What to Expect for Fuel Economy on Mercury 650

[scottwoodward]

I'm close to being out on the water for my first trip with the Mercury 650 engine (1970, 4 cylinder). Engine is now running great. I'm trying to get a sense of what kind of distance I can cover one way on 1/3 tank of fuel at cruising speed (although I plan to bring a spare 2 gallon container). The boat is a 1985 tri-hull SportCraft, just under 16 feet. I can't find anything in the manual about mpg's, nor have I found anything on the web for this engine. Anyone out there with similar vintage Merc motors who can speak to the in's and out's of fuel economy?

 

[jbcurt00]

1 gal of fuel @WOT for every 10hp, or roughly 6.5gal per hour at wide open throttle for your 65hp Merc. Thats for an optimally running, modern outboard. Suspect your vintage Merc is not quite optimal or considered modern.

Better economy at @75% max throttle, on plane, but 70s outboards (similar to cars of that age) arent known as miserly in the fuel department.

Wind, waves and use of the throttle greatly effect mpg, so usually expressed as gph (gallons per hour).

 

[GA_Boater]

What JB said is spot on. After you get some time on the water, you will have a better feel for how long you have to gas her up again.

I'm guessing you only ran the motor on muffs and since you worked on the carbs, the idle screws will probably need fine tuning so it will idle in the water.

Also when you splash stay close to the ramp until you gain some confidence in how it runs. You meet the nicest people on the lake, they tow you back. I've been on both ends of the line. :smile: :blue:

Good luck on Splash Day.

 

[racerone]

A 2 gallon container won't do much.------Go boating with 3 full 5 gallon tanks j

 

[scottwoodward]

GA Boater -- good advice in sticking close to the ramp on splash day and being ready to tune the idle. Yes, it's only been on muffs up to this point.

Is there a formula for GPH at 75% of WOT? Based on a couple of posts I've read now, sounds like this motor should consume just about one GPH at idle and maybe around 1.5 GPH at cruising speed (https://forums.iboats.com/forum/eng...good-uses-too-much-fuel-while-in-gear-idling; https://forums.iboats.com/forum/eng...s/543284-1979-evinrude-200-hp-v6-fuel-economy)

 

[racerone]

It will be more than 1.5 GPH at cruising speed.----Boats needs lots more power than a pick up truck to move at planing speeds !!!

 

[The Force power]

I had a 1973 65 HP Merc.(on a 15' run about) used for tubing, now imagine yourself in a 1973 Plymouth sedan; you'll know the answer
approx. 3 hours of tubing = 25 litres gone

 

[jbcurt00]

Is there a formula for GPH at 75% of WOT?

Yep:
1gph X HP/10 X 75%

(1 X (65/10)) X 0.75

6.5 X 0.75

Equals 4.85gph

At 75% WOT its more efficient, so lets go w 3.75 to 4gph cruising, roughly..... again on an optimally running, modern outboard...

Heck, even 3.5gph is still not exactly fuel efficient.

and the $ difference per hour is about a 12pk of beer, more or less depending on your libation of chooce.

boating certainly isnt cheap and always carry as much fuel as you can reasonably accomodate safely.