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A sterndrive or inboard/outboard drive (I/O) is a form of
marine propulsion Marine propulsion is the mechanism or system used to generate thrust to move a watercraft through water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electr ...
which combines
inboard 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 prop ...
power with outboard drive. The
engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...
sits just forward of the transom while the drive unit (outdrive or drive leg) lies outside the
hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affi ...
.


Operation

The drive unit (outdrive) carries power from the inboard engine, typically mounted above the waterline, outboard through the transom and downward to the
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
below the waterline. The outdrive resembles the bottom half 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 motorised method ...
and is composed of two sub-units: an upper containing a driveshaft connected through the transom to an engine which transmits power to a 90-degree-angle gearbox; and the lower containing a vertical driveshaft receiving power from the upper unit gearbox, transmitted through another 90-degree-angle gearbox to the
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
shaft. The
boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats. Small boats are typically found on i ...
is steered by pivoting the outdrive, as on 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 motorised method ...
; no
rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adve ...
is needed. The outdrive is pivoted up for trailer travel and between uses to avoid fouling. The outdrive can be matched with a variety of engines in the appropriate power range; upper and lower units can often be purchased separately to customize gear ratios and
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
RPM, and lower units are also available with counter-rotating gearing to provide balanced torque in dual-drive installations. Sterndrive engines are similar to those used in true inboard systems. Historically the most popular in North America were "
marinised Marinisation (also marinization) is design, redesign, or testing of products for use in a marine environment. Most commonly, it refers to use and long-term survival in harsh, highly corrosive salt water conditions. Marinisation is done by many manuf ...
" versions of
Chevrolet Chevrolet ( ), colloquially referred to as Chevy and formally the Chevrolet Motor Division of General Motors Company, is an American automobile division of the American manufacturer General Motors (GM). Louis Chevrolet (1878–1941) and ou ...
and Ford V-8 automotive engines. In Europe diesel engines are more popular, ranging up to 400 hp in models such as the
Volvo Penta Volvo Penta was founded as Penta in 1907 with the production of its first marine engine, the B1. The Penta company soon became an established internal combustion engine manufacturer, which in 1927 delivered the engine for Volvo's first passenge ...
D6A-400. Other brands of sterndrive include
MerCruiser Mercury Marine is a marine engine division of Brunswick Corporation headquartered in Fond du Lac, Wisconsin. The main product line is outboard motors. It also produces the MerCruiser line of sterndrives and inboard motors. Some manufacturing ...
(produced by
Brunswick Corporation Brunswick Corporation, formerly known as the Brunswick-Balke-Collender Company, is an American corporation that has been developing, manufacturing and marketing a wide variety of products since 1845. Today, Brunswick has more than 13,000 employ ...
's
Mercury Marine Mercury Marine is a marine engine division of Brunswick Corporation headquartered in Fond du Lac, Wisconsin. The main product line is outboard motors. It also produces the MerCruiser line of sterndrives and inboard motors. Some manufacturi ...
, which also manufactures outboard motors). Mercruiser have a slightly larger market share in the US, while in Europe Volvo Penta has an approximate 80% market share.


History

The history of stern drive power begins in the First World War with the production of the MAS of Isotta Fraschini, with two counter-rotating propellers. In 1948,
Mercury Marine Mercury Marine is a marine engine division of Brunswick Corporation headquartered in Fond du Lac, Wisconsin. The main product line is outboard motors. It also produces the MerCruiser line of sterndrives and inboard motors. Some manufacturi ...
engineer Charlie Strang mated an aluminum car racing engine to the lower unit of an outboard motor, creating a marine propulsion system more powerful than the outboard motors available at the time. During the 1950s, he and fellow engineer Jim Wynne worked at Mercury under founder Carl Kiekhaefer, who was initially dismissive of and opposed to a technology that would later capture 80% of the market. In 1959 Wynne left Kiekhaefer and in less than 90 days developed his own stern drive, which he subsequently patented. The first commercial introduction of sterndrives was the Volvo Penta "Aquamatic" at the 1959 New York Motor Boat Show. Kiekhaefer introduced a MerCruiser outdrive in 1961 at the
Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interactive Map of Chicago , coordinates = , coordinates_footnotes = , subdivision_type = List of sovereign states, Count ...
Boat Show. By 1962, 16 manufacturers were producing sterndrives. There was also the Outboard Marine Corporation (O.M.C.); however, due to several patent wars, O.M.C. ceased production.


Characteristics

Advantages of the sterndrive system versus outboards include higher available
horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are t ...
per engine and a clean transom with no cutouts for the outboard installation and no protruding powerhead, which makes for easier ingress and egress for pleasure boat passengers, easier
fishing Fishing is the activity of trying to catch fish. Fish are often caught as wildlife from the natural environment, but may also be caught from fish stocking, stocked bodies of water such as fish pond, ponds, canals, park wetlands and reservoirs. ...
and easier retrieval in rescue situations. Advantages of the sterndrive system versus inboards include simpler engineering for boatbuilders, eliminating the need for them to design propshaft and rudder systems; ease of trailering and shallow water maneuvering (with the drive trimmed up); also, a significant space savings with the engine mounted all the way
aft "Aft", in nautical terminology, is an adjective or adverb meaning towards the stern (rear) of the ship, aircraft or spacecraft, when the frame of reference is within the ship, headed at the fore. For example, "Able Seaman Smith; lie aft!" or "Wh ...
, freeing up the boat's interior volume for occupancy space. This is of particular significance to
consumer A consumer is a person or a group who intends to order, or uses purchased goods, products, or services primarily for personal, social, family, household and similar needs, who is not directly related to entrepreneurial or business activities. ...
s who are interested in " pocket cruisers," boats which have amenities like a
head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals ...
, a
galley A galley is a type of ship that is propelled mainly by oars. The galley is characterized by its long, slender hull, shallow draft, and low freeboard (clearance between sea and gunwale). Virtually all types of galleys had sails that could be u ...
, and separate sleeping quarters in a boat less than 30 feet in length. The main disadvantages of sterndrives versus straight inboards is that they are more exposed. There are hoses, rubber bellows and oil lines in the water which can be damaged. There are also more components in the water which are exposed to corrosion. Maintenance on I/Os is more complicated than outboards in several ways, less in others. While sterndrive engines are not as readily removable as smaller outboards, larger outboards (which combine both large powerheads and outdrives) are bigger, bulkier, and heavier than separate engine and outdrive components on sterndrives. Sterndrive engine compartments may be cramped; however components are neither as tightly engineered nor physically as compressed and densely packed as outboards, which all must fit under a streamlined integrated cover. With both inboards and stern drives there is a fire and explosion hazard from gasoline fuel vapors within the engine compartment. Typically these boats must run a blower for several minutes prior to starting the engine, and when idling or moving below cruise speed. The blower exchanges the air within the engine compartment with fresh outside air. Because of their practical advantages and attractive cost, sterndrives have become extremely popular especially for use in pleasure boats, and there are many models of boats for which sterndrive power is the only available propulsion offered.


See also

*
Kitchen rudder The Kitchen rudder is the familiar name for "Kitchen's Patent Reversing Rudders", a combination rudder and directional propulsion delivery system for relatively slow speed displacement boats which was invented in the early 20th century by John ...


References

{{Reflist Propellers Marine propulsion