Fuel consumption

[Mark42]

The last trip I took in the boat was 40 miles and the load was 12gals gas and me and my brother. Cruised at varying speeds from 18 to 22 mph (WOT = 29), made 3 stops, multiple slow downs to take photographs, monster wake, etc. My fuel useage comes out to 7 mpg. Does that sound right for my rig (see below)? I would have thought it would have been more like 9 or 10. Is that wishful thinking?<br /><br />Mark

 

[petrolhead]

Re: Fuel consumption

I asked a similar question recently, it seems that a figure of approx 4 gallons per hour at WOT is the rule of thumb for your motor, arrived at by the following formula...<br />HP divided by 10 = Gals per hour at WOT. So in your case, 40hp divided by 10 = 4 gph. <br />If you do 29mph at WOT using 4 gallons per hour, one gallon should take you about 7 miles, bearing in mind you were slowing down, speeding up, stopping, starting, and idling your mileage is probably about right. I've found from experience that half throttle doesn't equate to halved consumption, best mpg seems to be attained at WOT, and the slower you go the worse the consumption per mile.

 

[Chinewalker]

Re: Fuel consumption

Hi Mark,<br /> I think a lot of folks would kill to have 7mpg on a small outboard boat with a 2-stroke for power. I think you're right on the money with your calculations - I get about 6mpg with my small runabout with a 75hp OMC triple.<br />- Scott

 

[Mark42]

Re: Fuel consumption

Thanks guys! Its nice to be able to estimate my crusing range before taking a trip. Right now I have two 6 gal cans. May go to a larger above floor permenant tank if 12 gallons turns out to be restrictive on travel.

 

[phatmanmike]

Re: Fuel consumption

my craft gets 2 at 3/4 throttle, and 1.5mpg at WOT....then again, i have 99ci of beast under my hood. 6 holes that constantly need gas <br />1976 merc 1150 inline 6 cylinder.hence why i am selling it to somebody with deeper pockets than i ! good luck on that rig of yours

 

[BillP]

Re: Fuel consumption

7mpg is way high mpg for a boat. It's like getting over 50mpg in a car. You done better than good!

 

[agrazela]

Re: Fuel consumption

I'd kill for 7mpg. I figure my 1978 70hp Mercury 3-cylinder 2-stroke gets me 2-3 mpg...though I do alot of low-speed and idling.

 

[Scaaty]

Re: Fuel consumption

I have 70 feet of boat that comes in 4 sizes. Only the small one get great mileage, but the big one makes those monster wakes! Someday I will learn that a Boston Whaler type boat at around 14 feet with 35 horse is what I aways wanted and need, but I aint there just quite yet. Somehow I would try and stuff a 350 Chevy in it anyway and there goes the gas mileage. I justify lousy boat mileage with great motorcycle mileage!

 

[swist]

Re: Fuel consumption

"I've found from experience that half throttle doesn't equate to halved consumption, best mpg seems to be attained at WOT, and the slower you go the worse the consumption per mile." <br /><br />I don't believe the above statement is generally true. Maybe there is something unusual about your setup, but all the test charts I've ever seen show best fuel economy at some point just above planing speed (3700-4300 rpm, typically), and then the MPGs drop as you continue onto WOT.

 

[Mark42]

Re: Fuel consumption

Swist,<br /><br />Apparently OMC beleives the same thing as they specify an "economy" setting that is about 3/4 throttle in my owners manual.

 

[sangerwaker]

Re: Fuel consumption

I'd kill (well, not literally) for that kind of mileage! I run my EFI 200 near WOT for footin, and I have never actually figured out my mpg, but I'm sure it is significantly less than 7. Now I'm curious and will have to figure mine out.

 

[petrolhead]

Re: Fuel consumption

What do OMC say about the most economical throttle setting for a displacement hull?<br />It makes sense that once on the plane a reduced throttle may be more economical, but with a displacement hull that may not be the case.

 

[BillP]

Re: Fuel consumption

Originally posted by Mark42:<br /> Swist,<br /><br />Apparently OMC beleives the same thing as they specify an "economy" setting that is about 3/4 throttle in my owners manual.

When they say 3/4 throttle it doesn't mean 3/4 to wot. Few obs get better mpg at wot no matter what the set up. Maybe there are exceptions but like swist says, not in any testing info I've read.

 

[Winger Ed.]

Re: Fuel consumption

Originally posted by petrolhead:<br /> What do OMC say about the most economical throttle setting for a displacement hull?<br />

For most big boats, If it'll never come up on plane, the faster you go from being a little over idle, the less mileage you'll get.<br /><br />The water resistance gets harder and harder to push through as speed increases. So, the faster and faster you go, the more and more horsepower-and fuel to generate that horsepower is required. <br /><br />Its like a truck pulling a trailer, the more and more weight you put in it the harder and harder it has to work,,,, so the less and less gas mileage it gets.<br /><br />Planing hulls get better mileage once they quit pushing the water out of the way as they move, and get(more or less)up on top of it and then-resistance goes down and gas mileage goes up.

 

[18rabbit]

Re: Fuel consumption

Manf’s spec: fuel capacity: 188-gal. Range: 180nm.<br /><br />Cost to fill the tank: $470.<br />Nm/gal: 0.96nm/gal.<br />Cost to watch the water go by at 12nm/hr: priceless.

 

[Mark42]

Re: Fuel consumption

Billp,<br /><br />I think they mean 3/4 throttle is economy compared to wot. For that motor, the performance gain from running wot is negligible compared to the performance at 3/4 throttle.<br /><br />At least that is my understanding of what they wrote.

 

[18rabbit]

Re: Fuel consumption

I read the engine evaluations in PBR. I’m of the opinion there is no std from one motor to the next. Some motors have their optimum fuel consumption (nm/gal) around 1750rpm, others closer to 5000rpm.<br /><br />We have a cat diesel. If we give the hull/prop parameters to cat they will submit it to their software and spit out a list of rmps/time that optimizes fuel consumption for a specific motor’s build. I would think other companies have similar software.<br /><br />Winger - From what I’ve read, a planing hull’s efficiency drops in proportion to the increase in surface disturbance (chop). The less flat the water is, the more fuel is needed to maintain speed. Something about the air space between contacts with the water causes the leading edge of the water (that is re-contacted) to grab and hold the hull back, where continuous contact with the water doesn’t have this resistance.

 

[seahorse5]

Re: Fuel consumption

Mark42,<br /><br />Here are some performance reports in different boats using a new Evinrude 40hp. You can see how the milage varies according to both rpm and boat type.<br /><br /> http://www.evinrude.com/e-tec/pdf/PE288.pdf <br /><br /> http://www.evinrude.com/e-tec/pdf/PE281.pdf

 

[Mark42]

Re: Fuel consumption

Seahorse,<br /><br />Thanks for those reports. I did notice that the 40 hp gained 55lbs over the 1992 version. (240 vs 185). Interestingly, both those tests yeilded around 7 - 8 mpg at around 3500 rpm. Nice to know that my setup is giving me similar results as the pro's. Makes me feel that all the changes/tests and advice from this website resulted in a good performance and were not a waste of time money and effort. Verification of something as unique as individual boat composition is difficult, to say the least.<br /><br />You made my day.<br /><br />Mark.

 

[seahorse5]

Re: Fuel consumption

The weight gain is from being a completely new design that is quiet like a 4-stroke, smoother than a traditional 2 cylinder, and has no smoke while running. It also has the larger commercial motor gearcase than the older 40 did, and uses V4 propellers.