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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Overpropped; What to do?
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Thread: Overpropped; What to do? Reply to Thread
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Topic Review (Newest First)
04-20-2013 07:49 AM
tommays
Re: Overpropped; What to do?

NO NO NO

It is a motor with mechanical fuel injection which is THE problem being resolved

The mechanical fuel injection system will input excessive fuel based on throttle setting regardless of the RPM the motor makes

The extra fuel does bad things to motor and it is as simple as that


Any displacement hull will have a Huge prop slip number due to the need to NOT drag a bucket size prop through the water when sailing
04-19-2013 02:07 PM
sony2000
Re: Overpropped; What to do?

The sweet spot is hull speed and maximum torque without current and head winds. Resulting in a smaller engine to supply the needs.
Max torque is at a lower rpm on electric motors, diesels and then gasoline engines, in that order.
I've been thinking that Bombardiers new passenger jet, is gearing the turbines prior to the combustion turbines, in an effort to reach maximum torgue at the desired cruising speed.
Pitch is a combination of the angle of attack of the blade and its surface area. If you cut only its surface area, theen the blade will achieve a lesser distance thru the water than before it was cut.
04-18-2013 01:43 PM
TQA
Re: Overpropped; What to do?

Quote:
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.
Quote:
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.
04-18-2013 09:46 AM
sony2000
Re: Overpropped; What to do?

casey1999,
to repitch a prop the blades should be twisted when heated up. More of an angle or less of an angle. Then compare it to a jig of angles for that particular prop.
Suppose you have a 12X9 prop. The 12" diameter is measureable by you. The 9" means that in a tub of Jello, one rotation of the prop will advance it 9" in the Jello. Lessen the angle of the prop blade, and it only will go 7" for example.
The prop pitch can also be changed by cutting of equal amount of the blade, but that is a road far less travelled.
04-18-2013 01:15 AM
MedSailor
Re: Overpropped; What to do?

Quote:
Originally Posted by casey1999 View Post
How does a prop shop re-pitch a prop?
In the old days it was with a hammer and a jig. Now it involves something similar but I believe there is more jig and hydraulics and less hammering.

Obviously from my response, I've never worked at a prop shop, but one thing that might be relevant information is that for a well made prop in good condition one can usually have 3 inches of pitch added or subtracted. Trying to add or subtract more will depend on a lot of factors and may or may not be able to be done.

I had my 12" prop changed to a 9" and now it's in the shop again to be changed to a 10". (turns out the perfect sweet spot is probably 9.6" but I want 10").

If you want to change your 22" pitch prop to a 9" you'll be out of luck.


MedSailor
04-17-2013 11:22 PM
sony2000
Re: Overpropped; What to do?

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.
04-17-2013 09:53 PM
OPossumTX
Re: Overpropped; What to do?

Most "prop shops" have a "bargain board" of props they have taken in trade or salvaged from damaged props left by customers. Often you can get very good deals swapping for something on the "board". It is also a great place to pickup a spare if you are inclined to keep such unnecessary clutter about.
04-17-2013 08:53 PM
TQA
Re: Overpropped; What to do?

Quote:
Originally Posted by sony2000 View Post
In quoting me you dropped the word "overall". This term is to encompass efficiency related to
1. maximum range on available fuel.
2. a level of RPM that causes above normal wear on the engine.
3. time to destination.
4. maximum torque.
5. hull speed.
An over propped engine ends up operating at a higher manifold pressure, resulting in a more frequent, destruction of head gaskets.
I am confused here. You talk about manifold pressure. Now this is relevent to a superchrged or turbocharged engine but NOT a normally aspirated diesel as there is no form of restriction to the inlet manifold opening; it runs wide open all the time.

Also I do not see how time to destination figures into this. I wonder if you are using data from an aircraft fitted with supercharged petrol engine fitted with a device to control mixture strength and boost pressure plus a variable pitch prop.

Finally in my experience almost all cylinder head gasket failures can be traced to engine overheating from coolant failure, some are corrosion related.
04-17-2013 07:04 PM
casey1999
Re: Overpropped; What to do?

How does a prop shop re-pitch a prop?
04-17-2013 04:59 PM
sony2000
Re: Overpropped; What to do?

In quoting me you dropped the word "overall". This term is to encompass efficiency related to
1. maximum range on available fuel.
2. a level of RPM that causes above normal wear on the engine.
3. time to destination.
4. maximum torque.
5. hull speed.
An over propped engine ends up operating at a higher manifold pressure, resulting in a more frequent, destruction of head gaskets.
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