T&T: Engine efficiency and overpropping

[Robin Brueckner]

Bob posted:" If a diesel engine is rated to turn 2500 rpm and we overload it
to the point
of only making 1550, then reset the governor to 1550 rpm, the engine is still
overloaded. Also, those exhaust temp numbers sound way low to me, even for a
DD, possibly his temp sender is in the wrong place ?" Such an engine isn't
"overloaded" at any RPM. (But it could be above the reduced 1550 reduced RPM
setting.) Compare engine manufacturer "Rated shaft HP" curves for any marine
diesel with the "Propeller HP" curve: you'll see the engine is providing only
a small fraction of the HP available until higher RPM and WOT where the curves
intersect and loading is maximum...Reasonable overpropping  uses some of this
available HP but there IS a downsside:loss of maximum RPM (HP) and with it
higher speeds. Manufacturer WOT is a compromise which does not emphasize fuel
efficiency. Overloading would be evident from overheating coolant and higher
than permitted turbo exhaust temperatures among other effects.  I can't offer
any real insight on the exhaust temp: the owner/mechanic said thatabout 575
degrees should be enough to insure clean fuel burn... Rob Brueckner1972
Hatteras YF
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