Fuel Consumption

[kentuckyguy]

I was wondering what the difference would be in the fuel consumption between a 21 foot pontoon with a 90hp carb 2 stroke verses a 19.5 tahoe with a 4.3 v6.. Both boats are 2010... Thanks

 

[H20Rat]

Re: Fuel Consumption

I was wondering what the difference would be in the fuel consumption between a 21 foot pontoon with a 90hp carb 2 stroke verses a 19.5 tahoe with a 4.3 v6.. Both boats are 2010... Thanks

the engine and boat make zero difference in fuel consumption. I 100% guarantee either one is more than capable of double the fuel consumption of the other, all depending on what you do and how you drive.

 

[Bob's Garage]

Re: Fuel Consumption

If you "google" the brands you may be able to get a performance report previously published by one of the magazines. But these figures will only be useful as reference, not to be confused with actual use.

SMC is correct, it will be very dependent on how you run them, and how you set them up.

 

[kentuckyguy]

Re: Fuel Consumption

If you "google" the brands you may be able to get a performance report previously published by one of the magazines. But these figures will only be useful as reference, not to be confused with actual use.

SMC is correct, it will be very dependent on how you run them, and how you set them up.

I have the pontoon now... It get about 5-6mpg at wot.. Just wondering what the tahoe would get..

 

[H20Rat]

Re: Fuel Consumption

I have the pontoon now... It get about 5-6mpg at wot.. Just wondering what the tahoe would get..

How is that measured? That is VERY good, especially for a pontoon. Pontoons are essentially (semi) displacement hulls, they are not very good at going fast and tend to get really inefficient at speed.

If that is a 100% accurate reading (fuel flow meter + gps), then you will have a hard time finding a boat that gets better than that. The tahoo 4.3 is rated at a best of 5.0 mpg at best cruise, and 3.7 at WOT.

**just looked up on boat test.. There are 3 different 21 ft pontoons with 90 hp 4 strokes on them. They will certainly get better mileage than a carbed 2 stroke. They get between 2.45 and 3.1 mpg at WOT, so yeah, you aren't close to 5 mpg... My guess, if it is an etec at least, is mid 2's.**

 

[QC]

Re: Fuel Consumption

I have the pontoon now... It get about 5-6mpg at wot.. Just wondering what the tahoe would get..

Bad data. Sorry to be black and white, but sumpin' ain't Kosher there . . .

 

[45Auto]

Re: Fuel Consumption

Haven't you guys learned yet that most people have no idea how bad of mileage their boat gets?

If he's only doubling it (thinking 5 or 6 while really getting 2 or 3), then he's actually doing better than the majority of boaters out there at estimating their fuel mileage!

To the OP, a much better source of MPG data than asking people on the intenet is independent testing such as Boattest.com like Smokingcrater said.

Asking people what kind of mileage they get is pretty useless. Most people seem to figure that cruising out at 25 mph for half an hour, anchoring and partying or fishing for an hour, then cruising back at 25 MPH for half an hour, means that they have been using their boat for 2 hours.

So when it takes 10 gallons to fill up the boat they calculate that 2 hours at 25 MPH means they went 50 miles on 10 gallons of gas. That's 5 MPG. 5 MPG seems pretty bad to them in relation to what their car gets, so when they tell you their mileage they bump it up a little to 7 or 8 instead of 5 so it doesn't seem quite as bad.

In reality, their speedo is probably 25% optimistic at that speed, so they were really only going 20 MPH, not 25. So they really ran at 20 MPH for an hour total on that 10 gallons of gas. Means they were getting 2 MPG, not the 7 or 8 they will tell you!

 

[southkogs]

Re: Fuel Consumption

Not to hijack the thread, but is it worth measuring MPG on a boat? I've always just figured average GPH because 5 miles with a current, following wind and calm water is so completely different than 5 miles against gusting winds and 3' swells.

 

[H20Rat]

Re: Fuel Consumption

Not to hijack the thread, but is it worth measuring MPG on a boat? I've always just figured average GPH because 5 miles with a current, following wind and calm water is so completely different than 5 miles against gusting winds and 3' swells.

boattest generally tries to measure in fairly calm conditions. Anything thrown into the mix is yet another variable that obviously is going to cause issues. Its still worth measuring for comparative purposes if conditions are documented and similar.

 

[sw33ttooth]

Re: Fuel Consumption

i timed my 90 hp 2 stroke 6 gallons in about 1 hour 45 minuites wot, at 30 mph you do the math 8.6?. did the same test sort of on my 5.8l omc this week i did 20 miles at 30 mph and i only used 4.2 gallons. so i got nearly 5 mpg

 

[southkogs]

Re: Fuel Consumption

boattest generally tries to measure in fairly calm conditions. Anything thrown into the mix is yet another variable that obviously is going to cause issues. Its still worth measuring for comparative purposes if conditions are documented and similar.

Gotcha'. Thanks.

 

[AndrewsArk]

Re: Fuel Consumption

Honestly, I know boats suck gas down like crazy so I don't even worry about it. If I was that concerned I wouldn't have a boat.

 

[QC]

Re: Fuel Consumption

i timed my 90 hp 2 stroke 6 gallons in about 1 hour 45 minuites wot, at 30 mph you do the math 8.6?. did the same test sort of on my 5.8l omc this week i did 20 miles at 30 mph and i only used 4.2 gallons. so i got nearly 5 mpg

I can't explain why this data is wrong, but it also is . . . It just is. There is not a 90 hp OB in existence that uses less than 8 GPH at WOT, and you're claiming less than 4

 

[H20Rat]

Re: Fuel Consumption

I can't explain why this data is wrong, but it also is . . . It just is. There is not a 90 hp OB in existence that uses less than 8 GPH at WOT, and you're claiming less than 4

depends what you define WOT as! Wide open on the controls up front might be half throttle on the butterflies if the cable is badly adjusted. That would be the ONLY way to get <4 gph on that engine...

In the car world, a good estimator of fuel requirements is to take HP and multiple .5 to get the fuel (in pounds) per hour required. (.5 is the Brake Specific Fuel Consumption, a 4 stroke NA engine is .45 to .55, a 2 stroke will be somewhat higher yet. Again, this is for highly fuel optimized car engines, outboards are years behind for efficiency and aren't going to get BSFC's that are as good as a car engine.)

Anyway, 90hp * .5 / 6.175 (pounds/gallon) gives you 8.16 gallons of fuel per hour AT A MINIMUM. Anything less and you violate the laws of physics and engine design.

If you want a more general rule that works out well for boat engines, its HP / 10. 90/10 = 9 gallons per hour at WOT.,

 

[Cannondale]

Re: Fuel Consumption

I can't explain why this data is wrong, but it also is . . . It just is. There is not a 90 hp OB in existence that uses less than 8 GPH at WOT, and you're claiming less than 4

So true. I've seen testing with a 75hp on a 17' aluminum bass boat using 9.9gph in fuel use. Cannot believe a 90hp on anything other than muffs, running at part throttle, would do any better.

 

[geeco1]

Re: Fuel Consumption

I have always heard that the rule of thumb is that an outboard will burn about 10% of its horsepower in an hour at WOT. That means that a 90hp will burn 9 GPH at WOT. A 115 will burn 11.5 GPH at WOT. Granted this is just a rough estimate and I don't know if it applies to 4 strokes, but it sures seems to work out correctly for my 85HP.

 

[BTMCB]

Re: Fuel Consumption

Are we talking MPG / GPH while the boat is on the water....or.....on the trailer?

 

[45Auto]

Re: Fuel Consumption

i timed my 90 hp 2 stroke 6 gallons in about 1 hour 45 minuites wot, at 30 mph you do the math 8.6?

Will you send me some of what you were smoking when you timed it and filled it up if I PM you my address?

6 gallons in 1 hour 45 minutes is 3.4 GPH at WOT

EPI is a company that designs engines and gearboxes. They have a webpage with elementary level explanations of the derivations of some basic power equtions:

http://www.epi-eng.com/piston_engine_technology/thermal_efficiency.htm

TE (Thermal Efficiency) of an internal combustion engine is .0226 x HP / GPH.
In your case that would be .0226 x 90 / 3.4 = .60, or 60%.

Here's a quote from the website above:

The value of this Thermal Efficiency relationship is that, by assuming a reasonable TE value (27% - 29%), you can estimate the amount of fuel required to produce a given amount of power.

Notice that 27% to 29% is "reasonable"? If you really have an engine that's running at 60% TE, you're about to make Bill Gates look like a beggar. Any auto, ship, or plane engine manufacturer in the world would gladly pay you BILLIONS of dollars for engine technology that could do that. My company spends millions of dollars each year trying to improve TE by just 1%. The 30% you're claiming over modern technology would literally be almost priceless.

BSFC (as discussed by Smokingcrater) = 5.92 x GPH / HP
In your case your BSFC is 5.92 x 3.4 / 90 = .22 pounds per hour per horsepower

As pointed out by Smokingcrater, an efficient auto engine will have a BSFC of .45-.55. A 2-stroke is worse and the BSFC will be higher.

Here's another quote from the page referenced above:

An excellent BSFC for a well-developed, 4-stroke naturally-aspirated, high-performance liquid-cooled engine at 100% power is in the neighborhood of 0.44 ? 0.45. Claims of gasoline engine BSFC values less than 0.42 at max power tend to be suspect.

If claims of less than .42 tend to be suspect, what do you think a claim of .22 is? Try "utterly ridiculous" maybe?

One more quote from the EPI webpage:

So if someone tells you that they have developed a 4-stroke piston engine which, at max power, makes 300 HP on 20 GPH of gasoline, you can quickly calculate a BSFC of 0.39 and a Thermal Efficiency of 34.4%. You should be highly suspicious of such a claim.

If we should be "highly suspicious" of a claim of .39 BSFC and a TE of 34.4%, I can't imagine what they would advise about your claim of .22 BSFC and a TE of 60%.

As has been pointed out by QC and Smokingcrater, what you are claiming is physically impossible. Pretty much the equivelant of me claiming that my 3/4 ton 4WD Suburban gets over 50 MPG while towing my 5000 lb boat uphill.