Calculating theoretical fuel consumption, endurance and range

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I wish to calculate the theoretical fuel consumption, endurance and range of my Nauticat 42, powered by a Yanmar 110hp 4LH-TE. I have the measurements of the boat, and the Yanmar manual which gives a graph of the specific fuel consumption in grammes per horsepower - hours so I can easily calculate the fuel consumption per hour at a given power level. I don't have the prop details so I cannot as easily work from revs - but I can't see that that is a problem?

What I am looking for is a formula or tables that will allow me to estimate the theoretical speed given the shaft power to the prop as this should allow me to make a direct plot of consumption, range and endurance for a wide range of speeds. I seem to remember that naval architects have a simple approximation for use at cruising power levels but I am interested in getting an idea of the extended range and endurance at speeds well below the normal cruise either for use in extremis or as a guide to consumption when motor-sailing.

Many thanks for any help. David
 
Your Yanmar manual should be able to give you a table which compares horsepower to revs and also to fuel consumption. You will then need to do a quick practical trial to establish revs to boat speed . Of course this will still provide an very inaccurate measurement as boat speed to revs will be very dependent on apparent wind speed and direction, but you could work up a chart with appropriate error bars to cope with variation in conditions. You can the compare one chart to the other and establish range/speed charts. The beauty of this sytem is that it is independent of prop size or boat characteristics.
 
6 Knots - about 5 litres and hour - less speed = less usage a little bit more speed = an awful lot more fuel usage.

But I guess you know that.

A simple plot made up 30 mins at x Kts = y Litres etc. 30 mins at x+1 kts = y1 litres this would be simple but when all the variables wind conditions/sea states are introduced....the whole business becomes tremendously complex.

I've thought about the calculations, asked advice but it still all comes back to about 5 litres and hour on average. Guess you probably know that too..

Ian
 
I have the curves but since the load is unknown I cannot get back to hp for a given RPM. The fuel consumption is a function of hp not RPM (and is pretty flat across the whole load range). A diesel 'throttle' sets a speed governor and as much fuel as is needed to keep that RPM is delivered to the injectors. So for a given RPM the fuel could vary by a factor of 10:1
 
Easiest solution...

Easiest solution would be to pop a changeover valve in the fuel line, rig up a little day tank with a graduated 1 litre section, then take the boat out, get it going at your chosen speed, flip the changeover valve and time how long it runs on a litre at the chosen speed. Repeat as required for different speeds.
 
I've heard from others that it is around 10 litres per hour, which sounds too high.
 
Re: Easiest solution...

It's not as simple as that because you have the fuel return to take account of.

Even so, you would have to have a clean bottom and a representative weight on board. Quite an exercise, whereas if I can establish a theoretical curve I can plot a few actuals as the months go by which should give me quite an accurate overall curve.
 
Ian, [ QUOTE ]
6 Knots - about 5 litres and hour - less speed = less usage a little bit more speed = an awful lot more fuel usage.

[/ QUOTE ]Did you mean this for ANY yacht or for a Nauticat 42?
 
Re: Easiest solution...

If I was going to do that I would fit a fuel-flow meter which is very handy - tells you when you've got a bit of something round the prop long before you'd notice otherwise, dirty bottom, etc. Hmmmm...... maybe I will, but it is quite a project to do properly with the wiring, plumbing, and mounting the instrument in a professional manner. But then where do you stop with instrumentation? Everything would be nice. Meanwhile, I'll be happy if I can sketch a theoretical power vs speed curve for calm sea, no wind conditions.
 
On the web-site at bottom ...

Click on my web-site at base of this post and you will get a table of average consumption .... as Ian says .... it seems that whatever boat you have unless you have a huge beast of an engine - they drink remarkably similar !!!

/forums/images/graemlins/tongue.gif
 
Re: On the web-site at bottom ...

Thanks for that, Nigel. We are 42' 16,500kg displacement and have a 110hp engine. The nearest (and only similar) vessel to us on your list is the Maxi 120 40' with a displacement of 15,000kgs, which only has a 63 hp engine. He says that he consumes 1.5 l/hr at 1500 RPM but speed is unstated so it isn't very helpful.

However, this is indicative of what I am saying. I am not just interested in the fuel consumption at cruising speed, I am interested in the speed/hp curve. Not surprisingly, fuel consumption drops dramatically at low RPM and the max range and endurance can increase enormously.

The figures in your table vary enormously - I'm not sure that one could draw any sort of conclusion from it.
 
While a naval architect can predict fuel consumption quite accurately when designing with modern tools, in the end on larger vessels it is always proven during sea trials over constant speed runs (usually with modern bigger diesels power and fuel consumption are available from the monitoring system).

In my own case, with a yacht, I have calibrated the fuel tank and roughly monitor use against hours, but in the end fuel consumption can vary alot (as I think you allude to in your posts) according to sea/wind conditions encountered and also for charging hours. So I just use a conservative guess from that experience when predicting duration (in our case roughly 4 litres/hr cruise over a variety of conditions, and something less than 2 litres/hr for charging for 50 hp drive). Roughly speaking, on average, the consumption at cruise revs seems to be close to the engine makers fuel consumption versus revs curves.

I would suggest monitoring use over a variety of conditions from a calibrated fuel tank and use a conservative figure from that to calculate duration. Allowing that achieved duration depends on actual conditions encountered and run nature (charging versus cruise hours ratio) so cannot be predicted with any level of certainty in a small vessel.

John
 
[ QUOTE ]
The fuel consumption is a function of hp and that is pretty flat across the whole load range

[/ QUOTE ]
Yes a bit of a problem. I was drawing a corollary with my own engine which does have a nice rpm/hp curve - but is a smaller engine.

If you have a Navman 5500 or 5600 chart plotter, they have a nifty device which measures fuel flow from and to the engine and will give you fuel consumption figures.
 
Navman - only for petrol...

I had a Navman 5600. They only measure fuel flow for petrol engines unfortunately - there's no way of measuring the return flow of diesel to the tank.
 
But here\'s one which measures diesel...

The FloScan gauges measure diesel consumption accurately, using 2 flow sensors. The 7000 Series is available in a metric version. The 9000 Series, also available in a metric version, interfaces with the GPS so it can also give a "miles per gallon" readout. For a 100HP diesel, the 7000 Series is about £600.
 
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