[PUP] Over propping

[Mark]

Bob, help me understand why this is the case.

As I read the engine and prop curves on: http://www.ossapowerlite.com/tech_library/fuel_efficiency/fuel_efficiency.htm
they say: "It turns out that by shifting our propeller curve upward, we operate in the peak efficiency range of the engine over a much greater power range. Comparing the new prop to the original one, we see that the fuel economy of the engine would improve by an average of 13% in the low and middle speed ranges."  

He does go on to say that there is a problem that you can damage the engine by exceeding the point at which you overload the engine.  However if one is willing to give up some high RPM operation for increased efficiency in the range in which the engine is used most often (or use a VPP) that seems reasonable, no? I think Beowulf used a VPP and they dialed up the prop until the exhaust temp indicated that the engine was properly loaded.

It alway seemed odd to me that the engine/prop system is optimized for operation at WOT.  I never have a run a boat at the engine red line. I've been told that cruising sailboats are routinely overpropped since they can't make use of the top end of the rpm range.

I'd appreciate some insight as to why those charts are wrong or I'm mis reading them.  

Thanks.


bob england <bob_england@hotmail.com> wrote: 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.
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