A sterndrive or inboard/outboard drive (I/O) is a form of marine propulsion which combines inboard power with outboard drive. The engine sits just forward of the transom while the drive unit (outdrive or drive leg) lies outside the hull. 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 below the waterline. The outdrive resembles the bottom half of an outboard motor 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 shaft. The boat is steered by pivoting the outdrive, as on an outboard motor; no rudder 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 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" versions of Chevrolet 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 D6A-400. Other brands of sterndrive include MerCruiser (produced by Brunswick Corporation's Mercury Marine, 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 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