Sterndrive
Encyclopedia
A sterndrive or inboard/outboard drive (I/O) is a form of marine
propulsion
. The engine
is located inboard just forward of the transom
(stern
) and provides power to the drive unit (outdrive) located outside the hull
.
, and is composed of two sub-units: the upper unit contains a driveshaft that connects through the transom to the engine and transmits power to a 90-degree-angle gearbox; the lower unit bolts onto the bottom of the upper unit and contains a vertical driveshaft that transmits power from the upper unit gearbox down to another 90-degree-angle gearbox in the lower unit, which connects to the propeller
shaft.
The outdrive carries power from the inboard engine, typically mounted above the waterline, outboard through the transom and downward to the propeller
below the waterline. 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
RPM, and lower units are also available with counter-rotating gearing to provide balanced torque in dual-drive installations. The boat
is steered by pivoting the outdrive, just like with an outboard motor
, and no rudder
is needed.
The engine itself is usually the same as those used in true inboard systems, historically the most popular in North America were "marinised
" versions of Chevrolet
and Ford
V-8
automotive engines. In Europe diesel engines are more popular with up to 370 hp available with Volvo Pentas D6A-370. Brands of sterndrive include Volvo Penta
(part of the Volvo Group) and MerCruiser (produced by Brunswick Corporation
's Mercury Marine
, which also manufactures outboard motors). Mercruiser have a slightly larger market share in the USA but in Europe Volvo Penta have a very large market share around the 80% mark. In 1999 Brunswick was sued by Volvo Penta for monopolizing the stern drive business in the USA.
, with two counter-rotating propellers.
Charlie Strang patented it by Jim Wynne. During the 1950s, both engineers worked at Mercury Marine
under Carl Kiekhaefer
, who was initially dismissive of and opposed to the idea of sterndrives, but would later capture 80% of the market. In 1958, Wynne left Kiekhaefer and in less than 90 days "invented" the stern drive. The first commercial introduction of sterndrives was by Volvo Penta with their "Aquamatic" at the 1959 New York
Motor Boat Show. Kiekhaefer would introduce the first MerCruiser outdrive in 1961 at the Chicago
Boat Show. By 1962, 16 manufacturers were producing sterndrives. There was also the Outboard Marine Company (O.M.C.); however, due to several patent wars, O.M.C. ceased production.
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 and for easier fishing
.
Advantages of the sterndrive system versus inboards include simpler engineering for boatbuilders
, eliminating the need for them to design propshaft and rudder systems; also, a significant space savings with the engine mounted all the way aft
, freeing up the boat's interior volume for occupancy space. This is of particular significance to consumers
who are interested in "pocket cruiser
s," boats which have amenities like a head
, a galley
, 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. For example, oil changes can require complicated pumping mechanisms, and engine repairs are often far more complicated than outboards because of incredibly tight spaces in the engine compartment. In some boats the entire engine must be removed to perform otherwise trivial repairs whereas, with an outboard, all one must do is pop off the 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.
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
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...
. The engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
is located inboard just forward 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...
(stern
Stern
The stern is the rear or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite of the bow, the foremost part of a ship. Originally, the term only referred to the aft port section...
) and provides power to the drive unit (outdrive) located outside the hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
.
Operation
The drive unit (outdrive) resembles the bottom half of an outboard motorOutboard 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 and are the most common motorized method of propelling small watercraft...
, and is composed of two sub-units: the upper unit contains a driveshaft that connects through the transom to the engine and transmits power to a 90-degree-angle gearbox; the lower unit bolts onto the bottom of the upper unit and contains a vertical driveshaft that transmits power from the upper unit gearbox down to another 90-degree-angle gearbox in the lower unit, which connects to the 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...
shaft.
The outdrive carries power from the inboard engine, typically mounted above the waterline, outboard through the transom and downward to the 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...
below the waterline. 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
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...
RPM, and lower units are also available with counter-rotating gearing to provide balanced torque in dual-drive installations. The 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...
is steered by pivoting the outdrive, just like with an outboard motor
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 and are the most common motorized method of propelling small watercraft...
, and no 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...
is needed.
The engine itself is usually the same as those used in true inboard systems, historically the most popular in North America were "marinised
Marinised
Marinised is when products have been specifically designed, redesigned or tested for use and long term survival in the harsh marine environment...
" versions of Chevrolet
Chevrolet
Chevrolet , also known as Chevy , is a brand of vehicle produced by General Motors Company . Founded by Louis Chevrolet and ousted GM founder William C. Durant on November 3, 1911, General Motors acquired Chevrolet in 1918...
and Ford
Ford Motor Company
Ford Motor Company is an American multinational automaker based in Dearborn, Michigan, a suburb of Detroit. The automaker was founded by Henry Ford and incorporated on June 16, 1903. In addition to the Ford and Lincoln brands, Ford also owns a small stake in Mazda in Japan and Aston Martin in the UK...
V-8
V8 engine
A V8 engine is a V engine with eight cylinders mounted on the crankcase in two banks of four cylinders, in most cases set at a right angle to each other but sometimes at a narrower angle, with all eight pistons driving a common crankshaft....
automotive engines. In Europe diesel engines are more popular with up to 370 hp available with Volvo Pentas D6A-370. Brands of sterndrive include Volvo Penta
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 to Volvo's first passenger car....
(part of the Volvo Group) and MerCruiser (produced by Brunswick Corporation
Brunswick Corporation
The Brunswick Corporation , formerly known as the Brunswick-Balke-Collender Company, is a United States-based corporation that has been involved in manufacturing a wide variety of products since 1845. Brunswick's global headquarters is in the northern Chicago suburb of Lake Forest, Illinois...
's 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...
, which also manufactures outboard motors). Mercruiser have a slightly larger market share in the USA but in Europe Volvo Penta have a very large market share around the 80% mark. In 1999 Brunswick was sued by Volvo Penta for monopolizing the stern drive business in the USA.
History
The history of stern drive power begins in the First World War with the production of the MAS of Isotta-FraschiniIsotta-Fraschini
Isotta Fraschini represents two Italian manufacturing companies which produce, respectively, marine engines and luxury goods. In the early 20th century it was famous worldwide as a luxury car manufacturer.-History:...
, with two counter-rotating propellers.
Charlie Strang patented it by Jim Wynne. During the 1950s, both engineers worked at 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...
under Carl Kiekhaefer
Carl Kiekhaefer
Elmer Carl Kiekhaefer was the owner of Kiekhaefer Marine , and also a two-time NASCAR championship car owner.-Kiekhaefer Mercury founder:...
, who was initially dismissive of and opposed to the idea of sterndrives, but would later capture 80% of the market. In 1958, Wynne left Kiekhaefer and in less than 90 days "invented" the stern drive. The first commercial introduction of sterndrives was by Volvo Penta with their "Aquamatic" at the 1959 New York
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
Motor Boat Show. Kiekhaefer would introduce the first MerCruiser outdrive in 1961 at the Chicago
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
Boat Show. By 1962, 16 manufacturers were producing sterndrives. There was also the Outboard Marine Company (O.M.C.); however, due to several patent wars, O.M.C. ceased production.
Relative merits
Advantages of the sterndrive system versus outboards include higher available horsepowerHorsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
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 and for easier fishing
Fishing
Fishing is the activity of trying to catch wild fish. Fish are normally caught in the wild. Techniques for catching fish include hand gathering, spearing, netting, angling and trapping....
.
Advantages of the sterndrive system versus inboards include simpler engineering for boatbuilders
Boat building
Boat building, one of the oldest branches of engineering, is concerned with constructing the hulls of boats and, for sailboats, the masts, spars and rigging.-Parts:* Bow - the front and generally sharp end of the hull...
, eliminating the need for them to design propshaft and rudder systems; also, a significant space savings with the engine mounted all the way aft
Aft
Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...
, freeing up the boat's interior volume for occupancy space. This is of particular significance to consumers
Consumer
Consumer is a broad label for any individuals or households that use goods generated within the economy. The concept of a consumer occurs in different contexts, so that the usage and significance of the term may vary.-Economics and marketing:...
who are interested in "pocket cruiser
Pocket cruiser
A pocket cruiser, microcruiser, trailer sailer or pocket yacht is a small lightweight sailboat with a cabin, designed for recreational cruising. Pocket cruisers can be readily loaded on a trailer and towed by most passenger automobiles. Both commercially made and designs for home built pocket...
s," boats which have amenities like a head
Head (watercraft)
The head is a ship's toilet. The name derives from sailing ships in which the toilet area for the regular sailors was placed at the head or bow of the ship.-Design:In sailing ships the toilet was placed in the bow for two reasons...
, a galley
Kitchen
A kitchen is a room or part of a room used for cooking and food preparation.In the West, a modern residential kitchen is typically equipped with a stove, a sink with hot and cold running water, a refrigerator and kitchen cabinets arranged according to a modular design. Many households have a...
, 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. For example, oil changes can require complicated pumping mechanisms, and engine repairs are often far more complicated than outboards because of incredibly tight spaces in the engine compartment. In some boats the entire engine must be removed to perform otherwise trivial repairs whereas, with an outboard, all one must do is pop off the 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.