A sterndrive or inboard/outboard drive (I/O) is a form of marine propulsion. The engine (or upper unit) 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 or lower unit) 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.
Sterndrive.JPGThe engine itself is usually the same as those used in true inboard systems, historically the most popular in North America was marinised versions of Chevrolet and Ford V-8 automotive engines. In Europe diesel engines are more popular with up to 370hp available with Volvo Pentas D6A-370. The most popular brands of sterndrive are 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.
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