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Author Topic:   Fuel Economy--Johnson 2-stroke
joenewton posted 08-02-2003 03:53 PM ET (US)   Profile for joenewton   Send Email to joenewton  
I have a queston about the motor on my 15ft center console. I have a 1998 Johnson 50 h.p. oil injected motor and I was wondering about how many "miles to a gallon" I should maybe be getting with it. I have the red fiberglass fuel tank that holds about 15 gallons. Running a little below wide open, what kind of miles to a gallon should I get. It seems like I'm putting a lot in and not really going all that far before I fill up. Thanks for any input.
Steve Leone posted 08-03-2003 01:48 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
Approx 3 miles to the gallon on average, if you are lucky. Boats, unlike autos, are constanly "under load". The resistance of the hull to the water is quite substantial. Fuel injected and four-stroke outboard motors get considerably better fuel economy. Steve
joenewton posted 08-03-2003 07:38 AM ET (US)     Profile for joenewton  Send Email to joenewton     
Thank you for the info.
Sal DiMercurio posted 08-03-2003 11:36 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
The reason your not getting decent fuel economy is, your running that engine far to hard if your close to wot.
Any time you run any engine at maximum or near maximum, figure 10% of your hp is gallons burned, example.
100 HP engine = 10 gph, 150 hp = 15 gph, 200 hp engine = 20 gph.
In your case, 50 hp engine, = 5 gph which only leaves you with 1 gallon left in a 6 gallon tank.
Back off the throttle & watch the difference.
joenewton posted 08-03-2003 02:43 PM ET (US)     Profile for joenewton  Send Email to joenewton     
Thanks, I'll slow it down some. Maybe I should look at getting a 70hp ehgine in the future for that boat. I've seen several 15ft's with that hp on the back. What is the cost of a new 70hp Johnson, four stroke? They do give you money for trade-in motors as well don't they, thanks!
Barryc posted 08-04-2003 04:43 PM ET (US)     Profile for Barryc  Send Email to Barryc     
I also have a 50 HP Johnson (1996 2 stroke/3 Cyl)- on my 1997 15' Dauntless. I can go 3 1/2 - 4 hours on a 6.6 gal tank and I usually change over to my 2nd tank w/ almost a gallon remaining- I don't want to run dry at a bad time. I spend most on the time crusing @ 3600-4000 RPM (around 22 MPH)- which is very easy on the motor and me ( WOT is 5600 and IMO, way too fast. Is your 50 a 3 cylinder model, most , I am told are 2 Cyl? Overall I am very happy w/ the performance but I wish the fuel consumption was a little better.


joenewton posted 08-04-2003 08:09 PM ET (US)     Profile for joenewton  Send Email to joenewton     
I have two on my motor. I have that old 15gallon red tank that sits under the seat. It does not have cracks and makes for a nice out of the way fit. I heard the motor kinda stall the other day when the little amount of gas I had drifted to the other side of the tank. I'll keep the rpm's down and see how the gas flows, while keeping the tank above the 4gallon mark. Thanks.
jimh posted 08-04-2003 10:23 PM ET (US)     Profile for jimh  Send Email to jimh     
Your fuel economy will be determined by

--your engine type (gasoline, 2-stroke)
--the horsepower being used
--the hull efficiency of your boat.

A typical two-stoke gasoline engine will consume about 0.5 pounds of fuel for each horsepower for each hour.

If you are operating your engine at 50-HP rating, you will consume

0.5 X 50 lbs. = 25 lbs. of fuel each hour.

Gasoline weighs about 6-lbs./gallon so you will burn about

25/6 = 4.16 gallons each hour when operating at 50-HP.

If 50-HP makes your boat go 25-Miles an Hour, you will have a fuel economy of about

25 / 4.16 = 6 MPG

That is probably a generous figure. Your mileage may vary.

Operating an engine at 100-percent of its rated horsepower should only be done for short periods. I used to have a small diesel engine. It was rated about 15-HP. At 3000 RPM it was rated for continuous service. At 3300 RPM it was rated for maximum of one hour service.

Bigshot posted 08-05-2003 10:59 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
A 50 2cyl should burn about 2.5GPH at 4k rpms. A 70 will up it to roughly 4gph but speed will increase. You should get 2-3 hours from a 6gal tank, I did with my 48spl.
Bigshot posted 08-05-2003 11:01 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
PS...a 70 4 stroke will stay at roughly 2.5gph. I would not however put that on your boat.....too heavy. Get the Merc or Yamaha 60hp 4 stroke or any brand 50hp for weight reasons.

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