Boat MPG ? 120HP Evinrude

[QC]

Re: Boat MPG ? 120HP Evinrude

BTW, Silvertip, you and I are in complete agreement. My point has always been that GPH is totally meaningless in all apllications unless you have something additonal to make it relevant. Without accurate speed, GPH tells you very little So my reaction to slasmith's comment and the previous ones you and I have discussed is only regarding "MPG doesn't matter, GPH does". GPH without speed is only a nice to know . . .

The last thing I will point out is that in a small boat on a small lake, with similar loads and similar speeds MPG won't vary that much unless something is broked. Annnnnnd, that is why all of us generally use 1/3 out, and 1/3 in, and 1/3 reserve. It helps with the variables . . .

There is never a substitute for good, accurate and complete data. Realtime data is even better. Without it we do the best we can with what we have, in this case other people's experience

 

[ondarvr]

Re: Boat MPG ? 120HP Evinrude

You will burn around 12 gph at WOT. Do you know it tops out at around 50 MPH, or is that a guess? If that's correct your looking at less than 100 miles on a full tank at top speed.

 

[Nandy]

Re: Boat MPG ? 120HP Evinrude

I do 4.8 average in the cobia in my signature/avatar. I will be surprised if you do better with an outboard. Fill your tank, log your trip with a gps trying to keep a predetermined speed, then fill your tank again. That will tell you how much gas you burned. Then you have your gps tracks that will tell you speed, distance and time. You can make any calculation for that given speed. Repeat.... I have to agree, you will find your sweetspot somewhere from 3k to 4k. Fuel flow meter is the best tool to help you know when to turn around..

 

[slasmith1]

Re: Boat MPG ? 120HP Evinrude

BTW, Silvertip, you and I are in complete agreement. My point has always been that GPH is totally meaningless in all apllications unless you have something additonal to make it relevant. Without accurate speed, GPH tells you very little So my reaction to slasmith's comment and the previous ones you and I have discussed is only regarding "MPG doesn't matter, GPH does". GPH without speed is only a nice to know . . .

In any mathematical equation you need at least 1 constant the only constant in the fuel consumption equation is gph. At any given rpm your motor will consume x gallons of fuel if you alter weight , wind or current your speed at a given rpm will change but the gph will remain relatively constant. If I know I have 210 gallons of fuel and burn 11gph at 3200 rpm I know I can run 18 hours with a 1 hour reserve I can then look at the current speed I am traveling and know where on my trip I need to plan fuel stops this can all change as conditions change but I still have my 19 hours to change my plans. mpg doesn't matter if you turn to avoid a log and that alters your course for the trip by 1-2degrees to get back on you course to destination this can add miles to a trip that you thought was a fixed distance but your gph is still constant are you now understanding why gph is the most important number to know. It is because everything else changes from moment to moment......

 

[QC]

Re: Boat MPG ? 120HP Evinrude

We are saying the same thing, but I am stubborn . . . If you have speed, you know MPG . . . Without speed, GPH is meaningless. As I once said to Silvertip, I don't care when I run out of fuel, I care where . . . BTW GPH is not constant . . . Add more weight and it goes up, in fact, burn off fuel and it goes down if you incrementally back out of the throttle for the same boat speed.

To answer your question, no, I do not understand why GPH is the most important number. Alone it is unusable to determine if I am going to make it home. There is no most important number in this equation, all thee (fuel level, GPH and speed) are required to make any decisions regarding range, and once I change course I still have to reevaluate my plan again using all three numbers . . .

I actually hate this stuff, as I end up looking petty, but damnit, damnit, damnit, GPH is only part of the equation, no more or less valuable than the others. In fact you only ultimately need two pieces, MPG (realtime) and fuel qty.

 

[Silvertip]

Re: Boat MPG ? 120HP Evinrude

QC -- you said ". . If you have speed, you know MPG . . ". Without some sort of fuel measurement you do not know MPG. Yes, if you know fuel flow and speed you can compute MPG. But armed with only speed you will not be able to calculate MPG until you know how much fuel you actually used -- and that means you don't know that until fill up time. You can certainly use the fuel gauge as a measure but that certainly is not an accurate measure. Since more boats are being fitted with better instrumentation including GPS, the engine management systems are now able to display MPG, GPH, fuel used, fuel remaining and a host of other data. But regardless, MPG at any point in the trip is still a trip average. Continue to be stubborn by the way. Never ending discussions are fun. If MPG pleases you, there is no way any of us can convince you otherwise. Ever wonder why airlines calculate fuel load in pounds and fuel consumption in pounds/hour? The main reason is that the airplane has to actually lift the load so not having to convert thousands of gallons of fuel to pounds is an unnecessary step. Since you know how many pounds of fuel you have, the moment you lift off, climb, level off and set the throttles for cruise, you check the flow rate in #/hr and you know in an instant if you have an issue. We agree sir -- we just find different value in the flow meter. And I so enjoy the banter. Cheers.

 

[QC]

Re: Boat MPG ? 120HP Evinrude

QC -- you said ". . If you have speed, you know MPG . . ". Without some sort of fuel measurement you do not know MPG.

Since GPH was the topic I didn't add that. I agree totally. I have never contended MPG does not require fuel flow AND speed. There is no dispute there. I also understand your point about realtime data, I have agreed on that completely. My point is and has always been that GPH alne gives you very little. MPG assumes you have speed and GPH, no other way to get it . . .

Continue to be stubborn by the way. Never ending discussions are fun.

No problem there

If MPG pleases you, there is no way any of us can convince you otherwise. Ever wonder why airlines calculate fuel load in pounds and fuel consumption in pounds/hour? The main reason is that the airplane has to actually lift the load so not having to convert thousands of gallons of fuel to pounds is an unnecessary step.

Same as my comment to slasmith about GPH not being constant, load matters too . . . They also use speed over the ground so they know how far they are able to go . . . Ultimately all they care about too

We agree sir -- we just find different value in the flow meter

Yes, we agree, that has been my frustration, because we also agree on the value of the flow meter. I just think it has little value without speed, because then I have MPG and I then know how far I can go in the plane or the boat on my pounds or gallons or milliliters of fuel remaining. Cheers to you too . . .

BTW, the other reason they use pounds is because it is constant for energy content regardless of fuel temp

 

[slasmith1]

Re: Boat MPG ? 120HP Evinrude

Same as my comment to slasmith about GPH not being constant, load matters too . . . They also use speed over the ground so they know how far they are able to go . . . Ultimately all they care about too

Once again you only read half of what I said so you could disagree I said gph at any given rpm is constant 3200rpm is 3200rpm no matter how you are loaded but 3200rpm may = 32mph in one set of conditions and loading and only 28mph in another drastically changing mpg so your only measurable constant in the equation is gph.

round and round is fun.

 

[45Auto]

Re: Boat MPG ? 120HP Evinrude

I said gph at any given rpm is constant 3200rpm is 3200rpm no matter how you are loaded

LMAO!!!

Put your boat in nuetral and crack the throttle (careful, it won't take much) till it's turning 3500 RPM. Look at your fuel flow meter. It will show very little flow.

Now leave your boat tied to the dock. Put it in gear and open the throttle till it's doing 3500 RPM. Look at the fuel flow gauge. You will find that it's using A WHOLE LOT MORE FUEL than it was at 3500 RPM in nuetral!!!

What the example above is trying to show you is that GPH is NOT a constant at any given RPM. It depends entirely on load.

 

[Nandy]

Re: Boat MPG ? 120HP Evinrude

Fer Christ sake, you two!!!! Both MPG or GPH are subject to multiple influences that can make them accurate or inaccurate... The op wanted MPG...

 

[oregonboatnewb]

Re: Boat MPG ? 120HP Evinrude

LOL, knew this question would draw such attentions, either way, great read !

 

[QC]

Re: Boat MPG ? 120HP Evinrude

Once again you only read half of what I said so you could disagree

That's not true, I disagree only because I see an error

I said gph at any given rpm is constant 3200rpm is 3200rpm no matter how you are loaded

Actually not true either. The amont of boat in the water (load) increases drag so heavier that same 3200 RPM uses more fuel as is indicated by decreased speed. Think about it this way. With an isochronous governor (many diesels use these now) whatever RPM you select it will maintan on it's own. You hit a swell and the load increases (same as heavier boat) and the governor must increase the fuel rate to maintain that same 3200 RPM, so no 3200 RPM is not always the same fuel rate although in marine it is pretty close. But defnitely not a constant.

To help you understand my persistence on these type of debates, I have done nothing but sold, applied, spec'd, managed sales, managed product support for engine distribution for the last 31 years. I have done this primarily in truck applications, but also industrial pumps, generator sets and yes, marine. I have studied prop load curves for that same 31 years. I can specify an 18 speed heavy-duty truck with a pencil and pad of paper, I can draw shift charts from scratch, the variables are endless. This is also why I typically argue the relative unimportance of high peak torque values in pleasure craft engines (more important for some commercial applications). I have done nothing else since I was 18 except I help my wife make hot dogs at the local little league snack bar I am obsessed with understanding fuel rates, fuel properties, lug curves, heat rejection, efficiency etc. etc. etc. Again, this stuff has been my entire career, and I have lot of technical resources. When I am confused I consult with our engineers, and when I get in a real pickle I talk to my technical mentor who used to be the VP of Engine Development at Cummins and was also a past President of SAE. I share tyhis only to note that I don't argue this stuff just because I am stubborn

 

[grego]

Re: Boat MPG ? 120HP Evinrude

Good read!

 

[slasmith1]

Re: Boat MPG ? 120HP Evinrude

QC thanks for giving me something to entertain myself with for an entire day.

 

[triumphrick]

Re: Boat MPG ? 120HP Evinrude

I share tyhis only to note that I don't argue this stuff just because I am stubborn

HA!!

 

[alumi numb]

Re: Boat MPG ? 120HP Evinrude

well i'm AFU right now.

 

[Jdeagro]

Re: Boat MPG ? 120HP Evinrude

What I find very interesting about this thread is the amount of attention we are now seeing with respect to fuel economy. Good Thread.

If I may I would like to add a few points to the good advise that has already been offered.

There are some simple things that anyone can do to improve fuel economy, and they can make a significant difference.

1) remove anything from the boat that is not required for your trip. In short, reduce the weight.
2) Try to keep the boat balanced
3) Try to run the boat at the most efficient hull speed. The hull design dictates the efficiency. Unlike a car the water is more resistant, and the more hull in the water the more drag.
4) Keep the hull clean.
5) Make sure that the prop is suited to the boat and your activities with the boat.
6) I woun't make the last suggestion since it might be concidered advertisment.

 

[tashasdaddy]

Re: Boat MPG ? 120HP Evinrude

add smart tabs????

 

[slasmith1]

Re: Boat MPG ? 120HP Evinrude

add smart tabs????

I know some of you guys love reccomending smart tabs due to cost but there are some cases where they are absolutley the wrong product. if you ever operate your boat in areas with following sea conditions (any body of water with strong currents) you want adjustable tabs as a strong current pushing from behind your tabs can have an adverse and dangeruos affect on steering as well as a strong current combined with 1 good wave from your stern can stuff you under the wave. before reccomending this product you should ask where people boat.

 

[Silvertip]

Re: Boat MPG ? 120HP Evinrude

Two points of clarification and I'll bail on this thread as it is one of those that easily goes on forever. Someone pointed out that RPM is RPM regardless of conditions. It was pointed out a couple times that this is not true but the reason was not fully clarified. Any engine on any boat with and any load will operate at a specific RPM at a particular speed. Load that boat up, change water or wind conditions, and while RPM will stay the same, it will take more throttle opening to maintain that RPM. Throttle opening and RPM and not directly linked. The same principle applies when towing your boat. You need more throttle opening to main the same speed when not towing. Hence more fuel is consumed even though you are traveling at the same speed.

My second point of clarification is not about QC and my discussion because I know he has a fuel flow monitor. I only want to point out that most recreational boats do not have a way to measure "real time" fuel flow so even though you have a speedometer and a GPS, there is no way to calculate fuel flow to determine MPG on the run. You can only do that after you fill up and provided you know how far you traveled. MPG also has absolutely no bearing on speed other than the faster you go the worse it gets. MPG is fuel used over distance. Hence any fuel consumption discussion must include the question "at what speed, what load, what conditions". Without that data, MPG is also just another number.