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