Piers
Well-Known Member
Hi Daka - From memory, even Passage Maker had no reference to one engine buring less than two for a given speed. So agreed.
IS USING JUST ONE ENGINE FALSE ECONOMY
- Thread starter robyonfrome
- Start date
jfm
Well-Known Member
Yup I know that heat "balances the books" but that doesn't support your analysis. I still say x=y=z, ignoring the frictional stuff
jfm
Well-Known Member
[ QUOTE ]
is the revs / fuel consumption relationship fixed on any given engine ? What I mean is, if an engine uses 10lph at 1000 revs, can those numbers fluctuate
[/ QUOTE ]
This is an important point. It is absolutely not fixed. If an engine uses 10lph at 1000revs per the manufacturer's curve, that data is only valid for certain crankshaft torque, invariably loaded. If you spin the engine 1000rpm in neutral, it will burn just a fraction of 10lph. So yeah, the numbers can fluctuate loads, which is a big factor in this debate (and is the root of the disagreement between me and Mapis on the x/y/z thing)
is the revs / fuel consumption relationship fixed on any given engine ? What I mean is, if an engine uses 10lph at 1000 revs, can those numbers fluctuate
[/ QUOTE ]
This is an important point. It is absolutely not fixed. If an engine uses 10lph at 1000revs per the manufacturer's curve, that data is only valid for certain crankshaft torque, invariably loaded. If you spin the engine 1000rpm in neutral, it will burn just a fraction of 10lph. So yeah, the numbers can fluctuate loads, which is a big factor in this debate (and is the root of the disagreement between me and Mapis on the x/y/z thing)
Observer
Well-Known Member
Yes - but my experience was based on "substantially-less-than-hull-speed" - in my case <5kts against hull speed of ~7.5-8kts. It seems quite plausible to me that at close to hull speed two will be better than one, but at much less than hull speed, one will be better than two.
mjf
Well-Known Member
Good point on the rudder drag and sideways movment throu the water.
On the odd occassion i have run up our approach channel on one engine (to even up the hrs) I was surprised at the amount of helm needed and the odd 'leeway' progress made.
On the odd occassion i have run up our approach channel on one engine (to even up the hrs) I was surprised at the amount of helm needed and the odd 'leeway' progress made.
jfm
Well-Known Member
Gludy i dont dispute that a windmilling prop drag> braked prop drag, but it is absolutely not for the reasons you give.
"It takes the same energy to turn drive 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" is self evidently (wildly)wrong. The resistance (and hence power required) to spin a windmilling prop at 500rpm is the friction etc in the drive. The resistance to spin the 500rpm driving prop is the frictional load of pushing the boat at 6kts. At a detailed level this difference is manifested by a much larger delta between the water pressure on the front/back faces of the blades of the driving prop, compared with the windmilling prop.
Not disagreeing the answer, just how you get there (And not agreeing the answer either - I find it VERY counter intuitive to say that a windmilling prop has more resistance than a braked one, especially with high-pitch powerboat props. Obviously if the pitch were infinite, ie a jabsco pump impeller, it would not be true to say windmilling has more resistence. So if this counter-intuitive statement is true, there must be some point, some level of pitch, where there is a crossover and the windmilling prop has less drag than the fixed prop)
"It takes the same energy to turn drive 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" is self evidently (wildly)wrong. The resistance (and hence power required) to spin a windmilling prop at 500rpm is the friction etc in the drive. The resistance to spin the 500rpm driving prop is the frictional load of pushing the boat at 6kts. At a detailed level this difference is manifested by a much larger delta between the water pressure on the front/back faces of the blades of the driving prop, compared with the windmilling prop.
Not disagreeing the answer, just how you get there (And not agreeing the answer either - I find it VERY counter intuitive to say that a windmilling prop has more resistance than a braked one, especially with high-pitch powerboat props. Obviously if the pitch were infinite, ie a jabsco pump impeller, it would not be true to say windmilling has more resistence. So if this counter-intuitive statement is true, there must be some point, some level of pitch, where there is a crossover and the windmilling prop has less drag than the fixed prop)
nonitoo
Well-Known Member
If you intend using one engine as opposed to two for extended periods then ideally you should have inward turning screws (port screw right handed and starboard screw left handed).
This greatly reduces the 'crab' effect on the hull and the amount of correctional helm necessary to maintain a course and was a standard feature of many warships designed for extended patrols at slow speed while on station (at the cost of close quarter manoeuvring).
In addition with the inward turning screw configuration it does not matter which engine you stop - the transverse thrust of the working screw always tends to counter the off centre turning effect and thus the amount of correctional helm needed.
Conversely, merchant vessels always used outward turning twin screws (when fitted) to facilitate manoeuvring by utilising the turning effect of having both screws' transverse thrust working together (one ahead and one astern) to tighten the turn both to port and starboard.
As an aside, it is a fact that on a ship, slowing from full speed to dead slow is achieved more efficiently by slowing the engine speed rather than stopping it. Never understood why but it works (only a simple sailor rather then a naval architect).
I am enjoying this interesting discussion.
Tom
This greatly reduces the 'crab' effect on the hull and the amount of correctional helm necessary to maintain a course and was a standard feature of many warships designed for extended patrols at slow speed while on station (at the cost of close quarter manoeuvring).
In addition with the inward turning screw configuration it does not matter which engine you stop - the transverse thrust of the working screw always tends to counter the off centre turning effect and thus the amount of correctional helm needed.
Conversely, merchant vessels always used outward turning twin screws (when fitted) to facilitate manoeuvring by utilising the turning effect of having both screws' transverse thrust working together (one ahead and one astern) to tighten the turn both to port and starboard.
As an aside, it is a fact that on a ship, slowing from full speed to dead slow is achieved more efficiently by slowing the engine speed rather than stopping it. Never understood why but it works (only a simple sailor rather then a naval architect).
I am enjoying this interesting discussion.
Tom
MapisM
Well-Known Member
[ QUOTE ]
on our Fleming, travelling at a displacement speed of 8kts (hull speed is 9.5kts), we burn more fuel on one than on two (to keep 8 kts).
[/ QUOTE ]Good point, but you're now looking at this matter from a different perspective.
A hull built for low(ish) speed is in a different league compared to a planing boat used at hull speed.
I am no surprised at all to understand that your fine boat at displacement speed burns less fuel with both rather than just one screw, because - being much more optimized for those speeds compared to a planing boat - she's proportionally more affected by the negative effects which you mentioned, than she is by the advantage of feeding less cubic inches.
I never made any proper test, but I guess that my boat would behave very similarly at those speeds. Even more so if approaching max hull speed - actually I doubt that I could reach 9kts on one engine alone, in spite of the fact that each engine can produce 350 max hp, whilst it takes overall just 120 hp or so (60+60) to keep her happily cruising at 9kts/24lph.
On the other hand, the lower the speed, the less those negative effects are relevant. Again, I never made specific tests on that, but I'd bet that both mine and your boats would burn less on one engine, if trolling at 4 or 5 knots.
on our Fleming, travelling at a displacement speed of 8kts (hull speed is 9.5kts), we burn more fuel on one than on two (to keep 8 kts).
[/ QUOTE ]Good point, but you're now looking at this matter from a different perspective.
A hull built for low(ish) speed is in a different league compared to a planing boat used at hull speed.
I am no surprised at all to understand that your fine boat at displacement speed burns less fuel with both rather than just one screw, because - being much more optimized for those speeds compared to a planing boat - she's proportionally more affected by the negative effects which you mentioned, than she is by the advantage of feeding less cubic inches.
I never made any proper test, but I guess that my boat would behave very similarly at those speeds. Even more so if approaching max hull speed - actually I doubt that I could reach 9kts on one engine alone, in spite of the fact that each engine can produce 350 max hp, whilst it takes overall just 120 hp or so (60+60) to keep her happily cruising at 9kts/24lph.
On the other hand, the lower the speed, the less those negative effects are relevant. Again, I never made specific tests on that, but I'd bet that both mine and your boats would burn less on one engine, if trolling at 4 or 5 knots.
D
Deleted User YDKXO
Guest
[ QUOTE ]
This is an important point. It is absolutely not fixed. If an engine uses 10lph at 1000revs per the manufacturer's curve, that data is only valid for certain crankshaft torque, invariably loaded. If you spin the engine 1000rpm in neutral, it will burn just a fraction of 10lph. So yeah, the numbers can fluctuate loads, which is a big factor in this debate (and is the root of the disagreement between me and Mapis on the x/y/z thing)
[/ QUOTE ]
A bit out of my depth here but does that not depend on the type of engine? An older design with mechanically driven fuel pump would deliver the same amount of fuel to the cylinders at a set rpm irrespective of load. Some would get burnt and the unburnt fuel would be exhausted but the fuel consumption would remain the same. Whereas for a modern electronically controlled engine, the fuel is metered according to load and fuel consumption at a particular rpm would depend on load. Or is this bollox?
This is an important point. It is absolutely not fixed. If an engine uses 10lph at 1000revs per the manufacturer's curve, that data is only valid for certain crankshaft torque, invariably loaded. If you spin the engine 1000rpm in neutral, it will burn just a fraction of 10lph. So yeah, the numbers can fluctuate loads, which is a big factor in this debate (and is the root of the disagreement between me and Mapis on the x/y/z thing)
[/ QUOTE ]
A bit out of my depth here but does that not depend on the type of engine? An older design with mechanically driven fuel pump would deliver the same amount of fuel to the cylinders at a set rpm irrespective of load. Some would get burnt and the unburnt fuel would be exhausted but the fuel consumption would remain the same. Whereas for a modern electronically controlled engine, the fuel is metered according to load and fuel consumption at a particular rpm would depend on load. Or is this bollox?
gjgm
Well-Known Member
This is an important point. It is absolutely not fixed. If an engine uses 10lph at 1000revs per the manufacturer's curve, that data is only valid for certain crankshaft torque, invariably loaded. If you spin the engine 1000rpm in neutral, it will burn just a fraction of 10lph. So yeah, the numbers can fluctuate loads,
ok sure, but I wasnt really considering trying to run the boat in neutral ! Let me ask another way...if you run your boat at 3000revs, can the fuel consumption (significantly) change, and if so, by what causes?
I realise of course that sea conditions,junk on board etc might mean that the speed varies, but will the fuel consumption at those constant revs? Would the torque really change that much.. I mean you arent accelerating, for example
ok sure, but I wasnt really considering trying to run the boat in neutral ! Let me ask another way...if you run your boat at 3000revs, can the fuel consumption (significantly) change, and if so, by what causes?
I realise of course that sea conditions,junk on board etc might mean that the speed varies, but will the fuel consumption at those constant revs? Would the torque really change that much.. I mean you arent accelerating, for example
D
Deleted User YDKXO
Guest
Thanks for that, Nonitoo. That's interesting
Gludy
Well-Known Member
Good points JFM but I disagree.
[ 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 ]
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?
Observer
Well-Known Member
Drive a car at 3000rpm in any given gear on a flat road and fuel consumption will be far lower than the same rpm and same gear on an incline. The wheel/road speed will be the same.
Must be the same (more complex) with a boat. Flat seas = lower fuel burn, big seas = more fuel burn.
Presumably it's about the amount of work the engine is being asked to do, not simply the speed at which it's spinning the prop.
Must be the same (more complex) with a boat. Flat seas = lower fuel burn, big seas = more fuel burn.
Presumably it's about the amount of work the engine is being asked to do, not simply the speed at which it's spinning the prop.
MapisM
Well-Known Member
[ QUOTE ]
I know that heat "balances the books" but that doesn't support your analysis.
I still say x=y=z, ignoring the frictional stuff
[/ QUOTE ]Tut, tut.
You surely knew that, but didn't seem to consider it when you said that X, Y and Z "absolutely MUST be" the same number, else I would be "re-writing the laws of physics and thermodynamics".
Anyway, back to the point. Am I correct in understanding that what you're saying is:
1) let's assume that there are no differences in frictional losses and other side effects due to asymmetrical thrust etc. (as correctly reported by Piers).
2) Under this assumption, the power required by a hull to keep a given speed is always equal to the power actually generated by the engine(s) - regardless of their number, power curves, etc.
If my understanding above is wrong, care to better explain what else do you actually mean?
Alternatively, if my understanding is correct, I'll give you another chance to rethink your x=y=z equation...
...Mind, that's just because it's friday and I feel generous, having a boating weekend ahead /forums/images/graemlins/smile.gif
I know that heat "balances the books" but that doesn't support your analysis.
I still say x=y=z, ignoring the frictional stuff
[/ QUOTE ]Tut, tut.
You surely knew that, but didn't seem to consider it when you said that X, Y and Z "absolutely MUST be" the same number, else I would be "re-writing the laws of physics and thermodynamics".
Anyway, back to the point. Am I correct in understanding that what you're saying is:
1) let's assume that there are no differences in frictional losses and other side effects due to asymmetrical thrust etc. (as correctly reported by Piers).
2) Under this assumption, the power required by a hull to keep a given speed is always equal to the power actually generated by the engine(s) - regardless of their number, power curves, etc.
If my understanding above is wrong, care to better explain what else do you actually mean?
Alternatively, if my understanding is correct, I'll give you another chance to rethink your x=y=z equation...
...Mind, that's just because it's friday and I feel generous, having a boating weekend ahead /forums/images/graemlins/smile.gif
gjgm
Well-Known Member
yes, I see your point, but presumably the boat/prop slips more than a care tyre.Does the energy required to turn the prop really vary that much at a given, set,revs?I can see it will alter, but is it significant in terms of lph (not in terms of mpl)
scottie
Well-Known Member
A word of warning in some instances when running on one engine for longer periods (ie running overnight on a pssage at displacement speeds) it is posible to get a water flow into the other engine if the seacocks have fwd facing scoops and the water gets past the pump and fills the exhaust and then the engine
Observer
Well-Known Member
Hey - I'm not holding myself out as an expert but I would also think the amount of slip will vary depending on the angle of the boat relative to the horizontal. If the boat is climbing a wave there will be more slip compared to flat water. If engine speed is the same, the engine is working to achieve vertical acceleration as well as horizontal acceleration = much more work so more fuel. That would be mitigated by larger slip so the actual distance moved through the water is less hence much lower mpg but also substantially more gph.
As I said, I'm no expert.
As I said, I'm no expert.
Its_Only_Money
Well-Known Member
[ QUOTE ]
So if this counter-intuitive statement is true, there must be some point, some level of pitch, where there is a crossover and the windmilling prop has less drag than the fixed prop)
[/ QUOTE ]
I think that this point is where the fixed prop is stalled. Take an auto-rotating helicopter, windmilling blades can generate lift (which is effectively drag preventing a freefall to earth), stopping the blades stalls the airflow hence the lift is removed and a freefall results.
If you imagine a rotating prop, it has a positive angle of attack to the water flow, if you fix the prop and cause water flow then the angle of attack changes and moves fwds, I >think< enough to cause a stalled condition. If you allow the prop to rotate then the angle of attack moves back to an unstalled condition and "lift" astern on the prop is the result.
So whether a fixed prop generates more or less drag I think depends on the boat speed, if the fwd speed of the boat is sufficient to change the angle of attack to stall the blades. So a slow raggie may be going slow enough that even a fixed prop is not stalled and hence is generating substantial drag, a little faster and the same boat/prop may have a stalled fixed prop and hence less drag. This >might< account for why there is different experience in the benefits or otherwise of fixing the props on the drag experience of different boats.
It may or may not be possible for the boat to go fast enough to stall a windmilling prop, if my thoughts are correct (no guarantees, I'm not a hydrodynamics specialist /forums/images/graemlins/smile.gif )
If there is a hydrodynamics expert could he please step forward and advise???
So if this counter-intuitive statement is true, there must be some point, some level of pitch, where there is a crossover and the windmilling prop has less drag than the fixed prop)
[/ QUOTE ]
I think that this point is where the fixed prop is stalled. Take an auto-rotating helicopter, windmilling blades can generate lift (which is effectively drag preventing a freefall to earth), stopping the blades stalls the airflow hence the lift is removed and a freefall results.
If you imagine a rotating prop, it has a positive angle of attack to the water flow, if you fix the prop and cause water flow then the angle of attack changes and moves fwds, I >think< enough to cause a stalled condition. If you allow the prop to rotate then the angle of attack moves back to an unstalled condition and "lift" astern on the prop is the result.
So whether a fixed prop generates more or less drag I think depends on the boat speed, if the fwd speed of the boat is sufficient to change the angle of attack to stall the blades. So a slow raggie may be going slow enough that even a fixed prop is not stalled and hence is generating substantial drag, a little faster and the same boat/prop may have a stalled fixed prop and hence less drag. This >might< account for why there is different experience in the benefits or otherwise of fixing the props on the drag experience of different boats.
It may or may not be possible for the boat to go fast enough to stall a windmilling prop, if my thoughts are correct (no guarantees, I'm not a hydrodynamics specialist /forums/images/graemlins/smile.gif )
If there is a hydrodynamics expert could he please step forward and advise???
jfm
Well-Known Member
All agreed, though it is a different point from the more/less drag of a windmilling prop
Your comment "it is a fact that on a ship, slowing from full speed to dead slow is achieved more efficiently by slowing the engine speed rather than stopping it. Never understood why but it works " is explained by stalling of the prop blades
Here's an attempt at explaining: lets say you have a 10knot ship and the captain wants to slow. Let's say that at 10knots the prop will spin at 500rpm if windmilling. Captian reduces engine speed. This is like applying a shaft brake, like an engine-braking car. You apply a certain brake force and the rpm falls to say 400. Apply some more brake and it falls to 300rpm, and so on.
Now remeber the prop blades are a foil. When they are windmilling they are flapping like a barndoor, but as you apply the shaft brake they generate energy, in this case heat in the brake/pumping of gas through the engine. As you slow the prop more and more the angle of attack between the foil and the water flow increases. But there comes a point, say 200rpm, where the foil stalls. The manifestation of the stall is that less brake force is needed. Lets say the shaft brake guy had to apply 75% brake on the brake lever to get to the point where the stall occured (in practice is doesn't happen all at once. The outer tip of the blade stalls before the root end, but let's ignore that). Once the stall has occured he could release the brake to say 50% and the prop would still not increase its rpm. A stalled prop has less pressure delta between the two sides of its blade than a working non stalled prop.
So turning to your ship, the right answer is (exactly as you said) to reduce the engine/props to 300rpm or <u>whatever is the point when they are just about to stall, but haven't, to get max brake force</u>. If you overcook it and slow to say 290rpm and stall the blade foils, the braking effect suddenly reduces (quite significantly)
Sorry if that's a hard-to-follow expl but your ship story is 100% correct, for this reason.
I'm glad you made the ship point becuase writing the above has caused me to think this is indeed the reason why Gludy's point (about windmilling props = higher drag) is correct in some circumstnaces, and why the theory has some currency.
If the cruising speed in question were one where the prop is just about to stall, then you'd reduce drag by braking the prop, for sure. So if that's where you're at, Gludy is right and this is the reason why (though I'm open to better offers on explaining it...!)
But if the prop were nowhere near stalling at the cruise speed concerned, braking it will INCREASE drag compared with letting it windmill. In a fast power boat (including Gludy's old Trader which is capable of 20+kts) cruising on one engine at say 8kts the blade pitch and angle will be high so you'd be nowhere near stall speed at 8knots. If you had a Trader like Gludy's but got small engines so max speed was 10knots and it had small pitch propellors, you might well be at stall speed at say 7 knots and it would reduce drag if you braked the idle propshaft. But for general fast powerboats this wouldn't be the case.
So that's where I'm at: Gludy is right but only in a narrow set of circumstnaces, otherwise it is better to windmill.
This is all theoretical of course. The messy non laminar waterflow around a prop, and differences in props, pitch (influenced by gearbox ratios), friction in the drivetrain, will all have relatively large effects and so you have to find the right answer for each boat by trial and error. But I would say that Gludy's theory, while right in certain circumstances, is not universally correct.
Your comment "it is a fact that on a ship, slowing from full speed to dead slow is achieved more efficiently by slowing the engine speed rather than stopping it. Never understood why but it works " is explained by stalling of the prop blades
Here's an attempt at explaining: lets say you have a 10knot ship and the captain wants to slow. Let's say that at 10knots the prop will spin at 500rpm if windmilling. Captian reduces engine speed. This is like applying a shaft brake, like an engine-braking car. You apply a certain brake force and the rpm falls to say 400. Apply some more brake and it falls to 300rpm, and so on.
Now remeber the prop blades are a foil. When they are windmilling they are flapping like a barndoor, but as you apply the shaft brake they generate energy, in this case heat in the brake/pumping of gas through the engine. As you slow the prop more and more the angle of attack between the foil and the water flow increases. But there comes a point, say 200rpm, where the foil stalls. The manifestation of the stall is that less brake force is needed. Lets say the shaft brake guy had to apply 75% brake on the brake lever to get to the point where the stall occured (in practice is doesn't happen all at once. The outer tip of the blade stalls before the root end, but let's ignore that). Once the stall has occured he could release the brake to say 50% and the prop would still not increase its rpm. A stalled prop has less pressure delta between the two sides of its blade than a working non stalled prop.
So turning to your ship, the right answer is (exactly as you said) to reduce the engine/props to 300rpm or <u>whatever is the point when they are just about to stall, but haven't, to get max brake force</u>. If you overcook it and slow to say 290rpm and stall the blade foils, the braking effect suddenly reduces (quite significantly)
Sorry if that's a hard-to-follow expl but your ship story is 100% correct, for this reason.
I'm glad you made the ship point becuase writing the above has caused me to think this is indeed the reason why Gludy's point (about windmilling props = higher drag) is correct in some circumstnaces, and why the theory has some currency.
If the cruising speed in question were one where the prop is just about to stall, then you'd reduce drag by braking the prop, for sure. So if that's where you're at, Gludy is right and this is the reason why (though I'm open to better offers on explaining it...!)
But if the prop were nowhere near stalling at the cruise speed concerned, braking it will INCREASE drag compared with letting it windmill. In a fast power boat (including Gludy's old Trader which is capable of 20+kts) cruising on one engine at say 8kts the blade pitch and angle will be high so you'd be nowhere near stall speed at 8knots. If you had a Trader like Gludy's but got small engines so max speed was 10knots and it had small pitch propellors, you might well be at stall speed at say 7 knots and it would reduce drag if you braked the idle propshaft. But for general fast powerboats this wouldn't be the case.
So that's where I'm at: Gludy is right but only in a narrow set of circumstnaces, otherwise it is better to windmill.
This is all theoretical of course. The messy non laminar waterflow around a prop, and differences in props, pitch (influenced by gearbox ratios), friction in the drivetrain, will all have relatively large effects and so you have to find the right answer for each boat by trial and error. But I would say that Gludy's theory, while right in certain circumstances, is not universally correct.
Piers
Well-Known Member
Hi MapisM.
What boat do you have, and what sort of hull?
Mine is a semi-displacement hull, and has twin 450 Cummins.
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?
What boat do you have, and what sort of hull?
Mine is a semi-displacement hull, and has twin 450 Cummins.
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?