Fuel economy question

[Jerry Ohmer]

hello, I have a question about my engins fuel economy. If it seems to be using way too much fuel. I have a 1971 mercury 500 50horsepower thunder bolt ignition. I just got this boat so I am not familiar with what fuel mileage to expect. the boat is a 1971 skeeter 15 ft. I have had it on the water 2 times, the first time I burn 12 gallons of fuel to go 40 miles with a mixture of 3/4 throttle and full throttle. with only 2 people and fishing gear. the next time I was on the water for 3 hours mostly at no wake speed and 3 quarter throttle and burn 10 gallons of fuel in 3 hours. is this fuel consumption normal or is there an issue with my engine? I have had the carburetors apart to clean them and the engine seems to be running good. thank you in advance for your help and I am open to any suggestions to improve fuel economy.

 

[Silvertip]

Re: Fuel economy question

If you own a boat you cannot get concerned about fuel economy because marine engines are not economy champs. Actually they are but because they are attached to a boat, fuel economy is far different than in a car that doesn't have to push water out of the way to move forward. Look at it this way -- any outboard typically burns at or very near 10% of its rated HP at wide open throttle. Your 50 HP engine therefore at wide open throttle will burn about 5 gallons of fuel per hour. Yes you say -- but I didn't run wide open. Well -- even at 1/2 to 3/4 throttle the engine is still requiring about 3 - 3-1/2 gallons/ hour. It is just the way it is. So if you know your speed and distance covered you can calculate miles per gallon. If you used GPS to determine distance and measured fuel use accurately then your MPG was about 3.3. The second trip you burned 3 gallons per hour at a mix of off-plane and 3/4 throttle. That sounds pretty much like 3 gallons/hour which also sounds pretty normal. Just be glad you don't have a boat with two or three 300 HP engines. But if you had one of those you probably don't need to worry about fuel consumption huh?

 

[Jerry Ohmer]

Re: Fuel economy question

Thank you for your help. I was wondering because my mercruiser seems to be better on fuel with a larger boat and 170 hp. I guess it could be that I just don't notice as bad with a larger fuel tank.

 

[Chris1956]

Re: Fuel economy question

MerCruisers can burn 1/2 the fuel of a comparable outboard. I show your fuel mileage is as Silvertip commented. Not real bad. Make sure the carbs are not leaking, while underway.

 

[jimmbo]

Re: Fuel economy question

My I/O goes thru 130 liters(35 us gallons) in about 90 minutes @ WOT

 

[Chris1956]

Re: Fuel economy question

Jimbo, That is a lot of gas for any motor. It is about what twin 7.4L Mercruisers would use at full throttle. Are you sure of the numbers?