| The sterndrive, or stern drive, is also | | | | propshaft and rudder systems; also, a |
| called inboard/outboard (I/O), and is a | | | | significant space savings with the |
| form of marine propulsion. The engine is | | | | engine mounted all the way aft, freeing |
| located inboard just forward of the | | | | up the boat's interior volume for |
| transom (stern) and delivers power via a | | | | occupancy space. This is of particular |
| shaft that goes through the transom to | | | | significance to consumers who are |
| the drive unit (often referred to as the | | | | interested in "pocket cruisers," boats |
| outdrive) located outside the hull, | | | | which have amenities like a head, a |
| which resembles the bottom half of an | | | | galley, and separate sleeping quarters |
| outboard. This unit contains the gearing | | | | in a boat less than 30 feet in length. |
| for the system and carries the | | | | Disadvantages of sterndrives versus |
| propeller. The boat is steered by | | | | straight inboards include the fact that |
| pivoting this unit, just like with an | | | | power must pass through two 90-degree |
| outboard motor, and no rudder is needed. | | | | gears to get to the propeller, which is |
| The engine itself is usually the same as | | | | less efficient; also, since the |
| those used in true inboard systems, | | | | mechanism is more complex it is more |
| historically the most popular of which | | | | likely to experience mechanical failure |
| being marinized versions of Chevrolet | | | | and require more maintenance, plus with |
| and Ford V-8 automotive engines. The | | | | the mechanism located out in the water |
| most popular brand of sterndrive is | | | | it is more subject to damage from |
| MerCruiser, produced by Brunswick | | | | submerged hazards. |
| Corporation's Mercury Marine, which also | | | | This gearing is highly sensitive to |
| manufactures outboard motors. In 1999 | | | | various operating conditions. For |
| the company was sued by competitor Volvo | | | | example, it is critical to not engage |
| Penta for monopolizing the stern drive | | | | the drive with the outboard drive |
| business. | | | | trimmed fully upwards. Doing so could |
| History | | | | destroy components. Many boats have |
| The history of stern drive power begins | | | | separate controls for |
| in 1948, when Charlie Strang mated an | | | | fully-upward-trimmed and a lower upward |
| aluminum car racing engine to the lower | | | | trim setting, allowing boat owners to be |
| unit of an outboard motor, in order to | | | | more comfortable with trimming the lower |
| create a marine propulsion system more | | | | unit upward. The lower unit has an ideal |
| powerful than the outboard motors | | | | trim level where the gearing and joints |
| available at the time. The system would | | | | align the best. This point is easily |
| be patented ten years later by Jim | | | | identifiable as the drivetrain will |
| Wynne. During the 1950s, both engineers | | | | "feel" smoothest while at cruise speed |
| worked at Mercury Marine under Carl | | | | at this trim level. |
| Kiekhaefer, who was initially dismissive | | | | Maintenance with I/Os is more |
| of and opposed to the idea of | | | | complicated in several ways. For |
| sterndrives, but would later capture 80% | | | | example, oil changes can require |
| of the market. In 1958, Wynne left | | | | complicated pumping mechanisms, and |
| Kiekhaefer and in less than 90 days | | | | engine repairs are often far more |
| "invented" the stern drive. The first | | | | complicated than outboards because of |
| commercial introduction of sterndrives | | | | incredibly tight spaces in the engine |
| was by Volvo Penta at the 1959 New York | | | | compartment. In some boats the entire |
| Motor Boat Show. Kiekhaefer would | | | | engine must be removed to perform |
| introduce the first MerCruiser outdrive | | | | otherwise trivial repairs whereas, with |
| in 1961 at the Chicago Boat Show. By | | | | an outboard, all one must do is pop off |
| 1962, 16 manufacturers were producing | | | | the cover. |
| sterndrives. There was also the Outboard | | | | There is also a hazard of explosion from |
| Marine Company (O.M.C.); however, due to | | | | fuel vapors within the engine |
| several patent wars, O.M.C. eventually | | | | compartment. Typically, I/O-equipped |
| went under. | | | | boats must run a blower for several |
| Relative Merits | | | | minutes prior to starting the engine, |
| Advantages of the sterndrive system | | | | and when idling or moving below cruise |
| versus outboards include higher | | | | speed. The blower exchanges the air |
| available horsepower per engine and a | | | | within the engine compartment with fresh |
| clean transom with no cutouts for the | | | | outside air. |
| outboard installation and no protruding | | | | Because of their practical advantages |
| powerhead, which makes for easier | | | | and attractive cost, sterndrives have |
| ingress and egress for pleasure boat | | | | become extremely popular especially for |
| passengers and for easier fishing. | | | | use in pleasure boats, and there are |
| Advantages of the sterndrive system | | | | many models of boats for which |
| versus inboards include simpler | | | | sterndrive power is the only available |
| engineering for boatbuilders, | | | | propulsion offered. |
| eliminating the need for them to design | | | | |