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Old 14-06-2012, 23:06   #1
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Engine Sizing and Fuel Consumption

Hi all,

I'm looking in to choosing an engine and I've come up against a rather interesting question. Here goes...

I'm looking at a repower from an 18hp diesel. The boat needs around 10hp by my estimation to be pushed at a nice cruising speed. I've been looking at Beta engines between 16-25hp.

To produce a given HP output, the larger engines need to run at lower RPM, ie. with a greater pitched or larger propellor. However, the larger engines seem to have lower fuel consumption to produce a given power. I know fuel consumption is not linear with RPM, but this seems surprising.

To put out the 10hp (which I estimate will produce a nice 5kt) these are the figures. The graphs supplied aren't the best, so these need to be treated with caution.

Beta 16 = 1900 RPM, 0.7 litres per hour
Beta 20 = 1700 RPM, 0.5 LPH
Beta 25 = 1400 RPM, 0.5 LPH

A little surprising the larger engines don't use more fuel, but hey ho. Putting out 16hp is where it gets interesting though.

Beta 16 = 3600 RPM, 3.5 LPH (full power)
Beta 20 = 2600 RPM, 1.8 LPH
Beta 25 = 2100 RPM, 1.2 LPH

The fuel consumption is lower with a bigger engine. Not just a bit - a lot. To prove a point, here's 20HP.

Beta 20 = 3600 RPM, 4.5 LPH
Beta 25 = 2600 RPM, 2.2 LPH
Beta 30 = 1900 RPM, 1.0 LPH

That last one particularly got me. Going from a 25HP to a 30HP engine HALVES the fuel consumption needed to produce 20HP.

I appreciate that an engine running at full RPM is not at its most economical power. But this seems to be true over the whole range.

So am I right? Because fuel consumption and RPM aren't linear over any part of the RPM range, it will within reason be more economical to fit a larger engine, and by a big margin? Surely not! Am I just reading too much in to sketchy graphs?

Here are the data sheets.
http://www.betamarine.co.uk/seagoing...6-SDS-0311.pdf
http://www.betamarine.co.uk/seagoing...0-SDS-0311.pdf
http://www.betamarine.co.uk/seagoing...5-SDS-0311.pdf
http://www.betamarine.co.uk/seagoing...0-SDS-0311.pdf
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Old 15-06-2012, 00:34   #2
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Re: Engine sizeing and fuel consumption question

My John Deere 4045DFM uses about 9 litres/hour at 7 odd knots and 2000rpm. I estimate that I'm using about 40 hp under those conditions.

If you scroll down the specification in the above link and look at the power and fuel consumption graphs for that speed you'll see that I should be using 15 litres/hour.

If, however, you look at the propeller power required at 2000rpm and then the fuel consumption for that engine speed it should look pretty close to 9 litres/hour.

The Beta graph graph mentions "Fuel Consumption based on theoretical
propeller loading - matched at full speed", "Max Output" and "ISO-8665".

When the brochure figures are translated to the real world I would not be surprised to see a fuel consumption of rather more than 0.5 litres/hr to develop 10 hp.

I'd also expect the smaller engines to be more efficient at producing 10hp than the larger ones, but not to any sort of order of magnitude.

However, much more important than fuel consumption is matching the engine/transmission/propeller to your boat and your intended use.

You don't say what sort of boat you have, but assuming a heavier boat about the 30' mark to be used for cruising I'd look at the 30hp engine if I were choosing.
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Old 15-06-2012, 09:51   #3
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Re: Engine Sizing and Fuel Consumption

Can't speculate without knowing the boat. A 16 hp in a 6,000 pound boat that's skinny is a better choice than a 20 hp in a 11,000 pound full keeler...but even then it's a question of the prop size and pitch and the need to use max power to maneuver or because you have to go against a strong tide in a river or something.

We haven't even gotten to the issues of the extra weight and size of any new engine that's a little bigger. "Can it fit without me training a helper monkey?" is Question No. 1, not fuel use, in my view. We also haven't considered the possibility of a thicker prop shaft, which is a way uglier prospect than pennies for diesel.

Betas are generally a good choice, however, because they have a focus on ease of access.

On the basis that the two extra horses can spin a slightly higher output alternator, I would, without considering any of the above, opt for the 20 HP.
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Old 15-06-2012, 10:02   #4
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Re: Engine Sizing and Fuel Consumption

europaflyer,

I agree a larger engine at low RPM's is more fuel efficent than a smaller engine running higher RPM's.

My boat comes standard with 2 x 20 hp
I purchased it with 2 x 30 hp. I am using less fuel than the 20 hp boats.
I am much faster than them also. I can run on one engine with little impact on speed.

Go with the bigger engine. You will be happy you did.
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Old 15-06-2012, 10:22   #5
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Re: Engine Sizing and Fuel Consumption

Thanks for the replies!

Cotemar, that's pretty much along the lines of what I was thinking. The boat is a 9,500lb 32ft fin keeler. Normal engines would be 18-20hp. Looking at the figures, I was thinking that the 25hp would at best not hurt the fuel consumption, and if anything improve it. Your experience seems to back this up. The 25 isn't that much bigger than the 20, access shouldn't be an issue.

Boracay, you seem to disagree that the larger engine would be more efficient. Is this experience, engineering knowledge, etc?

Anyone else want to chime in?
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Old 15-06-2012, 10:22   #6
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Re: Engine Sizing and Fuel Consumption

I agree on the larger engines. These newer engines are rated at high rpm.... if they were rated like the old days they would be rated at half their HP rating! I'd pick an engine that gets you hull speed at 2300-2500 rpm.
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Old 15-06-2012, 10:31   #7
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Re: Engine Sizing and Fuel Consumption

There is always a lot of confusion with this because there are so many variables and few common misconceptions.

You need to look at the engines Specific Fuel Consumption. This is how much power an engine produces for a specific fuel burn rate. Or in other words, the engine efficiency.

It is true that as a generalization that larger engines are more efficient, but there are also exceptions. On the other hand, larger engines cost more, take up more space and they weigh more.

You only need an engine that has enough horsepower to go as fast as you want or one that takes you to hull speed for a full displacement boat, whichever comes first.

You don't need or want an engine that can make you go faster than what I described.

It is not really correct that purchasing an engine larger than what you need is going to save you money in fuel costs. You will spend more on the engine, more on engine maintenance and more on depreciation. Your boat will also weigh more making it slower under sail and less efficient under power.
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Old 15-06-2012, 10:42   #8
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Re: Engine Sizing and Fuel Consumption

I do not know where you calcs went wrong, but I think they may have.

IMHO any engine uses roughly 200 grams of fuel for 1 hp delivered. The number varies slightly, but not as dramatically as in your research.

An example:

An engine asked to deliver: 10 hp.
Fuel consumption expected: 2 liters per hour.

Again, expect some variation, depending on how big an engine you used, say 10, maybe 20%.

BTW Pick the engine so that it is optimally used - say .5 to .75 of top revs most of the time. This is supposed to keep the engine loaded and thus avoiding apparent damage from cold running / overheating (not all engines are built to run at full revs over longer periods).

If you are into strong tidal/very choppy sailing, use 4 hp of available continuous power per 1000 kg of displ., more, if the hull is very full.

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Old 15-06-2012, 10:48   #9
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Re: Engine Sizing and Fuel Consumption

and dont forget about windage. 35 knots of wind and 4-6 foot seas take a lot of power!
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Old 15-06-2012, 10:50   #10
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Re: Engine Sizing and Fuel Consumption

Error on the side of having slightly too much
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Old 15-06-2012, 10:56   #11
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Re: Engine Sizing and Fuel Consumption

I thought I was looking at the specific fuel consumption - the fuel used for a given power output. So, the specific fuel consumption decreases in larger engines, ie. they use less fuel to produce the same power. This seems to be what the data is showing. Is this plausible?

Basically, the data shows the 25 uses less fuel than the 20 when producing 16hp and 20hp, and is the same when producing 10hp. On the strength of this, other factors aside, to reduce fuel consumption especially at higher power outputs I should fit the larger engine. True or false?
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Old 16-06-2012, 00:10   #12
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"Bigger is better" is an opinion not some fact. I have a Beta 20 in a 16000# boat. Fin keel. Works fine cruising to Mexico and bashing back. No I do not get hull speed if there is any adverse sea or wind. But who cares? I don't.

I think there is something inaccurate or misunderstood regarding the Beta charts. I recall having the same observation about the fuel consumption charts. The Beta engines are very economical at all power settings ... Given a proper prop.
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Old 16-06-2012, 00:47   #13
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Re: Engine Sizing and Fuel Consumption

My expectation is that an engine will have an optimum speed where it is most efficient.

A larger engine may be most efficient when developing it's designed power. If that's more power than you need then it will be less efficient. On the other hand if a smaller engine is required to produce maximum power continuously then it may be less efficient.

If my John Deere is running at low speed I can smell the unburnt diesel fuel. It's not going to be efficient at low speed.
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Old 16-06-2012, 02:13   #14
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Re: Engine Sizing and Fuel Consumption

Aim for 70% rpm at hull speed!
(approx)

This wil give you a reserve of power
For the odd time that you need it.
The engine will then spend the
major part of its time about 70%
Of its rpm meaning good fuel economy,
whilst still having a good load on the engine.

Don't forget to run the engine at full throttle
Ocaisionaly to help stop the bore's getting
Glazed which keeps the rings bedded, can
Some times will stop oil burning as well

The above for you would be about
22 - 25 hp guesstimated

I run kubota based 27hp - 3cyl & get about 2lt per hr
At 2200-2300 rpm (3000 max)
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Old 16-06-2012, 02:28   #15
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Re: Engine Sizing and Fuel Consumption

Running an engine for long periods at low speed can cause carbon problems - one of many reasons not to overdo the power. If you end up cruising at, say, half power then every couple of hours run the RPMs up for a little while to prevent buildup.

OTOH sizing an engine based on performance in calm conditions might leave you underpowered. Many times I have wished for more power when trying to get through waves against the wind. Making just one knot to weather was not unusual, so I bought a larger engine when it was time. Carina tips the scales at 23k#, and the engine was a Sabb 18hp with variable pitch prop - which allowed me to get optimum pitch for conditions. The new engine is a *nominal* 38hp Volvo, with a larger 3-blade prop.

Also consider the effect of the alternator: if the batteries are low when you turn on the engine, the power available to the prop will be less. Even more so with a smart regulator or large alternator, or both. On Carina the effect could be a knot or two, with a 65A alternator. The new Volvo has more power, and a 130A alternator. One of the advantages of a large alternator is that a larger load is put on the engine when the engine is run just to charge the batteries (better for the engine, and potentially shorter run time). If the alternator does end up taking too much power on occasion, then rig up a smart regulator and add a switch to the power lead so the alternator can be turned off and all of the power available to drive.

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