posted 03-22-2004 08:49 AM ET (US)            

Of course these are estimates. Assuming you have the right prop for your purposes. If you load the boat with 7 people and gear, full tank etc, you'll use a lot more.
If you have a clean bottom, less than 1/2 tank and just 2 people, little wind, etc, you'll get better results.

Over the years, with carbed 2-strokes, these estimates have been pretty darn close. These include 13'SS, 17'Nauset, 20'Revenge, 25'Revenge. Only Whalers.....of course!

posted 03-22-2004 01:59 PM ET (US)            

At 225-HP, you will burn 113 pounds of gasoline in an hour. That is about 20 gallons per hour. This is fairly close to the estimate given about as 22.5 gallons per hour.

There will be quite a difference between 4000 RPM and 5000 RPM.

posted 03-22-2004 10:32 PM ET (US)            

Thus gasoline weighs:

0.67 grams      1000 ml      3.7854 litre    1 kg         1 lb.        6.8 lbs. 
___________  X _________ X  _____________ X ___________ X _________ =  ___________
1 ml            1 litre      1 gallon       1000 grams    0.373 kg.    1 gallon

posted 03-22-2004 10:57 PM ET (US)            

Weight of Gasoline
This reference give the weight of a gallon of gasoline from 5.8 to 6.5 pounds:

http://www.santacruzpl.org/readyref/files/g-l/gasoline.shtml

It must vary depending on the temperature and the grade of gasoline.

Let's call it 6 lbs./gallon.

Fuel Consumption Per Hour Per Horsepower

Here are some figures from:

http://www.prime-mover.org/Engines/GArticles/bsfc.html

---begin quoted material----

Typical BSFC ranges are as follows. As usual, there will be exceptions, but this should serve as a general guide:

0.26-0.34 Large industrial four-stroke diesel engines (very small hp/weight ratios)

0.28-0.36 Other four-stroke diesel engines

0.32-0.38 Two-stroke diesel engines

0.37-0.44 Fuel injected four-stroke gasoline aircraft engines

0.40-0.48 Fuel injected four-stroke gasoline automobile engines

0.43-0.48 Carburetted four-stroke gasoline aircraft engines

0.48-0.60 Carburetted four-stroke automobile engines

0.55+ Two stroke gasoline engines

0.55-0.70 Four-stroke aircraft engine takeoff fuel flows

Engines don't maintain a fixed BSFC over their entire range. Typically an engine's BSFCs when producing only a fraction of its rated power are quite high. This is due to thermodynamic factors which limit the engine's efficiency when it runs cold. BSFCs typically reach their lowest value for the engine in the 50-80% power range. Then then begin to trend upward again as friction begins to play a dominant role and/or the mixture must be enriched to provide for adequate engine cooling.

------end quoted material----

With these new numbers, let us re-calculate the fuel consumption of a 225-HP 2-stroke gasoline engine in pounds per hour:

225 X 0.55 = 123.75 pounds

If we are using 6-pound/gallon gasoline that will be

20.625 gallons per hour.

Note that the reference above suggests that the lowest value is obtained in the 50-80% range of the engine power. This may explain why "cruise" is the most efficient speed. At WOT the engine would probably burn more fuel per hour than calculated using the 0.55 figure

At 50% throttle we would only be making 113 HP thus burning about

posted 03-24-2004 01:09 PM ET (US)            

I OWN a 225 Ocean Runner and I do burn 23.5gph at WOT. She will consume 11-14gph at cruise(35-4200) and will consume a gallon of oil to every tank of fuel I burn(85gal capacity) which is usually about 8-10 hours per tank. I run 89 or 93 octane and do not run 87 in anything I own. If cost to run it is a deciding factor....you own the worst engine being its appetite for fuel. For $.10/gal, run 89 if possible, not gonna kill ya being it is only another $5-8 per fill-up. Run good quality oil as well, not the Sears brand.