2 trawlers, same engine, different fuel consumption???

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seattleboatguy

Senior Member
Joined
Nov 2, 2013
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327
Location
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Vessel Name
Slow Bells
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Marine Trader 38
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?
 
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!
 
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.
 
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.
 
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.

David
 
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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..
 
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.
 
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.
 
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...
 
Xsbank said:
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...
Click to expand...

What you said, yet many seem to have a big concern about about a few tenths of a gallon per hour.
 
Maybe one has a foul bottom
 
too many variables and or lack of info to comment seriously....
 
Lehman 120 on a MT40. You get the picture:rofl:
 

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I had Floscans on my last boat, but they're spendy. Too far down my list of goodies for now. :)
 
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.
 
Moonstruck said:
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.
Click to expand...
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?
 
manyboats said:
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.
Click to expand...

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.
 
seattleboatguy said:
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?
Click to expand...

It was a single in a 36' trawler.
 
SS that's a nice feature of SD hulls and one that is dear to some or most.

I'm just trying to say you can save more fuel from going even slower as some of us do all the time and you will still get to Bremerton. But this time of the year it may be in the dark Haha.
 
We've been down this road before and there comes a time when simply having the motors both running works against you. I don't have Floscans but my logic is somewhere just below hull speed any boat (sail, one motor or two) will operate at optimum efficiency (trading MPG for GPH). Where those two S-curves intersect is the sweet spot and my gut feel for mine is at about 8-8.5 kts.

Your LWL is a bit less and 7 kts would be a good guess from me as well.
 
After all this talk, I don't feel so bad about my 18GPH at 18kts!!!:blush:
 
Baker said:
After all this talk, I don't feel so bad about my 18GPH at 18kts!!!:blush:
Click to expand...
Exactly! And it didn't take you all day to get to wherever you were going! I fully understand that going slower & slower will increase your mpg. Some of us, however, would actually like to get somewhere at a decent time!

Going 50mph on a freeway is going to save hundreds of thousands of gallons (if not millions) of fuel but sometimes there is a pressing need or desire to get to your destination faster.

If your boat is getting 4 mpg at 4 knots, just think what your mpg would be at 1 knot! Fantastic, huh!:blush:
 
Baker said:
After all this talk, I don't feel so bad about my 18GPH at 18kts!!!:blush:
Click to expand...

Indeed.

I almost bought a slow boat and am glad I didn't. I can poke around like everyone else(see avatar) but have gotten out of more trouble than I've gotten into having the speed available.

Ironically great anchorages are about a half mile apart where I'm at so slow is not really a "penalty" :blush:
 
"4mpg at 4knots"

Walt's got it! Go w a 3knot tide and you'd be going faster than we generally go on Willy. I wonder if I'd burn a quart an hour at 4 knots?
Fuel burn was a big thing w me when I was shopping for Willy and I out did myself. I don't need to limit my fuel consumption to 1 gph. I went overboard. Life would be just fine w twice as much burn or thereabouts especially if I could find cheaper moorage.

But I don't think we'll kick Willy out of the slip for not consuming enough fuel. If we ever go on a long trip again it will come in handy.
 
some of us are on the other side of the "fuel's no big deal fence"....

some of us when we cruise have no schedule or need for speed, boat in an area where there's something to see and do every couple miles so travelling only 10-15 miles a day is just fine, burn more than 1000 gallons of fuel a year so economy and price of fuel can start to make an impression and are on a tight budget overall...

add those things up and yes some of us give a darn about 10ths of a knots and small increases in NMPG....

even small differences can mean the difference in how many times or where one can dine out or fix a broken system immediately instead of putting it off.

a new member even has a cruising kitty on her blog so anyone can help out another cruiser enjoy....
 
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I agree with psneeld. The voyage is the primary purpose when I boat (I don't fish). So, why be in a hurry. Destinations are more often best approached using car, plane, (sometimes a bus), or commercial ship if the voyage isn't important. Trains are also an option if the voyage is primary.
 
psneeld said:
some of us are on the other side of the "fuel's no big deal fence"....

some of us when we cruise have no schedule or need for speed, boat in an area where there's something to see and do every couple miles so travelling only 10-15 miles a day is just fine, burn more than 1000 gallons of fuel a year so economy and price of fuel can start to make an impression and are on a tight budget overall...

add those things up and yes some of us give a darn about 10ths of a knots and small increases in NMPG....

even small differences can mean the difference in how many times or where one can dine out or fix a broken system immediately instead of putting it off.

a new member even has a cruising kitty on her blog so anyone can help out another cruiser enjoy....
Click to expand...

I fully understand. I honestly look forward to the day I slow down. My last boat was my first power boat with a bit of speed(15kts@6GPH....not bad for a planing 30 footer). It kinda got me hooked on speed. Being that I still work and like to cover ground when I am off. But one day, I will slow back down!!!...:)
 
The best boats for fuel economy are of course FD boats. And of course light long and narrow is also by far best.

People that are really pressed to cover their fuel costs clearly have the wrong boat.

John Baker wrote " My last boat was my first power boat with a bit of speed(15kts@6GPH....not bad for a planing 30 footer)" I suspect it's "not bad" because it's light and it's light because it's wood. That planing boat w a small diesel would be at least as economical as a FG FD boat just because of it's lightness. Build a 40' canoe w plywood and power it w 30hp and cruise at 8 knots perhaps. Light, skinny and FD. Can't beat it unless you go the catamaran route.
 
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