Calculating Fuel Consumption

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NauticalNovice

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Given any known configuration, is there a formula for calculating fuel consumption in relation to speed and can that formula be converted into an equation to produce a graph?

For example.....

If knots were represented by the X axis and fuel consumption represented by the Y axis then is there an equation to create a graph. Using Excel would be an added benefit.

Thanks

NN
 
As far as I know the formula is that revs, speed and width of grin are directly proportional but mutually inversely proportional to the health of your bank balance.

Beyond that it's best not to think about it.
 
As is often the case, unfortunately I don't think it's that straightforward.

Unless you just want very rough figures, there are a number of variables that need to be taken into account to give you true consumption figures at any given speed:-

1. Hull condition (fouling)
2. Wind
3. Sea state. More fuel used in a chop than a flat calm for the same speed.

..and no doubt a heap more that others will shortly add to.
 
Most engine manufacturers publish rev vs burn rate graphs as part of their sales promotion, so start there. If you get one treat it with a large dose of skepticism until you prove the curves from experience.
 
Bertramdriver said:
Most engine manufacturers publish rev vs burn rate graphs as part of their sales promotion, so start there. If you get one treat it with a large dose of skepticism until you prove the curves from experience.
Click to expand...

thats what I wanted to sugest.
sometimes this is published with the engine specs,

MAN publishes "max" and "prop" curves,
coincidently
my consumption (both engines together) at
cruising displacement speed 10kn /1100RPM, is 7l/nm
and
cruising planing speed 20kn / 2000rpm, is 15l/nm
this is exactly on this "prop" curve,

I sail almost alway's at 10kn or 20kn,
so have been able to check real consumption figures after each re-fueling

so with this prop curve I can produce a approximate consumption curve by taking note of engine RPM at
11, 12, 13, 14........21, 22, 23,25 kn
(all averaged, but a good indication)
on my todo list when I sail out next
 
have a look at the old MBY boat test articles to see if they have ever tested your particular boat - if you knock 5 to 10% off the claimed test figures you will be about right. As others have said, load, age, state of hull, sea, wind etc all affect it. or let us know what boat you have someone here may well be able to tell you what to expect.

regarding a formula thats very difficult, every model of boat behaves differently and different boats of the same make and model will behave differently. The mpg curve is very non linear nor is it easy to approximate with a more complex mathematical function. If you look at engine consumption speed curves with max load consumption and idealised prop demand consumption, the actual consumption will be anywhere between the two lines depending on the design of the boat the engine is in.
 
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BartW said:
and
cruising planing speed 20kn / 2000rpm, is 15l/nm
this is exactly on this "prop" curve,
Click to expand...
We've had this conversation before! Don't know about MAN but for Cat, the prop curves are always for displacement hulled boats and therefore not really accurate for planing hulled boats. In the case of my boat, the prop curve underestimates fuel consumption and the max power curve overestimates it. IIRC, we agreed in the past that if the prop curve is 3.0, then a more accurate curve for planing boats would be around 2.5
 
NauticalNovice said:
Given any known configuration, is there a formula for calculating fuel consumption in relation to speed and can that formula be converted into an equation to produce a graph?

For example.....

If knots were represented by the X axis and fuel consumption represented by the Y axis then is there an equation to create a graph. Using Excel would be an added benefit.

Thanks

NN
Click to expand...

Do you have a particular make/model of boat in mind?
 
Deleted User said:
We've had this conversation before! Don't know about MAN but for Cat, the prop curves are always for displacement hulled boats and therefore not really accurate for planing hulled boats. In the case of my boat, the prop curve underestimates fuel consumption and the max power curve overestimates it. IIRC, we agreed in the past that if the prop curve is 3.0, then a more accurate curve for planing boats would be around 2.5
Click to expand...

Mike,
the prop curve on the MAN spec sheet is 2.7
and as mentioned before, this curve is very very close to my measured consumption at 10kn (displacement) and 20kn (cruising planing speed)
so for me a very good indication of reality.
I don't know for other engines / boat combinations

the MAN engine model in my boat is a commenly used engine in planing boats, (as mentioned in the brochure)
so I assume the spec sheet is also for that application.

how are you getting on with your quest for finding a new boat ?
don't you want to share more of your findings ?
 
the prop curve on the MAN spec sheet is 2.7
Click to expand...
Ah OK, didn't realise, thats v useful

how are you getting on with your quest for finding a new boat ?
don't you want to share more of your findings ?
Click to expand...
I'm looking at a couple of boats at the moment and I hope to make a decision in Jan after we've inspected them again
 
Thanks for the many interesting replies.

I am trying to assimilate all of this and determine if I can convert this information into an Excel SS.

Should be an interesting exercise in playing with numbers.:encouragement:

NN
 
The above are all very well but to give a meaningful answer we need to know the type of boat, engine and drive type as well as fuel.

Telling someone exactly what the prop curve says and they go out in on a lumpy day with a pair of old petrol V8's and cant stay on the palane so spend all their time trying to get on the plane going up hill and they will get in serious do do's .

The most meaning ful advice is from someone the same boat or similar and the same engine/drive setup.

Some fuel gauges are not giving you a proper reading because it says there is a third of a tank full does not always mean that.

Always keep a third or so of a tank up your sleeve so as you don't get caught out depending on the boat course, I never drop below 20% on large tanks with a theoretical 5.5 hours running left.
 
longjohnsilver said:
Are you going bigger, Mike, or just want something different?
Click to expand...
SWMBO always wants a bigger one. I'm thinking one more flybridge planing boat in the Med before a trawler type yacht of some sort for more extended cruising but best laid plans and all that so I'm trying to choose a planing boat that could work as a semi-liveaboard cruising boat as well in the future if needs be
 
Deleted User said:
SWMBO always wants a bigger one. I'm thinking one more flybridge planing boat in the Med before a trawler type yacht of some sort for more extended cruising but best laid plans and all that so I'm trying to choose a planing boat that could work as a semi-liveaboard cruising boat as well in the future if needs be
Click to expand...

still looking at ferreti's or something else aswell ?
 
NauticalNovice said:
Thanks for the many interesting replies.

I am trying to assimilate all of this and determine if I can convert this information into an Excel SS.

Should be an interesting exercise in playing with numbers.:encouragement:

NN
Click to expand...

can you mention the boat size / model you're looking at ?
as Bandid say's, folks on here could give you accurate real live figures
 
NauticalNovice said:
Given any known configuration, is there a formula for calculating fuel consumption in relation to speed and can that formula be converted into an equation to produce a graph?
Click to expand...
Easy peasy, see below for a self-explanatory representation of the formula..
Oh, and needless to say, the first version is for a clean hull, while the second considers also fouling.
d1d2.jpg
 
MapisM said:
Easy peasy, see below for a self-explanatory representation of the formula..
Oh, and needless to say, the first version is for a clean hull, while the second considers also fouling.
d1d2.jpg
Click to expand...
I put the figures into your formulae for my boat and the answer is 42
 
BartW said:
still looking at ferreti's or something else aswell ?
Click to expand...
I was about to make an offer on a used Ferretti 630 but then a used San Lorenzo SL62 came up. They are 2 very different boats. The 630 is very much in the modern Ferretti style which is fine but it feels a touch bland. The SL62 has more of a feel of a little ship. We are planning to go out to Italy again after Xmas to look at these 2 boats again and hopefully make a decision. I think probably we will go for the devil we know ie the 630 as it will be a more sensible buy
 
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