I usually idle through the "no wake" zones here in the Intercoastal at about 800rpm -- with my fuel flow meter and GPS that is showing about 3.4 to 3.8 MPG on the integrated display.

If I am on plane at an economical speed -- say about 25-30MPH I typically get around 3.2 MPG

If I should attempt to speed up a little in the no wake zone --which I can do at about 1200 rpm if I drop my tabs down (with them up the wake would definitely not be acceptable to the local water police.) I am getting about 2.8 MPG.

So I use less fuel at 25-30mph, at planing speeds -- than I do trying to push it a little in the no wake zone. (actually mean get better fuel mileage -- I know the specific Gallons per hour burn rate is higher at higher speed)

Maybe you should run in reverse?

I have seen the exact same thing on my boat (my numbers are a bit lower overall though). That fuel flow meter has been a real eye-opener. I have saved 5 to 10 gallons on every trip (compared to last year) since I installed it.

Sadly though, now while boating, I think: "wow, cruising into that bay for a scenic tour is gonna cost me $16. Hmmm, better just follow the most direct line...". It's a double edged sword.

you got it. with a planing hull, your best burn rate (ie the fewest gallons burned per hour and the best mileage per gallon) will occur idling forward at hull speed. the next best burn rate will be with the boat just on plane and properly trimmed (tabs and outdrive if you've got 'em). any speed faster than just on plane will cost you more in fuel and trying to push the boat faster than idling and under planing speed will also cost ya more too. someone much smarter than me once called it something like physics.

when the boat is transitioning from 'displacing' to 'planing', it uses more fuel. These speeds are typically in the 1200 - 2000 rpm range.

From the work that I have seen, usually the best milage (or miles able to be cruised for a tank of fuel) is at dead flat idle in gear (around 550 rpm or 650 rpm). The boat at that engine speed should give alot more milage than anything else.

But no one wants to go 2-3-4 mph all day!

Phooey on "gallons per HOUR". I don't know who ever started that nonsense for measuring gas usage of things that move, but that's just what it is, a nonsense measurement... I mean, I have a great big boat, and at "cruise speed" I'm only burning 3 to 4 gallons an hour! WOW, how great is that! Why, I know boats that go thru 20 gallons an hour, how do they afford to use the thing??

Do the math-- My boat only went 7 miles in that hour, while the gas hog went 40 miles...

umm---exactly the same gas mileage.

Now, if you're talking say, GENERATORS, then gee, measuring their use by gallons per hour makes sense! ---They don't GO anywhere!

But, FR, what if I'm not worried about how far I go on a cruise but how many hours I can pull the kids in tubes?!?

That is why I included range per tank of fuel.

And...

Dale, I put a FloScan dual gauge on my boat when I got it and love it. I then did 2 trial runs (downstream and upstream following the same course) at every 200 rpm's from idle (1000) to WOT (4400).

At each rpm setting I trimmed out the boat to maximize the speed at each rpm, then recorded the GPS speed over ground. (Since I was making a run in both directions I didn't have to factor in the current at that point).

I entered the data into an Excel spreadsheet to calculate (a) the average speed of the two runs at each rpm, (b) the fuel consumption at each rpm, and that gave me the average fuel economy (miles per gallon).

I found that I got within a tenth of a mile per gallon of 1.0 at rpm speeds from 1000-1400. Above that the stern started to dig in and the mpg lowered as the boat started to transition from moving through the water to bein on plane. The higher the rpm's from that point until on plane, the mpg was horrible (.75)

Once the boat got on plane (3200 rpm) the mpg picked back up to 1.0 mpg (+/- .1) and stayed at that mpg until I got up to about 4200. At that rpm the mileage dropped off quickly and I attributed that to the carb secondaries opening.

The .1mpg variation in fuel economy while running on plane I attributed to variations in the wind, etc. That small an amount I'm not going to get too concerned about.

As a result of all that, I now travel on plane when I'm going somewhere. I get the same 1 mpg on plane as I get at 1000-1400 rpm's, and put 1/3 the hours on the boat while cruising on plane. Plus I don't have to be constantly tweaking the steering to maintain a course like I do when the boat is at idle speed and wandering around a bit.

See if you can do some runs at "idle, in gear" rpms and the boat speed for that engine speed (about 550 rpm)....Edited: this rpm is more for a typical I/O....

...another edit: most engines drop about a 100 rpms once in gear as the drive and prop drag down the rpm...

Our boat won't idle below 800 rpms, and hardly moves at all in idle. It need 1,000 rpms or more to really "idle". There was one time in a 15mph headwind that the boat went backwards at 800rpm idle.

I know a 1999 Merc 7.4 MPI idles at 750rpms with a B1 drive because I've driven it. Not necessarily true that ALL BB's idle that fast, but it is in at least one instance.

So, I think 550/650rpms in idle might be a little optimistic. It also depends on the propping of the boat. Dual prop drives seem to have lower idle speeds because less energy is wasted, thus hull speeds are greater and lower RPMS.

A boat powered by a 5.7L is not going to have the same prop/gears as the same boat with a 7.4L. In idle, the 7.4L boat is going to move faster, assuming the same idle speeds.

I think GFC's 1400 number is a telling argument. It's not really the engine speed as much as it is the hull speed that determines the MPG factor.

GFC, if you do not mind me asking, how fast was your boat going at 1,000 rpms? 1,400 rpms?

What I am wondering is if GFC's hull speed at 1,000 rpms or 1,400 rpm is similar to Seabuddy's old 268 with the 496 at 550/650rpms.

Sorry, I am probably over analyzing this, but it does arise my curiosity.

Since we're on fuel milage, is it alright if I derail this thread for a moment?

Does anyone have fuel burn numbers for two same-model, same engine boats with a single and DP/B3 drive? I wonder how much more efficient twin counter-rotators are.

Here is some info form the Four Winns Site -- they have this for all years and all boats and engines. You might look up a model with power similar to yours.

FUEL FLOW DATA - 240 HORIZON W/ VP5.7Gi/SX/280/209
& MC350MPI/B1/300/224
Engine Speed - RPM Boat Speed Fuel Flow
2 0 0 0 --------- 8 ------ 6
2 5 0 0 --------- 1 2 ------ 7
3 0 0 0 --------- 2 7 ------ 9
3 5 0 0 --------- 3 1 ------ 1 1
4 0 0 0 --------- 3 6 ------ 1 5
4 5 0 0 --------- 4 2 ------ 1 9
W O T --------- 4 8 ------ 2 3
FUEL FLOW DATA - 240 HORIZON W/ VP5.7Gi/DP/280/209
& MC350MPI/B3/300/224
Engine Speed - RPM Boat Speed Fuel Flow

2 0 0 0 ---------- 1 1 ------ 6
2 5 0 0 ---------- 2 0 ------ 7
3 0 0 0 ---------- 2 9 ------ 9
3 5 0 0 ---------- 3 4 ------ 1 1
4 0 0 0 ---------- 3 8 ------ 1 5
4 5 0 0 ---------- 4 3 ------ 1 9
W O T ---------- 4 8 ------ 2 3

As I would expect the DP idles at a higher speed and is more efficient in terms of gas used -- but is no higher in top speed
(I think of a DP as slightly lower in top speed - compared to an SX single prop)

edit worked on the spacing obove

Do they show "engine at its idle speed, in forward gear" numbers?