Fuel Consumption Calculator

[Alan de Enfield]

I would appreciate comments on the accuracy of the following 'formula' ( and my maths)

 

Diesel engines use approximately 1 gal (4.4 litres) per hour per 20 HP developed :

Fuel consumed is proportional to the power developed

Power developed is approximately proportional to the cube of the engine revs

Hence fuel consumed is proportional to the cube of the engine revs.

I have a twin engine set-up, and apparently twins will use slightly less than 2x 'a single', but I'll verge on the side of conservative and take a twin set up as being 2 x a single.

Engines :

WOT = 2600 RPM

WOT = 143 HP

Fuel Consumption = 0.195 Litres/HP/Hour ( equates to about 4.2 litres per 20hp)

Example 1 : At a 8 knot cruise at 2000 rpm that is 80% of WOT

So : 80% x 80% x 80% = 51%

ie 51% of 143hp = 71hp

71hp @ 0.195 lt/hp/hour = 13.8 litres per hour

Example 2 : At a 5 knot cruise at 1500 rpm that is 57% of WOT

So : 57% x 57% x 57% = 19%

ie 19% of 143 hp = 27hp

27hp @ 0.195 lt/hp/hr = 5.2 litres per hour.

Twin engines = 2x 5 litres per hour at 5 knots ( 10 litres per hour) About 2 litres per mile

[rustydiver]

I dunno I think you have far too much time on your hands to come up with fancy equations like that.

 

But it looks good.

[cuthound]

When I was working we used to say half a pint of fuel per kW per hour as a rule of thumb for diesel engines standby generators, which can at either 1000 rpm or 1500 rpm, depending on engine type to generate at 50Hz.

[johnnie1uk]

I use half a litre an hour. general 3 mph cruising , 34hp Vetus

[Sabcat]

I would appreciate comments on the accuracy of the following 'formula' ( and my maths)

 

Diesel engines use approximately 1 gal (4.4 litres) per hour per 20 HP developed :

Fuel consumed is proportional to the power developed

Power developed is approximately proportional to the cube of the engine revs

Hence fuel consumed is proportional to the cube of the engine revs.

I have a twin engine set-up, and apparently twins will use slightly less than 2x 'a single', but I'll verge on the side of conservative and take a twin set up as being 2 x a single.

Engines :

WOT = 2600 RPM

WOT = 143 HP

Fuel Consumption = 0.195 Litres/HP/Hour ( equates to about 4.2 litres per 20hp)

Example 1 : At a 8 knot cruise at 2000 rpm that is 80% of WOT

So : 80% x 80% x 80% = 51%

ie 51% of 143hp = 71hp

71hp @ 0.195 lt/hp/hour = 13.8 litres per hour

Example 2 : At a 5 knot cruise at 1500 rpm that is 57% of WOT

So : 57% x 57% x 57% = 19%

ie 19% of 143 hp = 27hp

27hp @ 0.195 lt/hp/hr = 5.2 litres per hour.

Twin engines = 2x 5 litres per hour at 5 knots ( 10 litres per hour) About 2 litres per mile

 

I've no idea about the validity of the formula but you've done some rounding so for example on the first one (using 2000 rpm, not 2080 which would be 80%) I get the consumption to be 12.68 litres an hour. The second one using 1500rpm I get 5.35 litres an hour.

[ditchcrawler]

How do you know what Hp you are using?

[Southern Star]

Way too complicated. All you need to do to calculate fuel consumption is to divide fuel used into engine hours or distance travelled and Robert is your mother's brother.

[Alan de Enfield]

How do you know what Hp you are using?

 

Power developed is approximately proportional to the cube of the engine revs

[Alan de Enfield]

Way too complicated. All you need to do to calculate fuel consumption is to divide fuel used into engine hours or distance travelled and Robert is your mother's brother.

 

But I have 2800 litre fuel tank, I don't necessarily want to fill it and then cruise a few hours and then have to refill it to work out my cruising range.

 

I realise that hull shape and various other factors will play a part but I believe that a 'rough guide' to your expected range, and then using the old saying :

"1/3 for on the way out, 1/3rd for on the way back, and 1/3rd for God" gives you plenty of safety margin.

 

My Narrowboat engine ( for whatever reason, but I believe that a 3:1 gearbox and possibly mismatched prop was responsible) used 2 litres per hour.

On a recent 340 mile trip whilst 'doing' 3mph+ when cruising I actually achieved almost exactly 2 mph when waiting time, locking time etc was added in.

So 1 litre per mile.

 

Edit to add :

Canal Planner has the trip at -

This will take 169 hours and 6 minutes which is 24 days, 1 hour and 6 minutes at 7 hours per day.

So, their default is 2 miles per hour which is the same as my 'actual'.

Edited January 19, 2016 by Alan de Enfield

[Laurie Booth]

I would appreciate comments on the accuracy of the following 'formula' ( and my maths)

 

Diesel engines use approximately 1 gal (4.4 litres) per hour per 20 HP developed :

Fuel consumed is proportional to the power developed

Power developed is approximately proportional to the cube of the engine revs

Hence fuel consumed is proportional to the cube of the engine revs.

I have a twin engine set-up, and apparently twins will use slightly less than 2x 'a single', but I'll verge on the side of conservative and take a twin set up as being 2 x a single.

Engines :

WOT = 2600 RPM

WOT = 143 HP

Fuel Consumption = 0.195 Litres/HP/Hour ( equates to about 4.2 litres per 20hp)

Example 1 : At a 8 knot cruise at 2000 rpm that is 80% of WOT

So : 80% x 80% x 80% = 51%

ie 51% of 143hp = 71hp

71hp @ 0.195 lt/hp/hour = 13.8 litres per hour

Example 2 : At a 5 knot cruise at 1500 rpm that is 57% of WOT

So : 57% x 57% x 57% = 19%

ie 19% of 143 hp = 27hp

27hp @ 0.195 lt/hp/hr = 5.2 litres per hour.

Twin engines = 2x 5 litres per hour at 5 knots ( 10 litres per hour) About 2 litres per mile

Are you related to Albert Frankenstein.

[ditchcrawler]

 

Power developed is approximately proportional to the cube of the engine revs

So a 100Kw generator on full load at 1500 rpm will use the same amount of diesel as one with no load? you are assuming that you are taking maximum load from the engine at all times

[GUMPY]

Similar single engine 120hp ford

Boat dispacement 35t

Size 60x12x3

Average fuel use v revs over 10 years

1100rpm 4L/hr 3kn

1400rpm 5.5 L/hr 4kn

1700rpm 7 L/hr 5kn

1900rpm 10 L/hr 5.5kn

Never went faster than 1900 and rarely more than 1400 as used to much fuel....

That should give you something to guess from

[Alan de Enfield]

So a 100Kw generator on full load at 1500 rpm will use the same amount of diesel as one with no load? you are assuming that you are taking maximum load from the engine at all times

 

Yes - I suppose that is an assumption I am making, however as I will only be using the engine under load ( I don't plan on sitting for hours with the engines running in neutral)

 

However I am not assuming .maximum load. as the load demanded is proportional to the revs and by taking the known fuel consumption at known revs and known HP it is possible to extrapolate backwards.

 

 

Similar single engine 120hp ford

Boat dispacement 35t

Size 60x12x3

Average fuel use v revs over 10 years

1100rpm 4L/hr 3kn

1400rpm 5.5 L/hr 4kn

1700rpm 7 L/hr 5kn

1900rpm 10 L/hr 5.5kn

Never went faster than 1900 and rarely more than 1400 as used to much fuel....

That should give you something to guess from

 

Thanks for that - although your water line length is longer and the boat considerably heavier (and no doubt a different hull profile) it does tend to support (roughly) my calculations.

 

Your 1400 RPM = 5.5 litres, my 1500 RPM = 5 litres

 

As you say, your cruise was 1400 rpm ( 4 knots) whilst mine is 1500 rpm (5 knots)

 

Just as a matter of interest which model engine do you have ( mine are 2725E 6.3 litre Dover engines)

If its any of the 2700 series I have operator and workshop manuals for most of the range of Dorset & Dover engines if you need them.

Edited January 19, 2016 by Alan de Enfield

[GUMPY]

It was a 2725? Dorset and later a Dover as we changed the engine as it was very smokey due to being used to charge the batteries .

New one came out of a fising boat that had only ever done flat out runs out to the fishing grounds so ran very clean.

We don't have Parglena now we downsized to a narrow boat 3.5 years ago. If you haven't found them Seahorsepower.co.uk is a good source of parts, all my spares went with the boat

[Tony Dunkley]

I would appreciate comments on the accuracy of the following 'formula' ( and my maths)

 

Diesel engines use approximately 1 gal (4.4 litres) per hour per 20 HP developed :

Fuel consumed is proportional to the power developed

Power developed is approximately proportional to the cube of the engine revs

Hence fuel consumed is proportional to the cube of the engine revs.

I have a twin engine set-up, and apparently twins will use slightly less than 2x 'a single', but I'll verge on the side of conservative and take a twin set up as being 2 x a single.

Engines :

WOT = 2600 RPM

WOT = 143 HP

Fuel Consumption = 0.195 Litres/HP/Hour ( equates to about 4.2 litres per 20hp)

Example 1 : At a 8 knot cruise at 2000 rpm that is 80% of WOT

So : 80% x 80% x 80% = 51%

ie 51% of 143hp = 71hp

71hp @ 0.195 lt/hp/hour = 13.8 litres per hour

Example 2 : At a 5 knot cruise at 1500 rpm that is 57% of WOT

So : 57% x 57% x 57% = 19%

ie 19% of 143 hp = 27hp

27hp @ 0.195 lt/hp/hr = 5.2 litres per hour.

Twin engines = 2x 5 litres per hour at 5 knots ( 10 litres per hour) About 2 litres per mile

 

That's fine as an alternative to using a formula derived from Propeller Law curves, or plotting fuel consumption versus Rpm on a graph, but in Example 1 you've got 2000 rpm as 80% of 2600 rpm and it's actually 76.9% . . . not a big difference in itself, but a bit more by the time you've cubed it. I make the fuel consumption at 2000 rpm 45.5%, not 51%, of the 2600 rpm figure.

Horsepower absorbed by the propeller is proportional to the cube of the Rpm and, leaving out frictional losses in the transmission and sterngear, the engine will be developing only the horsepower that the propeller can absorb, so you can say that fuel consumption is proportional to the cube of the Rpm.

Assuming that the propellers are matched correctly to the engines at rated maximum rpm, with the propeller power curve crossing the engine power curve at the rated maximum of 2600 rpm, then you could plot a graph for fuel consumption versus Rpm from the equation : ~

Fuel consumption = C(constant) x RPM ³ .

 

The constant C can be derived from the estimated fuel consumption at maximum rated Rpm and the maximum rated Rpm, ie : ~ C = Fuel consumption divided by RPM ³.

 

[Nb. For calculation purposes it's easiest to use Rpm figures in decimal units of 1000, ie 2600 rpm = 2.6]

Edited January 19, 2016 by Tony Dunkley

[Alan de Enfield]

Thanks Tony - appreciate confirmation that I'm on the 'right track' (even tho' I.ve rounded things on the side of 'safety')

 

I'll have a 'play' with your formula and see what differences it produces.

[Tony Dunkley]

Thanks Tony - appreciate confirmation that I'm on the 'right track' (even tho' I.ve rounded things on the side of 'safety')

 

I'll have a 'play' with your formula and see what differences it produces.

 

Just out of interest, have you got any figures for operating at various speeds on one engine only? . . . always remembering, of course, not to exceed the gearbox manufacturers advised maximum time for 'trailing' the gearboxes in the absence of a tailshaft brake. The time limit is due to the oil pump on most boxes being driven off the input shaft and the resulting lack of oil circulation when 'trailing' without the engine running.

This potential source of harm to gearboxes is also something to bear in mind when moored in a strong current or tide for an any amount of time.

Edited January 19, 2016 by Tony Dunkley

[Alan de Enfield]

 

 

Horsepower absorbed by the propeller is proportional to the cube of the Rpm and, leaving out frictional losses in the transmission and sterngear, the engine will be developing only the horsepower that the propeller can absorb, so you can say that fuel consumption is proportional to the cube of the Rpm.

 

That's what I'm basing my calculations on

 

 

Tony, on the power / torque / fuel usage graph in the manual, the max fuel consumption is shown as 22 litres per hour.

 

Fuel consumption = C x RPM³

 

Where C = Max fuel consumption / Max RPM³

 

Therefore C = 22 / (2.6 x 2.6 x 2.6) or 22/17.58 = 1.25

 

Therefore Consumption = 1.25 x RPM³

 

At 2000 revs fuel consumption = 1.25 x (2 x 2 x 2) = 10 litres per hour

 

Using 'my' formula with 2000 rpm being 76.9% of 2600 WOT and taking 76.9³ to give 45% of 143hp generated at 0.195 litres / hp / hr

 

At 2000 revs fuel consumption = 12.5 litres per hour

 

Just out of interest, have you got any figures for operating at various speeds on one engine only? . . . always remembering, of course, not to exceed the gearbox manufacturers advised maximum time for 'trailing' the gearboxes in the absence of a tailshaft brake. The time limit is due to the oil pump on most boxes being driven off the input shaft and the resulting lack of oil circulation when 'trailing' without the engine running.

This potential source of harm to gearboxes is also something to bear in mind when moored in a strong current or tide for an any amount of time.

 

No I don't have any figures.

 

I had never considered the gearbox 'free-wheeling' issues. On all my twin engine boats I have always used both.

 

I guess that's partly the reason (and drag) for yachts having folding props ?

[Tony Dunkley]

 

 

Tony, on the power / torque / fuel usage graph in the manual, the max fuel consumption is shown as 22 litres per hour.

 

Fuel consumption = C x RPM³

 

Where C = Max fuel consumption / Max RPM³

 

Therefore C = 22 / (2.6 x 2.6 x 2.6) or 22/17.58 = 1.25

 

Therefore Consumption = 1.25 x RPM³

 

At 2000 revs fuel consumption = 1.25 x (2 x 2 x 2) = 10 litres per hour

 

Using 'my' formula with 2000 rpm being 76.9% of 2600 WOT and taking 76.9³ to give 45% of 143hp generated at 0.195 litres / hp / hr

 

At 2000 revs fuel consumption = 12.5 litres per hour

 

No I don't have any figures.

 

I had never considered the gearbox 'free-wheeling' issues. On all my twin engine boats I have always used both.

 

I guess that's partly the reason (and drag) for yachts having folding props ?

 

If the specific fuel consumption figure of 0.195 litres/hp/hour is at the max rated 2600 rpm and full load, then the figure of 22 litres/hour, which indicates a developed 113 hp, suggests that the boat is a bit underpropped when running on both engines, probably intentionally, and as a result is almost certain to be capable of continuous operation at full power on one engine, and with that one engine developing it's full 143 hp

I make the fuel consumption at full load and max rpm [ based on 0.195 l's/hp/hour] to be 27.9 litres/hour, which will increase the value of the constant 'C' when running on one engine.

The 0.195 l's/hp/hour is a very good specific fuel consumption figure, . . . 23.3 hp for an hour on one gallon of juice, instead of the usual 'rule of thumb' of 20 hp for an hour for each gallon burned.

Edited January 19, 2016 by Tony Dunkley

[Laurie Booth]

Fuel consumption increases in direct proportion to the need to get to a diesel supplier urgently.

[Alan de Enfield]

Fuel consumption increases in direct proportion to the need to get to a diesel supplier urgently.

 

Yup - there are not many between Hull and Stavanger.

[StephenA]

I would appreciate comments on the accuracy of the following 'formula' ( and my maths)

 

Diesel engines use approximately 1 gal (4.4 litres) per hour per 20 HP developed :

 

That sounds wildly inaccurate for a BMC 1.5.- that would mean we'd burn 8 to 10 gallons a day which would empty our tank in less than a week...

[Alan de Enfield]

 

That sounds wildly inaccurate for a BMC 1.5.- that would mean we'd burn 8 to 10 gallons a day which would empty our tank in less than a week...

 

Its actually pretty accurate - don't make the mistake that because your engine is capable of producing 30, or 40 hp that it actually does. It may well be that at canal speeds you may be only producing 'low single figures' horsepower. Remember that 1hp will move 10 tonnes.

 

If you are producing 5hp then that's only (roughly) 1 litre per hour.

5Hp is more than enough for a NB

 

Have a look at the manufacturers / suppliers graph for your engine - you may be surprised.

 

At 1200 rpm my 6.3 litre 143 hp engine is ONLY using 2.16 litres per hour

[StephenA]

 

Its actually pretty accurate - don't make the mistake that because your engine is capable of producing 30, or 40 hp that it actually does. It may well be that at canal speeds you may be only producing 'low single figures' horsepower. Remember that 1hp will move 10 tonnes.

 

If you are producing 5hp then that's only (roughly) 1 litre per hour.

5Hp is more than enough for a NB

 

Have a look at the manufacturers / suppliers graph for your engine - you may be surprised.

 

At 1200 rpm my 6.3 litre 143 hp engine is ONLY using 2.16 litres per hour

 

OK - wasn't thinking straight. However calculating how much HP you are using on a canal is going to be pretty much impossible...

 

Even pushing up the Tidal Trent against the flow (plus some tidal) we got no where near that consumption level which suggests even then we weren't pushing the engine too much.

Edited January 19, 2016 by StephenA

[Alan de Enfield]

 

OK - wasn't thinking straight. However calculating how much HP you are using on a canal is going to be pretty much impossible...

 

Even pushing up the Tidal Trent against the flow (plus some tidal) we got no where near that consumption level which suggests even then we weren't pushing the engine too much.

 

HP produced is a function of engine revs and fuel used.

If your engine is doing (say) 2000 RPM it will generate exactly the same HP & use the same amount of fuel - It doesn't matter if you are on a tidal river, going with or against the flow or aground in a shallow canal, 2000 rpm is 2000 rpm, will generate the same HP and will be using the same amount of fuel.

 

You may well be going at different speed over the ground (or not moving at all) but your HP & fuel usage is the same.

 

I don't have the performance curves for the BMC 1500 (39 BHP at 3500 rpm) but if you look on the (very informative) Beta Marine website for the performance curves for the BV1305 engine (35 bhp at 3600 rpm) you will see that actual "propeller law" figures show that at 1600 rpm it is using 1 litre per hour, whilst at 2200 rpm it is using 2 litres per hour. WOT (3600 rpm) is a tad under 6 litres per hour.

 

Look at the BV1505 (37.5hp at 3000 rpm and almost 7 litres per hour)

 

The formula I have been using ( and the one provided by Tony) are applicable to any sized of engine as long as you know the WOT revs, WOT HP and WOT fuel consumption.

Edited January 19, 2016 by Alan de Enfield