Best cruise RPM on VolvoPenta MD2040 for fuel economy

[jonase]

Next summer we leave for an extended cruise including two ocean crossings. One thing that I still havent figured out is our fuel economy. Both the optimal RPM for fuel economy as well as the fuel consumption per hour at the optimal RPM.

During the 3 months we are sailing from Northern Europe to Gran Canaria before our first ocean crossing I will have calculated the fuel consumption. But I would like to get some help in figuring out the optimal cruise RPM without having to keep refilling and testing back and forth since the tank is not fully rectangular (follows the shape of the hull) and I cannot trust the accuracy of the fuel gage on the tank.

VolvoPenta recommends in the owners manual that the best comfort and fuel economy is to avoid running on full throttle (should not be any surprise). Instead they recommend a cruise RPM 300-500 below the full throttle. In the manual the full throttle range for the MD2010-2040 is 3200-3600 RMP and when we have tested our full throttle it was around 3300 RPM. I cannot find any more information in the workshop manual either regarding fuel economy.

If I would use the most conservative values that would give med a cruise RPM of 2700-2800 RPM (3200-500). Usually I cruise around 2200 RPM and consider 2700-2800 a fairly high RMP.

Does anyone else have a VolvoPenta MD2040 and have figured out the best cruise RPM for fuel economy?

[Dsanduril]

This is so specific to each boat/engine/propeller/weather/sea state combination that it is nearly impossible to answer. Below is the Volvo's fuel usage curve for our D2-40, which is a similar sized engine, with similar performance/RPM ratings. You'll see that above 2000 RPM fuel usage starts to go up significantly. This is under only one set of test conditions, so has to be taken with a very large grain of salt. Your boat may not demand the same power/RPM curve as the test. The specific fuel usage is nearly constant, but the amount of power required to add a knot to boat speed starts to go up exponentially as you get closer to hull speed.

If you look at the MD 2040 performance curve you'll also see that power output is essentially linear up to about 2400/2500 RPM, after that the slope decreases, indicating lower fuel efficiency above that RPM.

In real life we find 1800 to 2200 RPM to be the most fuel efficient, but that varies a lot with conditions. It is slow (about 4.5 to 5 knots for us), so if you have a one knot current against you it may be more fuel efficient to rev up (as an example). Too many variables, but overall that's where we have found the most miles per gallon (or liter).

[Kokanee]

For the best fuel economy, slower is better. Run at 1200 rpm if the only concern is fuel efficiency.

In reality, the "best" cruising rpm takes into account fuel efficiency, speed, engine life, noise, and vibration. With most diesel engines, I like to cruise somewhere near the engines peak torque rating, which is often about 60-70% of maximum rpm rating. Engines usually have a sweet spot close to that speed where there is little vibration, the noise is ok, fuel efficiency is good, and you are making decent speed.

Every boat is slightly different, even with the same engine.

[jonase]

Thanks a lot Dsanduril and Kokanee. This was what I was looking for. Of course everything is unique to each boat and the current conditions. That was why I was only asking about RPM and never any cruise speed.

The Torque curve was really useful and at peak torque I should also be around 60-70% of max RPM. It seems like the current cruise RPM that I use around 2000-2200 is also max torque something that I should keep using.

[a64pilot]

Really you can run it anywhere between a low RPM that will allow the engine to fully reach its operating temp and its max continuous RPM, but you don’t want to run it slower than it takes to fully warm up, usually about 1200 RPM is plenty to get one up to temp.
Then once or twice a day, rev her up and blow it out for a few minutes, then return to cruise speed.

As was said and I’ve graphed it on three different boats I guess or prove it to myself, but the slower you go, the Better your fuel mileage, which you need of know just in case your becalmed and you find out your next fuel stop doesn’t hav fuel or similar, for me I back down to 1200 and still get over 5 kts.
I run mine at 1800 RPM, on my engine and boat that gives my 6.5 kts smooth water and my engine is max continuous of 3500, but 1800 is nice and quiet and smooth, 2500 is my torque peak, but to me it sounds like I have a weed eater in the Salon, and I can’t stand that.

So pick an RPM that it’s smooth and not so loud, every boat has harmonic resonance frequencies, shaft curve anyway, and at those RPM’s it just vibrates, avoid those RPM, cause vibrations will wear stuff out.

[Scaramanga F25]

Obtain a fuel flow meter, that will give you your answer in all conditions.

[Hardhead]

Originally Posted by a64pilot

Really you can run it anywhere between a low RPM that will allow the engine to fully reach its operating temp and its max continuous RPM, but you don’t want to run it slower than it takes to fully warm up, usually about 1200 RPM is plenty to get one up to temp.
Then once or twice a day, rev her up and blow it out for a few minutes, then return to cruise speed.

As was said and I’ve graphed it on three different boats I guess or prove it to myself, but the slower you go, the Better your fuel mileage, which you need of know just in case your becalmed and you find out your next fuel stop doesn’t hav fuel or similar, for me I back down to 1200 and still get over 5 kts.
I run mine at 1800 RPM, on my engine and boat that gives my 6.5 kts smooth water and my engine is max continuous of 3500, but 1800 is nice and quiet and smooth, 2500 is my torque peak, but to me it sounds like I have a weed eater in the Salon, and I can’t stand that.

So pick an RPM that it’s smooth and not so loud, every boat has harmonic resonance frequencies, shaft curve anyway, and at those RPM’s it just vibrates, avoid those RPM, cause vibrations will wear stuff out.

I've always been an advocate of that as well. There's usually a difference in vibration between RPM ranges, and I generally settle in on the smoothest RPM within my target speed range. Keeps it simple.

[FULLCIRCLE331]

I have a MD2030 on a Beneteau 331. I track gph very closely. I find slower is better. I usually cruise at 2200 rpm and get .3 to .5 gph. At 2900 against wind and waved I get 1.5 gph.

[Pete7]

Originally Posted by Scaramanga F25

Obtain a fuel flow meter, that will give you your answer in all conditions.

Expensive since it needs to measure supply and return to calculate.

Like others have suggested slowing down does make a noticeable difference. For us 1900 - 2000 revs is a sweet spot giving 5 - 5.5 knots depending on sea conditions.

Pete

[a64pilot]

Originally Posted by Pete7

Expensive since it needs to measure supply and return to calculate.



Like others have suggested slowing down does make a noticeable difference. For us 1900 - 2000 revs is a sweet spot giving 5 - 5.5 knots depending on sea conditions.



Pete

They are, but two boats that I did the graph on were dual engines and had dual flow scans, one was a Diesel, one gas.
The third boat was an outboard dive boat with fuel injection and a Canbus architecture, it was a Mercury Verado and with a “mercy monitor” it allowed connection to a NMEA 2000 bus and just exactly like all the new cars their fuel use is not measured, but calculated based of course on the computer’s demanded fuel flow. Connected to my Garmin 740s it gave an instantaneous fuel MPG
However it was astonishingly accurate, I held over 100 gls and when I refueled it was within a gallon or two, which was within I think due to how exactly full was, was within measuring accuracy.

I bring that up because I’m certain that common rail motors can deliver the same fuel consumption data if there is a way to connect them to a NMEA 2000 bus, and surely there is.

[jonase]

One of the big drawbacks with the MD2040 is that it cannot connect to can or similar. There is no way to get data from this engine. The VolvoPenta model that replaced the MD2010-2040 has these options.

If could I would have bought the CAN module and integrated that one with NMEA2000.

[NorthernMac]

Do you have a fuel burn GPH for RPM?

Maybe wait till you’re in some steady water and start to advence the throttle, let the boat settle and record, next setting, etc then do the math.

With all the variables like prop, hull, weight, I don’t think it’s something you’ll be able to ask, going to have to record and do the math.

It’s crazy with how much these boats cost we don’t have default fuel transducers and with the GPS systems a range ring, let alone EGT/CHT, basic simple stuff that seems to not be found in most boats.

I’m debating installing a megasquirt with sensors just to get thmake we read outs (not for controlling anything)

[jonase]

Originally Posted by NorthernMac

Do you have a fuel burn GPH for RPM?

It’s the fuel burn GPH (or LPH) for RPM of the MD2040 I’m trying to find out so I can do the math.

I know that many engines has a Fuel Consumption vs Engine Speed (RMP) performance curves in either the owner manual or the workshop manual. This is what I am searching for the VolvoPenta MD2040.

If I had these performance curves I could do the calculations on my own based on boat speed at the different RPM’s (considering the different conditions at sea). But so far I have been unable to find these types of performance curves. No information about fuel consumption at different RPM:s is listed in the MD2040 manuals.

Since it’s a 150 liter tank and has a longe hose from the tank to deck I would consider running the engine at different RPM:s and the filling the tank to be fairly inacurate. Up until now we have just calculated a rough estimate based on engine hours between filling up the tank.

[NorthernMac]

Originally Posted by jonase

It’s the fuel burn GPH (or LPH) for RPM of the MD2040 I’m trying to find out so I can do the math.

I know that many engines has a Fuel Consumption vs Engine Speed (RMP) performance curves in either the owner manual or the workshop manual. This is what I am searching for the VolvoPenta MD2040.

If I had these performance curves I could do the calculations on my own based on boat speed at the different RPM’s (considering the different conditions at sea). But so far I have been unable to find these types of performance curves. No information about fuel consumption at different RPM:s is listed in the MD2040 manuals.

Since it’s a 150 liter tank and has a longe hose from the tank to deck I would consider running the engine at different RPM:s and the filling the tank to be fairly inacurate. Up until now we have just calculated a rough estimate based on engine hours between filling up the tank.

Non turbo?

Not ideal, but maybe this to get fuel burn at 100%
https://www.hardydiesel.com/generator-fuel-consumption-calculator.html

And divide max power from selected RPM to calculate burn at target setting?

Not pretty, it should be ball park

[Simi 60]

Originally Posted by Scaramanga F25

Obtain a fuel flow meter, that will give you your answer in all conditions.

We run a 14 litre engine in a 70 tonne boat and would never consider a fuel flow meter as it would take a lifetime to see a return on the $1000+ spent.

Better off spending the money on fuel, look at prop curve and take hand off stick.