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