At a given RPM for a given engine model, is fuel consumption independent of load? - Boating & Fishing Information and Reports Forum

Prodigal Son · #1 (**permalink**) 12-09-2007, 03:39 PM

Just curious...say I have the same engine setup in 2 different boats one placing much more strain or load on the engine, at a given RPM will the one with more load be burning more fuel to turn the exact same RPM?

glen e · #2 (**permalink**) 12-09-2007, 03:57 PM

yes...just go try to do a prop test on flat water vs 2 - 4's....

Prodigal Son · #3 (**permalink**) 12-09-2007, 04:54 PM

So yes? (I can't tell if you're being sarcastic with the second sentence or saying "even given that fact, still try to reduce loading to test a new prop")

ken2 · #4 (**permalink**) 12-09-2007, 08:22 PM

Yes, if its loaded more, yes, its going to be burning more fuel.

Trying different props is an easy way to see what happens to economy with load.

Here is some numbers off a 31 contender.
Enertia, 19 pitch.
RPM MPH GPH each
4000 28 10
4500 38 13.4
5000 43 16
5500 49.1 22.3
5900 50.1 27.6 max

Enertia, 17 pitch.
RPM MPH GPH each
4000 28 8.6
4500 36 11.9
5000 38 14.5
5500 41 15.2
6500 51.1 28.1 max

I believe your asking if the GPH is always the same at a given rpm regardless of load.
You can see with these two props, the engine is working harder to turn the 19's.
The GPH is higher across the board at any given RPM.

Going faster for a given rpm doesn't neccesarily mean less fuel burn.
With a lower pitch prop, you may turn a couple hundred rpm more to get the same speed, and actually burn less fuel then the higher pitch prop.
But its not a set rule, you just have to experiment and see what happens.

glen e · #5 (**permalink**) 12-09-2007, 08:29 PM

no sarcasm intended - no different than if you go to gramdmas house for Xmas loaded with gifts and the whole family on board.......

then turn around and come home alone/empty and the family flys home later...

you'll do much better mpg coming back, right?

Prodigal Son · #6 (**permalink**) 12-09-2007, 10:28 PM

Well yeah, I'd expect the better MPG in my truck w/out the boat behind it, but I'd also expect it to go up the hill leading to my townhouse at 2,000RPM in 3rd gear instead of 3,700RPM in 2nd gear ya know?

Like I realize the engine works harder to move a given load, but I didn't know if that translated into a higher fuel burn at a given RPM or a higher fuel burn DUE TO higher RPMs for a given task.

I guess what confused me is that the question I had in my subject line, if you'd answered "yes" then I was taking that to mean that the GPH at a given RPM were independent of load...when I think you said "yes" and meant they were dependent of load.

So, the consensus is that GPH vary at the same RPM based on conditions?

I really wasn't sure what I thought just based on my little mechanical knowledge...I figured it would vary, just not very much...but I'm thinking based on y'alls answers I was wrong in my guess.

glen e · #7 (**permalink**) 12-09-2007, 10:46 PM

sure becasue waves, load, gas levels are just like a big hill on the engine...if load was independant of fuel consumption, we would have no fuel crisis....

my original post was my boat on flat water can hit 37 mph using 21 gph total
but put me in 2 - 4's and I can still hit 37 mph but will be using more than 21 guaranteed as the load will increase on the motors...

Prodigal Son · #8 (**permalink**) 12-09-2007, 11:34 PM

Yeah, but I was thinking of the same thing differently...say you burn 21gph at 37mph at 4000RPM on flat water...in rough water say it takes you 4400 RPMs to get 37mph, so obviously fuel burn would increase...that's what I was getting at...does the increased load make the engine work harder for a given speed, but a given RPM would always burn the same (So in the example I gave, you would still burn 21gph at 4000RPM in rough water, just at 30mph instead of 37mph this time?)

Not doubting that load increases burn because it makes sense that it would, but I didn't know if maybe the burn at a given RPM stayed relatively constant and it was the RPM that changed with load (which changes the burn)? See what I'm getting at?

I also understand that we might be seeing eye to eye (you're answering my question as I'm posing it with a resounding "No, the RPM does not necessarily determine the burn" and I'm just being dense.)

glen e · #9 (**permalink**) 12-09-2007, 11:50 PM

I see what you are getting at...with increased load, the engine wants to slow down so yes, you have to give it more rpm to get to the speed...but as ken says I could prop differently to get the same rpm attained at 37 mph but it would put a whole lot more load on the engine and use more gas - 6 of one, 1/2 doz, etc...the simple fact is if the engine needs more torque or rpm, it does it by putting more fuel in the engine one way or another....

peterbo3 · #10 (**permalink**) 12-10-2007, 02:18 AM

In a nutshell, two identical hulls, trimmed & loaded equally, with the same motors & props running side by side should turn the same RPM for a given speed. They should burn the same amount of fuel.
Now, load one boat up with three extra people & a whole heap of extra gear & repeat the experiment. Theoretically, they will still stay abreast at X RPM, although the loaded boat will burn more fuel. In practice, the loaded boat may not keep up at X RPM, as the hull dynamics may have changed fractionally due to the extra load.