It is a common misconception that loading an engine at less than the speed for maximum power is damaging to the engine -- with modern engines (that use forced lubrication) this is usually not the case. There is a documented case involving military aircraft using propellers with variable pitch. The ground crew was puzzled why one officer's aircraft always returned with significantly more fuel in the tank than the other identical aircraft that flew the same mission. When the word got around the CO wondered why his plane used less fuel, they thought it must be the plane, but when questioned, It turned out that he admitted to ignoring "proper" operation procedure, and always operated his aircraft with the throttle wide open, and controlled speed with the prop by increasing the pitch to match the other aircraft flying in formation. The mechanic who "knew best" accused him of damaging government property by "lugging" the engine. The CO demanded that several engines with the same number of hours be torn down and the wear compared -- it was found that the officer's engine had much less wear than the engines operated according to procedure. I learned of this story (printed in "Sport Aviation" published by the experimental aircraft association) the operating procedure was quickly changed. As I recall the article said the result was 30% better fuel efficiency and about 30% greater service life. I realize that Diesel engines always operate with no throttle, and power is controlled by fuel delivery rate, so pumping losses are not as great as on a gasoline aircraft engine operated at part throttle. Most Diesels produce the best SFC (specific fuel consumption) at a load just under the "smoke limit" where the engine cannot get enough air to fully burn the fuel; AND at the RPM that produces the greatest mean effective pressure (just about corresponding to the torque peak). With a modern Diesel that has a wide speed range (where maximum power is significantly greater RPM than the torque peak RPM), this load is quite a bit greater than a prop sized to produce greatest speed can deliver. A controllable pitch prop can be effectively used to offer both best part throttle efficiency (by operating the engine at peak SFC) AND also achieve the top speed allowed by the maximum power the engine is capable of. It is true that some lightly built highly turbo-charged Diesel engines will be damaged (or the service life reduced) if operated at the torque peak for extended periods. A way to maximize MPG at a given speed is to obtain an efficiency map showing iso SFC lines on a percent load (throttle position) verses RPM graph, and select a prop that has a power absorption curve crossing the SFC peak. Daryl Oster (c) 2006 all rights reserved. ETT, et3, MoPod, "space travel on earth" e-tube, e-tubes, and the logos thereof are trademarks and or service marks of et3.com Inc. For licensing information contact: POB 1423, Crystal River FL 34423-1423 (352)257-1310, et3@et3.com , www.et3.com > -----Original Message----- > Date: Wed, 30 Aug 2006 01:25:20 +0000 > From: "bob england" <bob_england@hotmail.com> > Subject: [PUP] Over propping > To: passagemaking-under-power@lists.samurai.com > Message-ID: <BAY105-F441DDAC7B326CC3F73CAF943E0@phx.gbl> > Content-Type: text/plain; format=flowed > > I realize that we all drive cars, and overdrive is the greatest thing > since > sliced bread, but, it don't work with a boat. Nothing screams "I don't > have > a clue" more than overpropping a boat, especially on purpose. A diesel > engine burns x amount of fuel to produce x amount of power, rpm's makes > little difference except for a little more drag on the reciprocating parts > in the engine. An overloaded engine on the otherhand, burns more fuel to > do > the same work at a lower rpm than one spinning faster but not overloaded, > and the heavier loaded, lower rpm diesel engine will wear much faster. > Overdrive does not work in boats.