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?

CaptTom said:

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?

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

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.

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.

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.

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.

CaptTom said:

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?

Click to expand...

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.

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.

Spend the money and buy a FlowScan, some of the best boat funds I have spent.

Bill V said:

Spend the money and buy a FlowScan, some of the best boat funds I have spent.

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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.

Bill V said:

Spend the money and buy a FlowScan, some of the best boat funds I have spent.

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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.

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.

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.

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.

Marco Flamingo said:

I once searched around for "non-boat" flow meters, because like anything, the marine versions are 200% the cost of their terrestrial brothers.

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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.

Xsbank said:

My question is why do you care? Start with full tanks. Run the boat for a while and reload.

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Which has always been my approach. The geek in me likes more data, but it's certainly in the "non-essential" category.

Xsbank said:

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.

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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.

Soo-Valley said:

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.

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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.

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.

I toyed with the ideas of these once
One on fuel in and one on return.
$80 spend


https://www.amazon.com/Digital-Flow...229651&sprefix=fuel+flow+meter,aps,136&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.

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

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My thoughts exactly

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.

rslifkin said:

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.

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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.

stubones99 said:

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.

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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.

FlowScan has been sold, but they have a new web site now https://floscaninc.com/

fgarriso said:

FlowScan has been sold, but they have a new web site now https://floscaninc.com/

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I haven't seen anywhere with any of their stuff in stock, however.

I was quite excited to get real time fuel consumption on my new boat, which has a JD4045 M75. But after playing with it for a few weeks I realized that it really doesn't provide any meaningful insight for me. The fuel burn matches the prop curve provided by the mfg very closely.

A computer with an rpm and speed input could give a pretty accurate instantaneous mpg if using prop curve data, assuming you're propped properly. But you can also build a static version yourself by graphing speed * rpm and using prop curve. I did that with my previous boat and it was surprisingly accurate and informative.

here is one of the flow meters that I use.

That's the same fuel meter we also use, makes calculations without fuss!

These Australian fuel computers are the cheapest I could find.

Engine Fuel Consumption Gauge Kit (Single flow meter)

I had Floscan on my last boat, and they were great. They gave up to date consumption in changing sea conditions.

I installed FloScans on my boat because I carry 200 gallons of gas in 2 - 75 gal and 2 - 25 gal reserve tanks, all with accurate fuel gauges and I travel at times to a ports 150 -160 miles away. There are three fuel stops enroute and they all take a minimum of 1 hour time to use and cost me $1 more per gallon which I prefer not to waste the time or spend the extra funds if I don't have too. I generally can make the trip on about 140 - 155 gallons. The FlowScans in my case are their Twin Scan models and with gas engines have no return fuel requirement when used with carb'd engines. When I approach a fuel stop I can see the readout telling me fuel used and fuel remaining. There is very little "guessing" or errors. For the most part I feel very comfortable knowing that information. Some of the areas I travel in have very poor communications, even poorer support if problems develop and can have weather changes quickly. I truly understand that if cruising in a trawler that has 400-600 gallons of diesel that fuel levels are not as important to know every 2-3 hours nor is fuel mileage. I also believe that boats that travel the Inside Passage to Alaska are operating in far different conditions then traveling the Eastern US waters and may want to know more accurate data at times.

floscan

started using the flow meters in the 80's in aircraft. At the time was owner operator of an air freight company. These instruments were on all aircraft. Not only do you know your current fuel flow, but combined with exhaust gas temps, gave info about wear and tear on each engine. But, the most important info came when the fuel flows begin the change (and they were charted for each flight) something was wrong. This compared with EGT's prevented disastrous events. Eventually sold company and went into boating. Installed metering devices (floscan) on boats. Yes, they are expenses, but provide a lot more information other than the flow of fuel.

Martin J said:

These Australian fuel computers are the cheapest I could find.

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They look good, but at $1,000 US per engine, and no NMEA 2000 output, I'd have a hard time justifying this.

Jeff F said:

I was quite excited to get real time fuel consumption on my new boat, which has a JD4045 M75. But after playing with it for a few weeks I realized that it really doesn't provide any meaningful insight for me. The fuel burn matches the prop curve provided by the mfg very closely.

A computer with an rpm and speed input could give a pretty accurate instantaneous mpg if using prop curve data, assuming you're propped properly. But you can also build a static version yourself by graphing speed * rpm and using prop curve. I did that with my previous boat and it was surprisingly accurate and informative.

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My Floscan is also a digital tach, which is the major reason I bought it. However, it is always reassuring to see the same old GPH every time I put the boat up on plane at 3000 RPM. Were that number to be significantly different or begin to climb over time, I would be looking at some injector work. Since I only carry 150 usable gallons of diesel, I am also happy to have the totalizer window on the FS which I zero out when I fill up. I have a chart which I have developed giving MPG/GPH vs RPM, but I don't memorize any of the GPH numbers except the one at fast cruise. That 9.4 GPH staring me in the face sometimes induces me to slow up a hundred or two RPM to save up to 1.5 GPH. Anyway, I feel I get a heck of a lot of value for the investment I made in the FS.