Fuel consumption again

[Pizzazz]

I have been looking at a simple way to estimate fuel consumption on my boat, a 48 ft Riva, 2x 6CTA's at 430 hp, 32,000 lb displacement. The simplest formula is called Crouch's calculator that says speed = C*(hp/disp)^0.5, where C is just a constant to take care of different units used.

The boat does 30 knots, 32,000 lb of displacement at max power, at 2,600 rpms, which is the rated speed. If I plug in these numbers in the calculator above I get 30 knots = 180*(860 hp/32,000 lb)^0.5. So, the constant for my hull is 180. Then I look at the efficiency of my turbo diesel engines and it looks as if it is 19 hp/gallon at optimum rpm's and between 18 and 19 throughout the planing range. Using this information, I came up with the following table:

C 180 180 180 180 180
hp 860 600 500 400 300
hp/g 18.0 18.5 19.0 18.5 18.0
gph 45 32 26 22 16
Knot 30 25 23 20 17
nmpg 0.7 0.8 0.9 0.9 1.1

Is there anything dramatically wrong with this approach? If yes, what should be improved? I guess the specific fuel consumption of the engine is an approximation but can't be much different for modern turbodiesels. It would be helpful if other powerboaters calculate the C constant for their boats in actual conditions to compare. It seems to be that all modern improvements, such hull form, shafts/pods, etc. just change the C-constant +/- 5% and what really matters is displacement and how clean the hull is maintained.

[donradcliffe]

I remember running a similar boat up the east coast USA, and we got about the same fuel burn vs speed. On average it was 0.85 mpg. Of course if you back off the plane to 7-8 knots your range will go way up.

[skipperpete]

I’d opt for a Floscan fuel flow monitoring system with 4 flow meters and the twin display panel instrument, then you can see how much fuel you’re using in real time. It’s nice to calculate and adjust the endurance to avoid that embarrassing radio call for a tow. The old 6BT’s actually came with Floscan as an option, the only downside being that the early analog readout was in gallons per hour not litres, there might be a digital display on the newer models. At a guess, I reckon a good flow meter setup would pay for itself in fuel savings considering that at full noise it’s costing you about Aud $680/hr max and $272/hr minimum….. not including whatever the genset is drinking, on a Riva I doubt you’d be running without full AC power.

[Cargile]

Originally Posted by donradcliffe

I..................Of course if you back off the plane to 7-8 knots your range will go way up.

Okay. Doyou know if that holds true for a gasser? I have a 28' trailer trawler w 302 Ford and I/O. 150 gal tank. It planes say around 15 mph with WOT say 28 mph. I am looking for the least fuel consumption/max range at a reasonable cruising speed. I imagine that would be off plane. Have not found a graph that shows me this.
I apologize if this is thread hijacking.

[rslifkin]

A knot or so below hull speed will almost always burn less fuel than running on plane, although the difference is usually a bit smaller with gas engines than diesel (as gas engines lose more efficiency when load is reduced). The difference also tends to be smaller with smaller, lighter weight boats.

With my boat (~27k lbs, 38 feet, 2x 340hp gas engines) we burn around 30 gal/hr to cruise at 17 kts and about 5 gal/hr to cruise at 6.5 kts.