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Old 02-25-2021, 01:56 PM   #1
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Fuel Consumption Question

I am thinking about purchasing a trawler having put my sloop up for sale. (I am "a gentleman of a certain age"). But I have no idea of what fuel consumption is on a trawler. One 36 footer for sale has two Cummins 6BT5.9M engines rated at 210HP. Can anyone educate me as to fuel consumption of such a vessel? Thank you in advance for your input.


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Old 02-25-2021, 02:11 PM   #2
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Hi Ron, and Welcome to the "Dark Side" Well, with what you are thinking you are more than half way here.
I own(ed) a Nordic Tug with a Cummins 6BTA M3 330HP engine. Cruising at 7-8 knots we burned 2 gallons per hour (RPM varying between 1200-1400) which is about half throttle. Increasing speed, while possible, yielded poor fuel consumption results, meaning a large increase in fuel burned for very little increase in speed.
I would expect that with 2 smaller HP engines you would burn less than 4 gallons per hour but more than 2 gallons per hour at those same RPM's and speed. Hope that helps.
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Old 02-25-2021, 02:14 PM   #3
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Welcome aboard. Personally I don’t get hung up on fuel consumption since it will most likely not be your largest expense in owning a trawler. The difference in fuel burn isn’t going to be huge between similar trawlers with different engines. You will likely get between 1.5 to 2 miles per gallon at cruise. Good luck and have fun searching for your new boat.
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Old 02-25-2021, 02:22 PM   #4
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It really depends on how you use the boat; specifically how many horsepower you are using.

Any diesel engine consumes about 0.056 gallons/hour per horsepower used. You need about 4hp per ton to cruise at hull speed (about 7 knots on a 36 footer). A 36 foot trawler would weigh close to 10 tons so 10 x 4 x .056 = 2.2 gallons/hour.

Open the throttle to push the boat above hull speed and the fuel usage increases dramatically . Open up your wallet.
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Old 02-25-2021, 02:22 PM   #5
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Agree with Comodave. He's given you a workable number, with the caveat, your mileage may differ.

Just be careful when reading quoted gallons per hour; under identical conditions, a US boat will burn 20% more per hour than a Canadian boat.
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Old 02-25-2021, 03:02 PM   #6
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Talk engine brand and model. Then talk RPMs.
Some boats are more efficient/slippery moving through the water at a given RPM.
Also realize, any builder supplied data is taken on a boat 1/2 fuel load, 1/2 water load and fridge is running.
Then we load up out boats with our precious possessions, tools, clothing and various form of food, snack and 'other' liquids. Ah dont forget full load of fuel and also water, spare parts and more undefined stuff.
Figure 1 gph if you run the generator.
Sooooo your milage will most definitely vary.
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Old 02-25-2021, 03:11 PM   #7
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Talk engine brand and model. Then talk RPMs.
Some boats are more efficient/slippery moving through the water at a given RPM.
Also realize, any builder supplied data is taken on a boat 1/2 fuel load, 1/2 water load and fridge is running.
Then we load up out boats with our precious possessions, tools, clothing and various form of food, snack and 'other' liquids. Ah dont forget full load of fuel and also water, spare parts and more undefined stuff.
Figure 1 gph if you run the generator.
Sooooo your milage will most definitely vary.
"Some boats are more efficient/slippery moving through the water at a given RPM."
At a given speed not a given rpm.
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Old 02-25-2021, 03:18 PM   #8
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Dan,
Should be “talk hp and load”
But there’s a range of prop induced loads. Many here on TF overload and many don’t. Rpm will lead you astray.
And many/most calculate their gph by running time (as per hobbs) and gallons of fuel put in the tank. Some are somewhat close but most aren’t. The amount of time at a lower load than normal cruise is typically much more than most realize ... IMO.
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Old 02-25-2021, 03:23 PM   #9
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The amount of time at a lower load than normal cruise is typically much more than most realize ... IMO.

Unless making a long open water run, yes, this is a big factor.
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Old 02-25-2021, 03:32 PM   #10
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Fuel consumption isn't a huge part of the expense of owning a boat to many, depends on how much and how you use the boat. Twin engines is another expense that generates vast amounts of comments and expense, twice as much as a single. I'm on the low end of this discussion as I have very little money with which to supply my boat addiction. That said my two cents would be take a look at yourself and your bank account and armed with some honest info as you're read here make your own decision.
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Old 02-25-2021, 04:25 PM   #11
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I forgot, I am ignoring the prop pitch and cupping.

According to factory sea trials, running a know distance two ways,
American Tug 34, net 12ton
A single Cummins QSB 5.9 QSB 380hp
1000rpm, 6.5knts, 1gph.
1200rpm, 7.4knts, 1.4gph
1400rpm, 8knts, 2.1gph
The point is, the above is the theory of an empty boat, pretty much flat water, no or little wind no current or tidal effect.
So these numbers do not equate to the real world.
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Old 02-25-2021, 04:52 PM   #12
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Here is a more complete consumption curve, taken by me for the AT34 (which is about the size you are talking). Actual numbers slightly better as this was about 0.5 kn foul tide and 5 knots headwind. Note that there is a big knee in the curve at around 8 knots. How fast can you afford to go?

RPM %load G/H Boost SOG
1300 33 1.9 1.1 6.5
1400 34 2.3 1.4 6.9
1500 35 2.7 1.7 7.2
1600 35 3.6 2.3 7.7
1700 36 4.1 2.9 8.1
1800 36 5.0 3.7 8.5
1900 5.8 4.9 8.9
2000 43 7.4 6.6 9.3
2100 7.9 7.8 9.6
2200 51 9.0 10.4 10.4
2300 10.0 12.4 11.0
2400 11.2 15 12.3
2500 12.1 16.8 13.3
2800 80 15.8 22.3 15.7
3050 99 19.2 26.8 17.4
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Old 02-25-2021, 05:16 PM   #13
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Fuel consumption isn't a huge part of the expense of owning a boat to many, depends on how much and how you use the boat. Twin engines is another expense that generates vast amounts of comments and expense, twice as much as a single. I'm on the low end of this discussion as I have very little money with which to supply my boat addiction. That said my two cents would be take a look at yourself and your bank account and armed with some honest info as you're read here make your own decision.
Most things on a single w a total of say 120hp will be equal to twin of two 60hp engines. Saying “twins cost twice as much” only calculates when both the engines are the same power. That comparison shows nothing useful.

If you want to compare twins to singles you must compare boats w equal total power. Or nearly so if the comparison is to have useful meaning.
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Old 02-25-2021, 05:44 PM   #14
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Most things on a single w a total of say 120hp will be equal to twin of two 60hp engines. Saying “twins cost twice as much” only calculates when both the engines are the same power. That comparison shows nothing useful.

If you want to compare twins to singles you must compare boats w equal total power. Or nearly so if the comparison is to have useful meaning.

Agreed. Twins may cost a little more, but nowhere near double. If it's truly double in the same boat, then either then twins are over-powered or the single is underpowered, as one of them is the wrong engine choice.
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Old 02-25-2021, 05:50 PM   #15
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Most owners do less than 100 hours per year. You might burn 400 gallons per year. Given the other costs add up to around $15k for a boat in this size (slip, insurance, basic maintenance, etc), fuel expense will be about 8% of your costs.
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Old 02-25-2021, 06:01 PM   #16
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Would think regardless of all other factors due to transmission losses and increased frictional losses and increased parasitic drag from two sets of running gear twins of total output X would be less efficient than a single of equal power. This is assuming thrust efficiency of the two props is equal to the thrust efficiency of the single.
Even outboards tell you the engine hp and the lower hp at the screw. In short you need more engine hp to get the same prop HP with a twin set up.
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Old 02-25-2021, 06:04 PM   #17
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Would think regardless of all other factors due to transmission losses and increased frictional losses and increased parasitic drag from two sets of running gear twins of equal output would be less efficient than a single of equal power. This is assuming thrust efficiency of the two props is equal to the thrust efficiency of the single.
Even outboards tell you the engine hp and the lower hp at the screw.

There would usually be a slight loss from twins, although the shafts may be smaller than a higher powered single, which makes up some of it. And depending on running gear layout (struts, keel, etc.) the twins may have slightly higher prop efficiency due to cleaner water flow to the props. In general, the loss from twins wouldn't be a large one. Dragging a feathered or folded prop and shaft around for a wing engine is likely at least as bad.
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Old 02-25-2021, 06:08 PM   #18
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Here is a more complete consumption curve, taken by me for the AT34 (which is about the size you are talking). Actual numbers slightly better as this was about 0.5 kn foul tide and 5 knots headwind. Note that there is a big knee in the curve at around 8 knots. How fast can you afford to go?

RPM %load G/H Boost SOG
1300 33 1.9 1.1 6.5
1400 34 2.3 1.4 6.9
1500 35 2.7 1.7 7.2
1600 35 3.6 2.3 7.7
1700 36 4.1 2.9 8.1
1800 36 5.0 3.7 8.5
1900 5.8 4.9 8.9

2000 43 7.4 6.6 9.3
2100 7.9 7.8 9.6
2200 51 9.0 10.4 10.4
2300 10.0 12.4 11.0
2400 11.2 15 12.3
2500 12.1 16.8 13.3
2800 80 15.8 22.3 15.7
3050 99 19.2 26.8 17.4


It always comes down to how FAST can you afford to go.
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Old 02-25-2021, 07:07 PM   #19
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Would think regardless of all other factors due to transmission losses and increased frictional losses and increased parasitic drag from two sets of running gear twins of total output X would be less efficient than a single of equal power. This is assuming thrust efficiency of the two props is equal to the thrust efficiency of the single.
Even outboards tell you the engine hp and the lower hp at the screw. In short you need more engine hp to get the same prop HP with a twin set up.
Not so Hip,
Transmission losses will be mostly the same because the half power engine only needs half power transmissions, oil changes for two average small engines are the same as for one big engine. Oil filters ditto. Prop shafts will be smaller w less friction and less drag and even less weight. Even the larger motor mounts will be more expensive.

So if you compare power systems w double and half the power the results will be useless and meaningless.
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Old 02-25-2021, 07:10 PM   #20
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Fuel consumption on a 36 foot single engine trawler operating at normal trawler speeds will be less than 2 gallons an hour. That translates to about 4 miles per gallon.

I get a kick out of people who want to know all kinds of irrelevant facts before the will give you a fuel burn estimate. Things like prop measurements, weight on board, wind and wave conditions, are you towing a dinghy, rpm range, full displacement or partial, color of your trim and what type of toothpaste you use.

They really don't make any difference in the total scheme of things. And if they do make a tiny difference , so what? Fuel cost is the smallest expense of owning a trawler.

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