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An outboard motor is a propulsion

Marine propulsion

Marine propulsion is the act of moving an object on the surface of or through water. Such objects are commonly boats or ships. Propulsion devices can take many forms including: propeller, water jet, paddle wheel, sails, poles, paddles, oars and, experimentally, magnetohydrodynamic drive. They...

system for boat

Boat

A boat is a watercraft of modest 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...

s, consisting of a self-contained unit that includes engine, gearbox and propeller

Propeller

A propeller is a type of fan which 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 air or water is accelerated behind the blade...

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-jet 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 , or raked aft...

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 fluid . 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

SeaweedSeaweed has antioxidents. 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.

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 , have integral fuel tanks, and provide sufficient power to move a small dinghy at around
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, eg when fishing from a...

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.

 

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 115 horsepower suitable for hulls up to in length, to powerful V-6 and V-8 cylinder blocks rated up to ., with sufficient power to be used on boats of or longer.

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", 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." The collective name for a group...

and other freshwater species,

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

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 during the final stage of compression...

outboards are also available, but their weight and cost make them impractical for most purposes. An additional issue with diesel outboards is toxic exhaust emissions.

Pump-jet

Pump-jet

Pump-jet

A pump-jet 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.

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 five million Norwegian Americans according to the most recent U.S...

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....

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 industry legend Ole Evinrude. Elto outboard engines were produced in the U.S.A...

. 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 a 100 years, but again in fewer numbers.

Mercury Marine

Mercury Marine

Mercury Marine, founded in 1939, is a division of Brunswick Corporation of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury even has its own line of racing engines with less fuel efficiency and more focus at power and speed....

, 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, chiefly 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.

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 , or raked aft...

. 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 that oppose the relative motion of an object through a fluid . Drag forces act in a direction opposite to the oncoming 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 damage beyond economic repair.

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 , or raked aft...

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 that oppose the relative motion of an object through a fluid . Drag forces act in a direction opposite to the oncoming 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, the construction is dated from this event, with only the ship's launching considered more...

, 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 that measures 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 where further trim adjustment results in an RPM reduction. Motors not equipped with power trim are manually adjustable.

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 of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial cavitation, and noninertial cavitation. Inertial cavitation is the process where a...

, at which point no 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 of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial cavitation, and noninertial cavitation. Inertial cavitation is the process where a...

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.

Preventative 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 to increase the pressure and flow of a fluid.- Impellers in pumps :An impeller is a rotating component of a centrifugal pump, usually made of iron, steel, bronze, brass, aluminum or plastic, which transfers energy from the motor that drives the pump...

-- 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 time that a spark will occur in the combustion chamber relative to piston position and crankshaft angular velocity....

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 Vincent Hugo Bendix Vincent Hugo Bendix (December 12, 1881 – March 27, 1945 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).

 

An outboard motor is a

Marine propulsion

Marine propulsion is the act of moving an object on the surface of or through water. Such objects are commonly boats or ships. Propulsion devices can take many forms including: propeller, water jet, paddle wheel, sails, poles, paddles, oars and, experimentally, magnetohydrodynamic drive. They...

system for boat

Boat

A boat is a watercraft of modest 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...

s, consisting of a self-contained unit that includes engine, gearbox and propeller

Propeller

A propeller is a type of fan which 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 air or water is accelerated behind the blade...

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-jet 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 , or raked aft...

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 fluid . 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

SeaweedSeaweed has antioxidents. 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.

 

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 , have integral fuel tanks, and provide sufficient power to move a small dinghy at around
This type of motor is typically used:

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, eg when fishing from a...

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.

 

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 115 horsepower suitable for hulls up to in length, to powerful V-6 and V-8 cylinder blocks rated up to ., with sufficient power to be used on boats of or longer.

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", electric outboards are used

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." The collective name for a group...

and other freshwater species,

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

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 during the final stage of compression...

outboards are also available, but their weight and cost make them impractical for most purposes. An additional issue with diesel outboards is toxic exhaust emissions.

Pump-jet

Pump-jet

Pump-jet

A pump-jet 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.

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 five million Norwegian Americans according to the most recent U.S...

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....

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 industry legend Ole Evinrude. Elto outboard engines were produced in the U.S.A...

. 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 a 100 years, but again in fewer numbers.

Mercury Marine

Mercury Marine

Mercury Marine, founded in 1939, is a division of Brunswick Corporation of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury even has its own line of racing engines with less fuel efficiency and more focus at power and speed....

, 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, chiefly 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.

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 , or raked aft...

. 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 that oppose the relative motion of an object through a fluid . Drag forces act in a direction opposite to the oncoming 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 damage beyond economic repair.

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 , or raked aft...

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 that oppose the relative motion of an object through a fluid . Drag forces act in a direction opposite to the oncoming 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, the construction is dated from this event, with only the ship's launching considered more...

, 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 that measures 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 where further trim adjustment results in an RPM reduction. Motors not equipped with power trim are manually adjustable.

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 of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial cavitation, and noninertial cavitation. Inertial cavitation is the process where a...

, at which point no 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 of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial cavitation, and noninertial cavitation. Inertial cavitation is the process where a...

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.

Preventative Maintenance

 

Impeller

An impeller is a rotor inside a tube or conduit to increase the pressure and flow of a fluid.- Impellers in pumps :An impeller is a rotating component of a centrifugal pump, usually made of iron, steel, bronze, brass, aluminum or plastic, which transfers energy from the motor that drives the pump...

-- replace every two years (annually in a salt water environment).

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 time that a spark will occur in the combustion chamber relative to piston position and crankshaft angular velocity....

linkages, lubricate according to manufacturer recommendations.

Vincent Hugo Bendix

Vincent Hugo Bendix Vincent Hugo Bendix Vincent Hugo Bendix (December 12, 1881 – March 27, 1945 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:...