Reply
 
Thread Tools Display Modes
 
Old 09-03-2010, 03:26 PM   #1
Veteran Member
 
Dom61's Avatar
 
City: Grand Island NY
Country: USA
Vessel Name: Brandi Jo
Vessel Model: 1975 Marine Trader 34
Join Date: Aug 2010
Posts: 81
Fuel Mileage

I have a 34 Trader with a 120 any ideas about the best RPM to run for best fuel mileage , we are always with or aginst a curant so M/N per hr* will not work
__________________
Advertisement

Dom61 is offline   Reply With Quote
Old 09-03-2010, 06:02 PM   #2
Senior Member
 
coyote454's Avatar
 
Join Date: Oct 2007
Posts: 446
re: Fuel Mileage

Dominic,

Here's another area where everyone has their own opinions. *My 34' CHB, with the same 120 Lehman as you, normally will do about 8 knots (9.6 mph) with a fuel burn of 2.5 to 3 gallons per hour. *Of course, current, wind and general sea conditions will all affect these numbers.


Your own numbers should be close, but the size and pitch of your prop may change things a bit.


The only accurate method I've heard of for your best RPM is with a FloScan instrument, but they are a little pricey.


http://www.floscan.com/html/blue/ser...sid=14&catid=2




Mike
Brookings, Oregon
__________________

coyote454 is offline   Reply With Quote
Old 09-03-2010, 07:01 PM   #3
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Fule Mileage

Mike's suggestion of something that measures the fuel flow is about the only sure way of matching economy to throttle setting.

But I'm not sure in the case of a trawler-type boat with an FL120 the knowledge is worth the cost. The FL120 uses so little fuel--- perhaps 2.5 gallons an hour at something like 1600 rpm--- the fuel use difference within the "normal" range of power settings for this engine, which is generally considered 1500 to 1800 rpm-- is not going to be enough to care about. Running the engine hard--- 2000 rpm and up--- will result in a higher fuel burn but even then it's not going to be a staggering increase. Backing the power down below 1500 will probably drop the fuel burn a fair amount but now you're running too slow to maintain proper temperature so unless you've got a prop with a massive amount of pitch in it, it's not something you want to do anyway.

For the last twelve years we've run our two FL120s at about 1650 rpm (the SW tachs on the boat are not anything you want to set your watch by so 1650 is an approximation) and that gives us about 8 knots at a combined fuel burn of between 5 and 6 gallons an hour. This is with a 36', 28,000 pound boat and 23" diameter, four-bladed props with a 16" pitch.

-- Edited by Marin on Friday 3rd of September 2010 07:02:27 PM
Marin is offline   Reply With Quote
Old 09-03-2010, 08:04 PM   #4
Grand Vizier
 
Delfin's Avatar
 
Join Date: Jan 2010
Posts: 2,487
re: Fuel Mileage

I don't think this is quite as complicated as it would appear.* If you can find the power curve data from Lehman, they'll show 'prop demand data', indicating how many gph are burned at different rpms generating 'x' horsepower.* Just build a spread sheet that has rpms in the first column, prop demand fuel burn in the second column, and speed in knots absent current that you experience in the third column.* From that it is easy to calculate nm per gallon burned, and you can pick your most efficient balance between burn, speed and distance traveled.

For Delfin, we burn 2.75 gph at 7.5 knots or so, giving us a range of 6000 miles plus.* If I want to get there faster, I can go 8.25 knots at 4 gph, but for long distances, the increase in speed isn't worth it.

You indicated that NM/gph 'won't work'.* Regardless of the current, the water you travel through will be the water you travel through, even if you don't get anywhere because of current.* The efficiency calcs are the same, with the choice being do you want to travel through more water per unit of time, or travel the maximum distance regardless of the time it takes.
Delfin is offline   Reply With Quote
Old 09-06-2010, 03:22 PM   #5
Guru
 
Daddyo's Avatar


 
City: Cruising East Coast US
Country: USA
Vessel Name: Grace
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 2,441
re: Fuel Mileage

I know a guy who has a Flow Scan on his FL120 on a MT 40 sedan.

.75 gph @1400 rpm
1.5 gph @1600 rpm
2*** gph @1700 rpm
Daddyo is offline   Reply With Quote
Old 09-07-2010, 09:23 AM   #6
Veteran Member
 
Dom61's Avatar
 
City: Grand Island NY
Country: USA
Vessel Name: Brandi Jo
Vessel Model: 1975 Marine Trader 34
Join Date: Aug 2010
Posts: 81
re: Fuel Mileage

Thanks for the info I know the 1400 is not good for the engine 1600 sounds about right from what i have been hearing** Thanks Dom
Dom61 is offline   Reply With Quote
Old 09-07-2010, 10:19 AM   #7
Enigma
 
RT Firefly's Avatar
 
City: Slicker?
Country: Bumpkin?
Join Date: Oct 2007
Posts: 9,994
re: Fuel Mileage

Hiya,
** As mentioned, a fuel burn curve should answer your question.* From MY experience with a FL 120, to the best of my memory, 1750 RPM would give me about a 2 GPH burn and about 7 knots.* It seemed the 2 "sweet spots", that area where the FL "sounded" the best, were right in line with the two rivets on the face of the old Motorola tach'.* The*rightmost upper*rivet was 1750 RPM and I can't remember the RPM where the lower spot was.* I mentioned this to Bob Smith at one point and he agreed that the FL seemed most comfortable @ 1750.* But slowing down a bit WILL give you better "mileage".**
**Ocasionaly I would pin the throttle producing copious amounts of black smoke and get to 8 knots (2200 RPM) but I knew I was over propped.* You should be able to reach close to 2400 RPM (WOT) with the appropriate prop.
* As also mentioned, prop size will also determine what your burn will be at any given cruising RPM's.
** From Daddyo's post it would seem 1600 RPM's is what you're looking for.
RT Firefly is online now   Reply With Quote
Old 09-07-2010, 10:59 AM   #8
Guru
 
jleonard's Avatar
 
Join Date: Jun 2008
Posts: 2,735
re: Fuel Mileage

"You should be able to reach close to 2400 RPM (WOT) with the appropriate prop"

You need to reach 2500 rpm.
I can do that with my Albin 40 and get up to a whopping 8.3 knots.

Normally I run at about 1750 and burn approx 1.5 gph as close as I can figure.
jleonard is offline   Reply With Quote
Old 09-07-2010, 05:39 PM   #9
Veteran Member
 
Dom61's Avatar
 
City: Grand Island NY
Country: USA
Vessel Name: Brandi Jo
Vessel Model: 1975 Marine Trader 34
Join Date: Aug 2010
Posts: 81
re: Fuel Mileage

I can hit 2400-2500 and just suck the fuel right out of the tanks* Dom
Dom61 is offline   Reply With Quote
Old 09-09-2010, 11:27 AM   #10
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,630
re: Fuel Mileage

Dom, I don't think they are suggesting you do that on a regular basis. But that is a good measure of whether you're properly "propped".
Baker is offline   Reply With Quote
Old 09-13-2010, 09:37 AM   #11
Guru
 
jleonard's Avatar
 
Join Date: Jun 2008
Posts: 2,735
re: Fuel Mileage

"But that is a good measure of whether you're properly "propped"."

Exactly. I run it up to WOT maybe half a dozen times thruout a season. Always as I am approaching the harbor entrance on my return trip., just in case something goes wrong. I run at WOT for about a minute, maybe 2. This lets me make sure I get to 2500 rpm, make sure I do not have black smoke, and let's me check out my cooling system to make sure there is no overheat.
jleonard is offline   Reply With Quote
Old 07-06-2013, 09:42 PM   #12
Veteran Member
 
City: puyallup
Country: usa
Vessel Name: Neaptide
Vessel Model: tung hwa clipper 30
Join Date: Apr 2013
Posts: 40
fuel milage

We took our Tung Hwa Clipper 30 out today for a jaunt around Vashon Is. from Tacoma. It was the first time out with her for an extended run (6 hrs.) I ran at 1750 rpm for 2 hours using aprox 3 gals. or 1 1/2 gph at 7+ kns. Then ran 2 hours at 2000rpm using aprox.6 gals. or 3 gph at 7.5 to 8 kns. Its a fl 120 with 3380 hours on the clock and seems to be propped about right as it will wind up to just shy of 2500 rpm
amdavis is offline   Reply With Quote
Old 07-07-2013, 04:21 AM   #13
TF Site Team
 
Peter B's Avatar
 
City: Brisbane
Country: Australia
Vessel Name: Lotus
Vessel Model: Clipper (CHB) 34 Sedan/Europa style
Join Date: Jun 2008
Posts: 6,669
Send a message via Skype™ to Peter B
That sounds about right. I find our sweet spot about 1750-1800 rpm.
__________________
Pete
Peter B is offline   Reply With Quote
Old 07-08-2013, 07:37 PM   #14
Guru
 
motion30's Avatar
 
Join Date: Jan 2010
Posts: 740
I used to run my empty at 1650 rpm. 2.5- 2.8 gph. I slowed down to 1400 rpm and burned 1.75 gph. lost about 1 mile per hour
motion30 is offline   Reply With Quote
Old 07-08-2013, 10:12 PM   #15
yo
Veteran Member
 
yo's Avatar
 
Join Date: Sep 2011
Posts: 68
We also run at 1750 --1800 every thing feel right at that speed.
yo is offline   Reply With Quote
Old 07-08-2013, 10:51 PM   #16
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,706
I think this "sweet spot" stuff has little to do w the engine and all to do with how the boat was built. I also assume "sweet spot" specifically is the engine speed (rpm) that the least amount of vibration and noise is experienced. That mostly has to do w the dimensions and stiffness of bulkheads, hulls, cabin roofs, floors, fuel tank panels or the hull just above the props.

The engine "excites" such structural things, causing vibration and resulting in noise. Not too much unlike a tuning fork.

Everybody responds differently to noise and vibration. Some avoid it like the plague and others actually like it. I'm a "sweet spotter" too but Willy seems to have very little resonance. Smaller boat w possibly heavier structures and a much smaller engine. I suspect lots to most of the noise on Willy comes from the engine air intake. The intake of an engine needs muffling to a surprising extent like the exhaust. Haven't built a resonator intake box so I guess Willy seems quiet enough. Bigger engines (all other things being equal) make more noise.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 07-09-2013, 10:24 AM   #17
Guru
 
SomeSailor's Avatar
 
City: Everett, WA
Vessel Name: Honey Badger
Vessel Model: 42' CHB Europa
Join Date: Oct 2011
Posts: 784
I run at 1650 most times. It seems to be the spot where the boat "feels" right and nothing is working too hard. I have digital tachs and getting everything sync'd to within a few RPM really seems to cut down on the resonance and noise.
SomeSailor is offline   Reply With Quote
Old 07-17-2013, 02:13 AM   #18
Ben
Guru
 
Ben's Avatar
 
City: Wilmington, NC
Country: US
Vessel Name: Sand Castle
Vessel Model: 1986 MT Sundeck 35.5
Join Date: Dec 2007
Posts: 693
A fellow boater just posted his readings on another thread. I was compelled to graph them. His engine is a Perkins 6.354 (twins) which is roughly the size of my ADC Ford Lehman 135. These readings should not be far off from your FL120 in terms of the curves and sweet spots in the graph. His are twins, but as I read his numbers, fuel use figures appear to be per engine. So what I can extract is a sweet spot around 1800 RPMs.

All current, prop pitch/diameter, hull shape parameters will impact this, but use as you wish as a guide.
Attached Thumbnails
Click image for larger version

Name:	Perkins.jpg
Views:	150
Size:	39.3 KB
ID:	21364  
__________________
..................................
Ben

MV Sand Castle
Wilmington, NC
Ben is offline   Reply With Quote
Old 07-17-2013, 07:13 AM   #19
Guru
 
City: Pensacola
Country: USA
Join Date: May 2012
Posts: 748
Ben,

Yes they are twins and usage is per engine. Glad I could help.
Blue Heron is offline   Reply With Quote
Old 07-17-2013, 11:58 AM   #20
TF Site Team
 
FlyWright's Avatar
 
City: California Delta and SF Bay
Country: Sacramento, CA, USA (boat in Vallejo)
Vessel Name: FlyWright
Vessel Model: Marshall Californian 34 LRC
Join Date: Apr 2008
Posts: 10,169
If that's the case, shouldn't the GPH be doubled and the NM/Gal be halved? The sweetspot of 1800 RPM remains, but the data plots shift.
__________________

__________________
Al

Custom Google Trawler Forum Search
FlyWright is online now   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 09:53 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012