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.

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.

Passagemaking-Under-Power Mailing List

Sorry for my delayed response, been fishing. As it was explained to me by a
detroit diesel engine specialist that took care of oilfield pumps, some
small some had 16v154's, (pumps are very similar in power useage to full
disp. boats) it is the leverage ratio of the rod on the crankshaft
multiplied by time that determines load, which is a product of cylinder
pressure. Lower rpm means more leverage is excerted on the crankshaft to
produce the same amount of power that could be made at a higher rpm. What
compounds this is that at lower rpm cylinder pressure remains higher longer,
which increases the leverage even more. I think what we usually see in
boats, even full disp. boats, is that they are way overpowered for the
maximum speed they can make. Turbos compound the problems. Ideally, the cpp
and a pyrometer is the best approach. It can be tailored to every situation
and not overload an engine. For longest engine life (not neccesarily best
fuel economy) the engine needs to be able to get pretty close to rated rpm.
Doing so will guarranty that it is not overloaded. It's just a good rule of
thumb.

From: Mark mark424x@yahoo.com
Reply-To: Passagemaking Under Power
Listpassagemaking-under-power@lists.samurai.com
To: passagemaking-under-power@lists.samurai.com
Subject: Re: [PUP] Over propping
Date: Wed, 30 Aug 2006 14:43:24 -0700 (PDT)

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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