Sterndrive

The outdrive unit of a boat with sterndrive

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.

Operation

The drive unit (outdrive) resembles the bottom half of an outboard motor, 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.

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.

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.

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.

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.

See also

• Kitchen rudder