Hey guys I did some research and this is what I found, not sure if it is accurate but it seems like a good place to start:
"Here is a little metric I picked up recently (from where I cannot remember): Compute your engine's fuel consumption at wide-open-throttle (WOT) as approximately equal to HORSEPOWER/10 = GallonsPerHour (gph).
Then, approximate the fuel consumption at a particular throttle setting as a linear percentage of the wide-open-throttle consumption.
Let's try this out with my boat:
Engine Rated Power = 70 hp Est. Fuel Consumption WOT = 7 gph
At WOT the engine is turning about 5500 RPM. At cruise of 4400 RPM, the fuel consumption should be about:
7 gph * (4400/5500) = 5.6 gph
At 4400 RPM we are doing about 28 MPH and burning about 5.6 * 2 = 11.2 gph, giving us a Mileage of about 2.5 Miles per Gallon (mpg). (I had to double it because we have twin engines.)
So we are getting 2.5 mpg from our fairly low-powered set up of twin 70 HP engines.
Taking this same approach, I approximate LHG's fuel consumption, running a larger boat with bigger engines. He comes in like this:
200 HP --> 20 gallons per hour at WOT (6000).
Now for him to cruise at 28 MPH he is only running his engines about 2200 rpm, so he is burning about:
20 gph * (2200/6000) = 7.3 gph
Thus his fuel consumption is about 14.6 gph while making 28 mph, for a mileage of about 1.9 miles per gallon.
This was quite surprising to me! I thought I would would be much farther ahead with a smaller boat and smaller engines, but it turns out my fuel economy is only 0.5 mpg better, and that is only an approximation!
After cruising together for a week, it almost seemed like we were using about the same amount of fuel, implying that the fuel mileage was closer than shown above.
I also think that the approximation for fuel consumption versus engine speed is a little off. The fuel consumption is probably not linear with crankshaft speed, and it is likely lower at lower speeds than would be suggested by a linear approximation. This would again work in favor of the boat with larger engines running at lower speeds.
Comments and other observations welcomed,
--jimh"
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