Low RPM cruise

[Jim Cate]

Originally Posted by GaryBode

You all should become physics professors.

Surely not! There would be some frightful physics taught if they ALL became professors.

Jim

[Dockhead]

Originally Posted by Kenomac

No problems at all with our Yanmar since we began revving it six years ago. Our Maxprop had been over-pitched when we first bought the boat, so this was corrected when the new turbo was installed back in early 2013.

I have the same engine, and like Kenomac I run it hard for a few minutes every couple of hours, and at redline or near redline for a number of minutes when backing down on the anchor.



I use it at different speeds, from 1400 RPM motor sailing to 3200 RPM fighting head seas or something like that, but mostly at 1800 to 2000.


3200 hours (almost 2500 in my ownership over the last 9 years) and no problems other than smoke, which I've always had.


Overpitched prop is bad for any engine -- worth being sure about.


These engines like to have clean turbos -- worth not forgetting to wash the compressor wheel as required by the Yanmar maintenance schedule. And inspecting the hot section (turbine side) from time to time.

[Kenomac]

Originally Posted by Dockhead

I have the same engine, and like Kenomac I run it hard for a few minutes every couple of hours, and at redline or near redline for a number of minutes when backing down on the anchor.



I use it at different speeds, from 1400 RPM motor sailing to 3200 RPM fighting head seas or something like that, but mostly at 1800 to 2000.


3200 hours (almost 2500 in my ownership over the last 9 years) and no problems other than smoke, which I've always had.


Overpitched prop is bad for any engine -- worth being sure about.


These engines like to have clean turbos -- worth not forgetting to wash the compressor wheel as required by the Yanmar maintenance schedule. And inspecting the hot section (turbine side) from time to time.

1550 hours on ours since the turbo replacement and prop adjustment, but absolutely no smoke. The oil stays pretty clean too. It’s run mostly in the 1600-2200 range.

[Exile]

Originally Posted by daletournier

After approx 40k nms while using two yanmars and coming from a motor mechanics background I have to agree with A64. I'm not convinced with the low rpm glazing theory and I'm not convinced that you need to run your diesel at 80% load. Fuel consumption definitely goes up dramatically after approx 1800rpm with minimum gain.

My fuel consumption beliefs come from measuring my usage. My engine longevity beliefs come from what I've witnessed that conflict with what I used to believe (what I read).

I've seen yanmars with 14,000 hrs on them from owners that idle them morning and night to charge batteries and fridges etc, they get used to motor very regularly but very rarely over 2000 rpm. I ask other cruisers regular as it interest me and nearly 100% cruise under 2000rpm for fuel economy. Few I know regularly put their engines under load. Many don't read cruisers forum this don't know they should

Get your engine to operating temperature and I believe it will be fine.

Just my opinion.

I've heard this opinion echoed, but have also been advised by some marine engine mechanics that even my naturally aspirated engine should be run at WOT on occasion. Go figure . . . .

I have a generator so never idle my propulsion engine to charge batts. or run the fridge. When motoring or motorsailing I'll run my engine anywhere from 1400 to (occasionally) 2700 rpm (~80%) based on time or range constraints, but probably avg. 1800-2000. My engine manual says max rpm is 3500-3600 (yes, it cites a range) but, after re-pitching my prop, I've only been able to achieve 3400 rpm at WOT (calm conditions/no current). I figured that was close enough and hope I'm right.

Btw, when calculating what is 80% etc., it made more sense to me to apply my 3400 actual max rpm vs. the slightly higher potential max rpm per the manual. I hope I'm right on that one too. Anyone know? Frankly, the only time I start worrying about stuff like this is when I read these CF threads.

Hey Dale -- I have two questions about those cruisers (and others you've talked to) with 14K hrs. on their Yanmar's who idle them morning & night and cruise below 2000 rpm:

1. If you know, do their engines have turbo's? (I get the sense that turbo-charged engines may be a bit more vulnerable to sooting up).

2. If you know, do they ever run their engines at WOT as has been discussed?

[SailRedemption]

Originally Posted by transmitterdan

Try extending that curve all the way down to 0 MPH. You will cross over a peak at about 30-40MPH. It isn’t a simple always decreasing curve.

This. ^

Originally Posted by a64pilot

Yes, as I stated earlier on average a car will get its best fuel mileage at the lowest speed that the engine will run smoothly in the highest gear. Level, unaccelerated cruise that’s pretty much idle or slightly above.
Of course boats, most anyway, don’t have gears.

The engine has one gear and it has an efficient RPM to hull speed. You won't be efficient by just motoring in idle ahead.

[rgleason]

With a relatively new 2013 Yanmar 3YM30, I charge FLA in reverse at the mooring at 1700rpm and the manual does ask for racing 2 to 3 x's to 3000rpm in neutral, which I do.

Also the manual instructs to vary the rpm through a high range to meduium rpm when cruising. 3200 rpm is not continuos duty however.

Some of the differences in fuel use between boats may very well be related to length and hull speed. For example, our full federal 3 blade pushes this small boat very easily and quickly up to 5.7 knots. We do sail at 6.7 and sometimes hit 7.1-7.2.

The engine does push the boat uphill at 6.5-6.8 at about 2800-2900 rpm but we use fuel and have a big stern wave and are sunk in our own water hole..

Larger boats operate in a bigger range below their hull speed, requiring more horsepower, so the differences in fuel economy are perhaps are more noticeable.

I haven't read the entire thread, but should try to quantify the fuel use at lower rpm, and speed.

Its a good thread.

[FPNC]

In a displacement vessel, fuel economy is very relative to speed.

Hull drag in a displacement vessel increases very sharply over 4 kts. If you want very good fuel economy, do not exceed 4.5 kts speed. When you reach near hull speed, the forces against the hull again increase very sharply. If you want to waste a lot of fuel, try to push your vessel up to and beyond its hull speed. Which may or may not follow the classic speed length ratio constant of 1.34.

This is predicated on the engine HP & RPM and gear reduction and propeller being matched for the application. If you have a propulsion system that is mismatched to the application, the chances are good that you will get neither optimum performance or economy.

[a64pilot]

Originally Posted by SailRedemption

This. ^The engine has one gear and it has an efficient RPM to hull speed. You won't be efficient by just motoring in idle ahead.

Try it with a fuel flow measuring device and see.
It’s not about engine efficiency, any more than it is with a car or an airplane.
In all cases vehicle drag far exceeds any slight gain you might get with operating the engine in it max efficiency range.
Remember the engine efficiency map is based on efficiency per HP, not fuel consumption.
So yes, you make make HP for less fuel at 2600 RPM, but your boats drag is so high it way offsets that.

[Dockhead]

Originally Posted by a64pilot

Try it with a fuel flow measuring device and see.
It’s not about engine efficiency, any more than it is with a car or an airplane.
In all cases vehicle drag far exceeds any slight gain you might get with operating the engine in it max efficiency range.
Remember the engine efficiency map is based on efficiency per HP, not fuel consumption.
So yes, you make make HP for less fuel at 2600 RPM, but your boats drag is so high it way offsets that.

Indeed.


My boat -- measured with a precise fuel flow measuring device by Maretron -- uses less and less fuel per mile right down to and including idle.


You can figure this out without a Maretron fuel flow system just by comparing:


1. Engine specific fuel consumption curve for your engine

2. Hull resistance curve for your boat (free calculator here: Yacht Design: Hull Resistance Calculation)

3. Propeller curve.


As A64 correctly said -- hull resistance goes up exponentially and whatever efficiency gains are very small.

[DumnMad]

Originally Posted by Exile

I've heard this opinion echoed, but have also been advised by some marine engine mechanics that even my naturally aspirated engine should be run at WOT on occasion. Go figure . . . .

Seems to me a lot of mechanics just read the manual which is written for trucks cruising on level roads with occasional high loads to accelerate from start or to get up a hill.

The manual for my old Ford says to run at 1600rpm but the sweet spot is around 1100 and it reaches temperature quickly as long as its above idle and in gear.

[Kenomac]

When the turbo kicks in fuel efficiency increases along with mileage, which is why the fuel consumption curve isn’t a constant for all as some would like to believe... unless something is wrong with their engine of course.

”Pressure increase (or boost) A turbocharger may also be used to increase fuel efficiency without increasing power. This is achieved by recovering waste energy in the exhaust and feeding it back into the engine intake. ... The increased temperature from the higher pressure gives a higher Carnot efficiency.“. From wiki.

Here’s the entire wiki content: https://en.m.wikipedia.org/wiki/Turbocharger

[Dockhead]

Originally Posted by Kenomac

When the turbo kicks in fuel efficiency increases along with mileage, which is why the fuel consumption curve isn’t a constant for all as some would like to believe... unless something is wrong with their engine of course.

”Pressure increase (or boost) A turbocharger may also be used to increase fuel efficiency without increasing power. This is achieved by recovering waste energy in the exhaust and feeding it back into the engine intake. ... The increased temperature from the higher pressure gives a higher Carnot efficiency.“. From wiki.

Here’s the entire wiki content: https://en.m.wikipedia.org/wiki/Turbocharger

That is absolutely right, but you don't need to guess -- if you look at the specific fuel consumption curve for your engine, you will see the exact magnitude of this efficiency gain:





That's for a high pressure intercooled turbo engine, a 100hp Yanmar 4JH3 HTE, which will get the maximum gain from turbocharging.





Between 1800 RPM and 2400 RPM the difference is only a couple percent, in g/hp/hr, so not nearly enough to improve mileage. Optimum efficiency is between 2400RPM and 2600 RPM (around torque peak), and gets worse after that.

[Kenomac]

Originally Posted by Dockhead

That is absolutely right, but you don't need to guess -- if you look at the specific fuel consumption curve for your engine, you will see the exact magnitude of this efficiency gain:


Attachment 178889


That's for a high pressure intercooled turbo engine, a 100hp Yanmar 4JH3 HTE, which will get the maximum gain from turbocharging.





Between 1800 RPM and 2400 RPM the difference is only a couple percent, in g/hp/hr, so not nearly enough to improve mileage. Optimum efficiency is between 2400RPM and 2600 RPM (around torque peak), and gets worse after that.

So now it appears I am “absolutely right” instead of “mistaken” and wrong, the boat also covers more distance at the higher rpm. Our Perkins is even more efficient at a much lower rpm propelling our 62 along at 6 knots.

[Dockhead]

Originally Posted by Kenomac

So now it appears I am “absolutely right” instead of “mistaken” and wrong, the boat also covers more distance at the higher rpm. Our Perkins is even more efficient at a much lower rpm propelling our 62 along at 6 knots.

You are absolutely right that a turbocharged engine is more efficient when the turbo is working.


However, that efficiency gain amounts to a couple of percent -- see the curve posted. When the power required to move the boat one mile increases by 20% or 30% with every knot more speed even before you start to get into hull speed effects.


So fuel burned per mile increases with every knot of speed despite the fact that one horsepower/hour costs a few percent less when the turbo is working.


I hope that's more clear now.




Where I think you're getting confused is between efficiency as in grams of fuel per horsepower per hour, on the one hand -- and fuel burned per mile, on the other. You might burn less fuel per horsepower but still more fuel per mile, and you will burn more fuel per mile so long as hull friction is going up faster than fuel efficiency.



Everything that flies, rolls or floats is subject to this effect. Speed costs power (as A64 said) and therefore fuel. The only vehicle for which this is not true is a space ship traveling in a vacuum.




To illustrate the point -- say you burn 2 liters/hour at 3 knots. To get the same fuel burn PER MILE at 6 knots, you would have to burn no more than 4 liters/hour at that speed. But to burn just double the fuel per hour at double the speed, the power required to overcome hull friction would have to increase only linearly with speed. But it doesn't increase linearly, not for any boat, hull friction increases exponentially. With extremely narrow exceptions like planing hulls as they get up on a plane, and ships at some specific speeds where bow and stern waves interact.



If you can produce more horsepower for less fuel because the engine gets more efficient, that can cancel out part of the exponential increase in hull resistance. But compare the magnitude of this increase in efficiency, to the magnitude of increase of hull resistance, and that will tell you how much. Don't take my word for it -- this information is readily available. You can calculate hull resistance for your particular boat at different speeds using the free calculator I posted a link to.

[907Juice]

Dude, they are never gonna get it until they factor the time factor. In case anyone is still counting, I ran the numbers for 500 gallons. Quite an extreme number ran consistently just to show the difference. 5k @ 0.6gph gets you 4166 (n) miles 833hours. 6.5kn @ 0.8gph 4062 miles and 625 hours. So in 500 gallons, you only gain 100 miles but it takes you 200 hours to get there.