Fuel consumption query

[bobsadler]

Hi,

My sailboat has a Yanmar 4LH-HTE diesel engine. At 2100rpm I get about 6.9 - 7kts boat speed. I would like to know what the engine's fuel consumption should (liters/hr or gal/hr (US or UK) at 2100rpm but am not familiar with getting this information off the performance curves and wondered if someone would be good enough to calculate it for me

Prop is a Max Prop, 3 blade, 600mm/24inch

All help gratefully accepted (and performance curves attached)

Kind regards

Bob

[Kenomac]

There are way too many variables to give an answer. Hull design, prop blade angle/pitch, bottom growth, growth on prop blades etc.

[Wotname]

Taking a simple look at the power curve graph, it looks like the propeller should be absorbing about 35 hp at 2100 rpm.

Now looking at the fuel consumption graph, it looks like the fuel consumption should be 168 xxx (at 2100 rpm)

The attachment is just missing the units of the fuel consumption (it missed the edge of the scanner or whatever) but i'm guessing it is grams per Hp per hour.

If so, then the engine is using 168 grams x 35 (prop. hp) per hour. That is 5.88 Kg of fuel per hour.


The specify gravity of diesel varies quite a bit but usually is considered to between .8 and .95 (AFAIK) so using an average of .88, this tells us the volume of 5.88Kg is 5.17 litres

So fuel consumption at 2100 rpm is somewhere between 5 and 5.5 litres per hour.

But I could be wrong

[StuM]

A common rule of thumb is 0.225 litres per developed brake horsepower per hour.

Your file's "Propellor Power Curve" shows about 35HP at 2100 RPM, so your consumption should be around 7.8 litres per hour.

From experience, that sounds about right for a 15 ton, 42ft monohull at 7 knots assuming the prop is appropriately sized and the bottom is clean.

[StuM]

Originally Posted by Wotname

Taking a simple look at the power curve graph, it looks like the propeller should be absorbing about 35 hp at 2100 rpm.

Now looking at the fuel consumption graph, it looks like the fuel consumption should be 168 xxx (at 2100 rpm)

The attachment is just missing the units of the fuel consumption (it missed the edge of the scanner or whatever) but i'm guessing it is grams per Hp per hour.

If so, then the engine is using 168 grams x 35 (prop. hp) per hour. That is 5.88 Kg of fuel per hour.


The specify gravity of diesel varies quite a bit but usually is considered to between .8 and .95 (AFAIK) so using an average of .88, this tells us the volume of 5.88Kg is 5.17 litres

So fuel consumption at 2100 rpm is somewhere between 5 and 5.5 litres per hour.

But I could be wrong

You multiplied when you should have divided

5.88Kg of diesel at a Specific Gravity of 0.8 (a reason approximation for most marine fuel) is 7.35 litres (pretty close to the number derived from the .225 l/h/HP rule).

[Wotname]

Originally Posted by StuM

You multiplied when you should have divided

5.88Kg of diesel at a Specific Gravity of 0.8 (a reason approximation for most marine fuel) is 7.35 litres (pretty close to the number derived from the .225 l/h/HP rule).

Of course and if I had thought about it, I would have realised the litres had to be greater than the kilograms otherwise water would float on diesel.


And

And I was right about being wrong

[bobsadler]

Many thanks for the calculations (I came up with a similar figure of around 7.5L/hr at 2100rpm and, yes, consumption on graph is gm/hp/hr).


Over the past few days have motored marina to marina and seem to have burnt 190L fuel doing 103 miles (say 15.5 hours). With warming up and leaving the engine running while being assigned and getting into a berth and tied up (a somewhat slow process in Spain) probably another 3 hours of running could be added So, say, 19 hours running used 190L diesel which gives 10L/hr or about 45% more than expected.


The hull was antifouled within the last month and the Max Prop serviced (and we're getting the expected 6.9 - 7kts at 2100rpm)


The yard I used pulled and checked the injectors but since they proved incompetent at every job they did I wonder if there is a problem with the injectors or is consumption within the margin of error.


All thoughts gratefully received.


Regards


Bob

[Kenomac]

If you're not getting black smoke, then I don't think you're burning rich. Try lowering your revvs. On our boat, the fuel consumption drops in half if I reduce from 2200 down to 1800rpm. Even more of course if we motorsail.

[bobsadler]

Hi,

No black smoke and essentially no oil usage and engine sounds as it always has.

Regards,

Bob