IS USING JUST ONE ENGINE FALSE ECONOMY

Gludy I'm struggling here with your physics. Can you not see how utterly wrong your original statement is? ("It takes the same energy to turn ... a prop using an engine at say 500 rpm as it does to turn a loose prop at 500 rpm. So a dragged prop turning at X speed needs the same energy as driving one at X speed"). Nothing could be further from reality, you are miles off base with that statement. Agree?

Your "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" is not correct. What energy? The only energy beyond the friction is the foil drag of the prop blade. Nothing else. What you are saying implies that a glider wastes a load of its energy stores (being potential energy, due to gravity) in addition to overcoming the foil drag of its wings and the general fuselage friction. Well, no it doesn't

And no i dont agree at all "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"

First, you mean power not energy. But the answer is that, ignoring prop losses etc, it would take 100% of X power to drive the boat on one engine at 8 knots, just as it took X power to drive it on two engines at 8 knots. Indeed it would take X power to do 8 knots even with 100 engines. I think what you meant to say was if it takes X power at 8knots on 2 engines, then it would take say 95% or 105% of X on one engine, with the other dragging. Yes that could be true true, but it doesn't prove your theory at all. I'd say (ignoring the stall point) that it takes power of say 105% X with one engne running and the other windmilling, and 106%X if you apply the shaft brake ignoring engine inefficiency, but including prop drag. If you include engine waste, the answers could be say 94% windmill 95% braked
 
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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.

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I missed your reference to idling speed. If it's agreed that you can't extrapolate indefinitely, at idling speed or any higher speed, I have no argument with your limited conclusion. I'm not really following the rest of your debate with jfm so shouldn't have jumped in - sorry.
 
Mapis, I screaming at my computer screen here. You can maybe hear me in Italy? :-)

you said assume 1, and i said then 2 is self evidently true.

If you change the pitch on the props and so the boat's speed and mpg change, those changes are entirely due to (i) an alteration in the power developed by the engine (at the same throttle setting) and (ii) efficiency and frictional matters, which you specifically told me to ignore (under "assumption 1", remember!)
 
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If it's agreed that you can't extrapolate indefinitely, at idling speed or any higher speed, I have no argument with your limited conclusion.

[/ QUOTE ]Oh yes, this is 100% agreed.
In fact, my conclusions are indeed limited - at max rpm I would have agreed with jfm on the x=y=z thing.
And no need to be sorry btw - isn't debating all what forums are about? /forums/images/graemlins/smile.gif
 
Deleted User. The governor definitely controls the amount of fuel being injected. The idle and max revs mechanical stops decide high and low revs and the governor adjusts fuelling to suit. That is why you can over-prop an engine and it will not smoke in the intermediate rev range, only at max throttle when the governor is injecting more fuel and can't understand why the revs are not responding.
 
This is a very interesting thread.

I have a question to pop into the discussion.

I like to take a step back and look at the big picture.

If you consider a scenario of a single engine driving a windmilling prop.

If there is energy being absorbed by the windmilling prop - where to god is it going?

Surely, it can only go in turbluance and heat plus a little extra drag on the hull/rudders etc. If this is so, then the 64000 dollar question is does the waste energy amount to more or less than the extra energy required to run the overhead of the second engine.
 
I'm not saying it does'nt just that on older engines, the governor only controlled idle and max speeds, that's all otherwise I agree with you
 
So where are we:

1. Received wisdom is that a windmilling prop has more drag than a stopped prop

2. Empirical evidence of a number of posters (including me) is that at a given slow speed (well below hull speed), fuel consumption is less with one engine than with two.

3. However, other empirical evidence (Piers at least) is that at a higher speed (approaching hull speed), fuel consumption is less with two engines than with one.

I don't see any inconsistency between 2 and 3. We know it takes relatively little energy (or should that be power) to move even a heavy boat through the water at low speed. So isn't it quite likely that one engine is more economic than two at (boat) speed range up to (say) 70% of hull speed (drag from windmilling prop is negligible) but two engines more economic than one at higher speeds (drag from windmilling prop becomes more significant)? (Ignoring other factors for the moment).

JFM has explained (I think) that if the prop has stalled there is less drag than if it is windmilling. So can we reconcile 1 by observing: (i) that the received wisdom is likely to come from (mostly) yachties who are concerned about reduction in boat speed from a windmilling prop; and (ii) the boat speed of a yacht (combined with prop pitch) at which the effect of a windmilling prop is more likely to be a concern is when the boat is well powered up in a good breeze, so not especially close to stall speed, therefore 'stopping' the prop is better than leaving it windmilling. At lower boat speed, the prop would be stalled anyway, because of shaft resistance/friction, so whether its stopped or windmilling makes no perceptible difference.

Does that make sense?
 
That's precisely the question Mike, in deciding 1 or 2 engines. At low speeds, in a fast boat with a windmilling prop the fuel saved on the second engine overhead is not all wasted on the turbulence (heating and kinetic energy in the water), at least on some boats mine included. Other boats might go the other way eg Piers Fleming

The question of windmill vs shaft brake is of course a separate question!
 
I agree 2 and 3, makes perfect intuitive sense.

I didn't say "if the prop has stalled there is less drag than if it is windmilling". What I said was if the prop has stalled there is less drag than if it is windmilling and just about to stall. If it is windmilling and nowhere near a stall (eg a very fast mobo with coarse props but chugging at 10knots) it will have less drag than if you apply the shaft brake

It is only at the about-to-stall point that you get the step change in drag. Hence Nonitoo's ship-driver story above.

I didn't follow your yachtie analysis. Two things are basically relevant to stalling: A finer pitch prop will stall at slower boat speed than a coarse prop all other things being equal, and of course stall occurs as the boat speed increases and/or the prop shaft is braked. Generally in a yacht, the prop might stall at say 9 knots not 4, very approximatley. So in a good breeze and in a yacht with a fine pitch prop, it makes sense to brake the prop shaft becuase by so doing you might induce a stall and lower your drag

At lower boat speed I dont know if it would be stalled. With only a little drivetrain friction it might well be nowhere near stalled, in which case you would increase drag if you braked the shaft. Better to windmill. The only thing that will make a prop foil stall at low hull speed is either a very fine pitch or a lot of friction in the drive train
 
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an alteration in the power developed by the engine (at the same throttle setting)

[/ QUOTE ]Aha, bingo. This statement made me realise what the reason for our misunderstanding is. Not the changes in frictional matters, mind, I'm aware that there are some, but hey, how much of it for 1" difference in pitch at the same rpm?
The point is another one. Reading back your posts, you actually never said that the engines deliver, at any given speed, their max power (which in turn would have been the power required by the hull).
But that's what I understood for some reason - hence I envisaged the engines stopping because of the longer prop.
On the other hand, I did say that the engines deliver more power than the hull needs, but what I had in mind is that they can deliver more than that, at least at low-mid rpm.
Which was shown in the figures I posted re. prop demand vs. max power curves, btw.
There's also another figure which explains what I meant: the fuel consumption expressed in lb/bHp-hr.
That is in fact inversally related to the rpm (the lowest the rpm, the higher the consumption - except at the very top rpm). Even more so in prop demand, rather than in max power curves.
Which shows why a single engine running at mid rpm is more efficient than two of them running at a lower rpm, for a given total power - which is also pretty intuitive, actually.

PS: I hope we can agree now, I did hear some weird noises mixed with the strong N wind we have at the moment, can you really scream that loud? I'm scared... /forums/images/graemlins/grin.gif
 
I need to break this up into little bits.

I understand "a finer pitch prop will stall at a lower boat speed than a coarse prop".

I also understand "stall occurs as ... the prop shaft is braked".

But how does the above fit with "stall occurs as the boat speed increases"?
 
""stall occurs as the boat speed increases" is becuase of drivetrain friction.

Imagine a fully frictionless drivetrain, and a windmilling prop (cos the other engine is driving the boat) with infinitely thin blades. In this state, the prop is nowhere near stall whether the boat is doing 1 knot or 100knots. The prop spins at almost exactly* an rpm which matches the forward speed of the boat, ie it screws perfectly through the water leaving no wake

The flow of water over the blades (becuase they're spinning at the perfect speed) is ~parallel to the blade section, ie nearly zero angle of attack. In aero terms, that is the opposite of stalled

(* not quite, becuase you need a foil affect to make the prop spin at all, so there is always a slight angle of attack even in this perfect state)

Now introduce friction. Apply the prophaft brake, gradually more and more. The prop slows but the boat speed stays same. What this does (if you think of the prop blade as an aeroplane wing, which it is at any fixed radial line, though overall it is helical so much more complex maths than a plane wing) is it increases the angle of attack of the blade-water. Like rotating the nose up on a plane

There comes a point when the angle of attack is so much (at a guess, say 15 degrees at high speed) that you get a stall and lose all laminar flow over the blade. That's the step change point that Nonitoo's ship captain just didn't want to reach, and the point at which Gludy's theorem holds good, ie you should brake the shaft.

But the point is, this stall can only happen cos of drivetrain friction. Nothing else is going to create the required angle of attack of the prop blade to the water to induce a stall. If there were no friction, there would be no stall. In general in a boat the drivetrain friction increases as RPM increases, non linearly, so as the boat goes faster it's like turning up the shaft brake more than you are turning up the "lift" effect on the blades that is making the prop spin. Hence as you go faster you reach a point where the shaft brake effect on the windmilling prop makes it stall as described above. Where you reach that point is unique to the boat set up, but you reach that point at slower speed on a fine pitch prop boat than a fast mobo
 
Yes, there is a more than one value for power developed by an engine even at a fixed throttle setting. I mean you can fix the amount of fuel and air burnt per power stroke but vary the power developed by the engine.

you wrote "1 inch difference in pitch at the same rpm" but it is not possible to have the same rpm unless you make the engine develop more power. If you make the power constant, you MUST drop the rpm if you have increased the prop pitch. To get the same rpm you must have increased the throttle and fuel burn, hence you are not comparing like with like anymore

I think we agree. I'm still x=y=z over here! /forums/images/graemlins/grin.gif
 
I have 2 x D4-225 on a 37ft sportscruiser. On inland waterways, I sometimes run on one engine.

Speed decreases eg from 8 to 7.2 knots whereas one engines uses 8 litres and the other one about .5 at 7.2 knots. Whoever says that's not a saving ... but then again if it's only for 15 minutes then total saving in litres may be neglected. If it's for hours, then story is a little different.
 
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I think we agree. I'm still x=y=z over here! /forums/images/graemlins/grin.gif

[/ QUOTE ]'Salright.
And I'll be boating instead of trolling around here tomorrow morning, forecast gives 27°, sunny, wind F2/3.
To heck with engine consumption, mohitos here I come!! /forums/images/graemlins/tongue.gif
 
Me too! I'm hoping for Sunday-Tuesday on the boat. Forecast is good. Yes, to hell with fuel, I'll be two engines, two props spinning :-)

Hey Mapis, post some pics of boat and your cruisng location on here - would be nice to see some italian coast on here
 
No........all diesels have governors...You set the required speed at 3000 RMP the governor will ensure you get it....as the hull gets fouled up in the Autumn or if you are towing a buddy back to port then the governor will have to give more fuel to reach that 3000 RPM because the load has increased also the slip!
 
Agree with Piers. Exactly the same as for our Nimbus. We played around on trip to Paris. Running one engine actually used fractionally more fuel than running two to achieve the same boat speed.

I think Piers comments about rudder/boat angle are also correct.

So although there was nothing to be saved by running one engine we did run on one engine to save engine hours. We switched engines every two hours.
 
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