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Old 03-16-2016, 01:09 PM   #21
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It seems to me that comparing those two engines is an apples and oranges type thing. It would be more helpful if you could compare two naturally aspirated engines. At what RPM does the turbocharger spool up for example? Turbocharged engines can deliver more power per liter displacement, but I don't know that they are inherently more efficient as far as fuel burn goes.
OK, so if I comapre 4JH80 vs 4JH57, one Turbo, one natual, at 40HP, both seems to consume same amount of fuel, roughly 2.25GPH

I guess I don't fully understand when people say "Turbo charged engine is more efficient", what does that really mean? Does it mean turbo charge can get more max HP on a similar sized block? Or does it mean they can extract more HP on the same amount of fuel? If the answer is latter, then I just don't see it with above data.
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Old 03-16-2016, 01:17 PM   #22
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Originally Posted by chicagoq View Post
I did some research on engine fuel consumption, I found at low rpm, bigger/smaller engines' fuel consumption are more or less similar. Take for example:
Yanmar 4JH5E, naturally aspirated, rated 53HP, burns about 3GPH at 3000RPM,
Yanmar 6LY3, turbo charged, 380HP, at 1600RPM it burns roughly 3.5GPH to produce 68HP
I read the data from the manufacture graph, so it's just a rough estimation

Is my above conclusion correct?

Also, I thought naturally aspirated engines are supposed to be less efficient, but from above data, it doesn't seem to have noticable difference? 6LY3 is more efficient by a small margin. I read somewhere turbo can get roughly 30% more power?
Lots of apples and oranges.

For 2 engines of moderate size difference, you can get similar economy. You're not going to get similar economy if you need 100 HP and you have 450 HP. There is a cost in fuel to spin the much bigger engine. Newer engines tend to be more efficient producing the same HP. The newest engines (tier 3 and up) will suffer very slightly in economy as cleaner burning requires doing things that slightly impact efficiency. Generally the best efficiency comes at between 1/2 and 3/4 of full RPM. So, in a perfect world, having an engine operating in that range giving you the desired speed would get you very close to optimal economy.

In the real world, most of us are buying used and generally repowering won't pay for itself in fuel savings. There are of course exceptions. Unless you plan to do a lot of cruising, fuel is probably less than 20% of yearly operating cost.

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Old 03-16-2016, 01:28 PM   #23
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Turbocharging does not make the engine more efficient per se, but does indirectly.

Say you need 300hp continuously. A turbo engine around 8-10 liters can make that power and be in the "sweet spot" of the BSFC map. To do this without a turbo, it would take an engine over 15 liters to be in the same sweet spot. Or an engine aroud 15 liters operating near full power. Larger engines have more frictional losses and more thermal losses, so BSFC suffers. And running near full power is no good for BSFC either.

In Chicago's post, if the smaller engine is full power, that equates to 17.7hp/gph, which is typical for a NA engine at full. Note that no engine is particulary efficient at full power. In the second example the turbo engine is making 19.4hp/gph down at low power setting. If you took the turbo off the 380hp engine and got it making 68hp, it would probably burn about the same. But it would not be capable of 380hp.

That's why virtually all modern large commercial/industrial/marine engines are turbocharged. The whole package is more efficient for the job at hand.
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Old 03-16-2016, 02:16 PM   #24
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Wifey B: Well the boat I was on this morning gets over 10 nmpg at 9 knots and below. Of course it's a RIB with a 125 hp Textron (formerly Weber). At 25 knots we get 7 nmpg and at 35 knots we get 6 nmpg. Main point is that comparing is so freaking hard as every boat is different. The boat who's swim platform the RIB is back on now doesn't do that well. The best it does is at 10 knots it gets like just over 1 nmpg. It does 10 knots at 900 RPM and we haven't tried to check fuel usage any slower.
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Old 03-16-2016, 02:31 PM   #25
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3000 rpm is definitely not low on our Yanmar 4JH2 100hp engine. The gauge may go up to 5000, but there's no way I'm taking it there!

We cruise at 2700 rpm and the turbo kicks in somewhere over 1000 rpm or so, I think. At that rpm we average 7 knots, and over a year averaged 1.3 US gallons per hour.

Again...I think the source you found is whacked for saying 3000 rpm is low.
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Old 03-16-2016, 02:40 PM   #26
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Large engines have much more surface area exposed to the heat of combustion and thus have much more heat loss. These engines are heat engines and heat loss is power loss and efficiency loss.

I have a 100 cu in 4cyl NA engine in my 30' boat. She cruises at 50% load. Notice I did not mention rpm. If I put a NA Ford Lehman of 380 cu in my boat I would bet large sums of money my fuel consumption would go up pushing the boat at the same speed of 6 knots.

But it would not be dramatically different even though both engines are making 20hp. Lots of veriables. But this example is very close to apples and apples. Very little difference except for the size of the engines and the extra 1000lbs of the FL may need to be factored in. But that's only about 1/16th of Willy's displacement. So you could "factor" w a stroke or two of the pen.

Bigger engines doing little work is not efficient. If I said engines working hard is very efficient it would be closser to the truth.

This should be more true of the turbo engine ... considerably more true. I mention this as a turbo v/s non-turbo engine comparissom was made. Ideally for efficiency one would have a small engine with a good turbo working at about a 75% load. Notice I didn't mention rpm again for the same reason .. it's not relavant. But of course in my example given the rpm would be high working at a 75% load.

But basically all one has to do is look at the "specific fuel consumption curve" (I think that's what it's called) and see at what rpm the lowest fuel burn is per one hp. Then you get into the variables ... and there are many. So many that if one were to set out to disprove my post here it would be easy to find a comparison showing the opposite to be true. But if you take a little care to compare apples to apples larger engines working less will fail to be efficient.
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Old 03-16-2016, 05:16 PM   #27
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Efficiency, as stated by several posters, is the amount of power produced by a given amount of fuel. It is generally independent of engine size (displacement). Efficiency depends on compression ratio, friction and, in particular, how the fuel is metered into the engine. Modern electronically controlled injectors meter precisely the correct amount of fuel,
(and no more) producing high efficiency.

Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased.

Part of the efficiency equation is pumping losses and friction in an engine. A larger engine has more of both. So if a boat needs 50 HP to go at a certain speed at partial throttle there will also be some power expended on friction and pumping losses. A 200 HP engine has less of these losses than a larger, 400 HP engine, although both are putting out 50 HP. Therefore the smaller engine will get better fuel economy. However, if the 400 HP engine is the same size/displacement as the 200 HP engine and gets its power from turbocharging, its losses will be the about same as the 200 HP engine and both will get the same fuel economy.

A modern automobile trend is to use small, four-cylinder turbocharged engines. They have the advantage of both high power and low losses, thus get good mileage.

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Old 03-16-2016, 05:52 PM   #28
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40 ft, 15 tons, full displacement, @ 1500 RPM, 7 knots, 1.5 gl/hr. Fuel is the cheapest part of our cruising summer. Moorage, insurance, maintenance, etc.
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Old 03-16-2016, 06:07 PM   #29
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40 ft, 15 tons, full displacement, @ 1500 RPM, 7 knots, 1.5 gl/hr. Fuel is the cheapest part of our cruising summer. Moorage, insurance, maintenance, etc.
Hey, no fair! Your avatar photo is too vague...is this your boat?

Kasten's Coaster 40

If so, good choice
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Old 03-16-2016, 06:16 PM   #30
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Yes that's the Kasten Coaster 40, photo taken during my more artistic period, sunset in the Gulf Islands. This was the first one built, but a Canadian from Pender Harbor built his own, but himself. It's unpainted but virtually identical.
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Old 03-16-2016, 06:22 PM   #31
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Yes that's the Kasten Coaster 40, photo taken during my more artistic period, sunset in the Gulf Islands. This was the first one built, but a Canadian from Pender Harbor built his own, but himself. It's unpainted but virtually identical.
Very cool.
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Old 03-17-2016, 01:07 AM   #32
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I asked a question a while back about using knots and mph. I was surprised when the majority of you said you used mph. And when a question like this is asked the vast majority of you used knots....?

Just an observation....carry on!!!!
We are (just about) all using nautical miles, without stating the nautical part. Still reads as mpg, instead of nmpg :-) No one I know uses statue miles on the water... No one I know uses kph or kpg either...
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Old 03-17-2016, 01:45 AM   #33
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Actually in the poll that was taken, 59% use nautical miles and knots only, 38% use nautical and statute, and only 3% use only statute.
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Old 03-17-2016, 04:09 AM   #34
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The huge divergence in performance figures in this thread only illustrates how difficult it is getting accurate consumption numbers on diesels because of the fuel return pipe on the injector pump; a tiny error in the fuel flow meter can skew the results by a large margin.

To the original question I'd suggest a rule of thumb: at slow speeds on all boats smaller than 60' and larger than 32' is that it takes about 5hp to push a boat at 7-8kts or slightly less for every ton of weight.

So weight in tonnes x5 to give you the HP needed.

Diesels use 1 gal of diesel to produce 20 HP.

Divide the total HP needed by 20 to get gals/hr at 7-8kts.

I think you'll find the result surprisingly accurate!
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Old 03-17-2016, 05:35 AM   #35
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I asked a question a while back about using knots and mph. I was surprised when the majority of you said you used mph. And when a question like this is asked the vast majority of you used knots....?

Just an observation....carry on!!!!
Except for the Canadian Government who have kilometers per hour signage in canals and harbours , never seen a guage in kph on a boat.... metric sharts drive me nuts too.
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Old 03-17-2016, 08:43 AM   #36
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I've read on line that sailboats can get 8MPG under power(no sail) ~ at low speed like 5-6knots.

Obviously power boats have very different engine, hull design etc. I wonder in general what kind of MPG can a power boat get at low speed? Or could you tell me for your boat, what max MPG can you extract? At what speed and what kind of engine? Thanks!
I'm a bit surprised at many of the answers because in boating we usually use GPH (gallons per hour) not MPG (miles per gallon). Why? because the water is moving under us (current) so miles through the water and miles over land are usually not the same. If you are travelling with the current, the miles through the water will be less than the miles over land. If you are travelling against the current, the miles through the water will be more than the miles over land.

At 2K RPM, my boat burns about 1.9 GPH. At that engine speed, my speed over ground (GPS) can vary from 5 knots to 9 knots, depending on the current. I've seen as low as 3.8 and as high as 12.4.
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Old 03-17-2016, 09:36 AM   #37
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Peter:

User reported fuel consumption data is inaccurate for several reasons: Flowscan or similar meter inaccuracies as you note (does anyone rigorously calibrate their Flowscan?), Generator and docking time erroneously included in travel time vs fuel fill data, and probably most importantly- owner wishful thinking.

Your rule of thumb broadly works. I proposed a more comprehensive one that takes into consideration displacement, hull type and engine type in a related thread. See Trawler Forum - View Single Post - What is your mpg, gallons per hour etc... at various speeds?

By making the rule of thumb based on power to reach displacement speed, you automatically consider water line length. A longer water line leads to a higher displacement speed and better NM/gal. If you have two boats with the same displacement and hull type, the one with a longer water line length will achieve better fuel mileage.

But maybe the biggest variable is hull type. True displacement hulls are fairly consistent in hp per thousand pounds to reach displacement speed. The data shown in Bebe's book (edited by Leishman) shows a rather small variation of maybe 20%.

But semi-displacement hulls vary all over the map at low speeds. One poster on the referenced thread claimed he used twice the displacement hp or 3 hp per 1,000 lbs for a semi-displacement hull. My downeaster style Mainship Pilot 34 is notoriously inefficient at low and high speeds.

A less significant variable is engine type. No matter whether it is a 30 year old 5.8 liter 120 hp Lehman making 40 hp or a new 380 hp Cummins QSB making the same hp, they both make about 16 hp per gph of fuel. Only a modern smaller displacement engine like the 4.5 liter JD will do better because it has less iron to turn over and its fuel injection is optimized for the lower hp output.

So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption.

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Old 03-17-2016, 10:09 AM   #38
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David,
You keep talking about "reaching hull speed". I thought we made that expression illegal. It's valid however for SD hulls and about 93% here have SD. Many more have planing hulls than FD hulls. So it may increase clarity to state "(SD)" when you talk about power to reach hull speed. That's the best cruising speed for most SD boats but a FD skipper would be absolutely stupid to run at HS. And if not he clearly doesn't have a FD hull.

Rusty says;
"Diesels use 1 gal of diesel to produce 20 HP."
I think 15hp per gallon is closer to most of the engines on this forum.

WesK,
MPG is totally foreign to me on the boat.
GPH.

Paul Swanson wrote;
"Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased."
It is more efficient because the turbo pumps more air through the engine with less effort than the pistons do as a reciprocating air pump. And the CR is actually decreased to prevent over charging the combustion chamber w air creating too much pressure at compression.

David also wrote;
"So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption."
I don't even give fuel burn numbers that much credit on TF.
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Old 03-17-2016, 10:58 AM   #39
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I'll test Peter's math on my ride: 7.5t x 5hp/t = 38hp. My big engine is not very efficient at that low power, so figure 17hp/gph: 38/15 = 2.5gph.

At 7.5 kts my dipstick and tankfill gph avgs 1.9gph, well established. Although a few times we ran 8kts, and it was like 2.3gph.

We won't talk about running 20kts, that is the other speed of this dual purpose sled.

I might be better than most as the hull is relatively skinny, 38x12, and only about 10.5' at water line.

Or it could be my awsome job getting the bottom fair in construction!!??

I'd say Peter's math passes the sanity test.

Edit: most of our diesels are going to run around 15-17hp/gph depending on where we run them. The 20hp/gph is about the limit, only acheived when it is an efficient engine AND run at it's sweet spot on the BSFC curve. Really large engines of many thousands of HP can beat 20, but not ours in the 50-500hp class.
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Old 03-17-2016, 11:02 AM   #40
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David,
You keep talking about "reaching hull speed". I thought we made that expression illegal. It's valid however for SD hulls and about 93% here have SD. Many more have planing hulls than FD hulls. So it may increase clarity to state "(SD)" when you talk about power to reach hull speed. That's the best cruising speed for most SD boats but a FD skipper would be absolutely stupid to run at HS. And if not he clearly doesn't have a FD hull.
Eric, it is pretty much equally disadvantageous for an SD boat to run near hull speed as it is for a FD boat. David is speaking theoretically.

I have a question......why in the heck are Floscans so damn inaccurate when it measures the fuel being burned as well as the fuel being returned???
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