Bigger engines consume more diesel under cruising?

[rslifkin]

Originally Posted by DougR

My experience is that larger displacement diesel engines have lower BSFC than smaller diesel engines. That is, they burn slightly less fuel for h.p. produced than smaller engines. This is mostly due to improvements in combustion in larger cylinders and less heat loss into the surrounding metal.

Let’s look at the efficiency of the two engines discussed here. According to the link provided by the OP, the D4-300 makes 300 h.p. @ wot and consumes 55 liter/ hour. It is a 3.7 liter engine.
The D2-75 engine makes 75 h.p. @ wot and consumes 18 liters/ hour. It’s a 2.2 liter engine.
The D4 engine uses .183 liters per hour per h.p., but the D2 engine uses .24 liters per hour per h.p. That means the larger engine is 31% more efficient.

Now, I’m the first to admit this isn’t a completely fair comparison, because the difference is not totally based on engine displacement. But it does accurately depict the differences in consumption that could occur in a boat. The real reason that the difference is so stark is that the D2 engine is a mechanically controlled, IDI (indirect injection) engine, and the D4 is a modern DI (direct injection) electronically controlled common rail engine.

Yes, the larger engines are typically more efficient at WOT or their best efficiency point. Small-ish displacement diesels that run a lot of boost also tend to be more efficient. However, if we're talking about the fuel required to produce a constant, say, 50 horsepower, then assuming 2 engines of comparable basic design and control systems, the larger one will burn slightly more fuel to produce 50 hp, as it's got more mechanical losses.

[mvweebles]

I can't speak specifically to the Volvo power curves, and you have to correlate to the actual vessel. But the numbers sound roughly right. I have a friend with a 2014 Horizon Powercat PC52 that I've cruised with for a good 1000 nms around Keys and Gulf Coast. She has a pair of Cummins 6.7l (I think) diesels at about 550hp each. With digital controls, engine performance is accurately displayed. At 65%-70% load, she runs 18-kts and burns 36 gph total, which is pretty economical for a boat that sized. Last year, I bumped into someone with a 60-foot mono powerboat with large Cats that burned almost 60 gph at 20-kts.

Dialed back to 8.5 kts or so, and my friends PC52 burns around 7.5 gph (total). Every couple hours he'll bump it up on plane just as a precaution, but the engines do fine at lower RPMs and remain at operating temp.

Final comparison point is a Nordhavn 57 I delivered several years ago. She had a Lugger 300 hp straight 6 in her. We averaged 8-3/4 kts and 6 gph over 4500 nms from Dana Pt CA to Ft Lauderdale. Very impressive speed and relative economy.

So if the question is whether big engines can be slowed down for better economy, answer is yes - almost approaching traditional trawler economy. Is it bad for the engine? Newer common rail engines run fine, but it's still recommended to crank-up the RPMs every couple hours just to be sure.

Peter

[StuM]

Originally Posted by a64pilot

If I had a cruising Cat, I’d want my motors sized so that on one engine at 2/3 rated RPM and average conditions, it would give me a speed that I was comfortable with.
I would not size them so that it took both motors to cruise.

Running only one at a time will double the life of the motors or cut the hours run in half whichever way you look at it.

That sounds about right.

5.5 knots for me on one engine at 2/3 RPM burning about 3 lph. (Yanmar 40HP)

[Thumbs Up]

Pretty sure that the OP is asking about this one:https://www.multihullsolutions.com.a...63-phoenix-xi/
I an not so sure about this statement: "a typical rule of thumb is that a diesel will consume about 1 gallon/hour for every 20 hp produced." My Volvo md2030s burn about half a gallon of fuel per hour (each engine). They are rated at 29HP. I run them at around 2800 rpm. I make 5.5 knts with one and over 7 knts with both so am getting closer to double the efficiency than that rule of thumb on these non turbo engines.

[a64pilot]

Unfortunately common rail doesn’t increase efficiency all that much, it’s great for NVH, noise vibration and harshness, and to meet emissions, but it doesn’t do much for fuel efficiency.
It has however revolutionized the Diesel engine making it much more RPM flexible and clean burning and smoother and quieter, some of the latest common rails can have as much as five injection events per combustion stroke, injecting the perfect amount of fuel, and at the perfect time for emissions and clean burning.

In my opinion they are easier to maintain, but best carry a spare ECU and injector, something that some manufacturers make impossible because you have to have equipment that they won’t sell to program the ECU. All the magic is in the ECU and injectors, otherwise it’s sensors, a hydraulic pump and fuel pressure regulator.

However I’d rather not have one on my boat, but they can be as smooth and quiet as a gas motor.

[rslifkin]

Originally Posted by Thumbs Up

Pretty sure that the OP is asking about this one:https://www.multihullsolutions.com.a...63-phoenix-xi/
I an not so sure about this statement: "a typical rule of thumb is that a diesel will consume about 1 gallon/hour for every 20 hp produced." My Volvo md2030s burn about half a gallon of fuel per hour (each engine). They are rated at 29HP. I run them at around 2800 rpm. I make 5.5 knts with one and over 7 knts with both so am getting closer to double the efficiency than that rule of thumb on these non turbo engines.

No, you're not getting double the efficiency. If you're burning about 0.5 gph, you're probably only producing somewhere around 8 - 10 hp at your typical cruising load. They're rated at 29 hp, but they're only producing 29 hp at WOT (assuming they're propped to reach rated RPM at WOT). So at WOT, I'd expect them to burn around 1.5 gph (per engine).

[a64pilot]

Originally Posted by Thumbs Up

Pretty sure that the OP is asking about this one:https://www.multihullsolutions.com.a...63-phoenix-xi/
I an not so sure about this statement: "a typical rule of thumb is that a diesel will consume about 1 gallon/hour for every 20 hp produced." My Volvo md2030s burn about half a gallon of fuel per hour (each engine). They are rated at 29HP. I run them at around 2800 rpm. I make 5.5 knts with one and over 7 knts with both so am getting closer to double the efficiency than that rule of thumb on these non turbo engines.

My numbers are closer to .9 gallons an hour per HP, which is close enough.
However those numbers are pretty solid, what’s happening is that your not producing the 20 HP you think you are. Your making closer to 10 HP if your burning only a half gallon an hour.

On edit, find the fuel consumption graph for your engine, do not look at the HP the motor is capable of at x RPM, instead look at the propellor curve of how much HP the prop is using, on a normal installation the propellor doesn’t use anywhere near the HP the engine is capable of making, except at max RPM, at all RPM’s below max the engine isn’t loaded close to its max at all.

I’ll use my engine as an example, it’s capable of making 36 HP at 2800 RPM, But the prop is only pulling 20, so of course the engine only makes what the prop will absorb, or it would accelerate.

[MicHughV]

diesels, as said before, burn fuel in relationship to the work they do....a 29 hp diesel burning only 0.5 gal/hr indicates it is producing only about 13 hp....read " producing" to move your boat....which sounds about right for that engine...

there was another thread on here, about running 1 vs. 2 engines at the same time...running both engines, " reduces" the load on just a single engine, hence also reducing the fuel burn, it's not a simple math equation...where you divide 2 by a number.....doesn't work that way..

[a64pilot]

Originally Posted by MicHughV

diesels, as said before, burn fuel in relationship to the work they do....a 29 hp diesel burning only 0.5 gal/hr indicates it is producing only about 13 hp....read " producing" to move your boat....which sounds about right for that engine...

there was another thread on here, about running 1 vs. 2 engines at the same time...running both engines, " reduces" the load on just a single engine, hence also reducing the fuel burn, it's not a simple math equation...where you divide 2 by a number.....doesn't work that way..

It really is pretty close. If you take a twin engine boat, running both motors to make 6 kts, and shut one down and still make 6 kts, your fuel burn is pretty close, because the power required is the same.
What’s different is you in theory have half the friction of course, but two props are more efficient (less slippage) than one prop so it sort of washes out.
What’s different of course is the hours your not putting on the motor not running, cutting in half the hours, doubling the oil change interval etc etc.

[Thumbs Up]

A lot of people here report the same numbers (.5 gallons per hour per engine). I think that it takes a lot more than 20 hp to propel these boats at 7 knots, and a lot more than 10hp for 5.5 knots. These are real world numbers reported by members. We have had this discussion before.

[a64pilot]

Originally Posted by Thumbs Up

A lot of people here report the same numbers (.5 gallons per hour per engine). I think that it takes a lot more than 20 hp to propel these boats at 7 knots, and a lot more than 10hp for 5.5 knots. These are real world numbers reported by members. We have had this discussion before.

Well then I guess your engines are twice as efficient as everyone else’s then, because it can’t be that it only takes 10 hp to go 5.5 kts.

I guess you don’t like it, but it’s a fact that HP produced can be determined by fuel burned, and there are no magic engines that are way more efficient than others, if there were, don’t you think their marketing dept would be screaming about it at the top of their lungs?

On edit, using the chart above I cruise at 1800 RPM at 6.5 kts, and burn about a half gallon an hour. The chart above shows that I’m using 7.5 HP, but I have an autoprop and it will increase pitch to load the engine, so I’m probably using about 10 hp, to push a 24,000 lb 41.5’ mono at 6.5 kts.
Surely a Cat can do one kt less at the same power?

[MicHughV]

yes, the twin engine topic has been discussed at length before, it's important to have both engines running at the same rpm for optimum fuel burn...

As before, I would suggest reading the "propellor handbook" authored by Dave Gerr, it contains a ton of useful and relevant data useful to folks wanting to get a better understanding of diesel propulsion, fuel burn, horse power and torque curves, power ratings, prop selection, etc...

[Oceanride007]

Few of you would find these equations interesting.


VFC1/VFC2 = (V1/V2)^2 x (Disp1/Disp2)^(2/3) x (Dist1/Dist2)
Where VFC is Voyage Fuel Consumption V is velocity, Disp is Displacement, Dist is Distance
Its meant for similar ships or comparing a ship at loaded condition against a ballast voyage. Or just estimating how much fuel you need for a particular motored voyages.


Formula for Daily Fuel Consumption is:
DFC1/DFC2 = (V1/V2)^3.


From the above if everything else the same (Inparticular specific fuel consumption, Voyage Fuel Consumption would increase:
(new disp/old displ)^(2/3)


Answer 1% more fuel consumption.

[Oceanride007]

Haven't looked for many years but the Engine with the best Specific Fuel consumption was a very large Bore Marine diesel think it was a B&W, with a near 1metre bore and 3.5m stroke, Cylinder count not important here.Theory is that large combustion space is more efficient because of higher combustion temperatures in the centre of combustion space.
However that leads us to the Nitrous oxide debate, Environment v economy, who'd have thought the environment won, so one of major tools that a manufacturer has is in reducing Temperatures is in retarding the injection of fuel. Bad move I thought, but economies made else where, economy of modern engine is pretty good. But there is a serious BUT.

[StuM]

Originally Posted by a64pilot

Well then I guess your engines are twice as efficient as everyone else’s then, because it can’t be that it only takes 10 hp to go 5.5 kts.

5.5 knots for 10 HP is about right for me and around 3 lph

Roughly: 2000 RPM (1.5 lph) on two engines or 2500 RPM (3 lph) on one engine.