Twin Engines Fuel Consumption

Greg2

Greg2

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We have two fuel tanks in our Broom 41, one for each engine with no link pipes so they are completely independent of each other. We have had the boat since May and have been mainly on the river with only one short sea trip up untill last weekend when we visited the Orwell.

When we filled the tanks (both full) prior to the weekend the port tank took about 60 litres more than the stbd tank. I thought that maybe the previous owner had filled them unequally but after the outward leg of our trip (9 hours running with 2 1/2 hours on the plane and the rest on the river) the port tank took 24 litres more than the stbd tank.

This is the first boat we have had with completely independent tanks for each engine so my question is should I expect there to be a difference in fuel consumption or should they be pretty close? If the latter any thoughts on what the problem may be?

Worth saying that we ran at the same revs on each engine and whilst I haven't checked I think that the tachos are accurate and that the engines run in sync and give the performance I would expect.
 
I have a similar setup and have noticed differences too often 20-40litres, I put it down to the difficulty in actually filling the tanks and when they overflow. You might think it is full but depending on the boats trim when you are doing it there maybe air trapped in the top of the tank that accounts for the difference.
 
Most likely cause is your propellers are slightly different pitch giving different loading, possibly like this from new.

Next time the boat is out of the water check props carefully for damage and pitch and serial number to see if one has been damaged or replaced.
 
This is normal for my father’s Broom Ocean 38/38Cl. Probably any combination of what has already been suggested is the cause. I would say diesel heating is the main cause for us though as the generator isn’t used too often. I think we filled up early July with about 360 litres or so and about 40 more went into the port tank.
 
I have noticed this on a day to day basis when cruising with no heating or genset, for me anyway it is definitely a filling issue. Boat fuel tanks unlike cars have square tanks and only a slight list toward the filler for instance could leave a large area of trapped air on the high side especialy with a large tank, mine are 2 x 375litres and I have these disparites all the time moving from one engine to the other.
 
My Broom also has separate tanks. I've only filled up a few times since getting her but can't say I've noticed much difference between the tanks other than after the winter when I'd run one heater more than the other.
 
The bloke i bought my boat from told me not to drive by tachos, but by boost pressure, as then the engines are pulling the same load, and alleviates from discrepencies in props etc....
It makes sense to me and he has 30 years working on sunseekers...
 
Robg71 said:
The bloke i bought my boat from told me not to drive by tachos, but by boost pressure, as then the engines are pulling the same load, and alleviates from discrepencies in props etc....
It makes sense to me and he has 30 years working on sunseekers...
Click to expand...

Similar response.

I match revs listening to the drum you get when both are perfectly balanced , then increase one side 25-50 rpm to stop the iterating drum.

Discrepancy op fuel could be rev counter (most likely) or could be where he and his crew are stood when filling up or how tight the mooring lines are causing the boat to tilt, sometimes on the boat watching, sometimes wandering round the pontoon ?
 
on every boat I have owned so far the port side engine always uses a bit more fuel than the starboard side. I could never find anything wrong and in the end put it down to the fact that the port side engine has to do more work because of the extra gear in the gearbox to make the props run the opposite way around to starboard engine. Don't forget that the "throttles" don't control the fuel usage but the engine speed. The governor in the injection pump controls how much fuel is used to maintain the desired speed. Check everything out but there is probably nothing wrong at all.
 
As I have said in other threads, most of my twin engine experience is on superyachts and other larger commercial vessels. Most of these have constant fuel flow meters fitted which give an instant readout of fuel being used by each engine. I have in fact fitted a Floscan system to my little 33ft Aquastar with 2 old but good condition Volvo TAMD41As. One of the advantages, apart from knowing exactly how much fuel you are using at given speed, is noting any differences between the engines. Also, normally these old lumps of iron don't have boost gauges either so it's very difficult to judge performance. What the floscan, and the now retrofitted boost gauges have told me so far is that in fact one engine was using marginally more fuel than the other. Not as much as you describe but nevertheless a noticable amount. Stripping and cleaning the turbo on the errant engine corrected this and both engies now show identical boost pressures at the same RPM and use the same amount of fuel. I have also seen a radical difference between two large caterpillar engines which showed the same boost pressure. In this case new injectors were the solution. To anyone who is seeing one thirsty engine, I would say look for the reason! You are probably burning more fuel than you need due to it being in need of some diagnostics and TLC.... Assuming of course that it IS actually burning more fuel which you won't know unless you either have a flowmeter or measure the fuel used from each tank accurately over a period. It is NOT normal to have one engine burn more fuel than the other!
 
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Thanks guys, some very helpful responses here. T clarify the genie and eBay run off the stbd tank so not the one that appears to be using more. Interestingly on the way back the stbd boost pressure was a bit higher than port (and fluctuating slightly) so it will be interesting to see what the score is when I fill up this weekend.
 
Greg2 said:
Thanks guys, some very helpful responses here. T clarify the genie and eBay run off the stbd tank so not the one that appears to be using more. Interestingly on the way back the stbd boost pressure was a bit higher than port (and fluctuating slightly) so it will be interesting to see what the score is when I fill up this weekend.
Click to expand...

Let us know what you find...... I'm following with interest.
 
Greg2 said:
Thanks guys, some very helpful responses here. T clarify the genie and eBay run off the stbd tank so not the one that appears to be using more. Interestingly on the way back the stbd boost pressure was a bit higher than port (and fluctuating slightly) so it will be interesting to see what the score is when I fill up this weekend.
Click to expand...

That sounds like either:

a) there is a turbo problem or an air leak. (check the boost gauge pipe and connection)

b) the rev counters are misreading and you are actually running one engine faster


Reading your post I think what you said was that the port engine is the thirsty one and it is lower on boost pressure though so I would suspect it is slightly off tune for some reason. If it's not the turbo sooted up try having the injectors out and get them tested.
 
Just noticed the typos on my last post.......it would appear that I have a genie aboard together with eBay! Damm this iPad! :-)
I meant of course gennie and ebby.

Just checked the log and on the outward leg the boost pressure readings (half hourly) were 1.0 for both for the first part of the passage and then 0.9 port and 0.8 stbd for the second part, which may support the one engine working harder than the other theory.

On the return leg for the first part they were both 1.0 and the second part, having increased revs slightly, they were 1.0 port for all readings and 1.2 and 1.4 stbd on the half hour and this was when the needle seemed to waver a bit. Following the turbo boost theory I guess it may be that the stbd engine has used more this time

On both legs when I increased revs slightly the port engine ran warmer than the stbd i.e. 86-90 deg C against a constant 80 deg C on stbd. Port engine has the calorifier. At slightly low revs both were rock steady at 80. Interestingly the stbd boost pressure gauge didn't do anything other than stick at 1.0 at the higher revs on the way out.

I'll update when I have filled the tanks again. On that point I did wonder about airlocks etc potentially affecting how much fuel each tank will take but we did get overflow from both breathers......
 
Doesn't sound as if there is much wrong with the turbos then. I would take those readings as within normal perameters. Port engine having the calorifier should not make it run hotter. I would suspect a slight restriction in the raw water cooling. Check filters etc. While its not conclusive I would say that if the offending port engine is running hotter and consuming more fuel it requires a little TLC somewhere. Have you checked that your shafts are aligned properly and running free? Are you sure you don't have a damaged or fouled prop on that side?

From a more abstract point of view isn't it great to have two engines to compare? If one starts to behave less well than the other you can look for little problems before they turn in to big ones! With one engine who would know?
 
Yes, I guess the combination of fuel consumption and running warmer may indicate something slightly amiss and I take your point on having two engines to compare.

I was chatting with a fellow Broom 41 owner today who has the same symptoms but the other way round i.e. the stb engine is the one that is misbehaving a little.

Will probably top te tanks up tommorrow followed by a short sea trip so I will update after that.
 
Turbo boost and engine temps rock steady during our short run today. Stbd boost gauge did go higher for a very short time then stayed the same as the port guage at 1.0.

Didnt get to top the tanks off as the price wasn't competitive so will probably do that next weekend and we will see whether there is still an apparent difference in consumption.
 
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