Independant fuel tanks - port engine consumes more fuel

[BruceK]

Can anyone point me in the direction of what to look for first to remedy the problem. I've posed the question to a number of mechanics as to why the port engine uses more fuel and the response has always been there's always a lead engine. But 30% is a bit of a big ask imo.

Aging 34 foot cruiser running twin KAMD42's. She's an old boat, nothing is perfect, but we try getting there


Each engine has an independant fuel tank with no balancer between tanks.
Each refill I can expect to put roughly 30% more fuel into PORT tank.

further information:

propellors recently changed so not props. DP290 duo props

Tacho's probably not that reliably accurate but I balance engines to harmonics as much as possible.

I carry about 150kg of ballast on port side. This was put in firstly as the boat always had a list to starboard of about 3 degrees. I initially attributed this to removed equipment such as inboard genny, AC, vacuum and icebox that used to be located on the port side. However I'm not so sure the list was not created by the imbalance in fuel tanks. (I used to put in equal quantities per tank, i.e. 250 litres each side)(Each tank is ~400l)

underway at cruise, port side trim is often activated slightly more than starboard

Possible toe-in alignment issue between drives. To check on season lift

Starboard engine carries domestic calorifier

Starboard engine trim sensor not very well callibrated (i.e. possible small difference in trim angles)
Starboard engine does tend to run cooler on average according to guage Sub 80C whereas the port is 80C

No hull obstructions

Engines were completely rebuilt 4 years ago including new injectors etc. The engines when underway have always received favourable comments from those in the know for how good they sound for what it's worth. i.e. visually and audibly they sound very healthy.

Turbos both kick in at same times (no boost pressure guages to verify balance though) and harmonics dont change as throttle is applied so I'm assuming they're in the same boost range.

 

[A_8]

If you open them up, full throttle what rpm's are each engine running at?

 

[Danfarry]

Bruce, sorry to state the obvious but do you have diesel heating?

 

[BruceK]

No diesel heating.

As for WOT, both engines easily excede max revs of 3900 as I'm running C3's instead of C4 prop sets so that is an unknown. idle to cruising revs in under 20 seconds. 2k rpm to 3400rpm in under 6

 

[jfm]

"Lead engine" = old wives tale. It isn't the calorifier.

I think you should check the rpm thing eg with a Halfords strobe, to eliminate that one, and match the trims, just to eliminate that. If that checks out ok, and assuming it is not another consumer like heater or genset, then port engine is doing more work. This could be bigger prop pitch on port, prop slip on starboard, much gearbox friction on port (longshot). You might also have a power steering pump on port engine and a non functioning alternator on stbd, but those aren't 30% items

 

[BruceK]

I'll invest in a strobe.

Not gearbox, port side engine gearbox just replaced and still fuel issues.

Port side alternator was underperforming, I have just this week replaced it. However it's load was just the starter battery. The starboard engine which charges the house batteries so I would have expected it to have a larger drain on power.

Steering pump is on port. Could this be placing that much extra load because the drives are not 100% aligned? The legs appear to have a slight toe out rather than toe in.

 

[tico]

Just a thought, could it be that each engine draws from it's own tank BUT the fuel return goes back to only one tank. Seems a bit daft but you never know?

 

[oldgit]

Having kept a log of every last hour on all my boats for 30 odd years,every twin I have owned has produced a difference in fuel consumption,however this rarely ever exceeded 10% or so.This was usually put down to contra rotating props and odd ball gearbox ratios/prop pitch combinations to achieve same propeller revolutions on both shaft.Even on my present boat one prop needs tweeking by 1.5 inches for perfect match.
Is there a balance pipe between tanks?
Are the rev counters reading correctly perhaps due to voltage irregularities.
Although on the dial it may indicate that the engines are rotating at the same RPM one may not be producing the same power.

 

[BruceK]

Hi Tico. I haven't checked that but I seriously doubt it. If both tanks were full that'd leave a trail of diesel down the estuary from the overflow.

 

[BruceK]

Hi Oldgit.

Definitely no balance pipe. As I said, it would not surprise me if the rev counters were not accurate to the closest 100rpm. The boat is old, the electrics have certainly had time to get a few bad connections. I'll get a strob as per JFM's suggestion.

As an aside. I have no detailed knowlege of the boat's entire history. If one gearbox reduction ratio was different that would be hugely noticable wouldn't it?

Could a combination of added port trim tab and a toe out combo account for so much extra load? The steering does feel a bit tight. I know my 8 year old boy cant steer the boat with any finness ( heave an oversteer) but was quite good in the cuddy which only had a single inboard.

 

[Aardee]

As a (very) rough and ready check, would anything be learned by running on one engine at a time using the same revs and seeing what the difference in boat speed is?

 

[BruceK]

I can certainly try that. Never run on just one engine before. Certainly wouldn't get on the plane though and that leaves a bit of a large dead spot in the hole. Could I damage the steering doing that as only port engine has the steering pump on?

 

[Seastoke]

bruce i have a tacho mechanical one you can borrow if any use ,icould post it to you

 

[Greg2]

Not sure this is going to help much but we had the same issue a couple of years ago. Port engine was using anything between 10% and 30% more than stbd. Had the injectors checked (okay) and I now know that the props are okay. We had a bit of an issue with it running warm but that was resolved and then re-appeared. Strange thing is that the fuel use seems to have resolved itself as this year it has been pretty balanced.

Haven't be able to put my finger on it but my best guess is imbalance in RPM. Not a lot but maybe enough to explain it...perhaps....not sure!

 

[A_8]

A couple of things, you really should try to get the toe out fixed, there is usually an adjustable rod between the drives that you use to do this. It will affect top end performance and maybe even cause some unnecessary cavitation, could also be why your steering is heavy. The tab and toe out will however put the same load on both engines so its not why one is consuming more diesel.
The other thing is why did you go from C4 to C3, the boat should most likely be suited to C4 if you have the 1:78 gears. You should find a way to verify rpm's but assuming for now they are correct what was the wot engine rpm on each engine with the C4's? I am asking as it may indicate a problem with the engines. On the same topic you said they both now get over 3900 rpm but where do each engine stop?

 

[Ex marine industry]

Do you have valves to allow either engine to draw from either tank, they may have been left open? The other possibility is the boat was piped wrong from new and each engine is returning unused fuel to the wrong tank, this may only become an issue when running on only one engine.

 

[Momac]

It may or may not not be any help to know I have a 33 ft boat - with kad32 and 290dpe's (so not so much different in principle).
Based on long journeys with no heater used the fuel use is very even between the engines - so a big difference in fuel use is not normal.
My 4 cylinder engines are not so smooth as your six cylinders but they do balance up by resonating nicely pretty much with the tachometers reading both the same. You could swap your tachometers over and see if that made any difference but I doubt it.

I assume engine service including new fuel and air filters - has made no benefit .

Could it be the props are not the same on both engines - causing one engine to take more load? If one of the duoprops is damaged it doesn't necessarily cause much or any vibration but may well affect performance - or if the wrong pitch prop has been fitted on one leg? Swapping the props over might be an interesting experiment.

Stiff steering doesn't sound at all correct - something not quite right there.

 

[BruceK]

Hi A8. I am running C3 sets because one of the compressors is damaged so I have removed the belts for them from both engines while I wait for replacements. With C4s on without the compressors I just can't quite get over the hump to spool the turbos up. Winter scheduled repair.

As for stiff steering don't over read into that. I don't struggle, the boy does. It's stiff at low speed but probably best described as notchy to move from neutral position. Bit like driving a old classic car that does not have power steering at crawl speeds. It's noticeable by comparison to the cuddy.

Roy if I could borrow your tacho that would be great. When next are you down?

 

[vas]

just to note that return rate of diesel on tanks is WAY TOO MUCH to consider a wrong return pipe or something. If both engines were returning fuel to say the stbrd tank, after 5-10mins running at planning speed there would be no fuel on port tank and overflowing from the stbrd!
So return pipes cannot be the problem.

V.

 

[Ex marine industry]

just to note that return rate of diesel on tanks is WAY TOO MUCH to consider a wrong return pipe or something. If both engines were returning fuel to say the stbrd tank, after 5-10mins running at planning speed there would be no fuel on port tank and overflowing from the stbrd!
So return pipes cannot be the problem.

V.

Both engines returning to one tank and you're right. However each engine returning to the wrong tank and it would be Ok for a bit given same RPM etc. Some designs allow both engines to run from one tank and return to that tank. If the return valve is left in the open position then the fuel takes the line of least resistance and goes where it pleases. 30% is a big difference without other problems, black smoke etc?