Boats and Transmissions
Country: United States
Date: August 2008
Speed on a boat is directly connected to
the amount of throttle applied. Why aren't
(cannot?) boats equipped with automatic
transmissions for fuel efficiency at higher speeds?
The propeller on a boat is already optimized to drive the boat
at the greatest efficiency possible, at the boat's intended design
speed. Adding any kind of a transmission would not result in greater
fuel efficiency. In fact, since no transmission is 100% efficient, it
would actually result in reduced fuel efficiency.
The size of a boat's propeller, its diameter, and number of blades, and
its pitch, are all optimized to result in maximum efficiency at the
intended motor and propeller RPM. Adding a transmission will only drive
the propeller at a different RPM than it was designed to operate at,
resulting in inefficient operation, or even greater problems such as
On a boat, the engine and propeller are typically selected for overall best
performance, and this works quite well for the majority of users.
Some boats may have a two-speed transmission, but this is rare. If it is
necessary to obtain maximum performance from the engine over a range of boat
speeds (e.g., a ski boat), then a controllable-pitch propeller may be used,
which is equivalent to a transmission. Typically, the issues that most
affect boat performance and concern propulsion designers are propeller
diameter, pitch, and rake, rather than transmission.
Because water resistance is high, it is necessary to operate the engine at
maximum power at maximum boat speed - with maximum consumption of fuel. At
lesser boat speeds, less power is required from the engine, but little gains
are possible by adjusting the engine/propeller drive ratio. This situation
is different than the case of road vehicles, where air drag is relatively
small, and higher gears can be used for efficiency.
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Update: June 2012