In preparation for a 30 hour trip. What is the fuel consumption (gal/hr) of the Yanmar 3GM pushing a F-38 at moderate speed?
To answer this I need to assume two things. Firstly that you have the performance curves for your engine and secondly that the propeller is sized to allow the engine to develop full power at full rpm.
There should be four curves on your chart. Power (Kw/hp) Torque, brake specific fuel consumption and propeller law curve.
First determine your cruising rpm and locate on the horizontal axis (0-max rpm) The vertical line from that point will intersect the other curves. Torque is irrelevant in this case. Power is the power that the engine is capable of generating at those rpm and again is not relevant. The propeller law curve gives the power that the propeller is capable of absorbing at those rpm, this is the power required to drive the boat. The BSFC curve gives the fuel consumption per unit of power produced, by weight. Thus grams per kilowatt hour or pounds per hp hour. Multiplying the power absorbed by the weight of fuel per unit of power hour, give the weight of fuel consumed per hour under those load conditions. One needs to know the relative density of the fuel to convert the weight to a volume measurement. In the metric system (very approximately) the RD of gas oil is about .8 (if I remember correctly) so 1 litre of diesel would weigh 800 grams. I cant remember even approximately the US system.
So, an example might be as follows. An engine with maximum rating of 30hp (22.4 Kw) @3600 rpm will run at cruising speed 2800 rpm. At these rpm the engine can develop (say) 15 Kw but the propeller will only absorb 12 Kw. The BSFC at 2800 rpm is 320g/Kwhr. So the fuel consumption would be 12Kw x 320g = 3.84 kg/RD 0.8 = 4.8 litres per hour.
I hope this will be some help. Best wishes on your voyage.
Oh my God, I thought I asked a simple question ! 
Hi Phillip,
I have a F35 with a 3gm30f. I burn .4 gph @ 2500 rpm about 1gph @ 3200 rpm. In general if I am cruising, say 2800 rpm, I use .7 gph to calculate usage. I did find a fuel consuption table somewhere on the net that gave consuption for most Yanmars, but that is on the boat right now. Next time I get down I’ll try to bring it back and let you know.
Fred
Hi Phillip,
I didn’t mean to bamboozle you! I found some 3GM30 curves and Fred is just about spot on at .7 gal per hour at 2800 rpm - 3 litres per hour. This assumes that the prop is correctly sized. My earlier estimates were a bit conservative. Power absorption by the prop is about 12 kw, specific fuel consumption at 2800 rpm is about 220 grams per Kw/hr, and relative density is nearer .84 than .8.
I also found a later fuel consumption curve which states consumption against rpm but this is not very helpful or reliable. Fuel consumption varies according to how much power is being used, not how many rpm the engine is doing. So for reasonably accurate projections you can’t get away from BSFC and propeller law curves.
Regards,
My 3GM30F ,in Freedom 33 # 68 burned 3 liters/hr @ 2800m rpm,logging 6 kt at that rpm.
Thank you. I motorsailed for 24 hours at around 2200rpm and only consumed 10 u.s. gal…pretty economical.