MONTAUK 17: Fuel Economy with 90-HP Two-stroke

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Author Topic:   MONTAUK 17: Fuel Economy with 90-HP Two-stroke

wjriling posted 05-07-2005 11:15 AM ET (US)
Hello All, I am going to be relacing the 19--gallon fuel tank on my 1987 Montauk and was thinking about using two 6-gallon tanks. My engine is an 1987 Evinrude 90-HP.
Would anyone have an idea on how long I can expect to cruise around on 12 gallons of gas? I'm not sure if it's measured by MPG or hours, sorry.
Any advice would be appreciated! Regards, Wendy
jimp posted 05-07-2005 11:43 AM ET (US)
Wendy -
Years ago, with my 1982 Montauk and carbed 1982 Evinrude 90 I usually got at least 4 mpg, I counted on 36 miles out of a 12 gallon tank.
JimP
Teak Oil posted 05-07-2005 12:17 PM ET (US)
12 gallons in a Montauk is pretty useless, you better find a gas station every three hours or so.
My 1985 90 Evinrude does pretty well IMO, about 5mpg if I am real easy on it and the seas are calm. If it gets rougher I drop to about 4 mpg.
If you had a four stroke or E-tec engine that gets 7-8mpg you may be able to go to 6 gallon tanks, but with what you have now I think its a bad idea.
I carry 24 gallons of fuel and usually dont touch the second 12 gallons, but I sure am glad its there.
wjriling posted 05-07-2005 01:00 PM ET (US)
Thanks Jim. I don't travel very far in my Montauk, so 36 hours is fair time. This year I plan on using a slip so I figured two smaller tanks would be easier to fill individually (I plan on removing the tanks to fill them at the local gas station rather than the marina pumps).
Regards,
Wendy
jimh posted 05-07-2005 11:39 PM ET (US)
Most two-stroke engines will burn 0.5 pounds of fuel per horsepower per hour. Figure out how much horsepower you are using, and you can estimate the fuel consumption from that. A gallon of gasoline weighs about 6-pounds.
Example: Running at 50-HP for an hour will burn 50 X 0.5 = 25 pounds of fuel. That is about 4.2 gallons.
Another rule of thumb: divide horsepower by ten to get gallons per hour. If running with 50-HP, that would be 5 gallons per hour.
The rate of fuel burn depends on how much horsepower you are using.
Clark Roberts posted 05-08-2005 10:14 AM ET (US)
1980 Newport (same hull as Montauk) with 1995 Merc 90 swinging a 20"(could have been a 21"??) Quicksilver High Five prop> Top speed approx 48mph (50mph in a light top and 5700rpm) steady cruise mpg approx 5.5mpg (30mph)...if memory serves me correctly! Spruce Creek Navy
Tollyfamily posted 05-08-2005 10:53 AM ET (US)
I have the same boat as you and get about 4 MPG at 4000 rpm. I have 2 12 gallon tanks under the seat.
Dan
Bayoumontauk posted 05-08-2005 10:20 PM ET (US)
My 88 hp Johnson was BAD on fuel. A 26 gal tank would get me about 74 miles at about 5k rpm. That is why I am currently "in between" motors at this time. I'll either get the 90 etec or one of the fourstrokes. I've been hearing the 4 strokes are too heavy for my 96 Montauk hull. So basicly as JimH said there is not much you can do with 2 stroke GPH based on his formula.
Landlocked posted 05-09-2005 09:52 AM ET (US)
I've done several trials to calculate miles per gallon. I've found that my montauk/2002 Merc 90 2stroke will consistantly get 4.5 mpg over an average day of running with most time at a comfortable cruise - ~25-30 miles per hour.
bamatenn posted 05-09-2005 12:09 PM ET (US)
While on the subject of economy, I have noticed in the Performance Reports published by Evinrude and Johnson that the 90 HP E-tec has the same "Best Fuel Efficiency" as the 90 HP Johnson 2 stroke. Both are at 6.5 MPG at 3,500 RPM on a 995 LBS. test boat. That makes it hard to justify the $2,200 diference in price between the motors.
Ken
Teak Oil posted 05-09-2005 05:25 PM ET (US)
BayouMontauk, you would run at 5000rpms constantly for the entire 26 gallons? Good Lord thats WOT, I think 3mpg at WOT is pretty darn good. A fourstroke isnt going to be much better than that going wide open everywhere.
If you want to go 40mph everywhere you go you need to look into a bigger engine so you can run 40mph at 4000rpms or less
Bayoumontauk posted 05-10-2005 10:35 PM ET (US)
Teak oil, Yea, there's been some days when I've followed other boats with bigger engines and had to push it. WOT was usually 5500 or 5600 Rpms though.

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Author	Topic: MONTAUK 17: Fuel Economy with 90-HP Two-stroke
wjriling	posted 05-07-2005 11:15 AM ET (US) Hello All, I am going to be relacing the 19--gallon fuel tank on my 1987 Montauk and was thinking about using two 6-gallon tanks. My engine is an 1987 Evinrude 90-HP. Would anyone have an idea on how long I can expect to cruise around on 12 gallons of gas? I'm not sure if it's measured by MPG or hours, sorry. Any advice would be appreciated! Regards, Wendy
jimp	posted 05-07-2005 11:43 AM ET (US) Wendy - Years ago, with my 1982 Montauk and carbed 1982 Evinrude 90 I usually got at least 4 mpg, I counted on 36 miles out of a 12 gallon tank. JimP
Teak Oil	posted 05-07-2005 12:17 PM ET (US) 12 gallons in a Montauk is pretty useless, you better find a gas station every three hours or so. My 1985 90 Evinrude does pretty well IMO, about 5mpg if I am real easy on it and the seas are calm. If it gets rougher I drop to about 4 mpg. If you had a four stroke or E-tec engine that gets 7-8mpg you may be able to go to 6 gallon tanks, but with what you have now I think its a bad idea. I carry 24 gallons of fuel and usually dont touch the second 12 gallons, but I sure am glad its there.
wjriling	posted 05-07-2005 01:00 PM ET (US) Thanks Jim. I don't travel very far in my Montauk, so 36 hours is fair time. This year I plan on using a slip so I figured two smaller tanks would be easier to fill individually (I plan on removing the tanks to fill them at the local gas station rather than the marina pumps). Regards, Wendy
jimh	posted 05-07-2005 11:39 PM ET (US) Most two-stroke engines will burn 0.5 pounds of fuel per horsepower per hour. Figure out how much horsepower you are using, and you can estimate the fuel consumption from that. A gallon of gasoline weighs about 6-pounds. Example: Running at 50-HP for an hour will burn 50 X 0.5 = 25 pounds of fuel. That is about 4.2 gallons. Another rule of thumb: divide horsepower by ten to get gallons per hour. If running with 50-HP, that would be 5 gallons per hour. The rate of fuel burn depends on how much horsepower you are using.
Clark Roberts	posted 05-08-2005 10:14 AM ET (US) 1980 Newport (same hull as Montauk) with 1995 Merc 90 swinging a 20"(could have been a 21"??) Quicksilver High Five prop> Top speed approx 48mph (50mph in a light top and 5700rpm) steady cruise mpg approx 5.5mpg (30mph)...if memory serves me correctly! Spruce Creek Navy
Tollyfamily	posted 05-08-2005 10:53 AM ET (US) I have the same boat as you and get about 4 MPG at 4000 rpm. I have 2 12 gallon tanks under the seat. Dan
Bayoumontauk	posted 05-08-2005 10:20 PM ET (US) My 88 hp Johnson was BAD on fuel. A 26 gal tank would get me about 74 miles at about 5k rpm. That is why I am currently "in between" motors at this time. I'll either get the 90 etec or one of the fourstrokes. I've been hearing the 4 strokes are too heavy for my 96 Montauk hull. So basicly as JimH said there is not much you can do with 2 stroke GPH based on his formula.
Landlocked	posted 05-09-2005 09:52 AM ET (US) I've done several trials to calculate miles per gallon. I've found that my montauk/2002 Merc 90 2stroke will consistantly get 4.5 mpg over an average day of running with most time at a comfortable cruise - ~25-30 miles per hour.
bamatenn	posted 05-09-2005 12:09 PM ET (US) While on the subject of economy, I have noticed in the Performance Reports published by Evinrude and Johnson that the 90 HP E-tec has the same "Best Fuel Efficiency" as the 90 HP Johnson 2 stroke. Both are at 6.5 MPG at 3,500 RPM on a 995 LBS. test boat. That makes it hard to justify the $2,200 diference in price between the motors. Ken
Teak Oil	posted 05-09-2005 05:25 PM ET (US) BayouMontauk, you would run at 5000rpms constantly for the entire 26 gallons? Good Lord thats WOT, I think 3mpg at WOT is pretty darn good. A fourstroke isnt going to be much better than that going wide open everywhere. If you want to go 40mph everywhere you go you need to look into a bigger engine so you can run 40mph at 4000rpms or less
Bayoumontauk	posted 05-10-2005 10:35 PM ET (US) Teak oil, Yea, there's been some days when I've followed other boats with bigger engines and had to push it. WOT was usually 5500 or 5600 Rpms though.