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Old 07-11-2013, 09:39 PM   #41
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Originally Posted by markpierce View Post
At my normal cruising speed of 6.3 knots at 1800 RPM, I'm getting 85% of the JD's 80 horsepower consuming about 1.7 gallons an hour.
You may be running at 85% of rated RPM, that doesn't mean 85% of rated HP.
Wide open throttle for me is 1650 rpm with 150 hp using close to 7.5 gph at 11 kts.
At 1400 rpm I use 2.75 gph at 8 kts. My guess is that is around 60 hp.
A small throttle decrease can mean a large fuel savings.

By the way, that's 3 mpg for 87 tons. I was surprised I did that well with a Detroit 6-71.
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Old 07-12-2013, 01:11 AM   #42
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PG,

How'ed you arrive at 6gph? Sounds about right though.
Uh... I leave the math to my honey pie and his spreadsheets. He plugged the fuel consumption and hours into his spreadsheet and told me the gph and $$ we've spent on diesel. I just say "yes dear". Isn't that what the admiral is supposed to do?
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Old 07-12-2013, 09:49 AM   #43
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That's OK with me. (I looked at the wrong power curve.)
No, your repeated posting of those figures in numerous threads since you bought that boat indicates that you don't know how to read the performance graphs for a marine engine. If you didn't post in threads like this it wouldn't matter, most people don't but they don't pretend to either.

Normally it wouldn't matter one iota to anyone and I could care less how much you know or don't but when you choose to post in a thread titled "engine efficiency" that is read by a few thousand readers, it is not unreasonable to hope that what you post as factual information is at least close to reality.

If you are interested in efficiency and posted to contribute or question published or observed data then perhaps it should not surprise you when you repeatedly claim your engine is somewhere around twice as efficient as the most efficient marine engines on the planet, that that claim should attract some attention.
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Old 07-12-2013, 10:17 AM   #44
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No, your repeated posting of those figures in numerous threads since you bought that boat indicates that you don't know how to read the performance graphs for a marine engine. .
Interesting... as I don't know how to determine the efficiency of my engine but would like to. What parameters do you need to determine the efficiency or do you just read it off the engine spec sheet?
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Old 07-12-2013, 10:38 AM   #45
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Interesting... as I don't know how to determine the efficiency of my engine but would like to. What parameters do you need to determine the efficiency or do you just read it off the engine spec sheet?
The spec sheet will tell you the amount of fuel burned to produce a given amount of power as determined by operating that engine in a test cell. Period.

The numbers Mark posted are just the numbers supplied by JD on sheet 2 of the engine specific data sheet for his engine. Those figures may or may not have any association with the reality of that engine installed in his boat. The power figure he supplied is so far off that it should be obvious to anyone reading the data sheet in order to calculate boat performance or to share information about engine efficiency, hence my post.


You can use that sheet to estimate boat performance based on a theoretical propeller that absorbs the engine's rated output at its maximum rated rpm. What you do from there is most likely purely subjective or based on empirical data obtained during use or by recommendation of an expert or experienced user of the same boat and engine.


My take on it is that engine efficiency is only one small part of a much larger system which includes useage and subjective performance goals. You can have the highest possible fuel efficiency in terms of Brake Specific Fuel Consumption but put that engine in a poorly designed hull or use an efficient engine in an efficient hull in a manner for which it was not designed and can not perform then system efficiency is moot.

Do you want the greatest distance over the ground for the least fuel? The least time? The most weight carried? The best ride? What do you want from your boat?
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Old 07-12-2013, 10:43 AM   #46
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Walt I wonder why you are interested in fuel efficiency?

I suspect your'e just interested in talking about it. Like me. If any of us were seriously interested in significantly more fuel efficiency we'd need to get different boats .. including me.

Engine efficiency is easy to determine as sunchaser and Rick point out but not very meaningful because the other part of the picture is the boat and totally accurate numbers are not possible as there are too many variables.

One way to come very close however is to accurately measure fuel consumption at a given speed and use the charts to determine how much hp is produced. But the numbers are only good for WOT at rated rpm. There are other charts that come closer but none will be "dead nuts". You can't isolate the resistance of the hull. Too many unmeasurable variations. But by just looking up numbers we can come close ... closer than we actually need.
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Old 07-12-2013, 11:04 AM   #47
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...
The numbers Mark posted are just the numbers supplied by JD ...
Regardless, the engine burns 4 GPH at full throttle (2400 RPM), and less than half that at 6.3 knots (1800 RPM).
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Old 07-12-2013, 11:55 AM   #48
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My boat 40hp 2gph WOT 1 gal hr cruise.

Same speed half the power and half the burn.
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Old 07-12-2013, 12:08 PM   #49
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Walt I wonder why you are interested in fuel efficiency?
It's an interesting subject although efficiency is almost at the bottom of my list. I simply wanted to know how it is arrived at.
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Old 07-12-2013, 12:48 PM   #50
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You may be running at 85% of rated RPM, that doesn't mean 85% of rated HP.
Wide open throttle for me is 1650 rpm with 150 hp using close to 7.5 gph at 11 kts.
At 1400 rpm I use 2.75 gph at 8 kts. My guess is that is around 60 hp.
A small throttle decrease can mean a large fuel savings.

By the way, that's 3 mpg for 87 tons. I was surprised I did that well with a Detroit 6-71.
Nice to see a real world user of marine diesel fuel reporting in. I often marvel at how fuel efficient tugs are, say vs a 1/3 the weight yacht. The recent edition of PM has a good article on the ATW N40 and how it accomplished its very fuel efficient task
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Old 07-12-2013, 01:54 PM   #51
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Tom to me it's strange that anybody would think of Tugs as fuel efficient. Engines way way too large, very excessive weight and large wetted surface.

Perhaps if you looked at it from a power per ton requirement they wouldn't look so good.

Why do you suppose Dirk's boat is fuel efficient? He says "A small throttle decrease can mean a large fuel savings." Indeed but most all of that difference comes from the difference in hull resistance not engine efficiency.

I'm jealous of anybody w a DD. Dirk isn't the rated rpm of a 6-71 2300?
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Old 07-12-2013, 02:21 PM   #52
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Wonder of the fuel consumption of this near-Coot-sized tug equipped with azipod.

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Old 07-12-2013, 02:37 PM   #53
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Some of us LOVE fuel efficiency and could care less about engine efficiency...oh no...what is what???? Rick seems to get it.

It doesn't matter one little bit what your engine burns or what your flo scan says...it matters what you actually burn on your trip.

Some here will argue till they are blue in the face (and I'm sure they look just as comical when replying) that you can't or shouldn't as it's worthless to plan around tidal currents. Some will say...hey..it's just money and a small fraction of my boating budget..."who cares?" Well for some of us fuel can be a BIG chunk because we USE our boats a lot every year. Some will discuss how it doesn't matter because you don't have a FULL displacement boat yet at certain speeds ANY hull form gets realistically close to the efficiency of "full displacement" designs....cough....cough.

I could go on for quite a bit....but after a long life of cruising boats, 3 liveaboards, and lots of commercial miles where economy mattered to someone...there are lots of tricks to keep your annual fuel bill down...how you do it is up to you....just referring to an engine graph and posting how great your engine is while a tropical rain forest grows on the bottom of your boat while underway serves no purpose other than to make you feel good.
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Old 07-12-2013, 02:40 PM   #54
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Tom to me it's strange that anybody would think of Tugs as fuel efficient. Engines way way too large, very excessive weight and large wetted surface.

Perhaps if you looked at it from a power per ton requirement they wouldn't look so good.

If that tug were put in perspective it would look very different. When delivered it made 10 knots from a 100HP gasoline engine that directly turned a prop at 350 rpm. That engine weighed close to 6000 pounds.


Modern yachts don't do too badly. A moderate size (40 meters) boat that weighs 740,000 pounds can get better than 9 knots out of 20 gallons per hour and keep 20 people in luxury for about 21 days nonstop while traveling nearly 5000 nautical miles. I wouldn't call that inefficient.

You might look at it as it takes that yacht 11.6 gallons to transport each person onboard 223 miles per day in absolute comfort and luxury.

What does efficiency really mean when you are not hauling cargo and hoping to make a profit?
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Old 07-12-2013, 03:17 PM   #55
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Wonder of the fuel consumption of this near-Coot-sized tug equipped with azipod.
For starters, it has a pair of DD8V71s turning 44" diameter props the old fashioned way.

It is the old Cates #20 built in 1969 and is just over 41' long. If you wound it up to its full bollard pull it would probably burn around 30 gallons per hour.
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Old 07-12-2013, 03:28 PM   #56
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Tom to me it's strange that anybody would think of Tugs as fuel efficient. Engines way way too large, very excessive weight and large wetted surface.

Perhaps if you looked at it from a power per ton requirement they wouldn't look so good.

Why do you suppose Dirk's boat is fuel efficient? He says "A small throttle decrease can mean a large fuel savings." Indeed but most all of that difference comes from the difference in hull resistance not engine efficiency.

I'm jealous of anybody w a DD. Dirk isn't the rated rpm of a 6-71 2300?
My answer to that is "It depends"
DD 71's range from the ancient two valve head models to the turbo charged monsters.

Mine is an "N" with a lower speed blower and the smallest injectors they make.

As for the T.B. McClintic being a tug, it just looks that way. It was built in 1932 as a floating surgeons office. It's heavy but very streamlined. A 50" prop on a 4-1 gear helps keep the fuel bill low. I agree that hull resistance at a given speed is the primary factor overall efficiency.
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Old 07-12-2013, 03:46 PM   #57
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Nice to see a real world user of marine diesel fuel reporting in.
?? No more "real world" than all the other members of this forum who run toy boats with the same engines and displacement for the fun of it.

tugboat efficiency is measured by the amount of fuel consumed to develop a given bollard pull. If they aren't working hard they are just wasting a ton of fuel hauling around an engine far far larger than they need to move the boat from the dock to the job.

That is why the newest approach is to go hybrid and shut off the diesel except when they need the power.
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Old 07-12-2013, 04:53 PM   #58
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Real world numbers Perkins 6.354:

Anacortes to Ketchikan 897 miles 180 gals
Ketchikan to Sitka 847 miles 183 gals

Needless to say we didn't go by the shortest route and between Ketchikan and Sitka and we bucked the tide and winds a couple of times. I generally tried to run between 1500 and 1600 RPM but really focused more on a SOG target so as to get to a place in time for the tide or to end the day at a decent hour. A couple of times I got into the throttle pretty deep for a couple of stretches in 5-6 foot following seas to maintain a little better control and then I was running in the 1800 to 1900 RPM range.

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Old 07-12-2013, 07:55 PM   #59
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Real world numbers Perkins 6.354:

Anacortes to Ketchikan 897 miles 180 gals
Ketchikan to Sitka 847 miles 183 gals
That is my idea of "efficiency" and a good return for the quantity of fuel burned on a boat large enough to provide a good standard of comfort.
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Old 07-12-2013, 08:04 PM   #60
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How's about 8 inches per gallon?

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