Diesel consumption and Revs.

[BrendanChandler]

<span style="color:blue"> I would be interested for any rule of thumb guidance on how fuel consumption varies with engine revs. I realsie that it will depend on prop size and type, cavitation etc etc.

As a rule of thumb, if at maximum engine speed of say 3,000 rpm I use 3 litres of fuel per hour, how much am I likely to use at say 2,000 rpm and at say 1,500 rpm.

Should the question be phrased as "if I use 3litres of fuel per hour at 7 knots, how much would I use at 6knots and at 5knots.

This info would be of use in ocean sailing where fuel is often limited and there is a balance of speed against consumption.

As always, thanks for sharing your knowledge

Brendan /forums/images/graemlins/laugh.gif </span>

 

[Landale]

The method I use (I found it somewhere - I think the RNLI handbook) which seems to be reasonably accurate on the Volvo 2003 in my Moody 31 is-

If the rated power of the engine is say 28hp at 3200 rpm and using the approximation of 1 gallon per hour per 20hp produced:

((((actual revs/rated power revs)^3)*rated power)/20) gallons per hour

for petrol I think the figure mentioned is 1 gallon per hour per 15hp developed.

Looks complicated but I've put it into a spreadsheet on my pda worked out for 1000, 1500, 2000, 2500 (unlikely to rev that high!) and 3000 (or even this high!!)

 

[ChrisE]

Here\'s a website for you

Brendan,
If you look at the Beta Marine website you'll find a series of graphs showing fuel consumption vs revs. We have recently installed a 37.5 hp and can verify that the figures they give are spot on for their kit perhaps the engine you have has similar site?

We spent a fair bit of time ensuring that the engine, gearbox ratios and prop were matched for the type of craft. In calm conditions, 1500 revs gives us under 2 litre per hour and 5.5 knots 2000 revs gives us 6.5 knots and 2.2 litres per hour. Of course the rate of consumption doesn't change with sea state but the knots go up or down depending on whether we are motor sailing with the wind or bashing into a sea.
Chris

 

[Talbot]

fuel consumption will vary from engine design and size as well as boat design and size. You will need to start at a consumption/rev (should be available from the engine manufacturer, and then compare revs to boat speed (again this will vary due to wind and water conditions). Thus for your own purposes of finding the most economical revs for a becalmed boat, you assume zero wind and minimal wave interaction, thus most efficient is probably go to be around 0.5 - 1 knot below boat hull speed, and most economical abt 2 knots below hull speed.

 

[office_sailor]

It's more of a large ship method, but the formula:

Fuel coefficient = (Displacement^(2/3)*Speed^3)/(Daily fuel consumption)

can be used to find the fuel coefficient of your vessel, which can generally be assumed to be fairly constant for small changes in speed. Then you can use:

Daily fuel consumption= (Displacement^(2/3)*Speed^3)/(fuel coefficient)

to estimate the daily fuel at given speeds.

Hope this helps a little!

 

[TigaWave]

Between the UK and the Canaries you'll get a few chances to do accurately timed runs under engine at different settings, we did this to find our optimum cruise speed in case we needed to know exactly how much distance we had in the tanks, we also carried three large plastic cans and had worked out an easy clean way to refil the tank at sea (well worth practising)

Use a foot pump into the plastic can to pump in pressure to force fuel out and start syphon, also we had a long length of wide fuel pipe with several lines securley attached so that it wouldn't move whilst you're re-filling. Its sure to jump out if it can rule!

 

[Becky]

Roughly it is 1 gall per hour per 20 hp. If you drop the revs to two thirds the consumption drops to about a half. The most modern design of engine may be a bit better than this, but it is a good method for estimating the fuel usage. You can't talk in terms of miles, because this depends upon sea state.

HWMBO
/forums/images/graemlins/wink.gif

 

[Chris_Robb]

Revs is irrelevant

It is misleading to look at the revs you use and relate this to fuel consumption. A dielse engine has a governor which limits engine revs to the throttle setting. so in a flat sea/calm - the govenor will be holding the revs to that set, and be using little fuel.

Punching into an 8 the goveror will still hold the revs to that set, but in order to maintain those revs - will be pouring in a load more fuel. This can double consumtion in heavy weather over a flat calm.

This is much more noticeable in boats with bags of reserve power - ie 80 HP on a forty footer. a 40 ft with 30 hp will be driving flat out even in calm - so you wont notoce anything (if you are going anywhere at all).

 

[pohopetch]

What are the units in this formula? Tonnes, lbs, litres, gallons? Do you have any references to more info on these sort of calculations? Thanks, Peter.

 

[BrendanChandler]

Thanks everybody. Isn't life complicated.

I have a Volvo Penta M2040 on a 38ft yacht.

I cannot see that Volvo publish any table of revs to consumption.

The fuel tank is shallow but very wide and long. There is no electric fuel gauge, probably because of the shape of the tank in that a very small incline towards or away from the gauge would have a very large effect on the reading.

I am limited to calculating the fuel remaining by assuming that I use 2.2l per hour at about 2,500 revs on a calm day. As you can imagine, I am surprised from time to time and keep a secret can of fuel for emergencies, only happened once in the last 10 years despite a couple of Atlantic crossings and many ocean legs.

I would like to refine this estimate by relating it to the revs or speed on a calm sea with no wind and then adjust this estimate for weather etc to arrive at the most economical consumption for long distances.

Thanks

Brendan