Fuel usage graph way off.

[silverback]

My Yanmar 6lpa 315 hp at 2400 revs, actual not the 2500 rev counter shows, burns about half what the graph in the manual says. I believe the prop is correctly pitched. I have noticed this wi other, smaller, motors in yachts over the years. Can anyone shed some light?
Thank you

 

[O C Diver]

Are you running at wide open throttle?
The propeller absorption curve on the graph is a rough approximation. There's no way the graph can reflect all the different hull types and their power requirements.

Wide open throttle with the proper propeller pitch should get you near the engine's maximum fuel burn, but again their approximations. The fuel consumption numbers are generated on a test bench, not in a boat.

Ted

 

[FF]

For accurate boat set up only a fuel map or BMEP graph will allow one to get away from the usual prop dance "3rd time lucky".

The normal prop load graph is a load of BS as it does not show efficient areas of engine operation.

Usually easier to find plans for the latest nuke weapon than a fuel map.

Boat builders usually chose engines by lowest cost to them,,, not your operational or repair costs.

 

[DavidM]

Please tell us how you are measuring rpms and maybe more importantly fuel burned and then give us a few data points from your measurements. Are you looking at the prop curve not the wot curve?

David

 

[rgano]

My Yanmar 6lpa 315 hp at 2400 revs, actual not the 2500 rev counter shows, burns about half what the graph in the manual says. I believe the prop is correctly pitched. I have noticed this wi other, smaller, motors in yachts over the years. Can anyone shed some light?
Thank you

I have the same engine in a different boat. My Floscan says at 2400 RPM, which is an extremely poor RPM to run my particular boat, that I use 5.0 US gallons per hour.

 

[DavidM]

The Yanmar data sheet for the 6LP2 shows it produces 95 hp at 2,400 rpm and consumes about 5.1 gph. See attachment and be sure to look at the PROP curves.

Well, I couldn't attach that data sheet, probably too big. So here is the link: https://www.yanmar.com/marine/product/engines/6lpa-stp2-6lpa-stc/. Click on Download Specification Datasheet link about 1/3 of the way down the page just to the right of Performance Curves. You could look at the curves themselves in the link but it is tough to blow them up on my browser, but the pdf download blows up easily.

David

 

[MYTraveler]

Fuel consumption of a diesel engine is very largely a function of how much power it is producing. Those curves typically show how much power a particular engine can produce at various rpms, and how much fuel is consumed at that power. Any given fixed propeller needs disproportionately more power as rpms increase. If a prop is pitched such that the power required to turn it when the engine is at max rpms is very close to the max power its engine can produce at those rpms (any more pitch would create an "over propped" condition), it will not need nearly the power that the engine can produce at lower rpms (there are exceptions, for example the discontinuity at the speed where the boat is coming on plane), and will consume correspondingly less fuel than the engine would consume if max available power were demanded of it at those lower rpms.

 

[High Wire]

The Yanmar data sheet for the 6LP2 shows it produces 95 hp at 2,400 rpm and consumes about 5.1 gph. See attachment and be sure to look at the PROP curves.

Well, I couldn't attach that data sheet, probably too big. So here is the link: https://www.yanmar.com/marine/product/engines/6lpa-stp2-6lpa-stc/. Click on Download Specification Datasheet link about 1/3 of the way down the page just to the right of Performance Curves. You could look at the curves themselves in the link but it is tough to blow them up on my browser, but the pdf download blows up easily.

David

Yes the prop curve is the blue curve in the link above. That should be +/- 10% of real life.

 

[silverback]

Thank you for those helpful comments. I bought a photo rev counter to get that correct. And the only way to measure consumption is fill up to fill up less abit for generator. Crude i know.
At 2400 rpm it appears i use about 10.5 lph ( say 2.8 us gph) and achieve about 7 knots. I calculate hull speed at about 7.5 knots. So Around those revs looks fairly efficient for this boat. I think the original owner mistakenly believed he could use the huge hp efficiently.

 

[silverback]

Thanks Highwire looked again at prop curve and it would appear that she only uses about 65 hp. Guess that explains low consumption. Not that i am complaining. Just a shame to claber around such a lovely big engine knowing it is not much use. She will push her up to 10 knots flat out but i shudder to think of which middle eastern prince i would be enriching!

 

[rgano]

I got this off my FloScan today.

 

[silverback]

Thank you all. I think that i am happy with 2.6 gph at 2200 giving me 6.6 knots on a boat with theoretical hull speed of about 7.5knots. Just happy to out there! Fuel at my marina is A1.50 a litre. So averaging close to 1 1/2 litres a nautical mile is pretty good, i think.

 

[FF]

"I think that i am happy with 2.6 gph at 2200 giving me 6.6 knots on a boat with theoretical hull speed of about 7.5knots."

This is the reason some folks will install a "cruising prop".

If your boat only uses 2.6 GPH at the cruise speed you enjoy that would be under about 50HP . I am sure you can locate an RPM on you prop graph that produces 75 HP that will be at a lower RPM.

IF you prop to cruise at that lower RPM the engine will last longer and probably be smoother and quieter , and burn less fuel too.

You will NEVER be able to see the mfg. WOT RPM , no problem , just use the tach , and don't try to tow water skiers..

 

[silverback]

Thanks, i think you are suggesting a coarser pitched prop. I did look into that and you are right this prop is not really optimum. But at a cost of A$5500 for a new one and given the existing economy i think it would take a hell of a long time to save that money. The boat is very heavy, about 14tonne and beamy with a flat semi displacement hull shape. With the engine howling wot she cant do more than 10 and a bit knots. The real solution is a 100 hp engine with correct prop. This would also give more space in the engine room. I suspect the original owner was a bit naive and allowed himself to be talked into this big motor. But it is a beaut.

 

[Barabus]

Seems that you might be able to add an inch of pitch to your existing props during next hull out. That might help.

 

[silverback]

Thank you all. As i understand it the horsepower put out is directly linked to fuel usage. FF, I think that if i am achieving less than a knot under hull speed using 50hp i am already very efficient? Ignoring the waste of a lovely 300hp motor!

 

[Ski in NC]

It is not a waste. Unnecessary, yes, but your engine will be plenty happy making 50hp. The turbo and aftercooler are not much in play then, so the engine is acting like a natural.

My personal boat is even worse in that regard: An 8.3liter engine rated at 450/2600 and often I run at hull speed of 7.7 at 950rpm and 1.8gph. So probably 30-35hp.

I have run long trips (seven days or so) at this speed and engine was fine. I do occasionally run it up to 1900/2000 to clear the gunk out. Also some long runs at a solid 1900/19kt/10-11gph.

You are not going to hurt your engine running light load. That engine was in Toyota land cruisers and running highway cruise those things were likely 1800rpm and 2-3gph. Close to where you are at.

Worry not.