Low RPM cruise

[a64pilot]

I’ve heard it over and over here that if you motorsail or simply cruise at lower RPM to extend range, save fuel, or just don’t like the Salon sounding like an airplane, it’s bad on the engine, that every hour or two you have to place the engine in neutral and race it wide open several times.
Well that has never made sense to me, especially the place it into neutral part. Well the other day while discussing recommended oil viscosities I had downloaded the operators manual, and I ran into this statement. Now this is an older 4JHE, but it matches my beliefs very well. screen shot is directly from the Yanmar manual.

[sailorboy1]

I’ve never read putting the engine in neutral. I have always ran it up hard while motoring except for the rare time I’ve run the engine at anchor/dock/mooring

Btw I’ve kept track of motoring Speeds and motor sailing the past 2 years. I don’t think motorsailing or motoring slow saves fuel. Far as mpg goes motoring anywhere between 5-6.5 knots seems almost the same, except for the engine noise.

[Kenomac]

And our Yanmar manual instructs to either run above 2500 for five minutes OR five high revvs blasts in neutral, which I’ve posted many times.

The manual for our 4jh3 hte is right on front of me and reads as follows:

“When operating the engine at low speed for long periods of time, race the engine once every two hours.

Note: Racing the engine
With the clutch in NEUTRAL, accelerate from the low speed position to the high speed position and repeat this process about 5 times. This is done to clean out the carbon from the cylinders and the fuel injection valve.”

“Neglecting to race the engine will result in poor exhaust colour and reduce engine performance.”

[denverd0n]

Originally Posted by a64pilot

...every hour or two you have to place the engine in neutral and race it wide open several times...

I've have absolutely never heard it suggested that you should put the transmission into neutral and race the engine. What I have heard suggested is basically what Yanmar suggests. The idea being that you want to put a load on the engine every few hours, by increasing your RPMs, but NOT with the transmission in neutral.

[Kenomac]

Originally Posted by denverd0n

I've have absolutely never heard it suggested that you should put the transmission into neutral and race the engine. What I have heard suggested is basically what Yanmar suggests. The idea being that you want to put a load on the engine every few hours, by increasing your RPMs, but NOT with the transmission in neutral.

You’ll find it located on page 29 & 30 of the Yanmar 4JH3 HTE owners manual. Here... let me show you.

[sailorboy1]

I can report that my 4JH3E engine even if it has been motoring along at 2000 rpms likes every few hours to go up to 3000+ for a minute or so. You can really tell it cleans out the exhaust etc and when you back back down to 2000 it sounds so much happier.

[a64pilot]

The real point of the discussion if you will, is define low speed.
Many take this to mean below 2500 RPM or so, I think it means literally low speed and unloaded, like sitting there idling, that underway at any significant speed isn’t the intent.
As far as fuel, on my boat running at lower RPM makes a large difference in fuel consumption. I too have tracked it, although not with a flow meter unfortunately, but the difference for me of running at 2500 RPM and 1800 RPM is about a kt., yet reduces fuel consumption by to about 2/3, to almost half.
Difference is 1 gl an hour to a little over a half a gl an hour.
I’ve not figured out the difference in mileage, it’s not twice as far of course, cause I’m running 6.5 kts vs 7.5

Now some of that is likely due to the Autoprop which changes pitch by itself, I’m sure it’s a higher pitch at the lower engine RPM.

2500 RPM is the sweet spot for my engine in fuel efficiency, it makes the most work for the least fuel at 2500 RPM.

So point of this thread is in my opinion if you motoring along at low RPM it hurts nothing at all to blow her out every so often, I do once or twice a day myself.
But it’s not required, and certainly not ever two hours. That’s if you idling in neutral for whatever reason you would do that.

[sailorboy1]

Yes if I also run at lower rpm and at say 5 knots I will use less gallons/hour. But, far aas miles/ gallon it makes little difference if I run at 5 knots or 6.5 knot (measured at calm conditions). So it’s worth knowing that if you have to motor then doing so at 6 knots doesn’t cost anything really (it does but s so little I can’t meadure it), but gets you there sooner.

Not looking to argue or debate as that’s what over 2 years of tracking tells me.

[Kenomac]

Originally Posted by a64pilot

The real point of the discussion if you will, is define low speed.
Many take this to mean below 2500 RPM or so, I think it means literally low speed and unloaded, like sitting there idling, that underway at any significant speed isn’t the intent.
As far as fuel, on my boat running at lower RPM makes a large difference in fuel consumption. I too have tracked it, although not with a flow meter unfortunately, but the difference for me of running at 2500 RPM and 1800 RPM is about a kt., yet reduces fuel consumption by to about 2/3, to almost half.
Difference is 1 gl an hour to a little over a half a gl an hour.
I’ve not figured out the difference in mileage, it’s not twice as far of course, cause I’m running 6.5 kts vs 7.5

Now some of that is likely due to the Autoprop which changes pitch by itself, I’m sure it’s a higher pitch at the lower engine RPM.

2500 RPM is the sweet spot for my engine in fuel efficiency, it makes the most work for the least fuel at 2500 RPM.

So point of this thread is in my opinion if you motoring along at low RPM it hurts nothing at all to blow her out every so often, I do once or twice a day myself.
But it’s not required, and certainly not ever two hours. That’s if you idling in neutral for whatever reason you would do that.

Required? Oh yes it is, unless you don’t mind replacing an expensive turbo that becomes plugged up with soot or broken fins. But hey... it’s your boat, your money. Do as you like, but I learned the hard (expensive) way.

[a64pilot]

So Ken if your motoring at 2500 RPM, every two hours, you place the transmission in neutral and race your engine five times?

[a64pilot]

Originally Posted by sailorboy1

Yes if I also run at lower rpm and at say 5 knots I will use less gallons/hour. But, far aas miles/ gallon it makes little difference if I run at 5 knots or 6.5 knot (measured at calm conditions). So it’s worth knowing that if you have to motor then doing so at 6 knots doesn’t cost anything really (it does but s so little I can’t meadure it), but gets you there sooner.

Not looking to argue or debate as that’s what over 2 years of tracking tells me.

Not arguing, just wondering how much my reduced fuel consumption is due to my prop is all.

[Kenomac]

Originally Posted by a64pilot

So Ken if your motoring at 2500 RPM, every two hours, you place the transmission in neutral and race your engine five times?

I never wrote that. Low rpms to me on the Yanmar are below 1800, and yes I do that every 1-2 hours when running low rpms like your thread title describes. No issues over the past six, but plenty of issues not doing it over the prior year.

Our Perkins Sabre 185hp engine does not have the same requirement.

[sailorboy1]

Originally Posted by a64pilot

Not arguing, just wondering how much my reduced fuel consumption is due to my prop is all.

Consumption and economy aren’t the same. If i motor slow I use less fuel per h,our. But who cares as I’m more interested in how far I can go per gallon.

[arsenelupiga]

as you all have sail boat consider this :

when you decided to motor, do it at lowest recommended RPM, 1600 for my 3jh5e, and then use sail to gain extra mileage. Race every 2 hours by going to idle. This is to avoid any pipe bursts in engine room or any other wear running it at max RPM. In this way, i use 1.3L per hour and still have nice sail. 90% of time I can push speed beyond 5kn. Motoring only on one engine is more like 4.3 kn.

[StuM]

Originally Posted by sailorboy1

Yes if I also run at lower rpm and at say 5 knots I will use less gallons/hour. But, far aas miles/ gallon it makes little difference if I run at 5 knots or 6.5 knot (measured at calm conditions). So it’s worth knowing that if you have to motor then doing so at 6 knots doesn’t cost anything really (it does but s so little I can’t meadure it), but gets you there sooner.

Not looking to argue or debate as that’s what over 2 years of tracking tells me.

Strange, everyone that I've seen giving actual data gets much better mpg at lower speed.



In my case it makes a big difference. Running on one engine:
5 knots - 2 lph = 2.5 nm /l = 9.5 nmpg
6 knots - 3.3 lph = 1.8 nm /l = 6.8 nmpg

6.5 knots - 5 lph = 1.3 nm /l = 4.9 nmpg


i.e Almost double the range or half the cost per mile at the slower speed.