Outboard motor
Overview
 
An outboard motor is a propulsion
Marine propulsion
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 a motor or engine turning a propeller, or less frequently, in jet...

 system for boat
Boat
A boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...

s, consisting of a self-contained unit that includes engine, gearbox and propeller
Propeller
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 blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

 or jet drive
Jetboat
A jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses a propeller in the water below or behind the boat, a jetboat draws the water from under the boat into a pump inside the boat, then expels it through a nozzle at the...

, designed to be affixed to the outside of the transom
Transom (nautical)
In naval architecture, a transom is the surface that forms the stern of a vessel. Transoms may be flat or curved and they may be vertical, raked forward, also known as a retroussé or reverse transom, angling forward from the waterline to the deck, or raked aft, often simply called "raked", angling...

 and 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.
Encyclopedia
An outboard motor is a propulsion
Marine propulsion
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 a motor or engine turning a propeller, or less frequently, in jet...

 system for boat
Boat
A boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...

s, consisting of a self-contained unit that includes engine, gearbox and propeller
Propeller
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 blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

 or jet drive
Jetboat
A jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses a propeller in the water below or behind the boat, a jetboat draws the water from under the boat into a pump inside the boat, then expels it through a nozzle at the...

, designed to be affixed to the outside of the transom
Transom (nautical)
In naval architecture, a transom is the surface that forms the stern of a vessel. Transoms may be flat or curved and they may be vertical, raked forward, also known as a retroussé or reverse transom, angling forward from the waterline to the deck, or raked aft, often simply called "raked", angling...

 and 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
Skeg
A skeg is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard...

 also acts as a rudder
Rudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...

 when the engine is not running. Compared to inboard motor
Inboard motor
An inboard motor is a marine propulsion system for boats. As opposed to an outboard motor where an engine is mounted outside of 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 adriveshaft.-History:The first...

s, outboard motors can be easily removed for storage or repairs.

When boats are out of service or being drawn through shallow waters, outboard motors can be tilted up (tilt forward over the transom mounts) to elevate the propeller and lower unit out of the water to avoid accumulation of seaweed
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

, underwater hazards such as rocks, and to clear road hazards while trailering.

Large Outboards

Large outboards are usually bolted to the transom (or to a bracket bolted to the transom), and are 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 V-6 and V-8 cylinder blocks rated up to 557 hp., with sufficient power to be used on boats of 37 feet (11.3 m) or longer.

Portable

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 pull start system, with throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

 and gearshift controls mounted on the body of the motor, and a tiller
Tiller
A tiller or till is a lever attached to a rudder post or rudder stock of a boat that provides leverage for the helmsman to turn the rudder...

 for steering. The smallest of these weigh as little as 12 kilograms (26.5 lb), have integral fuel tanks, and provide sufficient power to move a small dinghy at around 8 kn (15.7 km/h; 9.7 mph)
This type of motor is typically used:
  • to power small craft such as jon boats, dinghies, canoes, etc.
  • to provide auxiliary power for sailboats,
  • for trolling
    Troll (angling)
    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...

     aboard larger craft, as small outboards are typically more efficient at trolling speeds. In this application, the motor is frequently installed on the transom alongside and connected to the primary outboard to enable helm steering.

Diesel

Diesel
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 outboards are available but their weight and cost make them rare.

Electric-Powered

Commonly referred to as "trolling motor
Trolling motor
thumb|250px|A 12 volt electric trolling motor mounted on a 8 foot [[inflatable boat]]A trolling motor is a marine propulsion system consisting of 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...

s" or "electric outboard motor
Electric Outboard Motor
Electric outboard motors are an option for boat propulsion. Most electric outboard motors have 0.5 to 4 KW DC electric motors, operated at 12 to 60 volts DC. These systems have the propeller fixed directly to the motor which is mounted in the lower unit under water - see trolling motor. This setup...

s" , electric outboards are used
  • on very small craft or on small lakes where gasoline motors are prohibited,
  • as a secondary means of propulsion on larger craft, and
  • as repositioning thrusters while fishing for bass
    Bass (fish)
    Bass is a name shared by many different species of popular gamefish. The term encompasses both freshwater and marine species. All belong to the large order Perciformes, or perch-like fishes, and in fact the word bass comes from Middle English bars, meaning "perch."-Types of basses:*The temperate...

     and other freshwater species,

and any other application where their quietness, and ease of operation and zero emissions outweigh the speed and range deficiencies.

Pump-jet

Pump-jet
Pump-jet
A pump-jet, hydrojet, or water jet, is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle...

 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.

History and developments

The outboard motor, as a portable propulsion system for boats that would otherwise be powered manually by oars, was made possible by the experimentations of Cameron Waterman, a young Yale Engineering student. The Waterman outboard engine appears to be the first real gasoline-powered outboard offered for sale. It was four stroke. Between 1903 and his patent in 1905 he successfully created the outboard. Starting with two dozen built in 1907, the company went on to make thousands of the units in the next 5 years. The inboard boat motor firm of Caille Motor Company of Detroit were instrumental in making the cylinder and engines. Kiekhaefer eventually bought out Cameron Waterman and used magazine ads with references to the Waterman.

The creation of the first practical and marketable outboard motor is often miscredited to Norwegian-American
Norwegian American
Norwegian Americans are Americans of Norwegian descent. Norwegian immigrants went to the United States primarily in the later half of the 19th century and the first few decades of the 20th century. There are more than 4.5 million Norwegian Americans according to the most recent U.S. census, and...

 inventor Ole Evinrude
Ole Evinrude
Ole Evinrude, born Ole Evenrudstuen was a Norwegian-American inventor, known for the invention of the first outboard motor with practical commercial application.-Biography:...

 in 1909. 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 onto success with ELTO
ELTO
The Elto Outdoor Motor Company was the second major outboard motor business founded by Ole Evinrude. Elto outboard engines were produced in the U.S...

. The 1920s were the first highwater 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 in their exhaust, and louder noise.

In the 1990s, US and European exhaust emissions regulations led to the proliferation of four-stroke outboards. Though fewer in number, four-stroke outboards have always been available. For example, Honda Marine has been marketing small four-stroke outboards since the early 70s. Other brands have been produced for over 100 years, but again in fewer numbers.

Mercury Marine
Mercury Marine
Mercury Marine, founded in 1939, is a division of Brunswick of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury also has its own line of very successful racing engines tailored for power and speed. The company's primary...

, Mercury Racing, Tohatsu Outboards, Nissan Marine, Honda Marine, Suzuki Marine, and Yamaha Marine, China Oshen-Hyfong marine have all developed new four-stroke engines. Some are carbureted, usually the smaller engines. The balance are electronically fuel-injected. Some models benefit from variable camshaft timing, and multiple valves per cylinder. 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.

Outboard motors benefit from the use of a submerged pump to draw water for cooling, obviating the need for radiator
Radiator (engine cooling)
Radiators are used for cooling internal combustion engines, mainly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of such an engine....

s and cooling fans, thereby simplifying the design and lowering component weight, however constant usage in seawater is liable to cause corrosion.

For boats which are moored rather than trailered, bronze propellers are unsuitable owing to galvanic effects. Quite often sacrificial anodes are found which have been painted over. One can only assume that owners notice that these parts were corroding and thought that the factory forgot to paint them. Severe damage is usually the result.

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 and underpowering often results in a boat that is incapable of performing in the role for which it was acquired. 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
Transom (nautical)
In naval architecture, a transom is the surface that forms the stern of a vessel. Transoms may be flat or curved and they may be vertical, raked forward, also known as a retroussé or reverse transom, angling forward from the waterline to the deck, or raked aft, often simply called "raked", angling...

. If the shaft is too long it will extend farther into the water than necessary creating drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

, 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 damages.

Motor mounting height

Motor height on the transom
Transom (nautical)
In naval architecture, a transom is the surface that forms the stern of a vessel. Transoms may be flat or curved and they may be vertical, raked forward, also known as a retroussé or reverse transom, angling forward from the waterline to the deck, or raked aft, often simply called "raked", angling...

 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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 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
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

, with the motor in neutral trim.

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.





If the motor is trimmed out too far, the bow
Bow (ship)
The bow is a nautical term that refers to the forward part of the hull of a ship or boat, the point that is most forward when the vessel is underway. Both of the adjectives fore and forward mean towards 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
Tachometer
A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute on a calibrated analogue dial, but digital displays are increasingly common...

) 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

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 prop to spin fast enough to result in cavitation
Cavitation
Cavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...

, 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. 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.

Cavitation

Cavitation
Cavitation
Cavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...

 as it relates to outboard motors is often the result of a foreign object such as marine vegetation caught on the lower unit interrupting the flow of water into the propeller blades.

Preventive Maintenance

  • Lower unit gear lubricant—change annually.
    • Inspect the old oil for metal fragments and if found, disassemble gearbox for inspection and repair.
    • Inspect old oil for evidence of water intrusion and if found, replace seals at propshaft, drive shaft and shift rod.
  • Water pump impeller
    Impeller
    An impeller is a rotor inside a tube or conduit used to increase the pressure and flow of a fluid.- Impellers in pumps :...

     -- replace every two years (annually in a salt water environment).
    • Inspect pump housing, and replace if scored or damaged.
    • Inspect old impeller for missing pieces and if found, remove thermostat housing and water jacket cover if necessary, to recover the liberated material.
  • Powerhead—annual inspection
    • Inspect engine wiring for corrosion, burned/chafed/missing insulation. Check all connections for tightness.
    • Inspect fuel lines for signs of aging.
    • Inspect spark plugs and replace when necessary.
    • Check all fasteners for tightness, torque to manufacturer specification if necessary.
    • Clean and inspect throttle
      Throttle
      A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

      , shift, spark advance
      Ignition timing
      Ignition timing, in a spark ignition internal combustion engine , is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke...

       linkages, lubricate according to manufacturer recommendations.
    • Starter motor (if equipped) -- apply two drops of light oil to the bendix gear
      Vincent Hugo Bendix
      Vincent Hugo Bendix was an American inventor and industrialist. Vincent Bendix was a pioneer and leader in both the automotive and aviation industries during the 1920s and 1930s.-Background:...

       threads.
    • Test overtemp warning horn or light (if equipped).

Manufacturers

  • British Anzani
    Anzani
    Anzani was an engine manufacturer founded by the Italian Alessandro Anzani , which produced proprietary engines for aircraft, cars, boats, and motorcycles in factories in Britain, France and Italy.-Overview:...

  • Aquawatt Electric Outboard Motor
  • Bolinder
  • Briggs & Stratton
    Briggs & Stratton
    Briggs & Stratton is the world's largest manufacturer of air-cooled gasoline engines primarily for outdoor power equipment. Current production averages 11 million engines per year.-History:...

  • British Seagull
    British Seagull
    British Seagull was a British manufacturer of two-stroke outboard engines from the early 1930s until the mid 1990s. The company went out of business due to the motor design not being able to keep up with more modern boat engines and increasingly tight emissions regulations...

  • China Parsun Marine
  • ELTO
    ELTO
    The Elto Outdoor Motor Company was the second major outboard motor business founded by Ole Evinrude. Elto outboard engines were produced in the U.S...

  • Evinrude
    Evinrude Outboard Motors
    Evinrude Outboard Motors is a 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 , and is now owned by privately held Bombardier Recreational Products.-...

    /Johnson
    Johnson Outboards
    Johnson Outboards was a US based manufacturer of outboard motors. The original company to make Johnson inboard motors and outboard motors was the Johnson Bros. Motor Company of Terre Haute, Indiana, United States...

    , a division of Bombardier Recreational Products
    Bombardier Recreational Products
    Bombardier Recreational Products or BRP is a Canadian company that traces its roots back to the year 1942 when Joseph-Armand Bombardier founded L'Auto-Neige Bombardier Limitée in Valcourt in the Eastern Townships, Quebec.In 2003, Bombardier Inc...

  • Hidea
  • Honda
    Honda
    is a Japanese public multinational corporation primarily known as a manufacturer of automobiles and motorcycles.Honda has been the world's largest motorcycle manufacturer since 1959, as well as the world's largest manufacturer of internal combustion engines measured by volume, producing more than...

  • Honda Marine
  • McCulloch
  • Mercury
    Mercury Marine
    Mercury Marine, founded in 1939, is a division of Brunswick of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury also has its own line of very successful racing engines tailored for power and speed. The company's primary...

    /Mariner
    Mercury Marine
    Mercury Marine, founded in 1939, is a division of Brunswick of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury also has its own line of very successful racing engines tailored for power and speed. The company's primary...

  • Nissan Marine
    Nissan Marine
    Nissan Marine began their North American operations in 1984 in Memphis, Tennessee as a division of Nissan Industrial Equipment Co. Over the next 6 years, the Marine Division experienced explosive growth and in 1991 Nissan Marine was separated from Nissan Industrial Equipment into a standalone...

  • Oshen-Hyfong Marine
  • Parsun Marine
  • Selva Marine
  • Suzuki
    Suzuki
    is a Japanese multinational corporation headquartered in Hamamatsu, Japan that specializes in manufacturing compact automobiles and 4x4 vehicles, a full range of motorcycles, all-terrain vehicles , outboard marine engines, wheelchairs and a variety of other small internal combustion engines...

  • Tohatsu
    Tohatsu
    The Tohatsu Corporation of Tokyo, Japan, was founded in 1922. It manufactures and sells outboard motors, pleasure boats, portable fire pumps, small fire trucks, pumps for construction and drainage, refrigeration units for transportation and also does real estate property management in Japan.The...

  • Ul'yanovsk Motor Plant
  • West Bend
    West Bend Company
    The West Bend Company was a West Bend, Wisconsin company from 1911 to 2003. It manufactures aluminum cookware and electrical appliances, but also made two-stroke cycle engines including outboard boat motors. Art Ingels used a surplus West Bend engine to power the first kart...

  • Yamaha Motor Corporation
  • Yanmar Diesel Power

See also

  • Air engine
  • Electric Outboard Motor
    Electric Outboard Motor
    Electric outboard motors are an option for boat propulsion. Most electric outboard motors have 0.5 to 4 KW DC electric motors, operated at 12 to 60 volts DC. These systems have the propeller fixed directly to the motor which is mounted in the lower unit under water - see trolling motor. This setup...

  • Luxury yacht tender
    Luxury yacht tender
    A luxury yacht tender is a transport used to service and to provide support and entertainment to a private or charter luxury yacht. Known commonly as the 'yacht's tender' a luxury yacht tender will often be a rigid-hulled inflatable boat which features cushioned inflatable rubber inner tubes...

    s
  • Sterndrive
    Sterndrive
    A sterndrive or inboard/outboard drive is a form of marine propulsion. The engine is located inboard just forward of the transom and provides power to the drive unit located outside the hull.-Operation:...

    (inboard/outboard drive)

External links


Patents

  • - Marine propulsion mechanism
  • - Canoe and other small craft
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