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The sterndrive, or stern drive, is also called inboard/outboard (I/O), and is a form of marine propulsion. The engine is located inboard just forward of the transom (stern) and delivers power via a shaft that goes through the transom to the drive unit (often referred to as the outdrive) located outside the hull, which resembles the bottom half of an outboard. This unit contains the gearing for the system and carries the propeller. The boat is steered by pivoting this unit, 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 of which being marinized versions of Chevrolet and Ford V-8 automotive engines. The most popular brand of sterndrive is MerCruiser, produced by Brunswick Corporation's Mercury Marine, which also manufactures outboard motors. In 1999 the company was sued by competitor Volvo Penta for monopolizing the stern drive business.
History
The history of stern drive power begins in 1948, when Charlie Strang mated an aluminum car racing engine to the lower unit of an outboard motor, in order to create a marine propulsion system more powerful than the outboard motors available at the time. The system would be patented ten years later 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 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. eventually went under.
Relative Merits
Advantages of the sterndrive system versus outboards include higher available horsepower 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 cruisers," boats which have amenities like a head, a galley, and separate sleeping quarters in a boat less than 30 feet in length.
Disadvantages of sterndrives versus straight inboards include the fact that power must pass through two 90-degree gears to get to the propeller, which is less efficient; also, since the mechanism is more complex it is more likely to experience mechanical failure and require more maintenance, plus with the mechanism located out in the water it is more subject to damage from submerged hazards.
This gearing is highly sensitive to various operating conditions. For example, it is critical to not engage the drive with the outboard drive trimmed fully upwards. Doing so could destroy components. Many boats have separate controls for fully-upward-trimmed and a lower upward trim setting, allowing boat owners to be more comfortable with trimming the lower unit upward. The lower unit has an ideal trim level where the gearing and joints align the best. This point is easily identifiable as the drivetrain will "feel" smoothest while at cruise speed at this trim level.
Maintenance with I/Os is more complicated 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.
There is also a hazard of explosion from fuel vapors within the engine compartment. Typically, I/O-equipped 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
"Sterndrive"