Measuring fuel consumption

Fill the tanks, do a few trips recording engine hours, then fill again and do the maths. Will only give an average of litres/hour over the period and will obviously vary depending on what sort of speeds you have done, but in my admittedly very limited experience there appears to be no other way unless you have modern all electronic systems which measure fuel flow etc and do it for you?
 
Thanks, I do this already but I am trying to find the optimum speed/economy balance for my boat.
Thus I need to somehow measure actual fuel consumed at a particular speed.
Otherwise I proceed on the basis that just on the plane is optimum.
 
I think to measure real time consumption you would need fuel flow meters in the input and return lines for each engine, and some clever electronics to work out the answer in terms of litres per hour.
 
The GPH for given RPM's should be available from the manufacturer. The speed for each RPM can be ascertained by running the boat. So surely it's pretty straightforward to calculate and create a table illustrating the MPG's for given RPM's?
 
Broom 33 is a difficult one as it rides bow up, optimum rpm for your engine is 3000/3100 any lower the boost is low but fuel is going in and not being burnt , you need to play with boat at sea in calm conditions and watch where it starts to plane pushing a big bow wave eats fuel , I'd say from experience you need to be going at least 15 knots .
 
The key problem with measuring relatively flow rates of a fluid that changes viscosity with temperature. After several hours running the spill return from a diesel will markedly warm the tank, affecting fuel viscosity.

Even small (1%) errors on fuel flow meters can create compound errors between flow and return of +- 10%. A more typical 5% sensor will provide a net usage accuracy only +- 25%. A while ago I did the maths to show this. In practice sensor accuracy will vary with external movement and in particular fuel viscosity, exacerbating the accuracy at any moment. This is worsened further because spill return will be warmer than fuel to the engine.

However, once a fuel gauge system is installed, even if its overall accuracy was dubious, it should be effective at determining optimum boat speed, where a relative measurement is desired, just don't rely too much on the reported figures accuracy. I guess if you had a system that you could affect calibration adjustments, you could make compensatory adjustments from your longer term refill / run hour measurements.

Engine revs assume a clean hull, and also sea conditions will affect specific fuel consumption.

My conclusion of using the market leader for flow measurement - Floscan, was that a very expensive set up, was not going to provide much more than the method suggested of recording run hours, and refills. The cost of the Floscan was that I would have used in a two year period for the fuel I was trying to save, I figured there was no viable payback ever achievable, and likely this explains why most boats do not have them.

I guess however, if you have a computer controlled engine where fuel flow is being accurately measured to control the engine then this data would likely be the best source as you are only looking at fuel burn from one sensor rather then flow to on one sensor minus fuel return on another.

On my own boat I have installed a tank sight glass and I am in the process of trying to calibrate the scale for each tank each side of the glass. It is excellent for checking how much is left, but I have found even a minor error in synchronisation (< 50 rpm) has a major effect on individual engine consumption over say 30 or 40 miles, although not too much on overall consumption of both combined over a given distance, where speed through the water and hull state seem the largest variables.
 
go with filling up, do a few trips and fill up again etc. Its expensive to get the kit to measure with real accuracy and knowing what it is will not improve your boating pleasure. You end up spending your time monitoring fuel burn rather than enjoying the day. It also can make you paranoid that something is wrong with your engines when its just hull fouling etc. Fuel burn is also markedly affected by sea state too (it gets worse with bigger seas). Again, that affects how you drive the boat. If you are not careful, you end up driving to hit a fuel burn target rather than getting the best from the boat/sea state.
 
I have a Maretron fuel flow meter
https://www.maretron.com/products/ffm100.php
It has a supply and return flow sensor so can be used for Diesel engines, it even has a temp sensor which compensates for the Diesel expansion in the return system . The system is connected to my NMEA 2000 network and all data is shown on my Raymarine MFD.
It is very accurate and if it says I have used 50 liters for a trip, the tank is full if I top it with exactly 50 liters.
It shows me my real time consumption in liters/hr , my average consumption, my total fuel used , calculates remaining fuel in the tank and even my expected range until empty.
(Ps.i am trying to upload a few pics for you but i keep getting the message 'upload failed' )
 
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If its any help deliberating ,we have access to L/hr fuel burn on the same screen as "load" and EGT,s
I also have on the plotter a "time to destination "

Therefore using l/hr and "time to destination " -together - all I can say there is plateau ** of seemingly optimal planning speed whereby a few knots either side it cost ( if that,s what you want to control ?) pretty much the same .
Eg once up and planing for me it makes no diff wether 24 or 28 knots cruise ,the faster burns more a bit but them the time to destination decreases so they run for less time @ the higher rate ,if you see what I mean .
I only notice the L/hr --as it's on the same page as the more important IMHO stuff -EGT /load
Certainly don't phaff about with micro adjustments fearhering the throttles etc , we just blat about @ 28 knots all day ,somewhere between 17-1800 rpm --- cos what the boat is set up to do basically and I like that ,kinda knew that when I bought it and more than happy to go with the flow so to speak , burning about 90 L/side or lower if lightly loaded ?

Not really much saving if any easing off a bit to lower the L/hr TBO .

I Would fit EGT guages ahead of L/hr in the q for a sensible gadget to upgrade your boat .
If you the kinda bloke that fiddles with the steering buttons on a modern car -gwarping at the trip computer :cool:

** The plateau is kinda flat ish a slight upward slant ,however towards the top of the rated rpm it steepend s maybe gets a bit exponential . You can see the Hp /torque curves of the engine and the end bits of the graphs --stay out of the end RHS part of the graphs .

WOT for me is 2200 ish once we move over 2000 L/r starts to climb ,eg 2100 @ say 36 knots it's like 130 /140 per side
Don,t really go there TBO --for long maybe a10 min blast to shake off an idiot , there's plenty of wanna be Hero,s in the SoF
 
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benchmark said:
I have a Maretron fuel flow meter
https://www.maretron.com/products/ffm100.php
It has a supply and return flow sensor so can be used for Diesel engines, it even has a temp sensor which compensates for the Diesel expansion in the return system . The system is connected to my NMEA 2000 network and all data is shown on my Raymarine MFD.
It is very accurate and if it says I have used 50 liters for a trip, the tank is full if I top it with exactly 50 liters.
It shows me my real time consumption in liters/hr , my average consumption, my total fuel used , calculates remaining fuel in the tank and even my expected range until empty.
(Ps.i am trying to upload a few pics for you but i keep getting the message 'upload failed' )
Click to expand...
Nice solution but sadly about £1200+ per engine once sensors bought.
 
Thank you for your very helpful comments.
A fuel measuring system pays for a lot of fuel!
I find that the engines are pulling 3500 to get 16knots so needing more revs than desirable to ensure I am on the plane. The boat does not have such a clear cut transition when it climbs the wave then gets going. An S23 we had rode bow right up then reached planing and settled down in a very obvious way.
I have trim tabs fully down but they change the attitude but make little difference to the speed.
Max speed is 19 tops at 3800 and that was with new props which I feel are optimum.
Ah well c'est la vie! I shall have to shift the drinks cabinet into the anchor locker.
 
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