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Old 06-07-2020, 11:30   #1
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Bigger engines consume more diesel under cruising?

Anyone have any experience with
Volvo/1
Engine Type: Inboard
D300A1
It's a 300hp engine actually 2 of them on a 35,000lbs displacement full load.
Will an engine like this just totally be a pig even in cruising lower rpm conditions or will it be decent and just give me more power if called upon?
https://www.boat-fuel-economy.com/vo...umption-liters
interesting thing here to show consumption but can I run it at d300 1000rpm cruising for 3.4L/h yet if you bump it up to 1500rpm it jumps to 13.4L/h vs a d75 at 2000rpm at 4.9L/H?
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Old 06-07-2020, 11:40   #2
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Re: Bigger engines consume more diesel under cruising?

Isn’t the fuel burn proportional to the power produced

So if you only use 30% power...

Guess it depends on how much power that boat needs to make cruise speed
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Old 06-07-2020, 23:00   #3
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Re: Bigger engines consume more diesel under cruising?

Quote:
Originally Posted by Grimgrak View Post
interesting thing here to show consumption but can I run it at d300 1000rpm cruising for 3.4L/h yet if you bump it up to 1500rpm it jumps to 13.4L/h vs a d75 at 2000rpm at 4.9L/H?
RPM per se is meaningless.
Compare the HP output of the D300 @ 1500RPM to the HP output by the d75 @ 2000. It will probably be nearly 3 times as much.
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Old 06-07-2020, 23:43   #4
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Re: Bigger engines consume more diesel under cruising?

Just checked some spec sheets.
I don't know where you got your 13.4 number.

The curvers in the spec sheets I found at:
https://www.volvopenta.com/marinelei...ification.htmll
and
https://www.volvopenta.com/marinelei...ification.html
are very different.

They show approximately:

D400 @ 1500 RPM = 30 prop HP - 6 lph consumption
D75 @ 2000 RPM = 20 prop HP - 5 lph consumption.

Conversely,

The D75 gets 70 prop HP at 3000 RPM(WOT) at 17.5 lph.
The D400 gets 70 prop HP at 2000 RPM at around 13 lph.
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Old 07-07-2020, 05:54   #5
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Re: Bigger engines consume more diesel under cruising?

Yes. You are going to turn your boat into a pig. Multihulls tend to be much more sensitive to excess weight than monohulls. Putting giant, overweight engines into a multihull is just a bad idea all around.

Look at the specs that StuM provided. The important difference between those engines is a dry weight of 582 lbs versus a dry weight of 1345 lbs. Times two means that you'll be carrying an extra ton and a half in the back of your boat! Your fuel use will be more just because of that. Never mind the fact that the larger engines have more parasitic drag, and will therefore need more fuel to produce the same amount of power.

I think what you want is a high-speed, planing, power boat, not a sailboat. Trying to turn the latter into the former, by putting in grossly over-sized engines, is an exercise in wasting time and money. Just a bad idea all around.
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Old 07-07-2020, 05:59   #6
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Re: Bigger engines consume more diesel under cruising?

Also, diesel engines need/want to be run towards high end of their max output to stay happy. So if you have oversize engines you will be pushing along at max hull speed all the time with the associated discomfort.
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Old 07-07-2020, 06:09   #7
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Re: Bigger engines consume more diesel under cruising?

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Yes. You are going to turn your boat into a pig. Multihulls tend to be much more sensitive to excess weight than monohulls. Putting giant, overweight engines into a multihull is just a bad idea all around.

Look at the specs that StuM provided. The important difference between those engines is a dry weight of 582 lbs versus a dry weight of 1345 lbs. Times two means that you'll be carrying an extra ton and a half in the back of your boat! Your fuel use will be more just because of that. Never mind the fact that the larger engines have more parasitic drag, and will therefore need more fuel to produce the same amount of power.

I think what you want is a high-speed, planing, power boat, not a sailboat. Trying to turn the latter into the former, by putting in grossly over-sized engines, is an exercise in wasting time and money. Just a bad idea all around.

Where does the OP say anything about multihull?
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Old 07-07-2020, 06:10   #8
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Re: Bigger engines consume more diesel under cruising?

Bigger engines will burn a little more fuel for a given power output, but with diesels, it's not a drastic difference. Diesel efficiency doesn't drop off all that much at lower power output.

The bigger concern would be if you can ever load the big engines enough. If you run them lightly and never get a chance to work them hard, that could be an issue.
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Old 07-07-2020, 06:12   #9
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Re: Bigger engines consume more diesel under cruising?

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Where does the OP say anything about multihull?
An assumption since this is the multihull section.
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Old 07-07-2020, 06:38   #10
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Re: Bigger engines consume more diesel under cruising?

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Originally Posted by denverd0n View Post
Yes. You are going to turn your boat into a pig. Multihulls tend to be much more sensitive to excess weight than monohulls. Putting giant, overweight engines into a multihull is just a bad idea all around.
.
I didn't notice that this was in the multihull sailboat forum
I assumed that with 2 x 300HP he was looking at some sort of fast stinkboat but I see from other posts he's actually looking for a 45+ft sailing cat.

Putting 600HP on a sailboat is madness.

Even 2 x 75HP is a bit of overkill for a 35,000lb sailing cat.
That's about the weight of a Lagoon 570 which comes with 2 x 56HP engines!
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Old 07-07-2020, 06:47   #11
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Re: Bigger engines consume more diesel under cruising?

diesels are not like gasoline engines.....their fuel burn rate can differ even at the same rpm...depending on the load...

diesels also have different ratings....continuous...intermittent, etc....too involved to explain here...which affect fuel consumption....different diesels engines are designed to operate at specific rpm's...bigger diesel engines typically have much lower rpm's than smaller diesels.....it's a very involved science...


a 70 hp sailboat diesel at cruising rpm may only be producing ....say 40 hp...
a typical rule of thumb is that a diesel will consume about 1 gallon/hour for every 20 hp produced....therefore a 70 hp engine at cruise speed will burn about 2 gallons/hr..this is not set in stone, but a rough guideline....it's not uncommon for a 70 hp sailboat engine to consume less.....having a turbo charged engine is another variable.

diesels " burn" fuel, and this is a critical difference compared to a gasoline engine...for a diesel to " burn" fuel efficiently it must be loaded correctly....for instance, running a diesel engine to to turn the alternator to charge the batteries is not a very efficient way to run a diesel...

hope this provides some guidance...I have found the 1 gallon/hr per 20 hp produced is fairly accurate for sailboat diesels.......there is often a misconception about diesel engine horsepower figures....think of it this way....it's work produced....kinda like riding a bike....on a flat road,you can pedal at reasonable speed all day long, but going uphill at the same speed requires your legs to work much harder....and you will soon be exhausted....
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Old 07-07-2020, 06:52   #12
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Re: Bigger engines consume more diesel under cruising?

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Originally Posted by fivecapes View Post
An assumption since this is the multihull section.
Opps.....missed that.
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Old 07-07-2020, 07:15   #13
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Re: Bigger engines consume more diesel under cruising?

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Originally Posted by Hobiehobie View Post
Also, diesel engines need/want to be run towards high end of their max output to stay happy. So if you have oversize engines you will be pushing along at max hull speed all the time with the associated discomfort.

I see this all of the time and it’s simply not true.
On the 4th the Marina had a barbecue trailer over to feed us, on the tongue there was an Onan/Cummins generator purring away, It had an hour meter and had 7300 hours on it, and was purring happily away with a clean exhaust and no smoke at all.
Generators are fixed RPM, being as this was in the US I believe it was running at 1800 RPM, in Europe the same generator would be turning 1500 RPM, due to a generator being a fixed RPM, you can’t run a motor at the high end of their max output. That is the magic of inverter generators, they gain power but stay light by being able to turn up the RPM so that more power is available.
To answer the next statement, no they are not “special” engines designed so that max RPM is 1800, they are just common Diesels with max RPM usually around 3600 or so, just governed down to 1800 or 1500, and yet on average they last much longer than motors run near the max power output.

You know who runs Diesels near max power? Sportfishermen, and they go though motors like we do sails.

Commercial tugs and Commercial boats in general almost never run their motors hard, and they work every day and last for decades.

The disadvantage of a big motor on a sailboat is simple, it’s size and weight. That’s why turbo charging is so attractive to a manufacturer, they can have big motor HP in a smaller, lighter package.

However the BSFC or brake specific fuel consumption is pretty much flat, that means to make x amount of power it’s going to take x amount of fuel, there is a small efficiency increase at certain RPM and fuel flow, but it’s not huge, there is also greater friction in a larger engine, but again it’s not huge.

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.
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Old 07-07-2020, 07:18   #14
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Re: Bigger engines consume more diesel under cruising?

Most engines aren't rated to be run close to max output on a sustained basis, although some are. Some engines also have issues if run at too light a load for long periods of time without periodically being worked harder to get the combustion temps up, etc. It all depends on the engine in question. Basically, each engine design has a range of load / power output where it will live a long, happy life and operating it outside of that will produce less good results.
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Old 07-07-2020, 07:32   #15
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Re: Bigger engines consume more diesel under cruising?

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Bigger engines will burn a little more fuel for a given power output, but with diesels, it's not a drastic difference. Diesel efficiency doesn't drop off all that much at lower power output.

The bigger concern would be if you can ever load the big engines enough. If you run them lightly and never get a chance to work them hard, that could be an issue.
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.

Letís 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. Itís 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, Iím the first to admit this isnít 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.
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