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post #30 of Old 04-18-2013
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Re: Overpropped; What to do?

Originally Posted by sony2000 View Post
Lets say an overpropped engine lowers it's WOT rpm, but brings its operating RPM much closer to this lower WOTrpm, the result is a higher fuel consumption and greater wear per hour.
Time to destination, usually suffers if you are trying for the greatest MPG.
Detonation blows head gaskets.
I was listing a number of reasons to counter the statement that " to get maximum efficiency we should be overpropped". There are many maximums and some are opposites.
Sony2000 please be clear that I am not having a go at you personally but you are making statements here which are not correct.

All the following relates to a typical sailboat auxiliary normally aspirated diesel engine.

First of all you talk about WOT this usually means a wide open throttle. I do not like this used in reference to a diesel engine. Typically a throttle is a butterfly valve in the inlet manifold which coontrols the air flow into the engine. Diesel engines do not have them. The inlet manifold is not restricted in any way.

The engine speed lever connects to the governor fitted to the fuel injection pump. The governor controls the amount of fuel pumped into each cylinder. If the engine rpm differs from the one set by the engine speed lever then the governor either reduces the fuel supply or increases it to attempt to match the RPM.

Lets say you have a prop which allows an engine to achieve its maximum governed RPM . This will be beyond the point at which the engine generates maximum power.

With 'more' prop you might have a situation where at ' full fuel' the engine would attain the rpm at which it develops maximum power. This would be desirable if you were power boat racing.

With even 'more' prop you get to a situation where at ' full fuel' the engine would just attain the rpm at which it develops maximum torque. This is the rpm at which the engine is most fuel efficient. The engine wear rate will be less too. These are basic engineering facts based both on simple physics as well testing in real life situations.

In practice on a sailboat you would not select a prop which held the engine down to the rpm matching peak torque in flat water with no headwind. Normally it will be somewhere between peak power and peak torque.

Detonation. Basically this is not something that effects diesel engines. See

A supercharged petrol engine running lean with high manifold pressure and high cylinder head temp will be close to detonation.
The prop pitch can also be changed by cutting of equal amount of the blade, but that is a road far less travelled

No the prop pitch can not be changed by trimming the blade. If you reduce the diameter the pitch remains the same. If you make the blades narrower then by removing some of the leading and trailing edges, the pitch remains the same. If you were to make the blade narrower by trimming only the trailing edge you would get a very tiny change in pitch say 1%.

Oh yes I better enter a couple of qualifications on the above just in case we have any serious diesel heads around.

The governor type I refer to is the constant speed type found on almost all modern engines. If you have an old Gardiner you have something different.

If you have a pneumatic governor then you do have a throttle butterfly in the inlet manifold. I think the last one I saw outside the lab. was on a Merc 200D from the late 60s. They are an invention of the devil as most failures lead to an engine overspeed [ runaway ] which is pretty alarming. I used to create one in the lab. Students would run for cover. Anybody out there got one?

Gahh I used to teach and research in this stuff and spent hours running diesels on dynos. Retired now. No more trying to fill empty heads.

Last edited by TQA; 04-18-2013 at 01:06 PM.
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