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Old 11-02-2013, 09:00 AM   #1
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2 trawlers, same engine, different fuel consumption???

I saw these 2 trawlers in yachtworld, but I don't understand why their advertised fuel consumption is so different:

42' Bristol, New Bern, NC
displacement 36000 lb
engine = 120 hp Lehman (twin)
1.5 gph per engine (3 gph for both engines)
(no rpm info given)

41' Cheer Men, Newburyport, MA
displacement = 24000 (this is estimate from sister ship)
engine = 120 hp Lehman (twin)
5 gph (I assume this is for both engines)
1900 rpm

Are these 2 boats cruising at radically different speeds, or is there something else I'm missing?
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Old 11-02-2013, 09:06 AM   #2
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Go figure!....
One is 50% heavier than the other, revealing its probable full displacement nature.
Also you do not mention speed...so.....I would say that with the data that you wrote, it is very hard to say something valid. We can only guess, I think!
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Old 11-02-2013, 09:11 AM   #3
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Don't know about the speeds, but I would say the Bristol would be running about 15-1600 rpm. I found 1600 to 1650 to be a sweet spot for the Lehman I had. About 7 knots and 1.5 gal/hr. Pushing an extra 2-400 rpm increased the fuel consumption greatly.
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Old 11-02-2013, 10:00 AM   #4
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When I clicked on I thought this would be a hull form discussion. disappointed.
But yes just a load issue.
The range for the Lehman is 1.5 to 5gph. WOT is 6. News to few though.
And a lot of error is frequently added by using the hour meter for fuel consumption. Much engine time is not at cruising load. The highest load and consumption I've heard on this forum is 3gph for the Lehman.
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Old 11-02-2013, 10:28 AM   #5
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We should waste our time commenting on numbers published in Yachtworld?

Seriously, most owner supplied fuel consumption data is meaningless for the reasons that Eric noted. Much better to use the engine's propeller fuel consumption curve and a known cruising speed and rpm. You will get a much better answer than asking an owner how much fuel does he burn.

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Old 11-02-2013, 11:03 AM   #6
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At 8kts and 1650-1675 RPM I will burn 4GPH very predictably. Increase the speed and come up to 1800 RPM and I'll burn anout 50% more than that and make about 8.5kts and averaging about 6GPH.

We have twin Lehman 120s and we've made the 40 mile trip to Bremerton and back twice now on 40 gallons at 8 kts. For us that's about our best rate. I can squeeze another half knot out by coming up in RPM, but it's an expensive .5 kts. The 40 minutes we save costs $80-$90 in fuel.

I think of it as free moorage for the weekend if I can slow down about 150 RPMs..
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Old 11-02-2013, 11:40 AM   #7
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Some Sailor,

Ever tried it at 7 knots?

That's almost a knot faster than we go 99% of the time.

If you're concerned about the extra fuel you burn going faster why should you be any less concerned about what you could be saving going slower? How much would you not have to spend if you went 7 knots. I don't even have that choice as I can't go that fast.

Reducing 150rpm won't save you quite as much as not increasing 150rpm.
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Old 11-02-2013, 11:48 AM   #8
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The highest load and consumption I've heard on this forum is 3gph for the Lehman.

About right,, perhaps 50 HP , so they run forever there, and below.
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Old 11-02-2013, 12:54 PM   #9
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It's an easy relationship. HP = fuel. But it's definitely not linear. The difference between my usual cruise at about 125 HP and full rated HP 250 is not double, it's closer to quadruple the fuel consumption. Displacement hull. Oh, the speed goes all the way up to 9.5 knots! If you fuss about fuel, go slow.

By the way, fuel is definitely not the most expensive part of owning a boat. By a long shot. Wait until you look at your other bills...
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Old 11-02-2013, 04:10 PM   #10
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By the way, fuel is definitely not the most expensive part of owning a boat. By a long shot. Wait until you look at your other bills...
What you said, yet many seem to have a big concern about about a few tenths of a gallon per hour.
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Old 11-02-2013, 06:17 PM   #11
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Maybe one has a foul bottom
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Old 11-02-2013, 07:56 PM   #12
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too many variables and or lack of info to comment seriously....
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Old 11-02-2013, 08:25 PM   #13
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Lehman 120 on a MT40. You get the picture
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Old 11-03-2013, 06:48 AM   #14
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Lehman 120 on a MT40. You get the picture
Dadyo, you have the coolest instrument on the block. Very impressive, and very informative.
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Old 11-03-2013, 07:48 AM   #15
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Dadyo, you have the coolest instrument on the block. Very impressive, and very informative.
I wish. They're from another boat not mine
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Old 11-03-2013, 12:33 PM   #16
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I had Floscans on my last boat, but they're spendy. Too far down my list of goodies for now.
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Old 11-04-2013, 06:19 AM   #17
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Look at any engine sellers HP Fuel burn chart.

All have an imaginary prop graph used to represent the prop load.

Simply look at the HP required to spin the prop at different RPM to see why the engine burns more fuel at higher RPM

More power required = more fuel burned.
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Old 11-04-2013, 09:51 PM   #18
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Originally Posted by Moonstruck View Post
Don't know about the speeds, but I would say the Bristol would be running about 15-1600 rpm. I found 1600 to 1650 to be a sweet spot for the Lehman I had. About 7 knots and 1.5 gal/hr. Pushing an extra 2-400 rpm increased the fuel consumption greatly.
Hi Moonstruck. I assume you had 2 Lehman engines in your boat, so it was using a total of 3 gal/hr at 7 knots. Is that a correct assumption?
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Old 11-05-2013, 08:20 AM   #19
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Ever tried it at 7 knots?

That's almost a knot faster than we go 99% of the time.

If you're concerned about the extra fuel you burn going faster why should you be any less concerned about what you could be saving going slower? How much would you not have to spend if you went 7 knots. I don't even have that choice as I can't go that fast.
I stay right below hull speed and get the best trade between GPH/MPG. My hull speed calculates to 8.75 kts and I can push 10 or so, so for my engines and hull, something around 8 seems to work best.
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Old 11-05-2013, 08:49 AM   #20
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Hi Moonstruck. I assume you had 2 Lehman engines in your boat, so it was using a total of 3 gal/hr at 7 knots. Is that a correct assumption?
It was a single in a 36' trawler.
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