Best cruise RPM on VolvoPenta MD2040 for fuel economy

[jonase]

Originally Posted by Simi 60

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.

Thanks, just did a quick check on the fuel flow meter, with the Maretron FFM100 plus two sensors I would have to pay around 1600$ in my country. Too expensive to just find the optimal cruising RPM.

My aim is to calculate the amount of fuel I need to have in either the tank or jerry cans during ocean crossings for x amount of hours running the engine.

[NorthernMac]

Originally Posted by Simi 60

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.

It’s useful for more than just that.

Also what a rip off, what’s that engine cost? And they can’t even give you fuel flow? I think it’s just that folks don’t know what they don’t know and when many have never had that information, or know how to interpret it, not having it doesn’t seem like a big deal.
I’m a sailboat and I want it, and will be busting out my tap set and ordering some stuff to get it, if I only had my engine and no sails in a going distant places boat, I couldn’t imagine trusting a engine that gave me so little information, lots of things you could catch early on, or if there was a issue pin point much faster with proper instruments.

[a64pilot]

Originally Posted by NorthernMac

Non turbo?



Not ideal, but maybe this to get fuel burn at 100%

https://www.hardydiesel.com/generato...alculator.html



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



Not pretty, it should be ball park

I haven’t looked at that chart, but most Diesel engines fuel consumption is very close to the norm for all. There just isn’t one or two stellar performers out there, and there usually isn’t one or two really bad performers either.

However even if you find a fuel flow per RPM chart it too is only a guess, reason is because your boat and prop may load the engine more of less than the chart, and that of course affects fuel flow.

If you really want to know I’d look at something like this, I would want something that inputs directly into NMEA 2000 as many chart plotters can do a lot with that, The flowscan monitors we had 20 or 30 years ago were reliable and accurate, but didn’t have an interface.

I know nothing of this product, but suspect there are many. It shows one box, but there will have to be separate flow meters as Diesels have return fuel flow that has to be subtracted from total flow.
https://www.maretron.com/products/ffm100.php

However I suspect you will find that there is almost a direct correlation between speed vs fuel flow, I couldn’t find any sweet spot where the motor operated at its most efficient, it’s there and they graphs show you exactly where, but apparently it was overwhelmed by other factors like drag.

[Simi 60]

Originally Posted by NorthernMac

It’s useful for more than just that.

Is it?
Examples please.

Also what a rip off, what’s that engine cost? And they can’t even give you fuel flow?

.
I'm sure it could have been added in for a price
But, some of us don't need expensive gadgets to tell us what we already know

I think it’s just that folks don’t know what they don’t know and when many have never had that information, or know how to interpret it, not having it doesn’t seem like a big deal.

Quite the opposite.
Takes no intelligence at all to read numbers on a gauge.

Prop curve has all the information required.
That, the sight glass marked with electrical ties (slide up and down marking xxx litre increment) + observation has got our fuel burn as accurate as we need.
Watch the rpm and we burn x amount / hour.
Simple maths

[kas_1611]

If it helps over the 10 years I have had my Bav Cruiser 40 with a D2-40 I have tracked our fuel use versus engine hours and we average 1.89 litres / hour at 1750-1850rpm. That equates to 6kts average cruising speed but also includes close quarter manoeuvres in harbours/marinas, anchoring and the rare "sprint for the bridge". However the majority of our engine hours are steady 6kt cruising hours so I feel the 1.9l is very representative of average fuel use for our boat.

2litres per hour is approx 0.5galUS/hr (0.42 impGal/hr)

Or put another way is the equivalent of motorway cruising at 70 and getting 168miles (statute) to the Imperial Gallon!!!

[NorthernMac]

Originally Posted by Simi 60

Is it?
Examples please.
.
I'm sure it could have been added in for a price
But, some of us don't need expensive gadgets to tell us what we already know



Quite the opposite.
Takes no intelligence at all to read numbers on a gauge.

Prop curve has all the information required.
That, the sight glass marked with electrical ties (slide up and down marking xxx litre increment) + observation has got our fuel burn as accurate as we need.
Watch the rpm and we burn x amount / hour.
Simple maths

Off the cuff, issues with fuel delivery, once you have a baseline you can trend the engine and catch little issues before they become big issues, like how you send oil off for analysis and cut your filter at oil changes.

[a64pilot]

Originally Posted by kas_1611

If it helps over the 10 years I have had my Bav Cruiser 40 with a D2-40 I have tracked our fuel use versus engine hours and we average 1.89 litres / hour at 1750-1850rpm. That equates to 6kts average cruising speed but also includes close quarter manoeuvres in harbours/marinas, anchoring and the rare "sprint for the bridge". However the majority of our engine hours are steady 6kt cruising hours so I feel the 1.9l is very representative of average fuel use for our boat.

2litres per hour is approx 0.5galUS/hr (0.42 impGal/hr)

Or put another way is the equivalent of motorway cruising at 70 and getting 168miles (statute) to the Imperial Gallon!!!

Yes, but it’s actually 3 NM per gallon

[Simi 60]

Originally Posted by NorthernMac

Off the cuff, issues with fuel delivery, once you have a baseline you can trend the engine and catch little issues before they become big issues,

Off the cuff?
So you made a claim and can't support it.

As for issues with fuel delivery have you heard of maintenance?

. like how you send oil off for analysis and cut your filter at oil changes.

And why would I need to do that?
Engine doesn't work hard,
Oil and filters are changed on time and oil is still clear on removal.
Again, this thing called maintenance

[NorthernMac]

Originally Posted by Simi 60

Off the cuff?
So you made a claim and can't support it.

As for issues with fuel delivery have you heard of maintenance?



And why would I need to do that?
Engine doesn't work hard,
Oil and filters are changed on time and oil is still clear on removal.
Again, this thing called maintenance

maintenance? In that case why not just remove all instruments lol

And I proved my claim with my prior post, aside from dialing in your economy and exact burn rates, it will show you any issues with the fuel delivery system early

The maintenance on aircraft engines are to a much higher standard, yet you still have fuel flow, EGT/CHT/oil temp and pressure, tach and MP, volts and amps, etc, and you are always training for failure and training to see a issue before it becomes a major failure.

Outside of a lawn mower or dirt bike or something, I expect good instruments in anything that I’m putting over a certain amount of faith and risk in and blue water boat is one of those things.

Especially for a power boat, knowing exactly how far you go with your FOB seems like a basic request, crazy that this seems fringe.

[Simi 60]

Originally Posted by NorthernMac

maintenance? In that case why not just remove all instruments lol

Silly comment

And I proved my claim with my prior post,

You proved nothing apart from you having a reliance on electronic gizmos.
Google Floscan problems and you may realise they are not as infallible as you think.
I know several boat owners with them claiming economy that is simply not possible, but floscan says it is so it must be true.

One of the reasons we chose this boat was the old skool bulletproof no electronics required engine.

aside from dialing in your economy and exact burn rates,

Exact?
Nope, google shows plenty of times they are not exact and see above.

I reckon I have a far better idea from marked 500 litres on sight glass, run at cruising rpm until it runs out and take note of hours and miles.
Compare to prop curve and what do you know, pretty much bang on.
Simple maths and observation
Zero expense on floscan and installation.

it will show you any issues with the fuel delivery system early

So will the Vac gauge on top of the filters.
So will dropping rpm
I don't need a $2000 gauge to show me that.

But none of this should ever eventuate if you have proper fuel tanks, proper filtration and perform maintenance.

The maintenance on aircraft engines are to a much higher standard, yet you still have fuel flow, EGT/CHT/oil temp and pressure, tach and MP, volts and amps, etc, and you are always training for failure and training to see a issue before it becomes a major failure.

Did no one tell you?
We are on boats, not planes
Boats have travelled the oceans for decades without fuel flow meters and no one fell out of the sky yet.

[billknny]

Speaking of basic math, the power required to drive a displacement hull through the water rises with the CUBE of the boat speed. Under similar conditions it is always true that slower uses less fuel. ALWAYS. You don't need a fuel flow meter to know that, just basic physics.

[Differentroads]

Originally Posted by 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?

I spent a lot of time this summer working this out for my 10m sailing cruiser. I've a Yanmar 3GM30. Just 24HP so a bit less powerful than your Volvo Penta and my boat is likely to be a bit smaller. But the lessons might apply to you. Essentially, I surprised myself by discovering that for my fin keeled monohull sailing cruiser overall fuel consumption on a trip was the same anywhere between low cruising revs of 2000rpm and fairly high 3000rpm. At 3000rpm I used one third more fuel per hour but got to my destination one third faster, so burned fuel for one third less time.

The power/fuel consumption curve in my Yanmar manual is flat between about 2000rpm and 3000rpm. That is to say that 2000rpm produces less power than 3000rpm but uses less fuel in direct proportion. I did half a dozen trips this summer purely under engine (no wind) of 6 to 18 hours keeping steady rpm and monitoring fuel used. It showed me that the bench test fuel/power curves worked out in practice too.
At 2000rpm my boat does about 4kn in fairly calm conditions and consumes 2 Litres per hour
At 3000rpm it does 6kn and consumes 3LPH.
So for a 60nm trip at 2000rpm I'd take 15 hours and use 30L. At 3000rpm I'd take 10 hours and also use 30L. And be putting those hours saved to better use than pottering along at 4 to 5 knots like i used to, thinking that low revs meant I was saving fuel.

[jamhass]

You could rig a small (couple of gallons) day tank to gauge consumption quite accurately, refilling from the main tank as needed.