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Author Topic:   Outboard Engine Fuel Burn Rate
yellowlab posted 10-26-2014 08:51 AM ET (US)   Profile for yellowlab   Send Email to yellowlab  
I don't have a fuel flow meter on my [1992] Outrage, I was wondering if anyone can comment on what their gallons per hour burn is at cruise speed of 22 to 25-MPH. I have a 2-stroke 150 Yamaha
jimh posted 10-26-2014 11:43 AM ET (US)     Profile for jimh  Send Email to jimh     
The fuel burn rate of my boat at a speed of 25-MPH is about 9 to 10-GPH. I am not sure how that helps you as we have different boats and different outboard engines.

For a two-cycle classic outboard engine you can figure the fuel consumption will be about 0.55-lbs/HP-hour--at best. This is known as the brake specific fuel consumption.

To convert BSFC in units of lbs/HP-hour to gallons/hour, you need to know the horsepower and the density of the fuel. For gasoline you can use 6.25-lbs/gallon as a reasonably good figure for fuel density. This suggests the G/HP-hour will then be

0.55-lbs/1-HP-hour x 1-gallon/6.25-lbs = 0.088-gallons/1-HP-hour

To know the fuel burn you plug in the horsepower. For example, if you are running at full throttle and making 150-HP, the fuel burn would be

150-HP x 0.088-gallons/HP-hour = 13.2-gallons/hour

It is hard to say with any certainty what horsepower will be necessary to put your boat onto plane at a certain speed.

It is also common that BSFC varies over the engine speed and load range, and it can be higher than 0.55-lbs/HP-hour for some engine speed and load combinations. This is particularly true in older two-cycle engines.

A very simple approximation is to take the horsepower and divide by ten to get GPH for that power. Thus a 150-HP engine would burn 15-GPH.

jimh posted 10-26-2014 11:47 AM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: Not having any way to measure fuel flow rate is really a very liberating state. If you do not have any good measured data about the fuel flow rate of your engine, this sets you free to invent figures for it, and, if you are so inclined, to invent really good figures. Once you actually measure the fuel flow rate of your older two-cycle outboard engine, you are subject to reality and perhaps some depression. You will begin to be worried about the excessive amount of fuel being burned. It is much better to not measure it, and to continue to go boating with the mindset that your engine is quite fuel efficient. If you buy a $300 fuel flow instrument, all it will do for you is make you more likely to spend $15,000 on a new outboard engine.
DaveS posted 10-26-2014 11:16 PM ET (US)     Profile for DaveS  Send Email to DaveS     
You might want to try what I did with my old 1991 Johnson 120-HP outboard engine. To get a basic idea how much fuel it burned, I set up an second fuel tank, filled it with a known amount of fuel, and then went for a ride. I tried to keep a constant speed and measured my distance with the Global Positioning System (GPS). When the fuel ran out I was able to calculate about how much fuel I was burning. It wasn't exact, but it helped me plan my trips. Good luck--Dave
jimh posted 10-27-2014 11:28 AM ET (US)     Profile for jimh  Send Email to jimh     
The method proposed by Dave is a good one, but use of a second and usually smaller on-deck tank is essential. With a smaller on-deck tank you can more accurately measure the volume of fuel used than can be done with a large-capacity internal tank.

As for finding the distance traveled, using the Global Positioning System as a source of that data should be very accurate. I can't think of any better way.

Fuel economy overall depends on the pattern of use for the engine. The biggest surprise to most boaters about their engine use pattern will be to learn how little time they spend at full-throttle speed and how much time is spent at idle or very low speeds. Unfortunately, at these low speeds the classic two-cycle engine is at its very worst for fuel economy. While many like to look at figures of improved fuel economy at cruise resulting from a modern engine, the improved fuel economy at idle and low speeds will often be much greater. At cruise you may get a 25 to 50-percent improvement by using a modern engine, but at idle and low speeds the improvement can by ten-fold.

yellowlab posted 10-27-2014 11:35 AM ET (US)     Profile for yellowlab  Send Email to yellowlab     
Thanks all for the tips. I have a small 3 gallon tank that I can use for this purpose. I still need to track down the in-operable fuel guage!
jimh posted 10-28-2014 10:53 PM ET (US)     Profile for jimh  Send Email to jimh     
Do not depend on a fuel gauge. You should fill the on-deck tank to a level and mark the level. Run the boat and burn some fuel. Take the tank to a gasoline retail pump, and fill it back to the mark. Read the fuel volume off the retail gasoline pump dispensing the fuel.

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