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Name: Bob
Status: Other
Grade: Other
Location: N/A
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?

Hi Bob,

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 cavitation.


Bob Wilson

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.

Robert Erck

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