Fuel Consumption

[mystro]

Can someone point me to a chart or guide which shows fuel consumption at different RPM levels for this 4cyl I/O in a vintage 18' runabout? My fuel guage and tach need replacement but I have a close approximation of RPM when I used the dwell meter while underway.

 

[Haut Medoc]

Re: Fuel Consumption

Welcome to iboats......
Consumption will vary with hull design.......
3 mpg is usually a good estimate......
The best economy-wise is between 2800-3300 RPM on most boats.....
With a properly functioning tach, fuel gauge & GPS, you should be able to determine those specific values for your boat.....
Keep in mind that a properly tuned engine will give the best results.....
​

 

[Mischief Managed]

Re: Fuel Consumption

Drag is a function of the square of speed.
The amount of gasoline necessary to make 1 HP for one hour is typically .09 gallons.

Assuming the engine is running properly and propped right:

Find your WOT top speed using a GPS, or a reasonably accurate speedometer. Square this MPH number and divide by the rated HP of your engine. We'll call the result X

Take your cruising speed in MPH, square it and divide the result by X. This tells you the required HP to maintain cruising speed, lets call this result CHP

Multiply CHP by .09 to get the gallons per hour of fuel use. Call this result GPH

Divide cruising speed in MPH by GPH to get MPG.

Example:

My boat is propped perfectly, and runs perfectly. It will hit 50 MPH with a typical load and the engine is rated at 310 HP

50 MPH squared is 2500. 2500/310 = 8.06 so X=8.06

I cruise at 32 MPH 32MPH squared is 1024. 1024/X=127 so CHP=127

CHP*.09=11.43 so GPH=11.43

32MPH/GPH=2.79 so my MPG at 32 MPH is 2.79.

The numbers may be a little off, but this is as good as it gets without an accurate fuel flow tool.

 

[mystro]

Re: Fuel Consumption

Thanks all, this will sure help..I was sweatin alittle the last time I ran the main tank out and only had the 2 1/2 gallon. BTW, it's the 120hp but I've never been able to find out what the factory recommended prop was..it had a 14x14 and seems to be close to WOT rpm of 4400 at approx 38mph according to the speed censor.

 

[wcsparky05]

Re: Fuel Consumption

WOW! Mischief, you just blew my mind! that's pretty cool. what else you got?

 

[seven up]

Re: Fuel Consumption

Fuel Gauge. What a luxury.

Mine came with a mahogany stick to shove in the tank.

 

[I/O WALDO]

Re: Fuel Consumption

Great formula Engineer Mischief ! I ran the formula on my boat specs. and it is the same as my flow scan readings last summer!

 

[Mischief Managed]

Re: Fuel Consumption

WOW! Mischief, you just blew my mind! that's pretty cool. what else you got?

Thanks! What do you want to calculate? I live for this stuff.

Oddly, I was the idiot in high school algebra and geometry who said "why do I need to learn this? I'll never use this stuff in the real world..." Now I deal with composite third order harmonics, chromatic dispersion, milliwatts to dBm conversions, and stuff like that every day at work on long haul fiber optic systems. None of it requires anything more than freshman algebra, just seems complicated.

 

[Mischief Managed]

Re: Fuel Consumption

I need to clarify something with this formula. The formula assumes hulls have linear drag vs speed plots AND that engines have linear fuel usage vs HP plots. Neither do, especially stepped hull boats. The formula works fairly well, but it completely misses the possibility of substantial fuel savings through careful study of real data, not calculated data.

The formula I posted will not find the sweet speed where your hull has the least drag. It also will not show the engine's peak volumetric efficiency where it's likely to be making the best-HP-for-fuel-used either. For this, you really need real time data acquisition (flow scan and GPS)

When you can run your hull in it's sweet spot AND be propped/powered such that the engine is in it's sweet spot, you have reached MPG nirvana and there are substantial gains to be had. Reggie Fountain seems to know how to do this better than anyone. His stepped hulls and recommended engine packages are suprisingly efficient when compared to similarly sized/powered boats. Larson, Regal, and Stingray have made some good strides in this regard as well.