Fuel consumption - merc. 502?
 

Does anyone know the fuel consumption on a merc 502 at cruise speed contra full speed...? The boat is a 25 feet scarab and everything runs great except for the fuel gauge... 2 times I've ended up getting towed to the marina by nice people... both times I was sure the fuel would be enough for the trip...

fuel
 

cant give you many details,but this is what i know.at a cruise at about 3500 rpm i burn 30 gals/hour +/-

30 GPH? You are talking twins right?

I have a 454 Mag in a '24 Superboat and at 3500 RPM I average about 10 GPH. With a 502 and your boat is probably a little heavier than mine I would guess maybe 15 gph.

fuel
 

that would be for a single 502 magnum,i always thought that this boat was a pig.baja 27'dry , it weighs 4400 lbs.i would also like to hear from others to see how much fuel they burn.it always seems that i am re-fueling .engine is stock except for a holly carb that i installed.

Hey Mark,

I would say that your gas consumption sound high. The MPI motors are probably more fuel efficient. My boat weighs about 3500lbs. It also has a 7' beam. I can cruise at 50 mph all day long on about 10 GPH.

Dave M.

for 502 efi

3500 rpm about 15-17 gph,
about 32-34 gph wide open

depends how its propped if heavy say wot=4600 you will be on hi side of that range if light wot=5000 on low side

Temp, humidity etc also makes a difference but thats a good ballpark

I run a 242 Formula w/ 502 cruise alot (long distance) and with hydromotive quad and RPM about 38 to 4 I burn 16 gallons a hour
If they smile when you pull up to fill up you must be doin it right:D
Rocket

I have a 502 MPI in my 28' AT. The boat wieghs about 5,000 lbs.
I would guess I burn 15-17 gph at 3500 rpm.

Maybe a little more in this photo:D

fuel
 

well,after the last couple of posts,now i know that i have been calculateing my fuel useage all wrong.

Mark,
I am glad I don't have your fuel bill!

Mark, I have an '96 272 w/502 EFI and cruise around 3400-3800 and my consumption falls into where others are at 15-18 GPH. I don't believe the fuel you are burning is the boat/weight issue. Something isn't right with your set up. Prop/tune of motor. You are burning to much fuel.

1995 MPI 502. Boat weighs about 4500 lbs (guessing)

Half throttle - not very much
Full throttle - A LOT!!! :D

Thank you, fellow powerboaters for your input on the fuel-issue... I think 15 gph sounds reasonable at 3500 on a 502... next time I'll have to fill'er up a little more... In sweden (where I live) the fuel costs about one dollar/litre... on the other hand it's worth every penny to start up and go for a ride in the archipilago (did I spell that right???) outside Gothenburg... Do I love those waves from the passengerferrys...? YES!

Yeah, you guys burning 10/15 GPH in your 4000 lb deep V's also get 25 MPG with your tow vehicles too...
Isn't the furmula something like: 100 lbs fuel/hour per 100 HP ?

Yes, my 3500lb deep V does in fact burn approximately 10 GPH at 3500 RPMs. This is provided my hour meter and the meter on the gas pumps are working correctly. Do you think we are making these figures up? Unfortunately my tow vehicle only gets about 14 MPG.

How are you guys figuring this out?
 

I have no idea how you could figure this and be accurate. Close maybe, but not accurate. You would have to fill up, drive your boat at a certain rpm for a designated time and then go fill up again to be on the money. Using your fuel guage would not be very reliable.

By the way, my truck gets about 13 mpg. :( That could have something to do with how I drive it though! :rolleyes:

Uh...correction... I get 25.5 MPG towing my boat w/an SUV

I gotta get home and look at some gas bills. Im thinking I would be pretty happy to get 15-17. I have a 502MAG hooked to a 60 gallon tank and I can burn that up much faster than one would think.
My buddy has a 29 Outlaw w/ 454's and he doesnt burn much more than me. ???.. go figure.

1st of all GPH and milage are not the same thing a 502efi makeing 200hp at 3500rpm will use the same gph whether in a 15,000 lb cruiser or 2000lb flat bottom rocket. If the engine is proped heavy ( lugging) it will use a little more but if proped the same, i.e. both boats = 5000 rpm wot then gph will be close to the same.

Now mileage is another thing, the cruiser buring 16gph is going 16 mph, = 1 mpg. The rocket is going 48 mph = 3 mpg

as for pounds of fuel actually it takes about .5 lb of fuel per hp per hour so:

17 gph * 6lb/gal = 204hp at cruise
_____________
0.5lb/hp/hr

and

34 gph * 6lb/gal = 408hp at wot
_____________
0.5lb/hp/hr

go look at boattest.com at the gph numbers of a dozen dif boats, cruisers, sport, and high perf offshore, the 502 efi's will all be about 15-17 gph @ 3500 rpm and 30-34 gph at wot, 17.3 gph in a 15000 lb cruiser in the link above for e.g.

Driving habit can make a big difference to, going on plane takes a lot of fuel, if you are up and down a lot it makes a big dif versus if you get up and cruise for an hour then stop

I guess my calculations are not that scientific. Although the above formula and my findings work out pretty close. I simply divided my gallons used when I fill up by the amount of hours I ran since last fill up. Sure some of that time is at idle and some is at WOT, but most is at approx 3500 rpms. Knock on wood, I have never ran out of gas. Also when I am at the gas station I will usually know within 3-5 gallons how much it will take to fill. I rarely even look at my gas gauge. I record the numbers everytime I fill up and this average holds up over hundreds of hours.

Dave M.

ursus, thanks for the info and insight. very well said.

I have a flowscan on my velocity that only goes to 20gph. With the 7.4mpi at cruise 35-40 14-16gph. Doing 50mph I burn 20gph. I put 25gallons in the tank and went for a cruise ran 50mph the whole way there and back. 30min there 30min back. I burned 21gallons.

We don't put BIG carbs and BIG cams and BIG pistons and BIG blowers on motors to worry about fuel efficiency.

I usually try to figure how many miles we're going to run and then I assume if I'm going to be running mostly 3500r's, I'll calc for 1mpg, If I plan to run hard with the boys, I'll figure on .5mpg

Hell, the gas is the cheapest part of this stupid game.

ursus,

correct analogy for all "typical" planing hulls. Displacement or semi/displacement hulls, though, will use more fuel @ 3000rpm then a planing hull @ 3000 rpm.

a displacement hull's fuel usage curve will look like a curve that starts rising at low rpm and gets steeper and steeper as the rpms rise (there is a relationship between the waterline length and max efficient speed on a displacement hull that will affect this curve).

a planing hull's curve will start out with a steeper rise, then will peak and drop significantly, only to then mimick the curve of the displacement hull. Most, if not all, planing hulls will be on full plane by 3000 rpm and will be in the "nice" part of the now-rising curve. The displacement hull will be in the steeper part of the curve.

Since both, all if you will, motors will have the same fuel usage at WOT then there will be spots on the two curves where the difference is significant.

Since I assume that all of us here are speaking about planing performance hulls, then the numbers for a given motor (502 Mag EFI, etc) should indeed be similar.

All I know is that when my light comes on, and stays on, that means get fuel @#$hole. The other week the light came on and I came back to the marina and filled up the tank........turns out I had 2 gallons left . I should have gone to the next bar!!!:rolleyes: I can make a 110-120 run at 55mph/3600rpm on just under 65 gallons. I think that comes to around 1.8 miles to the gallon. I'm happy with that. :D

Twin 502 EFI's in my 34 Advantage. Hooked up the MerCruiser computer to it and at 3000 rpms I was getting/using 12 gallons per/hour per engine for a total of 24 gallons/hr.

GGAASS
 

Not sure about the boat but the truck has a 502 carb in it and it gets 6 miles to a gallon of fuel:D :D :D :D guess thats why it has a 45 gallon tank

This is just incorrect. @ 3000 rpm an engine will use the same ammount of fuel whether it is in a planing hull, or a tug if the load is the same and so it depends on the propping. If the planing hull is over propped say wot = 4000 rpm and the tugboat is under propped, it hits the rev limit of 5200 @ 3/4 throttle then the planing hull will use more fuel, Gallons per hour because the engine is under a heavier load, period. If the engine is propped the same in other words the prop is chosen correctly so that the engine is properly loaded then @ 3000 rpm they will both use the same ammount of fuel give or take a gallon per hour.

If you look at the fuel consumption GPH versus rpm graph it is a relatively straight line, Mileage curves as you point out our quite different for dif hull types/sizes/weights

You are describing the MPG curves. The MPG will be signifigantly different at dif spots on the curve but GPH is a function of rpm and load not the type of hull the engine is in. In other words @ 3000 rpm an engine uses a certain ammount of fuel per hour, mileage depends on how far you travel in that hour and so hull type weight etc

Relatively straightl ine fuel consumption curve for 502efi in a planning hull

typical MPG versus rpm for a planing hull, the curves you were describing

wow!!!! i cant even figure mine out in gph because basically wherever i go takes less than an hour to get there....cruise at 60 mph....i just figure i get 1mpg and go from there.......125 gal tank will get me there and back....

simialr Straight line consumption curve for a 502 in a displacement hull, note the displacement hull actually uses less fuel, gph, @ 3000rpm in this comparision because the planing hull was not quite on plane in the above example yet and so under its heaviest load @ about 3000 rpm

ursus,
What is the difference in fuel consumption between a 502 mag, 502 EFI or 502 MPI? There must be somewhat of a difference??
I would think the MPI would be allot more fuel efficient, right??

crap, ursus

can't argue with those charts.

Of course you are right about the "drop" in GPH for a planing hull - it does not occur. I was certainly letting the MPG stuff filter through on that coment.

As far as two similar motors using the same GPH at the same RPM with the same load - sure they use the same, but the key here was the "same load" comment. Rarely will you get a truly "same load" situation between two vastly differing hull types.

Your two hulls that are reflected in your graphs apparently DO keep fairly even parity between the load/rpm scenario.

I've owned houseboats with 454 carb motors and runabouts with 454 carb motors of the same flavor. I've used Floscan meters on them both as well and, like some others here have kept a fuel journal with Tripmeter, gals used, engine hours, and comments on all of my fuel fill ups (well, all of them on the houseboats and cruisers, and lots but not all of the playboat fillups). My numbers show my old houseboat being more evil to a tank of fuel @ 3000 rpm than my old playboat was @ 3000.

Not even trying to start some crap, cause we all (at least I) look forward to your input on OSO.

I just still think my old houseboat had the throttle plates open farther than my runabout did @ 3000 and that this indicates a more heavily loaded motor @ 3000 and as such, more GPH fuel consumption.

Both boats were propped for 4600 WOT, both boats used 24/25 GPH wot, and there was a 2 to 3 GPH difference at 3000 (11 vs 14).

No floscan on the Formula, but the fuel usage booklet I got when I bought the boat jives with everybody else's numbers on the 502's.

********************

From what I'm seeing, the carb 502 Mag and the 502 Mag EFI share similar usage curves.

Do you the numbers for us on that comparison?

I may have missed this but here it goes anyway
 

The hull speed is about

1.3 x sqrt (waterline length), measured in knots

so a 40 footer displacement can cruise nearly effortlessly to about 9 mph. Then the fuel is wasted with little improvement in speed.

:) sorry if my post seemed confrontational was not meant to be somtimes I get carried away,, see my signature :D:D:D , I was just trying to point out gph was dependent on load and rpm

as you pointed out the most efficient @ 3000 will be the runabout or sport that planes at 2000-2500 rpm (sorry I couldn't find a graph for a light sport boat that planes @ 2500)

next the most efficient will be a displacement hull which is designed to still be cutting water at those speeds, (the 2nd graph)

the worst will be a planer that doesn't plane till 3200 rpm because at 3000 it is plowing a ton of water and the engine is really loaded, ( the 1st graph)

sorry no numbers on carb vs efi, I have heard ancedotally that efi is more efficient and I belive my cruise mileage improved when I repowered with a 502efi versus a 330hp carb but a lot of variables there, dif prop etc. ( my total fuel usage has increased tho as I am on the throttles more for some reason ;)

ursus

no offense taken nor intended, just wanted to make sure I wasn't being perceived in the wrong tone, also.

I have a friend with a cruiser that will not plane fully until 3600rpm (can we all say "underpowered" together). It does then actually reach a nice top speed, but he has no way to cruise long distances economically (half into the 4 barrels at a decent plane). His fuel usage at anything above 12 mph (either GPH or MPG or any way you want to look at it) is dismal for a 29' cruiser.

I'd still like to find some info to compare fuel usage on a bigblock Mag motor carb vs multiport efi (502 or 454).

In my heavy 41 PC with twin 502's (8.2 MPI's), I average approx 17gph per engine running approx 3200rpm which is where my floscan gauges say I get the best fuel burn. Keep in mind my boat is heavy! Hope this helps!