Fuel consumption difference between two engines

[penpal]

When I last filed up the fuel tanks, I put in 177 litres in one tank and 137 litres in the other. I am about to fill up again and the fuel gauges would indicate this is going to be the same again. I recall on my original survey that the starboard shaft touches the stern tube. is it possible that this resistance could create the problem of higher fuel consumption? The engines are Volvo 140, 20 years old.

One engineer suggested the rev counters could be out which would mean I have not got the two engines in balance when under way.

Any ideas?

 

[kashurst]

quite a lot of reasons for this. If it was a 10% difference I wouldnt worry but thats a fair bit more:
assuming you measure this from full to full as boat fuel gauges are often a bit carp
1) get the tachos checked first
2) is the starboard engine the one with the higher fuel burn?
3) when were the heat exchangers last cleaned out?
4) fuel filters and air filters
5) tired turbo
6) props and shafts - damage/differences
7) possible fuel injection issue

 

[GBGaffer]

Return lines going into same tank?

 

[Deleted User YDKXO]

I don't think a misaligned shaft would increase consumption by so much.

Do you see more exhaust soot on one side of the transom than the other, which would indicate more unburnt fuel?
Is there a difference between the exhaust colour between one side and the other on acceleration?
Are the tanks interconnected and is the tap in the connection pipe closed?
Do both engines return fuel to their respective tanks only?
Are all other indicators the same ie oil pressure/temp, coolant temp?
Is there more vibration from one side or other?

Having said all that, my money would be on mismatched rev counters and failing that, an injector or fuel pump problem. When was the last time the injectors were serviced?

 

[Elessar]

When I last filed up the fuel tanks, I put in 177 litres in one tank and 137 litres in the other. I am about to fill up again and the fuel gauges would indicate this is going to be the same again. I recall on my original survey that the starboard shaft touches the stern tube. is it possible that this resistance could create the problem of higher fuel consumption? The engines are Volvo 140, 20 years old.

One engineer suggested the rev counters could be out which would mean I have not got the two engines in balance when under way.

Any ideas?

i had this recently and it was the rev counters - got to try that first

 

[penpal]

The smoke is the same, white and no soot, interesting idea about the fuel return I'll start with the rev counters but will also ensure the returns are okay. All the indicators are the same, oil, heat, there is no vibration. The injectors have not been serviced in the five years I have had the boat just a normal annual service.

 

[Deleted User YDKXO]

Sounds like rev counters is the place to start then! Interested to know what you find

 

[Piers]

Does one engine act as the domestic water heater?

 

[scottie]

Does one engine act as the domestic water heater?

Or have a second set of batteries or power steering pump working?

 

[jfm]

It's definitely not both fuel returns going to one tank because the one tank would be overflowing. That energy difference cannot be hot water or power steering. If the fuel system (injectors etc) is ok I'd guess (a) different rpm ie calibrate your tachos or (b) you have unmatched propellers (get them scanned).

Just sometimes gearboxes have different ratio each side but that difference is marginal and wouldn't explain your data

 

[MapisM]

The engines are Volvo 140, 20 years old.

It's easy to guess that on such engines you don't have any of the modern electronic gadgets which tell you everything about what engines are doing, including load, fuel burn, etc.

In which case, you might be interested in a trick for checking whether both engines are working equally hard.
And btw, the trick is effective regardless of what rpm counters show and whether gearbox ratio or props are matching or not.
It does require an autopilot though, and it must be a decent one, which automatically re-adjusts the rudders center based on the boat behaviour.

The starting point is that when both screws are pushing in balance, the average position of the rudder must be exactly in the center (leaving aside specific conditions like strong beam wind/current/waves).
Now, if one engine/prop is pushing stronger, let's say the port one, the a/p automatically compensates that by keeping the rudders (again, on average) steered to port by some degrees.
Trouble is, you might not even notice that, unless cruising in perfectly flat conditions, with no wind and current.
Otoh, any decent a/p, after sensing what is happening, automatically readjust the "zero" position of the rudder.
Let's say that after a while the a/p understands that it must, on average, keep the rudders steered 5 degrees to port (because the port prop is pushing more than the stbd one, hence making the boat steer to stbd unless corrected by the rudder).
By pressing some sequence of buttons (which depends on the a/p model) you should be able to see the rudder center that the a/p has recalculated.

In my experience, anything above a couple of degrees, either side, means that the engines/props are not balanced well enough.

As a countercheck, let's assume that after a few minutes of cruising with a perfectly balanced (on rev counters) rpm, you see that the a/p is working with a rudder center shifted 5 degrees to port.
As just explained, this tells you that you're running unbalanced, regardless of what rev counters are showing.
Therefore, you should slightly increase the rpm of stbd engine and/or reduce the port engine.
Very shortly, the a/p will sense the new situation and readjust the center rudder accordingly.
After a bit of trial and error, you will eventually achieve a balanced condition, without bothering about what the tachos are showing.

I understand that the above might seem overcomplicated, but in practice it's just a matter of understanding how your a/p works, if it has the automatic rudder center adjustment, and how to display it.
For instance, with my (pretty old) Furuno 330, it's just a matter of pressing three times the "SEL" button to show the automatically calculated rudder center.
I use this trick regularly, to check the engines alignment, particularly when helming from the f/b, where the engines can hardly be heard, thus making it difficult to understand whether they are in sync or not.

 

[jfm]

I understand what you're saying MapisM but that all assumes the autopilot knows what zero is, which aint necessarily the case. Especially on modern a/ps that don't care what zero is and during the set up of them there is no creation of a reference quality zero

 

[david_bagshaw]

one engine charges the domestic bank , the other doesn't perhaps

 

[dj43]

Have you got oil fired heating, if so this will affect the balance of the tanks ,as drawing from one tank only,

 

[MapisM]

I understand what you're saying MapisM but that all assumes the autopilot knows what zero is, which aint necessarily the case. Especially on modern a/ps that don't care what zero is and during the set up of them there is no creation of a reference quality zero

Yeah, I did mention the caveat about the a/p functionality, in fact.
Though in hindsight, rather than saying an a/p "which automatically re-adjusts the rudders center", I should have said one that integrates a rudder position sensor, and the capability to show it.
In fact, I'm not sure to understand what you mean by "modern a/ps that don't care what zero is", 'cause it's implicit in any a/p logic the need to sense the rudders position that makes the boat go straight at any given moment.
And that becomes the a/p "own" logical zero rudder - i.e. the position at which the a/p brings the rudders back and leaves them there for as long as no correction is again required.
The a/p might be unable to show at what physical rudder angle this "calculated zero" corresponds, either because it doesn't even have a rudder angle sensor, or (/and) because the control unit is designed with the typical Steve Jobs logic that the less the user knows of what the machine is doing, the better.
But this doesn't mean that the a/p doesn't readjust automatically the "calculated zero".

In fact, coming to think of it, the whole logic of my previous post would still stand also with an a/p that doesn't have the required functionality, as long as the boat has any other mean of monitoring the rudder angle.
Well, at least while cruising in ideal conditions, with a flat surface, no wind, waves, current, that is.
In fact, if the rudders are constantly correcting the course, it's pretty hard to see on a simple rudders indicator whether the a/p has compensated the unbalanced engines load with 5 degrees to port (or whatever).
In theory, it might still be possible to check that also in less than ideal conditions, if the rudder indicator could calculate and show the average zero of say the last three minutes of cruising, but I'm not aware of any rudder indicator with this sort of sophistication...


As an aside, I fully agree with your previous post ruling out fuel return, and also other marginal effects like hot water or power steering.
And I would include in the "marginal" list also an higher alternator load, as just suggested by d_b.
Of course all these things affect fuel burn, but none of them can be relevant enough to explain the difference which is being discussed.
Just for the records, one of my engine takes care of both an oversized alternator for domestic batteries charging AND the stabs hydraulic pump (arguably a stupid choice from the builder, but heyho!), and in spite of that I've never been able to quantify the fuel burn difference....!

 

[MapisM]

Have you got oil fired heating, if so this will affect the balance of the tanks ,as drawing from one tank only,

Yup, that might be an idea. As well as a genset feeded only by one tank.

 

[jfm]

Yeah, I did mention the caveat about the a/p functionality, in fact.
Though in hindsight, rather than saying an a/p "which automatically re-adjusts the rudders center", I should have said one that integrates a rudder position sensor, and the capability to show it.
In fact, I'm not sure to understand what you mean by "modern a/ps that don't care what zero is", 'cause it's implicit in any a/p logic the need to sense the rudders position that makes the boat go straight at any given moment.
And that becomes the a/p "own" logical zero rudder - i.e. the position at which the a/p brings the rudders back and leaves them there for as long as no correction is again required.
The a/p might be unable to show at what physical rudder angle this "calculated zero" corresponds, either because it doesn't even have a rudder angle sensor, or (/and) because the control unit is designed with the typical Steve Jobs logic that the less the user knows of what the machine is doing, the better.
But this doesn't mean that the a/p doesn't readjust automatically the "calculated zero".

In fact, coming to think of it, the whole logic of my previous post would still stand also with an a/p that doesn't have the required functionality, as long as the boat has any other mean of monitoring the rudder angle.
Well, at least while cruising in ideal conditions, with a flat surface, no wind, waves, current, that is.
In fact, if the rudders are constantly correcting the course, it's pretty hard to see on a simple rudders indicator whether the a/p has compensated the unbalanced engines load with 5 degrees to port (or whatever).
In theory, it might still be possible to check that also in less than ideal conditions, if the rudder indicator could calculate and show the average zero of say the last three minutes of cruising, but I'm not aware of any rudder indicator with this sort of sophistication...


As an aside, I fully agree with your previous post ruling out fuel return, and also other marginal effects like hot water or power steering.
And I would include in the "marginal" list also an higher alternator load, as just suggested by d_b.
Of course all these things affect fuel burn, but none of them can be relevant enough to explain the difference which is being discussed.
Just for the records, one of my engine takes care of both an oversized alternator for domestic batteries charging AND the stabs hydraulic pump (arguably a stupid choice from the builder, but heyho!), and in spite of that I've never been able to quantify the fuel burn difference....!

MapisM, sure a/ps know rudder angle relative to some approximate zero, and have a rudder position indicator (potentiometer) to do that. But this is not a reference zero and isn't good enough as a tool to synch the engines.

The only way to get a reference zero without hauling the boat out and setting up lasers and stuff is eliminate most friction in the rudder shafts then tow the boat in a straight line with both rudders free to flap like barn doors. That is a perfect zero. Then install the tie bar, then create your toe-in carefully with the same number of turns each side. Then you have a true zero, and if one engine is working harder you will see + or - some degrees on your a/pilot electronics

Nobody afaik bothers to create a reference zero by this means or anything similar, and of course these days with electronic engines it would be a totally pointless exercise anyway

If you let your a/pilot "find its own zero" and for some reason your engines are working 70% load on one side and 75% on the other, your a/pilot can never find a true zero on its own

 

[MapisM]

All well and good, but I don't understand what's the point you're trying to make.

We're talking of a boat which as I understand has no mean of checking whether the engines load is balanced, at any given moment. Other than non-electronic tachos, that is, but as we all know, they can be significantly misleading. And even if accurate, they still tell nothing about potentially unbalanced props.
So, given such assumption, IF (and I accept it's a big if) such boat has an a/p capable of showing the angle of its own calculated zero vs. the physical zero, in normal cruising conditions, the only reason for a significant deviation can be due to one prop pushing more than the other - hence requiring some rudders correction to stay on a straight course.
Consequently, this indication can warn of any meaningful differences in the work the engines are actually making, hence burning more/less fuel, regardless of rpm, possible differences in gearbox ratio, and even different props pitches.
If you can suggest any better (or even just alternative, not necessarily more accurate) way to achieve such result at zero cost, I'm all ears.

I understand/agree that if the rudder reference unit (or the tie bar, or whatever) is badly installed, there's no point in finding the throttle position that makes the a/p calculated zero as close as possible to the physical zero, but that's a different matter altogether.

Fwiw, with my old tub, I've experienced many times that whenever the a/p "trim", as Furuno calls it, is within 1.2/1.3 degrees either side (yes, the display even shows the decimal degrees of the rudder position), the engines are perfectly synced.
At least, they are synced well enough to not hear any cyclic noise from them, but according to Cat engineers, in such condition the actual rotation difference can't be more than 10/15 rpm, go figure. An accuracy I could only dream of, with the tachos.
Otoh, whenever the a/p shows a trim of 1.5 degrees or more, you can distinctively hear the non-synced cyclic noise from the engines, at any given rpm. With 3 degrees or more, you can also see the difference on the tachos.
Incidentally, it's nice to know of this behaviour, because it implies that the props are also balanced (since I know that the gearboxes have the same ratio, but that's easy to check on any boat). In fact, if one prop (let's say the port one) would have a higher pitch, with perfectly synced engines, the a/p would trim the rudders to port, to compensate the higher push of port prop.

 

[jfm]

All well and good, but I don't understand what's the point you're trying to make..

You said in post 11 that you had a trick to discover whether one engine is working harder than the other. You said check whether the a/pilot's normal drive-in-a-straight-line position is something other than 0 degrees, eg if straight line course for the boat requires 5% of rudders turned to port, then the port engine is working harder than stbd engine.

What I'm saying is that your a/pilot might well blurt out "Yeah the rudders are turned 5 deg to port" because it is a dumb bit of hardware, but you must not believe it. It does not know where zero is, and therefore it cannot know where 5 degrees is.

In your boat, you think everything is perfect but perhaps it isn't. Let's say you synch the engines to 15rpm by ear; that's fine. Let's then imagine that unbeknown to you the port prop has an inch more pitch or cupping that the stbd prop and is doing more work. Your rudders will be turned to port to keep the boat in a straight line. You "trim" reading isn't telling you about that, because it doesn't know it. You think your trim is giving you valid data at 1.2/1.3 degrees or whatever, but it isn't. Unless your trim measurement has been calibrated so that it knows where zero degrees is, it cannot know what 1.2/1.3 degrees looks like. You've been duped I'm afraid!

 

[MapisM]

Why, of course it knows where the (physical) zero is.
As much as I've got a few bits of friday afternoon installations on my boat (one of which I mentioned also in this thread, i.e. fitting both the bigger alternator and the stabs pump on the same engine), it's not like they installed every equipment without caring at all if it worked or not!
The boat has no other rudder indicator than the one of the a/p, and I've seen other boats with the very same setup.
Now, knowing how the yard which built my boat works, pretty sure they didn't use any laser for such installation.
But as I said, when the a/p trim is within one degree or so EITHER SIDE, the engines are synced - and viceversa - and this is a fact.
Accurate enough, I reckon. It is for me, anyway.

I seriously struggle to understand how you can think that this could happen with one prop longer than the other.
The only theoretical case I can think of is if the longer prop would be turned by a gearbox with a different reduction ratio.
Both engines are spinning at the same rpm, the shaft attached to the longer prop spins a bit more slowly, but the higher pitch exactly makes up for that.
Possible, of course. Even likely, if the boat is built with two different ratio gearboxes, to start with.
But as I already said, both my gearboxes have the same ratio.

Anyway, I'm not pretending that you take my word for what I'm saying.
I suppose that also the "ear syncing" trick can sound unreliable to anyone who never tried it, but also BartW for instance reported in other occasions to have found that useful and accurate. And he is in the same situation, with non-electronically controlled (and monitored) engines.
But in this case, I'm afraid the only way to try and test what I'm saying is to jump on a plane and come down here for a sea trial.
Glad to arrange that at your convenience, and take the opportunity for a swim in the process.

Btw, I had a VERY experienced forumite onboard till just a couple of days ago.
I could have shown him in practice what we are debating, and he could have added his first hand impressions.
But when we went at sea together, we rather discussed which type of aperitif to have before dinner while overnighting at anchor, selecting the best swimming spot for the wind conditions, and other similarly trivial things, you know...