Reply
 
Thread Tools Search this Thread Display Modes
 
Old 03-31-2021, 08:10 AM   #1
Guru
 
CaptTom's Avatar
 
City: Southern Maine
Vessel Model: Prairie 36 Coastal Cruiser
Join Date: Dec 2012
Posts: 2,205
Poor Man's Fuel Flow Meter - Monitor RPMs?

The cost of fuel flow monitors for two engines, supply and return, has never been justifiable for me. I just stick the tanks occasionally.

But at home I've implemented a different system for my oil-fired boiler. Knowing the fuel consumption rate of the burner on the boiler, by monitoring how log it runs each day I can very closely estimate the fuel used.

Is something like that possible on a diesel? My NMEA bus already has RPMs for each engine. I'd just need a way to total that up over time, and apply a formula of fuel consumption per RPM.

But is this ratio reliable? In other words, is it safe to assume that I'll always be burning about the same amount of fuel at a give RPM?
__________________
Advertisement

CaptTom is offline   Reply With Quote
Old 03-31-2021, 08:16 AM   #2
Guru
 
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft Catalina 381
Join Date: Aug 2019
Posts: 3,333
Quote:
Originally Posted by CaptTom View Post
But is this ratio reliable? In other words, is it safe to assume that I'll always be burning about the same amount of fuel at a give RPM?

On a governed diesel engine, no, it's not a reliable assessment. A couple barnacles on a prop will change that relationship, for example.
__________________

rslifkin is online now   Reply With Quote
Old 03-31-2021, 08:49 AM   #3
Guru
 
twistedtree's Avatar
 
City: Gloucester, MA
Join Date: Jan 2013
Posts: 6,441
I agree it won't be the most accurate thing in the world, but probably a good-enough estimator. There challenge is that you probably need a computer to do it, unless you want to spend all your time with a clip board recording and adding up numbers.


The first step would be to create a fuel map of your fuel burn rate across the engine's RPM range. The more data points the better, especially at higher loads where there can be a significant difference between RPM settings. You would need an accurately graduated, small burn tank so you can measure the fuel burn. Then run the boat, preferably across a reciprocal course, at each RPM point and record the fuel burn. Now you know how much you burn at each RPM settings. Those number will get worse if the bottom is fouled or if you are in bad seas, but otherwise should be pretty consistent and reproducible.


Then when operating the boat you need to tally time spent at each RPM, multiply those times by the fuel burn at each RPM, then add them all up. Simple, but very tedious. A perfect job for a computer.


By now you will likely be looking for a further simplification, and that's to do what people have done for ages. If you typically run your boat at the same RPM, then just figure out the fuel burn for that data point. Then use you engine hours multiplied by that single fuel burn number. It won't be as accurate, but will get you close if you run the boat in a consistent manner. If you operate across a wide range of RPMs, then forget it.
__________________
MVTanglewood.com
twistedtree is offline   Reply With Quote
Old 03-31-2021, 08:51 AM   #4
Guru
 
tiltrider1's Avatar
 
City: Seattle
Vessel Name: AZZURRA
Vessel Model: Ocean Alexander 54
Join Date: Aug 2017
Posts: 2,479
I’ve run my boat with a clean bottom and I’ve run my boat with a very dirty bottom. Always at 1750 rpms. The difference is .2 gallons per hour. The real difference is 1 knot less speed which turns into an additional hour at 10 gph.
tiltrider1 is offline   Reply With Quote
Old 03-31-2021, 08:57 AM   #5
Guru
 
City: Carefree, Arizona
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 9,238
CT
If you have sight tubes on your tanks you can easily calibrate them during fuel fill ups. Then measure/read off at the end of a day's run noting your RPMs and hour meter. I've found this type of tracking to be more accurate than flow scans.
sunchaser is offline   Reply With Quote
Old 03-31-2021, 09:18 AM   #6
Guru
 
High Wire's Avatar
 
City: Cape May, NJ and Englewood, FL
Vessel Name: Irish Lady
Vessel Model: Monk 36
Join Date: Dec 2007
Posts: 3,854
Agree the short answer is no for older mechanical injection engines. Huge difference between the fuel consumed in neutral at 2000 RPMs and fast cruise at 2000 RPMs. The newer computer controlled engines can.
So until you get some useful fuel data, you could build a simple RPM/Speed table and cruise about 1 knot below calculated hull speed.
__________________
Archie
Irish Lady
1984 Monk 36 Hull #46
Currently in Cape May NJ
High Wire is online now   Reply With Quote
Old 03-31-2021, 09:39 AM   #7
TF Site Team
 
koliver's Avatar
 
City: Saltspring Island
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 4,093
Quote:
Originally Posted by CaptTom View Post
The cost of fuel flow monitors for two engines, supply and return, has never been justifiable for me. I just stick the tanks occasionally.

But at home I've implemented a different system for my oil-fired boiler. Knowing the fuel consumption rate of the burner on the boiler, by monitoring how log it runs each day I can very closely estimate the fuel used.

Is something like that possible on a diesel? My NMEA bus already has RPMs for each engine. I'd just need a way to total that up over time, and apply a formula of fuel consumption per RPM.

But is this ratio reliable? In other words, is it safe to assume that I'll always be burning about the same amount of fuel at a give RPM?
Over the years I have kept a good record of hours of use and of fuel purchases. Once I determined the sweet spot of 8.0 to 8.2 knots boat speed, my consumption per hour, at each fill settled onto a very consistent number. If there is an event that should translate into higher or lower usage, I see it at the fill. I also use some of that diesel for the genset and the diesel stove, so those consumers also factor in, but not so much as to put the overall number into the trash.
In my overall averages, I get 2 nmpg.
Since I put the present pair of TAMD41s in, in June 2000, my numbers have been solidly consistent.
__________________
Keith
koliver is offline   Reply With Quote
Old 03-31-2021, 09:59 AM   #8
Guru
 
CaptTom's Avatar
 
City: Southern Maine
Vessel Model: Prairie 36 Coastal Cruiser
Join Date: Dec 2012
Posts: 2,205
I guess what I'm hearing is the answer is "no." Basically it would be no different from what I do now; estimate based on my average fuel burn at my typical cruising speed, and verify by sticking the tanks after each leg.

As for the hardware, a Raspberry Pi running OpenCPN would be able to handle it. It would even make a nice plug-in. But if there's no good Revolutions-per-gallon formula, it wouldn't be possible. Another over-thought technical solution to a problem that doesn't really exist.
CaptTom is offline   Reply With Quote
Old 03-31-2021, 10:05 AM   #9
Senior Member
 
Bill V's Avatar
 
City: Lynden
Vessel Name: Joint Venture
Vessel Model: 1978 GlasPly 2800 mid cabin
Join Date: Nov 2020
Posts: 113
Spend the money and buy a FlowScan, some of the best boat funds I have spent.
__________________
"Joint Venture" 1978 midcabin 2800, twin 2017 Vortec roller cam "bullet proof" 383/6.3L full roller 350hp engines
Bill V is offline   Reply With Quote
Old 03-31-2021, 10:16 AM   #10
Guru
 
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft Catalina 381
Join Date: Aug 2019
Posts: 3,333
Quote:
Originally Posted by Bill V View Post
Spend the money and buy a FlowScan, some of the best boat funds I have spent.

Sadly, those aren't an option these days. The company basically shut down, got sold, and the new iteration hasn't re-started production on a lot of the products yet from what I can find.
rslifkin is online now   Reply With Quote
Old 03-31-2021, 10:27 AM   #11
Guru
 
CaptTom's Avatar
 
City: Southern Maine
Vessel Model: Prairie 36 Coastal Cruiser
Join Date: Dec 2012
Posts: 2,205
Quote:
Originally Posted by Bill V View Post
Spend the money and buy a FlowScan, some of the best boat funds I have spent.
I couldn't justify the cost even when they were in business. And their competitors weren't any cheaper. Much as I'd love to know my fuel burn minute-by-minute, it really doesn't contribute to my safety or comfort in any meaningful way. Rough numbers are plenty for planning purposes, since I'd always plan on having sufficient reserves.
CaptTom is offline   Reply With Quote
Old 03-31-2021, 10:27 AM   #12
Senior Member
 
City: Dewatto
Vessel Name: CHiTON
Vessel Model: Tung Hwa Clipper 30
Join Date: Jan 2020
Posts: 337
I once searched around for "non-boat" flow meters, because like anything, the marine versions are 200% the cost of their terrestrial brothers. This was for a small (50 hp outboard) engine where the fuel tank did not have a gauge. I bought the one shown below but before installation I found a Faria flow pickup and gauge (now out of production) that fit my dash better. But search the brand name and the hundreds of available meters by Digiflow (gas, diesel, water, threaded/non-threaded, etc.) You might find what you want. They not attractive for the dash, but maybe for a conveniently hidden location.

The "mini" version (and the Faria) is accurate to under 1 gph, maybe not constantly when running (it might bounce between .01 and .04 gpm), but the total is generally really accurate over time. My outboard has a 23 gallon tank (85 liters when north of the border). I ran it dry when I had a little spare tank onboard and it predicted the shutdown within a mile.
Attached Thumbnails
Digiflow.jpg  
__________________
Marco Flamingo
Marco Flamingo is offline   Reply With Quote
Old 03-31-2021, 10:34 AM   #13
Guru
 
Xsbank's Avatar
 
City: Pender Harbour, BC
Vessel Name: Gwaii Haanas
Vessel Model: Custom Aluminum 52
Join Date: Sep 2013
Posts: 3,797
My question is why do you care? Start with full tanks. Run the boat for a while and reload. Divide it by the engine hours etc. running and there is your fuel consumption. Mine is 14.2 litres per hour with heater and genset use.

If you slow down to save fuel, instead of running in the "sweet spot," you run the engine longer and burn more fuel/trip but your hourly fuel rate is better. Likewise go faster, higher total consumption but a shorter trip and fewer engine hours.

Floscan gives you more information, but for what? Fuel usage for me is much less cost than moorage (1/5th) insurance (1/3rd) or capital costs/maintenance (just bought a diaphragm pump for $600) so I don't care about fuel consumption. Now carbon taxes I care for, they are voodoo like insurance rates but are simply the government screwing us out of our money for twaddle.
__________________
Don't believe everything that you think.
Xsbank is offline   Reply With Quote
Old 03-31-2021, 10:45 AM   #14
Guru
 
Soo-Valley's Avatar
 
City: Gulf Islands
Vessel Name: Soo Valley
Vessel Model: Grand Banks 36
Join Date: Jul 2019
Posts: 2,283
A flowscan would be nice to have. But once I found the sweetspot for operation would I look at it again. Some things are just obvious over time. A nice add on, another thing that can malfunction. Not today.
__________________
SteveK AKA Soo Valley
You only need one working engine.
That is why I have two.
Soo-Valley is online now   Reply With Quote
Old 03-31-2021, 04:49 PM   #15
Guru
 
CaptTom's Avatar
 
City: Southern Maine
Vessel Model: Prairie 36 Coastal Cruiser
Join Date: Dec 2012
Posts: 2,205
Quote:
Originally Posted by Marco Flamingo View Post
I once searched around for "non-boat" flow meters, because like anything, the marine versions are 200% the cost of their terrestrial brothers.
I did the same thing. I mean, factories are full of process control computers fed by all kinds of flow sensors. Surely they don't pay $500 each.

But the problem is, I have twin diesels. So that's four sensors (two supply, two return) and two computers to subtract the latter from the former on each engine. Then wiring and a gauge. At "marine" prices, it adds up quickly.

Quote:
Originally Posted by Xsbank View Post
My question is why do you care? Start with full tanks. Run the boat for a while and reload.
Which has always been my approach. The geek in me likes more data, but it's certainly in the "non-essential" category.

Quote:
Originally Posted by Xsbank View Post
If you slow down to save fuel, instead of running in the "sweet spot," you run the engine longer and burn more fuel/trip but your hourly fuel rate is better.
I may have a quibble with this. On all the boats where I've done the math, going slower (not counting dropping off plane) does result in measurable fuel savings. The only exception would be where there's a significant current. Obviously going 3 knots against a 4 knot current won't improve your MPG. I don't really look at GPH. I care about how much fuel it will take to go a given distance.

Quote:
Originally Posted by Soo-Valley View Post
A flowscan would be nice to have. But once I found the sweetspot for operation would I look at it again. Some things are just obvious over time. A nice add on, another thing that can malfunction. Not today.
Exactly. But personally, on boats with a flowscan I'd monitor it all the time. Not critical by any means, but as you say, a nice-to-have.
CaptTom is offline   Reply With Quote
Old 03-31-2021, 05:03 PM   #16
Senior Member
 
Spinner's Avatar
 
City: Seattle
Vessel Name: Spinner
Vessel Model: 2003 Nordic Tug 42
Join Date: Dec 2018
Posts: 399
My boat came to me with a fair amount of Maretron instrumentation, so I had the Maretron fuel flow sensors installed (one for the feed, one for the return). I find the data very handy while working the back eddies! Seems very accurate.
__________________
Regards

Sue
42 Nordic Tug Spinner
Spinner is online now   Reply With Quote
Old 03-31-2021, 05:31 PM   #17
Guru
 
Simi 60's Avatar
 
City: Queensland
Vessel Model: Milkraft 60 converted timber prawn trawler
Join Date: Jul 2016
Posts: 3,351
I toyed with the ideas of these once
One on fuel in and one on return.
$80 spend


https://www.amazon.com/Digital-Flowm...s%2C136&sr=8-3

Easier to use prop/fuel curve and sight tubes which we already have
Measure and run 1000 litres through, note rpm and do the maths
Close enough and almost exactly what the prop/fuel curve chart said.

Quote:

Originally Posted by Soo-Valley View Post
A flowscan would be nice to have. But once I found the sweetspot for operation would I look at it again. Some things are just obvious over time. A nice add on, another thing that can malfunction. Not today
My thoughts exactly
Simi 60 is offline   Reply With Quote
Old 03-31-2021, 07:03 PM   #18
Guru
 
City: Melbourne, FL
Join Date: Dec 2014
Posts: 1,664
If you have newer diesel engines (with electronics), they have a fairly good idea how much each engine is burning, since their computer adjusts the openings of the piezo injector valves for each cylinder. They are not as accurate as a flowscan, but they are included with the engine.

The problem with flowscan and older engines, is you have to subtract the return fuel to the tank from the fuel coming from the tank to get your fuel burn rate. On an older engine (without electronics and common-rail injection), find the RPM for max torque and ease off a bit... that's your sweet spot.
stubones99 is online now   Reply With Quote
Old 03-31-2021, 07:28 PM   #19
Guru
 
City: Carefree, Arizona
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 9,238
Quote:
Originally Posted by rslifkin View Post
Sadly, those aren't an option these days. The company basically shut down, got sold, and the new iteration hasn't re-started production on a lot of the products yet from what I can find.
There are several very accurate industrial type flow meters that I've used over the years. The types used can include positive displacement, magnetic, differential pressure or vortex. As best I recall the smallest flow measured was about 20 cc/minute and the largest about 3,000 gpm. For diesel flow meters start with McMaster Carr.

Any of the common and small industrial types can easily be used in a NEMA 2000 or Maretron backbone setup for small vessel marine application. A handy industrial experienced instrument tech or gas station tech company would be helpful if one is seriously looking at a workable setup.
sunchaser is offline   Reply With Quote
Old 03-31-2021, 07:52 PM   #20
Guru
 
rgano's Avatar
 
City: Southport north of Panama City
Vessel Name: FROLIC
Vessel Model: Mainship 30 Pilot II since 2015. GB-42 1986-2015. Former Unlimited Tonnage Master
Join Date: Oct 2007
Posts: 3,109
Quote:
Originally Posted by stubones99 View Post
The problem with flowscan and older engines, is you have to subtract the return fuel to the tank from the fuel coming from the tank to get your fuel burn rate. On an older engine (without electronics and common-rail injection), find the RPM for max torque and ease off a bit... that's your sweet spot.
There are two flow meters on my Floscan, and their difference is constantly displayed on the gauge. Once you calibrate the unit, it does all the math, accurately. I can also have the raw date from each flow meter displayed on the unit.
__________________

__________________
Rich Gano
FROLIC (2005 MainShip 30 Pilot II)
Panama City area
rgano is online now   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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



» Trawler Port Captains
Port Captains are TF volunteers who can serve as local guides or assist with local arrangements and information. Search below to locate Port Captains near your destination. To learn more about this program read here: TF Port Captain Program





All times are GMT -5. The time now is 06:29 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012
×