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Old 12-25-2013, 04:37 PM   #21
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Originally Posted by Portuguese View Post
Thank you all guys

Ted, thank you for your comments in particular.

I didn't mean to start a major discussion about the subject. It is obvious that we are far from a conceptual consensus about the issue.
I have no doubts whatsoever that fuel efficiency for a given vessel has to be translated in miles per gallon, distance traveled per volume of fuel burnt, which translates in work done/energy spent in the process. It does not matter if it considers speed over ground, speed made good, shaft rpm engine brand etc. This is just desk engineering technicalities which I am tired off after 33 years of deep water sub-sea engineering in the oil drilling industry.
What I would like to see here, is an arrow straight answer based on the boat data below, from your experience, assuming that the boat sails, with a 1.1 times the hull speed, 7 knots, in normal quite conditions of sea, current, wind etc, covering 2.4 miles per gallon, is this specific hull performing good or bad in terms of fuel efficiency? That is all I want to know.

Pertinent data of M/V “Rainha Jannota”
LOA 46’
LWL 40.2’
Hull Weight @ 75% 55000 lbs
Main Engine Deutz Turbo 145 HP @ 2500 RPM
Gear ZFW220 3.96:1
Propeller 4 Blade Bronze 34” x 26”
Shaft 2.25” 304 SS

Thank you all
While I'm no expert on this, nor am I familiar with Deutz engines, basd on pictures of your hull, your choice of Hp and drive train, if we were holding a lottery on your MPG @ 7 knotts, I would pick 3 MPG flat water no wind no current.

Ted
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Old 12-25-2013, 04:51 PM   #22
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Originally Posted by RickB View Post
Which is a number that may vary radically within hours and miles. It depends on the wind, current, waves, and ability of the driver to steer straight.
Of course. That's why averaging over longer runs is more accurate. And why good sea trials report the conditions and do two opposing runs to counteract these variables as much as possible.

But what more objective measurement is there?

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When the term "efficiency" is tossed in just for effect, it becomes even more of a gratuitous expression since the power and time to move an unknown weight that distance is not expressed or probably even known.
I disagree. The energy is represented exactly in the quantity of fuel consumed. There is nowhere else for it to come from. And the distance is whatever distance you decide to go. If one boat can get there on 5 gal and another on 7 gal, the 7 gal boat is less efficient. And their efficiency would be expressed in NM/G. It's not opinion, it's not pseudo science, it's physics.

Sorry to everyone for the sidetrack here. That was not my intent. I was just trying to add some science and objectivity to what is typically a pig wallow of apples and oranges comparisons that say nothing. There always seems to be someone who claims a super efficient boat, when really what they have is a boat that goes slower than everyone else's, or is lighter than everyone else's. When you normalize it to NM/G, and compare at the same speed, you will find there is much less difference between boats than you might think.
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Old 12-25-2013, 05:15 PM   #23
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Originally Posted by Portuguese View Post
Pertinent data of M/V “Rainha Jannota”
LOA 46’
LWL 40.2’
Hull Weight @ 75% 55000 lbs
Main Engine Deutz Turbo 145 HP @ 2500 RPM
Gear ZFW220 3.96:1
Propeller 4 Blade Bronze 34” x 26”
Shaft 2.25” 304 SS
To move that boat at 7kts will take 63HP according to the BoatDiesel calculator which is as good as any.

You will need to find the prop curve for the exact Deutz engine to get the correct answer, but as a substitute to show the calculation, consider a Deere 4045TFM50 which is M3 rated at 145HP. At 63HP load it burns 3.47 GPH. 7Kts divided by 3.47 GPH yields 2.0 NM/G

I hope that helps.
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Old 12-25-2013, 05:17 PM   #24
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Originally Posted by Portuguese View Post
Thank you all guys Ted, thank you for your comments in particular. I didn't mean to start a major discussion about the subject. It is obvious that we are far from a conceptual consensus about the issue. I have no doubts whatsoever that fuel efficiency for a given vessel has to be translated in miles per gallon, distance traveled per volume of fuel burnt, which translates in work done/energy spent in the process. It does not matter if it considers speed over ground, speed made good, shaft rpm engine brand etc. This is just desk engineering technicalities which I am tired off after 33 years of deep water sub-sea engineering in the oil drilling industry. What I would like to see here, is an arrow straight answer based on the boat data below, from your experience, assuming that the boat sails, with a 1.1 times the hull speed, 7 knots, in normal quite conditions of sea, current, wind etc, covering 2.4 miles per gallon, is this specific hull performing good or bad in terms of fuel efficiency? That is all I want to know. Pertinent data of M/V “Rainha Jannota” LOA 46’ LWL 40.2’ Hull Weight @ 75% 55000 lbs Main Engine Deutz Turbo 145 HP @ 2500 RPM Gear ZFW220 3.96:1 Propeller 4 Blade Bronze 34” x 26” Shaft 2.25” 304 SS Thank you all
Hmm, I've heard great things about Deutz, a lot of commercial boats run them. Also I saw a Hatteras 92 repower with them.
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Old 12-25-2013, 05:49 PM   #25
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Twistedtree said: I disagree. The energy is represented exactly in the quantity of fuel consumed. There is nowhere else for it to come from. And the distance is whatever distance you decide to go. If one boat can get there on 5 gal and another on 7 gal, the 7 gal boat is less efficient. And their efficiency would be expressed in NM/G. It's not opinion, it's not pseudo science, it's physics.


That’s exactly my point. Measuring distance per fuel burnt is the most logical way to define efficiency. I agree.
To move that boat at 7kts will take 63HP according to the BoatDiesel calculator which is as good as any.

You will need to find the prop curve for the exact Deutz engine to get the correct answer, but as a substitute to show the calculation, consider a Deere 4045TFM50 which is M3 rated at 145HP. At 63HP load it burns 3.47 GPH. 7Kts divided by 3.47 GPH yields 2.0 NM/G


I have done that exercise just before I initiated this thread. Comparing Boatdiesel results with the engines prop curve it was adjusted in 53hp. This power required takes me to 2.55 gallons per hour or 2.74 miles per gallon.
I am happy, and thankful to all of you who made me feel safe with my calculations
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Oliver said: Hmm, I've heard great things about Deutz, a lot of commercial boats run them. Also I saw a Hatteras 92 repower with them.


I saw a taxi boat with 2 of these babies with 9000 hours. Never made an overhaul, never even open heads or changed anything inside. It’s only diesel, oil, refurbishing the water pump twice in one engine, and 3 times in the other
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Old 12-25-2013, 06:06 PM   #26
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I have done that exercise just before I initiated this thread. Comparing Boatdiesel results with the engines prop curve it was adjusted in 53hp. This power required takes me to 2.55 gallons per hour or 2.74 miles per gallon.
I am happy, and thankful to all of you who made me feel safe with my calculations
I don't follow this. How did you "adjust" the prop curve to 53hp? Perhaps 53HP is the closes data point that Deutz provides on their prop curve chart? If so, you need to scale that to the 63HP required to move your boat 7kts. The easiest way to do it is to use the closest published data point and calculate the number Gal/hr/hp, i.e. GPH divided by the HP produced. Then multiply that times the desired HP of 63.
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Old 12-25-2013, 06:25 PM   #27
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While HP calculators are a good place to start, my very limited experience with the one on boatdiesel tends to require modestly to moderately more HP than real world numbers. If using modern more efficient engines, the numbers tend to be further off.

Ted
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Old 12-25-2013, 06:32 PM   #28
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If one boat can get there on 5 gal and another on 7 gal, the 7 gal boat is less efficient. And their efficiency would be expressed in NM/G. It's not opinion, it's not pseudo science, it's physics.
That paragraph is about as misleading and meaningless as it gets. One boat might use 1 gallon but take a month to make the trip with a payload of 3 lbs.
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Old 12-25-2013, 08:47 PM   #29
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What we have done in the past to figure fuel consumption is use a formula based on a known. So, if we know we burned say 3 g/hr at 10kts, then we use the cube to figure what we would burn at say 8kts. So 8 to the 3rd, divided by 10 to the 3rd, times 3, which was the last known gallons per hour, gives us an estimate of 1.536 gallons per hour, which is our new estimate of what we will consume at the new speed if all of the conditions remain the same. You don't need to know the miles or the slip or speed made good, but remember that this new figure is an estimate on what you started with, the 3g/hr at 10kts. We use it as a check and balance against fuel consumed and fuel remaining on board.
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Old 12-25-2013, 08:55 PM   #30
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I personally like dollars/nautical mile.....last year it was about $1/NM ...this year should be the same....

Makes trip planning easy....3000 NM trip is gonna cost me $3000 in fuel..
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Old 12-25-2013, 09:19 PM   #31
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About the most interesting thing is that a 40 ft lwl boat weighing about 50,000 lbs going 7 knots will get about 2.5 to 3 mpg. Hull type, number and size of engines or color of the bootstripe are insignificant. So, how do I now go about touting my trawlers assumed superior efficiency?
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Old 12-25-2013, 09:41 PM   #32
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About the most interesting thing is that a 40 ft lwl boat weighing about 50,000 lbs going 7 knots will get about 2.5 to 3 mpg. Hull type, number and size of engines or color of the bootstripe are insignificant. So, how do I now go about touting my trawlers assumed superior efficiency?
Call it all a Wallow of Fruit Salad?
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Old 12-25-2013, 10:18 PM   #33
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I like fruit salad. I cant tell you how many of those little cans of DelMonte fruit salad I can eat per mile, whether statute or knautical, but probably 2.5 to 3. Merry Christmas all, and to all a good night.
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Old 12-26-2013, 12:04 AM   #34
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So, how do I now go about touting my trawlers assumed superior efficiency?
My stupid statements for Christmas on this subject are:
  • I really don't care about "efficiency" as I'm confident Art DeFever did the hull design and engine/prop selection correctly.
  • My fuel bill is normally less than 15% of my yearly boating costs before depreciation so why worry.
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Old 12-26-2013, 02:22 AM   #35
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I think the boat diesel calculator is reasonably good. You do need to select the right hull type, and it might not be what you would think.

Some actual numbers below. I'm a little longer LWL ~46', a little heavier D 60,000# half load, semi-displacement hull.

For me, 1.1 is a bit under 7.5 kn. At that speed I am using about 60 HP and burning 3.3 gph. So NM/G is around 2.3.

As best as I can scale off my graphs from sea trials, at 1.0 I would be at 6.8 kn, using 44 HP burning 2.3 gph and have NM/G of a touch under 3.

Yes, speed makes a big difference as pointed out in earlier posts. Portuguese, to me you calcs look in the ballpark, although maybe a touch optimistic. I think that if you want, say, 2.5 NM/G then you will need to be a little below 7 kn.
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Old 12-26-2013, 06:25 AM   #36
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Without spending mega bucks , about the only control over the fuel bill per trip is the Throttle.

Clean bottom ,clean prop , GPS flowscan, and cruising prop can all help, but the throttle position is easiest to change.
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Old 12-26-2013, 08:06 AM   #37
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Thank you all for the opinions and wise words put in this thread.

Merry Christmas to all
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Old 12-26-2013, 08:09 AM   #38
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What's the origin of the figure of 1.1 x hull speed that's been mentioned here?

I'm getting 8.47 knots as hull speed for a 40' LWL boat, using the only formula I've ever known: 1.34 x sqr(40)). I'm assuming a "typical" monohull, and putting aside the debate about what that means.

1.1 times that would be over 9 knots, and most certainly wouldn't be as efficient.

Sorry if this is a dumb question, just that I haven't run into that 1.1 value before.

As for the other debates, I find myself agreeing with Twisted. The goal is to figure out how far you can go on a given amount of fuel. NMPG at a given speed is the ONLY important number when you're trying to plan your route, and your fuel stops.

You can factor in the effects of wind, waves and current, or even an inexperienced helmsman, but you need a starting point. That's NMPG.

Technically, a better number would be gallons per 100 NM. It's sort of the same thing, but it's a more useful way to think of it. A slight change in NMPG might sound significant, while the same figure expressed as a change in Gallons per 100 NM might not. I can't explain it any better than that, but try it and see if it works better for you.

One more thing, back to the hull speed calculation. Someone above was suggesting that hull configuration was irrelevant. While minor hull shape changes do have only minor impacts on efficiency, rest assured that a multi-hull will throw all these calculations out the window. Hull shape does matter.
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Old 12-26-2013, 08:49 AM   #39
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1.35 is as arbitrary a number as 1.1....I believe THAT number comes from the prismatic coefficient and well as a few other "design features" of your boat.

IT IS NOT A CONSTANT FOR ALL BOATS!!!!!!! Only people that have NOT really studied NA very much keep spouting off that 1.34 is a real number to use (usually (really have one person in mind) or that there is such a thing as hull speed that can be ACCURATELY CALCULATED for any given boat. I'm pretty sure it can be approximated and has to be tested with tank models then ACTUALLY run on your finished outfitted boat....which is why Rick keeps pointing out that most of all this is just mental you know what for a lot of different reasons!!!!

from.... Professor Robert B. Laughlin, Department of Physics, Stanford University

"Obviously this analysis is oversimplified. Naval architects and professional shipwrights perform much more sophisticated analyses of their craft to account for interaction with ocean waves and wind, the precise shape of the hull, the modeled shape of the wake, and other factors. However the concept of a speed to length ratio is so useful and fast that many professionals work out a value for a specific hull shape and then refer to it as they scale the model up or down in size. Values range from 1.18 (in nautical units) for barges to 1.42 for very long, sleek vessels. Most amateurs use 1.34 as a good approximation for most common hull shapes"
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Old 12-26-2013, 09:32 AM   #40
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What's the origin of the figure of 1.1 x hull speed that's been mentioned here?
I think it's just the max speed that a lot people target for a boat. You might use it when selecting the max power for the engine.

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You can factor in the effects of wind, waves and current, or even an inexperienced helmsman, but you need a starting point. That's NMPG.
I think NMPG is an interesting way (better way, in my opinion) to operate a boat for a passage. You know the distance, assuming you don't reroute for weather etc. And you know how much fuel you have. The big variables/unknows are wind, current, and waves, and the extent to which they will cause you to burn more or less fuel than planned.

But knowing the distance and available fuel, you can easily calculate the max allowable NMPG to reach your destination with an acceptable reserve. If you operate at or above that NMPG rate, you will get there on the available fuel. Maintaining the desired NMPG rate will mean adjusting speed to compensate for all the above variables, but I think it makes it very easy giving just one number to watch. Of course this depends on having accurate instrumentation for fuel burn and course made good, and preferable something to do the math for you, but newer chart plotter can do it, and new engines report their fuel burn very accurately without floscans and other such stuff.

Food for though.

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Technically, a better number would be gallons per 100 NM. It's sort of the same thing, but it's a more useful way to think of it. A slight change in NMPG might sound significant, while the same figure expressed as a change in Gallons per 100 NM might not. I can't explain it any better than that, but try it and see if it works better for you.
Sure, use whatever units works for you. I think the key it to have some form of damping for the numbers so you don't have to do it all in your head. But we are all probably pretty used to averaging out speed in our heads from a bouncing GPS reading..

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One more thing, back to the hull speed calculation. Someone above was suggesting that hull configuration was irrelevant. While minor hull shape changes do have only minor impacts on efficiency, rest assured that a multi-hull will throw all these calculations out the window. Hull shape does matter.
Yes, I said that. It was meant in the context of displacement trawlers.
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