Mileage vs Consumption

Merry Christmas Sailors:
Many ideas and opinions have been written in this forum about consumption, cruising speed, engine RPM, etc.
I am yet to find out what is economical and what is not for a specific boat size and weight. When I project the behavior of my boat, which is one of a kind because it is the first one in its size and design and there’s no brother vessel to compare, the best I can get is 2.4 miles/gallon at 7 knots cruising speed. According to the same calculations max speed will be 8.5 knots where the performance would be at its best 1.45 miles/gallon.
Anyway, having in mind that the cruising speed of 7 knots, in a 55 Klbs displacement hull sliding in 40.2’ of LWL, are 2.39 miles/gallon an economical number or could the it be better?
For you guys with boats ranging the same size and weight, is this a good number?
Thank you for your answers.
Portuguese

I'm looking at a 44' powercat that allegedly burns 2.5 GPH at 8 knots, which is 9.2 mph, so that's what, 3.68 mpg?

Is that how you figure that?

My miles reflect nautical miles. Nautical miles/hour are Knots.

Basicaly what you burn in 0n2 hour divided by what you run in the same period.

It's about that. There's a diference between road miles and nautical miles.

If one knot = 1 NMPH, just divide 8 knots per hour by 2.5 GPH is 3.2 NMPH?

My sweet spot for cruisin is determined usually by rpm's. However actual speed/distance traveled can be greatly altered by the wind and current. Therefore I have always referred to gph, regardless.

According to my engine electronic monitor our current trip is at an average of 7 mph getting an average of 2.3 mpg. Don't know why the monitor is in statute miles.



Dave

That's an easy one for me. I got a boat w such low consumption it dosn't matter. Haven't got the time to go far enough for mileage to be worth thinking about.

What .. me worry ... who cares.

But I've always thought of fuel consumption on a boat as GPH. Never been able to relate to MPG. Since we burn 1GPH I instantly convert that to 6MPG as we go 6 knots whenever you guys start talking about MPG.

I had a Cadillac that burned 6 GPH at 60 mph.

magicbus said:

According to my engine electronic monitor our current trip is at an average of 7 mph getting an average of 2.3 mpg. Don't know why the monitor is in statute miles.



Dave

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Dave, it was probably set for statute mpg because the ICW id denoted in statute miles.

With a 22 tons vessel, I can do 3.5gph at 7 knots, it means 2 mm/gal. I think thats good

A great deal of fuel consuption efficiency will depend on drive train setup. My guess is that your miles per gallon could be over 3. A lot will depend on what engine rpm you run at 7 knotts. Engine size will also be a consideration. At 7 knotts you may be under 40 HP. If you are generating that 40 HP out of a 450 HP motor, you won't get the efficiency you would get generating the same HP out of a 130 HP motor. Time will tell.

Ted

I have 2 turbo cats 3208, 375 HP each one. I think that I don't need 750 HP, the maximum speed that I cruise is 9.5 at 1,800 rpm. To cruise at 7 knots, i run at 1,200 rpm.

When looking at efficiency, which is Nautical Miles Per Gallon, not GPH, there are several factors that come into play. Some of those factors are really big, some are medium impact, and some have a small impact. In order of importance, they are:

1) Speed. The HP, and hence fuel burn, required to move a boat varies as the square of the speed. If you double the speed, you quadruple the fuel burn. This is by far the dominant factor in fuel burn and efficiency, and is why comparing numbers between boats is almost always a useless and misleading exercise.

2) Displacement. The HP, and hence fuel burn required to move a boat varies proportionally to the displacements. In this case, displacement is the actual weight of the boat, fuel, and gear, not the calculated displacement used by registries. The size of your boat matters, but not nearly as much as the speed you are trying to move.

3) Engine size, type of running gear, and hull form. This makes very little difference. The fuel burn of an engine, regarless of it's size, depends on how much HP it is generating, and that depends on the two factors above. A 100HP engine and a 400HP engine, both putting out 70HP to move a boat at a particular speed, will burn just about the same amount of fuel. Yes, there will be some difference, but it's the least important factor. Changing speed by 1/10th of a knot will make much more of a difference. Also, there will be slight differences between old engines and newer, common rail engines, but that too is small compared to the other factors. Hull shapes can make a difference, but that too is comparatively small.

There are lots of calculators that will estimate fuel burn, and they tend to be pretty accurate. It's the same calculators that are used to size a prop, which in turn determines the HP.

We tend to focus on things like "the most efficient RPM for my engine", and "4 vs 5 blade props". This can make a difference, but they are the small things, not the bog thing. The big thing is speed.

Moonstruck said:

Dave, it was probably set for statute mpg because the ICW id denoted in statute miles.

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Yes my wife plans our day by the ICW books and I think in terms of knots, which actually works out well because we end up with a cushion factor that - knock fiberglass - has so far managed to get us to a stopping point before dark!

Dave

twistedtree said:

When looking at efficiency, which is Nautical Miles Per Gallon, not GPH, there are several factors that come into play.

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Most of which are not even mentioned after that introduction.

Nautical miles per gallon is pretty much a useless measure except for ballpark comparison of trips from one day to the next and has zero to do with efficiency - of which type of efficiency never seems to be mentioned in these voodoo engineering threads.

If you count the shaft turns, the distance traveled over the ground, the distance traveled through the water, the weight of fuel burned, and the horsepower hours produced, then you might have a number which makes some sense or has a useful meaning.

Otherwise it is equivalent to looking at a photo of a transom and telling the world what kind of engines are best suited for the boat.

RickB said:

Most of which are not even mentioned after that introduction.

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True, I left the division out, assuming it's not hard to do.

Nautical miles per gallon is pretty much a useless measure except for ballpark comparison of trips from one day to the next and has zero to do with efficiency - of which type of efficiency never seems to be mentioned in these voodoo engineering threads.

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huhh? Efficiency is a comparison of work done per unit of energy. In this case NM moved per gallon of fuel burned. GPH is energy per hour, but leaves out the work accomplished, i.e. the distance traveled. Comparison of GPH is meaningless unless you also consider the distance traveled, ie the work done. One boat burning 10 GPH moving 10 kts is the same efficiency as a boat burning 5 GPH moving 5 kts. Both are getting work done at 1 NM/G

If you count the shaft turns, the distance traveled over the ground, the distance traveled through the water, the weight of fuel burned, and the horsepower hours produced, then you might have a number which makes some sense or has a useful meaning.

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Yes, and it distills down to NM/G

twistedtree said:

3) Engine size, type of running gear, and hull form. This makes very little difference. The fuel burn of an engine, regarless of it's size, depends on how much HP it is generating, and that depends on the two factors above. A 100HP engine and a 400HP engine, both putting out 70HP to move a boat at a particular speed, will burn just about the same amount of fuel. Yes, there will be some difference, but it's the least important factor. Changing speed by 1/10th of a knot will make much more of a difference. Also, there will be slight differences between old engines and newer, common rail engines, but that too is small compared to the other factors. Hull shapes can make a difference, but that too is comparatively small.

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This isn't really true. A DD 8-71 will burn significantly more fuel to generate 70 HP as compared to a John Deere 4045T (4 cyl). You may not see as much of a difference between 2 engines of the same series such as a Cummins 6BT 220 HP and 6BTA 375 HP. When Willard switched from the Cummins 6B 135 HP (6 cyl. ) to the John Deere 4045T (4cyl. ) for there last 40' trawler, they realized a 20% fuel savings at 7 knotts. Same hull, same speed, different engine. While speed is likely the biggest factor, engine size can make a significat difference. At $4 a gallon, fuel savings could be substantial to an active cruiser.

Ted

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

Quote This isn't really true. A DD 8-71 will burn significantly more fuel to generate 70 HP as compared to a John Deere 4045T (4 cyl).

This would be true be true because the DD is way oversize & 2 cycle.

Quote When Willard switched from the Cummins 6B 135 HP (6 cyl. ) to the John Deere 4045T (4cyl. ) for there last 40' trawler, they realized a 20% fuel savings at 7 knotts. Same hull, same speed, different engine. While speed is likely the biggest factor, engine size can make a significat difference.

Really, 20%?

twistedtree said:

Yes, and it distills down to NM/G

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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.

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.

Brooksie said:

Quote When Willard switched from the Cummins 6B 135 HP (6 cyl. ) to the John Deere 4045T (4cyl. ) for there last 40' trawler, they realized a 20% fuel savings at 7 knotts. Same hull, same speed, different engine. While speed is likely the biggest factor, engine size can make a significat difference.

Really, 20%?

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Willard's last 40' (Aloha) was custom built for Patrick Gerety a principle in the trawler division of Willard. At the time of it's building, it wasn't expected to be the last 40'. It was built with the JD 4045T and according to Patrick from the testing at Willard, " Expect to burn about 1.3 GPH @ 7.5 knotts in Aloha and 1.7 GPH @ 7.5 knotts in Annabelle (Willard 40 with Cummins 6BT)."

Ted

Portuguese said:

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

Click to expand...

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

RickB said:

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.

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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?

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.

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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.

Portuguese said:

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

Click to expand...

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.

Portuguese said:

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

Click to expand...

Hmm, I've heard great things about Deutz, a lot of commercial boats run them. Also I saw a Hatteras 92 repower with them.

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.

Click to expand...

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

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.

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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

Portuguese said:

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

Click to expand...

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.

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

twistedtree said:

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.

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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.

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.

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..