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