Luv-N-It

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

velocity260

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.

cigarette1

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.

mcollinstn

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.

R Addiction

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!!! 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.

LakeRacer

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.

ragtop409

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

ursus

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

ursus

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

ursus

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