jfm
Well-Known Member
Yes, for the reasons given by Observer. you can get a whole range of lirtes per hour at 3000rpm or any fixed rpm number. My neutral example is a perfectly valid example - it's just one end of the spectrum!
IS USING JUST ONE ENGINE FALSE ECONOMY
- Thread starter robyonfrome
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Piers
Well-Known Member
I thought a stopped prop was well proven to be less darg than a windmilling one. There must be some papers on it in the ether. Let's go looking.
jfm
Well-Known Member
It is indeed bollox Deleted User! Older engines use a mechanical governor which adjusts the fuel flow rate to maintain rpm. So as you load up the engine (at a fixed rpm) more fuel is squirted in. It is NOT the case that fuel squirt-in rate is a function of rpm
jfm
Well-Known Member
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The resistance (and hence power required) to spin a windmilling prop at 500rpm is the friction etc in the drive.
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The whole point is that it is also having to move the water out of the way of the spinning prop and this is what a driving prop does.
There is a lot more to force needed to spin a free prop than overcoming the frictional drag of the shaft. The forces on the spinning prop induced by the fact it is in water are very high.
Agreed?
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I very much disagree, and know it to be wrong. You are correct that the load on a windmilling prop is not only the gearbox/driveshaft friction. There is also drag from the blade foil itself, becuase it is acting as a windmill and so must have a (small) angle of attack to the water. The energy involved is manifested by a warming up of the water. But this is a small amount and I included it in the term "friction in the drive" becuase it wasn't worth itemising it separately. Your statement "a dragged prop turning at X speed needs the same energy as driving one at X speed" is still (wildly) incorrect
[ QUOTE ]
The resistance (and hence power required) to spin a windmilling prop at 500rpm is the friction etc in the drive.
[/ QUOTE ]
The whole point is that it is also having to move the water out of the way of the spinning prop and this is what a driving prop does.
There is a lot more to force needed to spin a free prop than overcoming the frictional drag of the shaft. The forces on the spinning prop induced by the fact it is in water are very high.
Agreed?
[/ QUOTE ]
I very much disagree, and know it to be wrong. You are correct that the load on a windmilling prop is not only the gearbox/driveshaft friction. There is also drag from the blade foil itself, becuase it is acting as a windmill and so must have a (small) angle of attack to the water. The energy involved is manifested by a warming up of the water. But this is a small amount and I included it in the term "friction in the drive" becuase it wasn't worth itemising it separately. Your statement "a dragged prop turning at X speed needs the same energy as driving one at X speed" is still (wildly) incorrect
Piers
Well-Known Member
Looking on the net, one commentary on a stopped prop says,
"If the propeller is not windmilling the drag is 0.5*rho*V^2*projected area of the propeller.
"When the propeller is allowed to windmill the velocity would be the net velocity vector (of prop tangential speed and water speed), and the area would be that projected into this vector.
"To calculate drag you need to take the component in the forward direction. So it would indeed turn out that the drag force is greater when the propeller windmills."
Another comment is,
"I suppose, in simple terms, you could say that a locked three blade prop creates three very turbulent wakes, the total of which might be 80% of the area of the prop disc area. The windmilling prop might create one large turbulent wake, of perhaps 120% of the disc area."
(Comment taken from an engineering forum, almost word for word)
"If the propeller is not windmilling the drag is 0.5*rho*V^2*projected area of the propeller.
"When the propeller is allowed to windmill the velocity would be the net velocity vector (of prop tangential speed and water speed), and the area would be that projected into this vector.
"To calculate drag you need to take the component in the forward direction. So it would indeed turn out that the drag force is greater when the propeller windmills."
Another comment is,
"I suppose, in simple terms, you could say that a locked three blade prop creates three very turbulent wakes, the total of which might be 80% of the area of the prop disc area. The windmilling prop might create one large turbulent wake, of perhaps 120% of the disc area."
(Comment taken from an engineering forum, almost word for word)
jfm
Well-Known Member
You wont find that I bet Piers, cos i bet it doesn't exist. At least not as a univerally true thing. It is true in the stall analysis above, but that's a subset of real life situations.
Piers
Well-Known Member
I suspect you are spot on. Reading more on the subject, it looks like three camps exist. The mor or less drag, and the one that says each boat needs to be tested, given its own characteristics such as props, shafts, stern gear etc.
So perhaps it's a 'get it tested and see what the results are'. Now, where's that test tank?
So perhaps it's a 'get it tested and see what the results are'. Now, where's that test tank?
Observer
Well-Known Member
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At idle with both, I do around 4 kts. I will see what happens with one - I haven't tried this. If I still get 4 kts, then it would make sense that I burn less fuel on one at that speed. perhaps it would be to do with inefficiencies at idle?
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If you get 4kts with both engines in gear at idle speed you'll never get 4kts at same engine speed on one engine. Have to try (say) 4kts boat speed both engines (= idle revs) then whatever revs needed to produce 4 kts with one engine, and measure fuel flow for each. Probably can't be done if you don't have fuel flow sensor system.
At idle with both, I do around 4 kts. I will see what happens with one - I haven't tried this. If I still get 4 kts, then it would make sense that I burn less fuel on one at that speed. perhaps it would be to do with inefficiencies at idle?
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If you get 4kts with both engines in gear at idle speed you'll never get 4kts at same engine speed on one engine. Have to try (say) 4kts boat speed both engines (= idle revs) then whatever revs needed to produce 4 kts with one engine, and measure fuel flow for each. Probably can't be done if you don't have fuel flow sensor system.
Piers
Well-Known Member
...which agrees with the fact that Play d'eau burns more on one.
jfm
Well-Known Member
But what's the source of that Piers? The first comment is quite clearly nonsense. Just cos something is on a forum doesn't make it right /forums/images/graemlins/grin.gif
Imagine a freewheeling prop with zero friction in the shaft and infinitely thin blades. It will (subject to the foil point below) spin at the precise rpm that will cause the water flow to be parallel to the blades. The drag component in "the forward direction" (to use the term in your cited para) will be precisely zero, like a plane wing with zero angle of attack. So whoever wrote that has completely mixed up his directions of forces and fluid flow
Actually to make the prop spin there always has to be an angle of attack and hence some drag, just as there has to be on a plane wing even if the plane is infinitely light, the wing is infinitely thin and the wing area is infinitely large. But in this discussion the drag from the foil effect of a windmilling prop is smallish and that doesn't change the fact that the first writer you quote has completely mixed up the directions of forces and flow.
The second comment might be true but doesn't take us anywhere that I can see
Edit - I've just remebered you're a pilot. Feel free to trash my foils/wing analysis
Imagine a freewheeling prop with zero friction in the shaft and infinitely thin blades. It will (subject to the foil point below) spin at the precise rpm that will cause the water flow to be parallel to the blades. The drag component in "the forward direction" (to use the term in your cited para) will be precisely zero, like a plane wing with zero angle of attack. So whoever wrote that has completely mixed up his directions of forces and fluid flow
Actually to make the prop spin there always has to be an angle of attack and hence some drag, just as there has to be on a plane wing even if the plane is infinitely light, the wing is infinitely thin and the wing area is infinitely large. But in this discussion the drag from the foil effect of a windmilling prop is smallish and that doesn't change the fact that the first writer you quote has completely mixed up the directions of forces and flow.
The second comment might be true but doesn't take us anywhere that I can see
Edit - I've just remebered you're a pilot. Feel free to trash my foils/wing analysis
nonitoo
Well-Known Member
Thanks JFM good explanation.
Tom
Tom
jfm
Well-Known Member
I'm finding it hard to follow your thinking Mapis. And i don't know what the tut tutting is for!
But yes, assuming 1 to be correct, statement 2 is self evidently correct. That's exactly why x=y=z.
And, um, thanks for giving me another chance /forums/images/graemlins/confused.gif /forums/images/graemlins/grin.gif
But yes, assuming 1 to be correct, statement 2 is self evidently correct. That's exactly why x=y=z.
And, um, thanks for giving me another chance /forums/images/graemlins/confused.gif /forums/images/graemlins/grin.gif
MapisM
Well-Known Member
Hi Piers,
my boat is a 53' pure displacement trawler, and she also idles at 4kts or so (600rpm) on both engines.
As I said I never tried that properly, and I don't expect her to keep the very same speed on just one engine (that would mean almost 50% lower consumption, which is unrealistic), but almost.
Dunno, maybe 3.5 kts or so? Still relevant in terms of lower fuel burn, anyway.
But I'm just guessing now, it's not really a practical speed, aside maybe for fishermen.
Re. what that has to do with, sure there are many factors, but imho the most important one - which is where jfm disagreed - is that at these speeds the engines generate more power than the hull requires, hence cutting down one of them the percentage of power wasted is reduced on the remaining one.
my boat is a 53' pure displacement trawler, and she also idles at 4kts or so (600rpm) on both engines.
As I said I never tried that properly, and I don't expect her to keep the very same speed on just one engine (that would mean almost 50% lower consumption, which is unrealistic), but almost.
Dunno, maybe 3.5 kts or so? Still relevant in terms of lower fuel burn, anyway.
But I'm just guessing now, it's not really a practical speed, aside maybe for fishermen.
Re. what that has to do with, sure there are many factors, but imho the most important one - which is where jfm disagreed - is that at these speeds the engines generate more power than the hull requires, hence cutting down one of them the percentage of power wasted is reduced on the remaining one.
Gludy
Well-Known Member
JFM
I will concede that the reality is more complex than my simple explanation.
In reality the free prop would turn much slower than the driven prop but would still use up the energy in spinning that was well in excess of just the frictional loss on the shaft. - the only way to increase its speed up to the driven prop would be to input more energy into it and drive it up to the driven prop speed and you would then have just two engines turning.
I am saying that all else being equal lets say it takes X energy to drive a boat at 8 knots with two engines. Then (assuming the engines have the power) it would take more than 50 per cent of that energy to drive the same boat at 8 knots with one engine if one prop was left freewheeling.
Can we agree on that as a base for further discussion?
I will concede that the reality is more complex than my simple explanation.
In reality the free prop would turn much slower than the driven prop but would still use up the energy in spinning that was well in excess of just the frictional loss on the shaft. - the only way to increase its speed up to the driven prop would be to input more energy into it and drive it up to the driven prop speed and you would then have just two engines turning.
I am saying that all else being equal lets say it takes X energy to drive a boat at 8 knots with two engines. Then (assuming the engines have the power) it would take more than 50 per cent of that energy to drive the same boat at 8 knots with one engine if one prop was left freewheeling.
Can we agree on that as a base for further discussion?
jfm
Well-Known Member
Simon, only just read your post, I think it went up as I was typing the same theory into my (later) post on stalling blades, but you got there first! I agree with what you said; the answer to this counter-intuitive thing is I reckon blade stall theory.
MapisM
Well-Known Member
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yes, assuming 1 to be correct, statement 2 is self evidently correct. That's exactly why x=y=z.
[/ QUOTE ]I frankly do not see where the self evidence is. Let me make an example.
Assume to increase the pitch of both props on your boat - say one inch - all other things being equal.
What do you think would happen when idling?
In my theory, the boat will go slightly faster, with correspondingly better mpg.
In your theory, the engines should stop, because if previously they were working exactly on the borderline, they can't cope with the higher requirement anymore.
Should we arrange a test?
yes, assuming 1 to be correct, statement 2 is self evidently correct. That's exactly why x=y=z.
[/ QUOTE ]I frankly do not see where the self evidence is. Let me make an example.
Assume to increase the pitch of both props on your boat - say one inch - all other things being equal.
What do you think would happen when idling?
In my theory, the boat will go slightly faster, with correspondingly better mpg.
In your theory, the engines should stop, because if previously they were working exactly on the borderline, they can't cope with the higher requirement anymore.
Should we arrange a test?
Observer
Well-Known Member
Errr, no, actually. AIUI, you observed that you use more fuel on one engine at 8tks (84% of hull speed) than both. Have you tried the same at (say) 4kts (42% of hull speed). You will need higher revs with one engine (not idle speed) but I'm guessing you will find that the consumption of one engine at whatever speed is needed for 4kts (say 1000rpm) is less than both engines at idle (say 700rpm), which also gives 4kts.
Observer
Well-Known Member
[ QUOTE ]
[ QUOTE ]
yes, assuming 1 to be correct, statement 2 is self evidently correct. That's exactly why x=y=z.
[/ QUOTE ]I frankly do not see where the self evidence is. Let me make an example.
Assume to increase the pitch of both props on your boat - say one inch - all other things being equal.
What do you think would happen when idling?
In my theory, the boat will go slightly faster, with correspondingly better mpg.
In your theory, the engines should stop, because if previously they were working exactly on the borderline, they can't cope with the higher requirement anymore.
Should we arrange a test?
[/ QUOTE ]
That's a false conclusion. If you increase the pitch of the prop the boat will go faster IF there is sufficient torque to accelerate the prop to the set engine speed. However, it's clear there must be some degree of pitch at which torque will be insufficient, so you end up being overgeared, the engine labours and burns more fuel for lower speed.
[ QUOTE ]
yes, assuming 1 to be correct, statement 2 is self evidently correct. That's exactly why x=y=z.
[/ QUOTE ]I frankly do not see where the self evidence is. Let me make an example.
Assume to increase the pitch of both props on your boat - say one inch - all other things being equal.
What do you think would happen when idling?
In my theory, the boat will go slightly faster, with correspondingly better mpg.
In your theory, the engines should stop, because if previously they were working exactly on the borderline, they can't cope with the higher requirement anymore.
Should we arrange a test?
[/ QUOTE ]
That's a false conclusion. If you increase the pitch of the prop the boat will go faster IF there is sufficient torque to accelerate the prop to the set engine speed. However, it's clear there must be some degree of pitch at which torque will be insufficient, so you end up being overgeared, the engine labours and burns more fuel for lower speed.
MapisM
Well-Known Member
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If you increase the pitch of the prop the boat will go faster IF there is sufficient torque to accelerate the prop to the set engine speed.
[/ QUOTE ]Precisely. So, what do you think would happen in a Squadron 58 with 1" higher pitch props, would those engines have sufficient torque at idle speed?
What I'm saying is that, unless you keep the boat tied to the dock, they would for good.
If you increase the pitch of the prop the boat will go faster IF there is sufficient torque to accelerate the prop to the set engine speed.
[/ QUOTE ]Precisely. So, what do you think would happen in a Squadron 58 with 1" higher pitch props, would those engines have sufficient torque at idle speed?
What I'm saying is that, unless you keep the boat tied to the dock, they would for good.
D
Deleted User YDKXO
Guest
Are you sure it's bollox? All a simple mechanical governor does is control idle speed and max speed. AFAIK it does not control fuelling in the intermediate range. Agreed that more sophisticated governors do control fuelling relative to load and rpm throughout the rpm range