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.


Ron

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.

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.

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.

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.

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.

OldDan1943 said:

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.

Click to expand...

"Some boats are more efficient/slippery moving through the water at a given RPM."
At a given speed not a given rpm.

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.

Nomad Willy said:

The amount of time at a lower load than normal cruise is typically much more than most realize ... IMO.

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Unless making a long open water run, yes, this is a big factor.

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.

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.

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

Fish53 said:

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.

Click to expand...

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.

Nomad Willy said:

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.

Click to expand...

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.

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.

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.

Hippocampus said:

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.

Click to expand...

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.

DDW said:

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

Click to expand...

It always comes down to how FAST can you afford to go.

Hippocampus said:

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.

Click to expand...

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.

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.

pete

Pete Meisinger said:

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.

pete

Click to expand...

This +++

Pete,
An FL makes 60hp at half load and burns 3gph because an FL at full bore making 120hp burns 6gph.

Are you saying the average trawler only needs 30hp to run 7-8 knots?
Flatswing posted a hint about this w his “+++”.

The 3 important things in determining fuel consumption. How long is the boat? How heavy is the boat? How fast do you plan to go?

A 35' diesel boat traveling at 6 knots should burn about .6 to 1.2 gallons per hour. Now if you are extremely heavy you could easily end up burning more fuel. If you try to go 7 knots you will burn a lot more fuel. If the boat gets longer but no more weight you will be able to go a fraction faster at the same fuel consumption.

It is really hard to tell some one what their fuel consumption will be do to all the variables. In my case I always seem to burn 1 gallon per mile no mater what boat I'm on. The difference of course is always my speed.

Flatswing said:

This +++

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Yeah, I went nuts about this coming from a sailboat as well.

The right answer for a 34-42' trawler seems to be 2nmpg for a twin and 4nmpg for a single when operating a bit below hull speed. Plus or minus 20% for a million other small factors that can be debated endlessly.

Bottom line is that it's a lot less efficient an aux engine sloop. My 40' Beneteau with a Perkins 4108 went the 170nm to Catalina and back on ~25 gallons of diesel. Our 41' trawler too 3x (~80gal) as much fuel last summer.

But on the Beneteau it was a pretty miserable slog up - 13hrs of slamming into the current and swell at 6.5kts, all my kids got seasick. Did it in the trawler in 10 hours at 8kts in comfort and they slept most of the way. We all felt it was well worth the extra $~200 in fuel.

My sailor principles mean I would love a super-efficient long and lean powerboat with a modern single diesel, but they're very hard to find, and expensive to buy and berth, and the actual fuel savings are really pretty paltry for coastal cruising.

Being new to these types of boats what i found most surprising is the non-linear fuel consumption. At around 8 knots depending on current were using 1.5-2 gph. Go to 9 were at 5 gph,10 around 10 gph and flat out at 12 about 17 gph. If a person is thinking of running above the sweet spot its going to get expensive and likely harder on equipment. I suspect this will apply to most semi-displacment trawlers so fuel burn is really nonsense. Its all about how fast you expect to go. I would guess a lot of folks coming into the trawler market are not recognize these boats are slow and efficient and are not speed boats.

Speed is hard on the credit card. A friend has a relatively new 52 foot power cat with Cummins 425 hp Diesels. She carries 700 g of diesel. Range at 18 kts is 350 nms. At 8.5 kts, range is around 900.nms.

One thing he really likes about trawler speed is his boat stays a lot cleaner. At 18 kts, there's enough spray and moisture from his wake that his boat gets pretty salty after a good run.

Just depends on what your priorities are.

Peter

"Its all about how fast you expect to go"

For many boats the Sq RT of the LWL is the sweet spot where wave making about equals skin friction .This is called SL.

1.34 times SL ( AKA as "hull speed") can cost as much as 3 times the fuel burn.

Want to go fast cheap? get a longer boat of the same weight.,

socalrider said:

The right answer for a 34-42' trawler seems to be 2nmpg for a twin and 4nmpg for a single when operating a bit below hull speed. Plus or minus 20% for a million other small factors that can be debated endlessly.

Click to expand...

A twin won't burn twice the fuel of a single at the same speed. It just doesn't work that way. The 2 - 4 nmpg range at around a knot below hull speed is a reasonable estimate though (depending on the hull, engines, etc.)

You will get a zillion answers but keeping it simple the trawlers here usually get 2.5- 4 NMPG depending on size if people keep the bow down. Faster boats at speed get about .5NMPG. As said above fuel is not the big expense.

In theory, my AT34 and 400 gallons of fuel, I should be able to go from Miami to about some place in NY. Most definitely if I jump into the Gulf Stream.
Of course, if I traveled in the ICW I would see many interesting things.
I cant figure out why I would want to end up in NY. LOL