelled by Bernoulli's principle and Newton's third law. Most marine propellers are screw propellers with helical blades rotating around a approximately horizontal axis or propeller shaft.


Bolinder's two-cylinder Trim outboard engine. Trim outboard engine.jpg
Bolinder's two-cylinder Trim outboard engine.
A Mercury Marine 50 hp outboard engine, circa 1970s Merc 50 hp.jpg
A Mercury Marine 50 hp outboard engine, circa 1970s
1979 Evinrude 70 hp outboard, cowling and air silencer removed, exposing its shift/throttle/spark advance linkages, flywheel, and three carburetors 79rude70.JPG
1979 Evinrude 70 hp outboard, cowling and air silencer removed, exposing its shift/throttle/spark advance linkages, flywheel, and three carburetors

In order to eliminate the chances of hitting bottom with an outboard motor, the motor can be tilted up to an elevated position either electronically or manually. This helps when traveling through shallow waters where there may be debris that could potentially damage the motor as well as the propeller. If the electric motor required to move the pistons which raise or lower the engine is malfunctioning, every outboard motor is equipped with a manual piston release which will allow the operator to drop the motor down to its lowest setting. [1]

General use

A motorboat with an outboard motor attached to it Motorboat Zanzibar.jpg
A motorboat with an outboard motor attached to it

Large outboards

Large outboards affixed to the transom using clamps and are either tiller steer up to approx 100hp. Generally 100hp plus is linked to controls at the helm. These range from 2-, 3- and 4-cylinder models generating 15 to 135 horsepower suitable for hulls up to 17 feet (5.2 m) in length, to powerful V6 and V8 cylinder blocks rated up to 557 hp (415 kW)., [2] with sufficient power to be used on boats of 37 feet (11 m) or longer.


Small outboard motors, up to 15 horsepower or so are easily portable. They are affixed to the boat via clamps, and thus easily moved from boat to boat. These motors typically use a manual start system, with throttle and gearshift controls mounted on the body of the motor, and a tiller for steering. The smallest of these weigh as little as 12 kilograms (26 lb), have integral fuel tanks, and provide sufficient power to move a small dinghy at around 8 knots (15 km/h; 9.2 mph) This type of motor is typically used:

A throttle is the mechanism by which fluid flow is managed by constriction or obstruction.


A tiller or till is a lever used to steer a vehicle. The mechanism is primarily used in watercraft, where it is attached to a rudder post or stock to provide leverage in the form of torque for the helmsman to turn the rudder. A tiller may also be used in vehicles outside of water, and was seen in early automobiles.

Trolling (fishing)

Trolling is a method of fishing where one or more fishing lines, baited with lures or bait fish, are drawn through the water. This may be behind a moving boat, or by slowly winding the line in when fishing from a static position, or even sweeping the line from side-to-side, e.g. when fishing from a jetty. Trolling is used to catch pelagic fish such as salmon, mackerel and kingfish.


Electric outboard motors are self contained propulsory units for boats, first invented in 1973 by Morton Ray of Ray Electric Outboards [3] . These are not to be confused with trolling motors, which are not designed as a primary source of power. Most electric outboard motors have 0.5 to 4 kW direct current (DC) electric motors, operated at 12 to 60 volts DC. Recently developed outboard motors are powered with an alternating current (AC) or DC electric motor in the power head like a conventional petrol engine. With this setup, a motor can produce 10 kW output or more and is able to replace a petrol engine of 15 HP or more. The advantage of the induction or asynchronous motor is the power transfer to the rotor by means of electromagnetic induction. As these engines do not use permanent magnets, they require less maintenance and develop more torque at lower RPM.

A trolling motor is a self-contained unit that includes an electric motor, propeller and controls, and is affixed to an angler's boat, either at the bow or stern. A gasoline-powered outboard used in trolling, if it is not the vessel's primary source of propulsion, may also be referred to as a trolling motor. Trolling motors are often lifted from the water to reduce drag when the boat's primary engine is in operation.

The watt is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second, and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by .

Direct current Unidirectional flow of electric charge

Direct current (DC) is the unidirectional flow of an electric charge. A battery is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current.


Pump-jet propulsion is available as an option on most outboard motors. Although less efficient than an open propeller, they are particularly useful in applications where the ability to operate in very shallow water is important. They also eliminate the laceration dangers of an open propeller.


Propane outboard motors are available from several manufacturers. These products have several advantages such as lower emissions, absence of ethanol-related issues, and no need for choke once the system is pressurized. [4]

History and developments

The first known outboard motor was a small 5 kilogram (11 lb) electric unit designed around 1870 by Gustave Trouvé, [5] and patented in May 1880 (Patent N° 136,560). [6] Later about 25 petrol powered outboards may have been produced in 1896 by American Motors Co [5] —but neither of these two pioneering efforts appear to have had much impact.

The Waterman outboard engine appears to be the first gasoline-powered outboard offered for sale in significant numbers. [7] Developed by Cameron Waterman, [8] a young Yale Engineering student, it was developed from 1903, with a patent application filed in 1905 [9] Starting in 1906, [10] [11] the company went on to make thousands of his "Porto-Motor" [12] units, [13] claiming 25,000 sales by 1914. [14] The inboard boat motor firm of Caille Motor Company of Detroit were instrumental in making the cylinder and engines.

The most successful early outboard motor, [13] was created by Norwegian-American inventor Ole Evinrude in 1909. [15] Between 1909 and 1912, Evinrude made thousands of his outboards and the three horse units were sold around the world. His Evinrude Outboard Co. was spun off to other owners, and he went on to success after starting the ELTO company to produce a two-cylinder motor - ELTO stood for Evinrude Light Twin Outboard. The 1920s were the first high-water mark for the outboard with Evinrude, Johnson, ELTO, Atwater Lockwood and dozens of other makers in the field.

Historically, a majority of outboards have been two-stroke powerheads fitted with a carburetor due to the design's inherent simplicity, reliability, low cost and light weight. Drawbacks include increased pollution, due to the high volume of unburned gasoline and oil in their exhaust, and louder noise.

Four stroke outboards

Although four stroke outboards have been sold since the late 1920s, particularly Roness and Sharland, in 1962 Homelite introduced a commercially viable four cycle outboard a 55-horsepower motor, based on the 4 cylinder Crosley automobile engine. This was called the Bearcat that was later purchased by Fischer-Pierce who are the makers of Boston Whaler for use in their boats because of their advantages over two strokes. In 1964, Honda Motor Co. introduced its first four-stroke powerhead. [16] In 1984, Yamaha introduced their first four-stroke powerhead. These motors were only available in the smaller horsepower range. In 1990 Honda released 35 hp and 45 hp four-stroke models. They continued to lead in the development of four-stroke engines throughout the 1990s as US and European exhaust emissions regulations such as CARB (California Air Resources Board) led to the proliferation of four-stroke outboards. At first, North American manufacturers such as Mercury and OMC used engine technology from Japanese manufacturers such as Yamaha and Suzuki until they were able to develop their own four-stroke engine. The inherent advantages of four-stroke motors included: lower pollution (especially oil in the water), noise reduction, increase fuel economy, and increased low rpm torque.

Honda Marine Group, Mercury Marine, Mercury Racing, Nissan Marine, Suzuki Marine, Tohatsu Outboards, Yamaha Marine, and China Oshen-Hyfong marine have all developed new four-stroke engines. Some are carburetted, usually the smaller engines. The balance are electronically fuel-injected. Depending on the manufacturer, newer engines benefit from advanced technology such as multiple valves per cylinder, variable camshaft timing (Honda's VTEC), boosted low end torque (Honda's BLAST), 3-way cooling systems, and closed loop fuel injection. Mercury Verado four-strokes are unique in that they are supercharged.

Mercury Marine, Mercury Racing, Tohatsu, Yamaha Marine, Nissan and Evinrude each developed computer-controlled direct-injected two-stroke engines. Each brand boasts a different method of DI.

Fuel economy on both direct injected and four-stroke outboards measures from a 10 percent to 80 percent improvement, compared with conventional two-strokes. Depending on rpm and load at cruising speeds, figure on about a 30 percent mileage improvement. [17]

However, the gap between two-stroke and four-stroke outboard fuel economy is beginning to narrow. Two-stroke outboard motor manufacturers have recently introduced new technologies that help to improve two-stroke fuel economy. [18]

LPG outboards

In 2012, Lehr inc. introduced some small (<5hp) outboards based on modified Chinese petrol engines to run on propane gas. Tohatsu currently also produces propane powered models, all rated 5hp. Conversion of larger outboards to run on Liquified petroleum gas is considered unusual and exotic although some hobbyists continue to experiment.

Outboard motor selection

It is important to select a motor that is a good match for the hull in terms of power and shaft length.

Power requirements

Overpowering is a dangerous condition that can lead to the transom accelerating past the rest of the vessel [19] and underpowering often results in a boat that is incapable of performing in the role for which it was designed. Boats built in the U.S. have a Coast Guard Rating Plate which specifies the maximum recommended horsepower for the hull. A motor with less than 75% of the maximum will most likely result in unsatisfactory performance.

Shaft length

Outboard motor shaft lengths are standardized to fit 15-inch, 20-inch and 25-inch transoms. If the shaft is too long it will extend farther into the water than necessary creating drag, which will impair performance and fuel economy. If the shaft is too short, the motor will be prone to ventilation. Even worse, if the water intake ports on the lower unit are not sufficiently submerged, engine overheating is likely, which can result in severe damage.

General dimensions

Different outboard engine brands require different transom dimensions and sizes, that affects performance and trim.

Outboard Brand - ModelTransom AngleMax Transom ThicknessTransom To Bulkhead
Yamaha - F35012°712 mm
Yamaha – F30012°712 mm
EVINRUDE – DE 30014°68.58 mm
EVINRUDE - G2 300 HP14°
SUZUKI – DF 300 AP14°81 mm
MERCURY – 300 HP14°
LEHR - 5.0HP14°
LEHR - 2.5HP14°

Manufacturers Mercury Marine Bombardier Evinrude Yamaha Marine Honda Marine Tohatsu Parsun Motors Lehr

Operational issues

Motor mounting height

Motor height on the transom is an important factor in achieving optimal performance. The motor should be as high as possible without ventilating or loss of water pressure. This minimizes the effect of hydrodynamic drag while underway, allowing for greater speed. Generally, the antiventilation plate should be about the same height as, or up to two inches higher than, the keel, with the motor in neutral trim.


Trim is the angle of the motor in relation to the hull, as illustrated below. The ideal trim angle is the one in which the boat rides level, with most of the hull on the surface instead of plowing through the water.
Neutral trim.JPG Trimmed in outboard.JPG Trimmed out outboard.JPG
If the motor is trimmed out too far, the bow will ride too high in the water. With too little trim, the bow rides too low. The optimal trim setting will vary depending on many factors including speed, hull design, weight and balance, and conditions on the water (wind and waves). Many large outboards are equipped with power trim, an electric motor on the mounting bracket, with a switch at the helm that enables the operator to adjust the trim angle on the fly. In this case, the motor should be trimmed fully in to start, and trimmed out (with an eye on the tachometer) as the boat gains momentum, until it reaches the point just before ventilation begins or further trim adjustment results in an RPM increase with no increase in speed. Motors not equipped with power trim are manually adjustable using a pin called a topper tilt lock.


Ventilation is a phenomenon that occurs when surface air or exhaust gas (in the case of motors equipped with through-hub exhaust) is drawn into the spinning propeller blades. With the propeller pushing mostly air instead of water, the load on the engine is greatly reduced, causing the engine to race and the propeller to spin fast enough to result in cavitation, at which point little thrust is generated at all. The condition continues until the prop slows enough for the air bubbles to rise to the surface. [20] The primary causes of ventilation are: motor mounted too high, motor trimmed out excessively, damage to the antiventilation plate, damage to propeller, foreign object lodged in the diffuser ring.


If the helmsman goes overboard, the boat may continue under power but uncontrolled, risking serious or fatal injuries to the helmsman and others in the water. A safety measure is a "kill cord" attached to the boat and helmsman, which cuts the motor if the helmsman falls overboard. [21]

Cooling system

Rotor of the impeller pump (cooling system) of an outboard motor Impellerrad.jpg
Rotor of the impeller pump (cooling system) of an outboard motor

The most common type of cooling used on outboards of all eras use a rubber impeller to pump water from below the waterline up into the engine. This design has remained the standard due mainly to the efficiency and simplicity of its design. One disadvantage to this system is that if the impeller is run dry for a length of time (such as leaving the engine running when pulling the boat out of the water or in some cases tilting the engine out of the water while running), the impeller is likely to be ruined in the process.

Air cooled outboards

Air cooled outboard engines are currently produced by some manufacturers, these tend to be small engines of less than 5 horsepower. Outboard engines made by Briggs & Stratton are air-cooled [22]

Closed loop cooling

Outboards manufactured by Seven Marine use a closed-loop cooling system with heat exchanger. This means saltwater is not pumped through the engine block as is the case with most outboards but still has to be pumped through the heat exchanger

Other outboard motoring method

Vietnamese-style "Shrimp Tail Outboard Motor" Schematic. Shrimp Tail motor.png
Vietnamese-style "Shrimp Tail Outboard Motor" Schematic.

In Vietnam and other parts of southeast Asia a long-tailed motor design is used. In Vietnam these they are called "May Duoi Tom"(Shrimp Tail Motor). The outboard motors, which can be smallish air-cooled or water-cooled gasoline, diesel or even modified automotive engines, are bolted to a welded steel tube frame, with another long steel tube up to 3 m long to hold the drive shaft, driving a conventional looking propeller. The frame that holds the motor has a short, swiveling steel pin/tube approximately 15 cm long underneath, to be inserted into a corresponding hole on the transom, or a solid block or wood purposely built-in thereof., [23] [24] [25] [26] [27] This drop-in arrangement enables extremely quick transfer of the motor to another boat or for storage - all that is needed is to lift it out. The pivoting design allows the outboard motor to be swiveled by the operator in almost all directions: Sideways for direction, up and down to change the thrust line according to speed or bow lift, elevate completely out of water for easy starting, placing the drive shaft and the propeller forward along the side of the boat for reverse, or put them inside the boat for propeller replacement, which can be a regular occurrence to the cheap cast aluminum propellers on the often debris-prone inland waterways. This design is utilized to propel Long-tail boats.


Former manufacturers

See also

Related Research Articles

Electric boat

While a significant majority of water vessels are powered by diesel engines, with sail power and gasoline engines also popular, boats powered by electricity have been used for over 120 years. Electric boats were very popular from the 1880s until the 1920s, when the internal combustion engine took dominance. Since the energy crises of the 1970s, interest in this quiet and potentially renewable marine energy source has been increasing steadily again, especially as solar cells became available, for the first time making possible motorboats with an infinite range like sailboats. The first practical solar boat was probably constructed in 1975 in England. The first electric sailboat which made a round-the-world tour, including the through the Panama Canal, with only green technologies is EcoSailingProject.

V4 engine piston engine with four cylinders in vee configuration

A V4 engine is a four-cylinder engine with its cylinders arranged in a "V" configuration.

Runabout (boat) boat type

A runabout is any small motorboat holding between four and eight people, well suited to moving about on the water. Characteristically 20' to 35' but under 35', runabouts are used for pleasure activities like boating, fishing, and water skiing, as a ship's tender for larger vessels, or in racing. Some common runabout types are bow rider, center console, cuddy boat and walkaround.

Yamaha Motor Company Limited is a Japanese manufacturer of motorcycles, marine products such as boats and outboard motors, and other motorized products. The company was established in 1955 upon separation from Yamaha Corporation, and is headquartered in Iwata, Shizuoka, Japan. The company conducts development, production and marketing operations through 109 consolidated subsidiaries as of 2012.

Motorboat boat which is powered by an engine

A motorboat, speedboat, or powerboat is a boat which is powered by an engine. Some motorboats are fitted with inboard engines, others have an outboard motor installed on the rear, containing the internal combustion engine, the gearbox and the propeller in one portable unit.

Volvo Penta

Volvo Penta was founded in 1907 in conjunction with the production of the first marine engine, B1. The Penta company soon became an established internal combustion engine manufacturer, which in 1927 delivered the engine for Volvo's first passenger car.

Ole Evinrude American inventor

Ole Evinrude, born Ole Andreassen Aaslundeie was a Norwegian-American entrepreneur, known for the invention of the first outboard motor with practical commercial application.

Evinrude Outboard Motors is a North American company that builds a major brand of outboard motors for boats. Founded by Ole Evinrude in Milwaukee, Wisconsin in 1907, it was formerly owned by the publicly traded Outboard Marine Corporation (OMC) since 1935, but OMC filed for bankruptcy in 2000. It is now working as a subsidiary of Canadian Muntinational Bombardier Recreational Products.

Outboard Marine Corporation 1998 power trim

Outboard Marine Corporation (OMC) was a maker of Evinrude, Johnson and Gale Outboard Motors boat motors and many different brands of boats. It was a multibillion-dollar Fortune 500 corporation. Evinrude began in Milwaukee, Wisconsin in 1907. OMC was based in Waukegan, Illinois. They also owned several lines of boats such as Chris Craft, Lowe Boats, Princecraft, Four Winns, SeaSwirl, Stratos, and Javelin. OMC was also a parent company to Ryan, which made lawn mowers.

Mercury Marine company

Mercury Marine, founded in 1939 as Kiekhaefer Mercury, is a division of the Brunswick Corporation. It is based in Fond du Lac, Wisconsin. The company manufactures marine engines.

British Seagull

The British Seagull® was a British manufacturer of two-stroke outboard engines in Poole, Dorset from the early 1930s until the mid-1990s. Although their "Classic" engines were for decades an exemplar of rugged simplicity and reliability, the company eventually failed when customer tastes changed and as tighter emissions regulations took effect. Production of complete engines stopped in 1996, and another company bought the rights to the brand in 1999. Spare parts continue to be produced for existing engines.


A sterndrive or inboard/outboard drive (I/O) is a form of marine propulsion which combines inboard power with outboard drive. The engine sits just forward of the transom while the drive unit lies outside the hull.

Tohatsu Corporation is a Japanese company manufacturing and selling outboard motors, pleasure boats, portable fire pumps, small fire trucks, pumps for construction and drainage, and refrigeration units for transportation. It also does real estate property management in Japan.

Nissan outboard motors are produced by Tohatsu Corporation of Tokyo, Japan. They are the second largest producer of outboard motors in the world and produce environmentally conscious TLDI series of two-stroke low pressure direct injection outboards that meet current United States Environmental Protection Agency regulations for the US. Mercury outboards from 30 hp and below are rebadged Tohatsus and all Nissan outboard engines in the US and Canada are Tohatsu's with a Nissan decal. Nissan Outboards sold in Japan are re-badged Hondas.

Carl Kiekhaefer NASCAR team owner

Elmer Carl Kiekhaefer was the owner of Kiekhaefer Mercury and Kiekhaefer Aeromarine and also a two-time NASCAR championship car owner.

Inboard motor

An inboard motor is a marine propulsion system for boats. As opposed to an outboard motor where an engine is mounted outside the hull of the craft, an inboard motor is an engine enclosed within the hull of the boat, usually connected to a propulsion screw by a driveshaft.

Ralph S. Evinrude was an American business magnate best known for being the Chairman of Outboard Marine Corporation, and the husband of singer and entertainer Frances Langford.

ELTO outboard motor business

The Elto Outboard Motor Company (Elto) was the second major outboard motor business founded by Ole Evinrude. Elto outboard engines were produced in the U.S. from 1921 to 1941.

G-W Invader

G-W Invader is a line of small recreational power boats that were manufactured by Arne Gray and George Wooldridge of Sharpsville, Indiana, United States.


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