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.
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.
General uses = Large Outboards=
Large outboards are usually 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 in length, to powerful V6 and V8
cylinder blocks rated up to 557 hp., with sufficient power to be used on
boats of 37 feet 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 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, have integral fuel tanks,
and provide sufficient power to move a small dinghy at around 8 knots 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 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.
= Electric-powered= Commonly referred to as "trolling
motors" or "electric outboard motors", 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 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 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 first known outboard motor was a small 5 kilogram electric unit designed
around 1870 by Gustave Trouvé, and patented in May 1880. Later about 25
petrol powered outboards may have been produced in 1896 by American Motors
Co—but neither of these two pioneering efforts appear to have had much impact.
The Waterman outboard engine appears to be the first real gasoline-powered
outboard offered for sale. Developed by Cameron Waterman, a young Yale
Engineering student, it was developed from 1903, with a patent application
filed in 1905 Starting in 1906, the company went on to make thousands of his
"Porto-Motor" units, claiming 25,000 sales by 1914. 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, was created by Norwegian-American inventor Ole Evinrude 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 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 technical superiority
and efficiency over two strokes. In the early 1980s Honda Motor Co. introduced
its first four-stroke powerhead. 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 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, noise reduction, increase fuel economy,
increased low end torque, and smoother operation.
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, boosted low end torque,
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.
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
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. 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 damages.
= General Dimensions= Different outboard engine brands require
different transom dimensions and sizes, that affects performance and trim.
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=
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 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. 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 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 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, 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. = Safety=
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.
= Cooling System= 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, 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
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 In Vietnam and perhaps other parts of
South East Asia such as Thailand, designs perhaps adapted from the
predecessors of one such as this Kohler Long Tail, using locally materials are
used until the present. In Vietnam, 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., 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.
Manufacturers Aquawatt Electric Outboard Motor
Bolinder Briggs & Stratton - USA - Up to 5 hp
Cimco Marine AB DBD Marine
ELTO Evinrude/Johnson, a division of
Bombardier Recreational Products - USA - Up to 300 hp
Hidea - China Honda Marine Group - Japan - Up to 250
hp Kohler Company
Jarvis Marine Maxus outboards
Mud-skipper Longtail outboard McCulloch
Mercury/Mariner - USA - Up to 300 hp Nissan Marine
Oshen-Hyfong Marine Parsun - China
Selva Marine - Italy - Up to 250 hp Seven Marine - USA - One model of 557
hp, utilising General Motors powerplant Suzuki Marine - Japan - Up to 300 hp
Tohatsu - Japan Tomos
Torqeedo - Electric outboards Zomair
Ul'yanovsk Motor Plant West Bend
Yamaha Outboards - Japan - Up to 350 hp = Former manufacturers=
British Anzani British Seagull
Yanmar Diesel See also
Electric outboard motor Luxury yacht tenders
Pneumatic motor Sterndrive
References External links
"How A Kicker Works, June 1951, Popular Science by George W. Waltz Jr - one of
the best basic articles on out board motors with lots of drawings and
illustrations The Antique Outboard Motor Club
International The Super-Elto Outboard Motor
Smithsonian Institution Libraries The Antique Outboard Wiki
Early Outboard Motor Boat Racing in British Columbia
Patents U.S. Patent 1,001,260 - Marine
propulsion mechanism U.S. Patent 1,011,930 - Canoe and other
small craft