Reply
 
Thread Tools Display Modes
 
Old 03-13-2013, 11:16 AM   #1
Guru
 
BobH's Avatar
 
City: Montgomery, TX
Country: USA
Vessel Model: None, but looking
Join Date: Feb 2012
Posts: 778
Guesstimating fuel consumption

The engine curves show 2.6 gph at 1500 RPM. With the transmission in neutral the fuel consumption would obviously be much less. So the question is, for the given 2.6 gph, is that at full throttle with enough load on the engine to hold at 1500 RPM?

Bob
__________________
Advertisement

BobH is offline   Reply With Quote
Old 03-13-2013, 11:26 AM   #2
TF Site Team
 
ksanders's Avatar
 
City: SEWARD ALASKA
Country: USA
Vessel Name: LISAS WAY
Vessel Model: BAYLINER 4788
Join Date: Feb 2011
Posts: 3,955
Quote:
Originally Posted by BobH View Post
The engine curves show 2.6 gph at 1500 RPM. With the transmission in neutral the fuel consumption would obviously be much less. So the question is, for the given 2.6 gph, is that at full throttle with enough load on the engine to hold at 1500 RPM?

Bob

If the manufacturers curve shows 2.6 GPH thats at full rated load.

If in a particular boat under way, the actual consumption is less then you are pulling less than full rated HP for that RPM.

If the fuel usage is more, then you are pulling more than full rated HP for that RPM.

Using the mfg fuel curve and comparing it to actual verified fuel burn is a good way to determine a particular boats loading of the engine.
__________________

__________________
Kevin Sanders
Bayliner 4788
Seward, Alaska
www.mvlisasway.com
ksanders is offline   Reply With Quote
Old 03-13-2013, 11:38 AM   #3
Guru
 
Brooksie's Avatar
 
City: Cape Cod, MA
Country: USA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 954
Usually, on a marine engine, the consumption curve is based on the theoretical propeller load. That is, with a propeller and gear ratio which will allow the engine turn up to it's full rated RPM underway at WOT. Then consumption anywhere else on the curve can be read off for that same propeller & gear.
This curve is made by calculation not tests and real results vary from vessel to vessel; but it is a close starting point.
Brooksie is offline   Reply With Quote
Old 03-13-2013, 12:48 PM   #4
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,712
The way to determine actual engine load is to know what the engine will burn at rated rpm and WOT. Then observe your fuel consumption at .. For example cruise rpm. The 120 Lehman is about 6gph at max so if you're burning 3gph you're at 50% load. It's that simple but getting accurate burn rate numbers is somewhat difficult.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 03-13-2013, 12:58 PM   #5
Guru
 
LaBomba's Avatar
 
City: Beaverton, Ontario
Country: Canada
Vessel Name: Airswift
Vessel Model: Ontario Yachts Great Lakes 33
Join Date: Nov 2012
Posts: 818
On my old boat I had a Northstar Chartplotter and one of the inputs was from a fuel flow meter. The meter merely was clamped in the fuel line (had to be vertical to keep air bubbles out) and read out constantly the actual fuel flow. Works good on planing boats for adjusting trim tabs etc. to get the best fuel efficiency and surprisingly sometimes increasing speed actually reduced fuel consumption. That theory likely wouldn't work on a displacement hull. These may be available for many chartplotters today with multiple inputs. Just another toy!
__________________
Allan & Ann
Airswift
LaBomba is offline   Reply With Quote
Old 03-13-2013, 04:31 PM   #6
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,712
LaBomba,
Sounds good to me but I wonder how accurate they are? I suppose if one was propped correctly one could go to WOT and observe the rate of flow indicated and see how close that comes to the manufacturers spec sheet. If it matched then one could have confidence in the instruments accuracy .... but only at WOT and that may be nit picking but most flow meters have a range of accuracy so measuring 1gph on a small boat like mine may be not very accurate at all compared to a boat that flowed 8gph. One should be able to get that information from the manufacturer.

But even if you get an accurate instrument once you've taken readings at speeds where they may be needed or wanted you may as well throw the thing over the side. Of what use would it be after observations up and down the scale have been made. Seems like just another toy to me. How many of us really need to know with great accuracy how much fuel we burn? Some do QI realize but I think it is a VERY small number. I know I sure don't. Willy burns a gallon an hour and she carries 100 gal of fuel.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 03-13-2013, 04:55 PM   #7
TF Site Team
 
Pau Hana's Avatar


 
City: Seattle, WA
Country: Good Ol' US of A!
Vessel Name: Pau Hana
Vessel Model: 1989 PT52 Overseas Yachtfisher
Join Date: Aug 2012
Posts: 1,647
The Northstar/Navman line were very accurate in determining fuel usage- but for gas engines only (no return line).

We were going to go with Floscan, but are now taking a hard look at Maretron and their FF100 system.
__________________
Peter- Marine Insurance Guru & tuna fishing addict!

1989 52' PT Overseas yachtfisher
Pau Hana is offline   Reply With Quote
Old 03-13-2013, 05:17 PM   #8
TF Site Team
 
ksanders's Avatar
 
City: SEWARD ALASKA
Country: USA
Vessel Name: LISAS WAY
Vessel Model: BAYLINER 4788
Join Date: Feb 2011
Posts: 3,955
I've had the floscans on several boats. with the diesel units we can get to within about 5% on average.

This is over a wide range of rpm settings.

I would suspect they would be more accurate if calibrated for a specific throttle setting.
__________________
Kevin Sanders
Bayliner 4788
Seward, Alaska
www.mvlisasway.com
ksanders is offline   Reply With Quote
Old 03-13-2013, 05:22 PM   #9
Guru
 
djmarchand's Avatar
 
City: East Greenwich, RI
Country: USA
Vessel Name: Bella
Vessel Model: Mainship Pilot 34
Join Date: Aug 2012
Posts: 2,879
Bob:

There are two types of horsepower and fuel consumption curves for diesel engines: full load wot hp and prop hp.

Full load hp and fuel consumption are measured by the manufacturer with the engine on a dyno and the throttle held wide open. Enough load is put on the engine to keep the rpm steady at each setting measured.

Prop hp is determined by a formula that uses an exponent somewhere between 2.5 and 3.0 depending on the manufactuer. It recognizes that except at wot rpm the prop hp will be less than the full load hp. The manufacturer then applies fuel consumption data to this hp to get prop fuel consumption.

So if you were looking at the prop fuel consumption curve, then it is a reasonable approximation of real world fuel consumption at 1,500 rpm. If you were looking at full load data, then it is way too high at prop loadings.

David
djmarchand is offline   Reply With Quote
Old 03-13-2013, 05:37 PM   #10
Senior Member
 
Bendit's Avatar
 
City: Auckland
Country: New Zealand
Vessel Name: Pioneer
Vessel Model: Westcoaster 53 converted to pleasure
Join Date: Nov 2007
Posts: 283
The power/prop calculator at boatdiesel.com is surprisingly accurate. It can give you fuel consumption estimates at the full range of engine revs.
Bendit is offline   Reply With Quote
Old 03-13-2013, 09:16 PM   #11
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,903
You can also rig a small fuel tank accurately marked and do trial runs of an hour or so at different RPMs. Very accurate...

But in reality accurate fuel consumption is only a fleeting concept for most cruisers...too many variables from quality of fuel to temperature to bottom growth, routing, idling, loading, bridges, etc..etc...

I find for long cruise planing, it all works out to be a rough estimate no matter how careful you want to be.
__________________

psneeld is offline   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 05:31 AM.


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