Outboard Fuel Economy Study

[Silvertip]

Re: Outboard Fuel Economy Study

BF -- these are three identical boats with three different engines, and a fourth boat that is very similar and a fourth different engine (selected only because that engine was not tested on the same boat as the other three). You simply cannot compare fuel economy in cars with boats for what should be very obvious reasons. Slippage and resistance are just two. And you like others, continue to use the word "efficiency" and "economy" erroneously. I also explained those terms earlier. No -- I am not leaning toward any brands or technology here. In fact today one is hard pressed to find an engine that is not fuel injected (for the same reason a car is by the way). And the primary reason is emissions, and secondary is economy at mid and slower speeds under certain conditions. Heck, I've been a Johnson/Evinrude person all my life and my current boat is powered by a Merc. How much more open minded can one get. Only time will tell whether 4-strokes will be around (untouched) in 20, 30, or even 40 years. I have two 1946 Johnsons neither of which have ever been opened up. I further contend that if you did plot fuel consumption at equal rpm across the board what you will end up doing is looking at the MPH column and saying -- "by golly look how much faster the two stroke engine is pushing the boat at that rpm." Your next comment would be, "well if I cut back the RPMs I would get nearly the same or in some cases better economy than the four stroke." It all boils down to the amount of work being done on the fuel being consumed. Once you make that connection all of this should make a lot more "financial sense". Yes we have a much cleaner burning engine with current technology. But thinking you will make up the cost difference in fuel savings is a stretch for the non-commercial boater.

 

[dajohnson53]

Re: Outboard Fuel Economy Study

... There are only two reasons one would take a trip of any duration and elect to run at an arbitrariily selected speed: 1) water conditions, and 2) the selected speed turns you on or happens to be a limit of sorts for that particular hull. That's why a tach or a fuel flow monitor is more valuable on a boat than a speedometer...

Unless I'm misunderstanding, what you're saying is that unless water conditions and / or hull determines the speed, one should choose traveling speed strictly on the basis of fuel efficiency in terms of GPH - not MPG. (implied by use of tach and flow meter and not speedo).

I disagree with that concept for the use I put a boat to. I generally choose the speed I travel on the basis of my time frame as well as conditions - just as I would in a car. I would tend to look for an engine that has the best economy (strictly in terms of MPG) at the speed I like to go, which is about 25-35 mph. I don't want to go, say, 15-20 mph when I have a 30-60 mile river trip to get to a cabin or camping place for the weekend. Doesn't matter that the economy is better at that speed (or rpms). Time is a factor right along with fuel usage.

It would not matter to me if engine X gets 5 mpg at 15-20 mph and engine Y gets 4 at that speed. I'd look at what they get at 25-30 mph. It could be that engine X is still more economical, but according to the rough data presented here, it could very well be that engine Y gets better mileage at my preferred speed.

Now if my only goal was simply to get as far as I could on the amount of fuel my boat can carry (e.g. cruising), I would indeed set my speed according to the most economical speed - but I would use MPG, not GPH. I would either use a gps and flow meter and do rough calculations myself, or I would get the type of GPS that takes flow meter data and displays MPG. GPH by itself is irrelevant for that, imho. I can't imagine why anyone would think that GPH is a more important number than MPG.

I would still have a tach because that's a critical gauge aside from fuel usage.

On a related note, idling/trolling economy doesn't matter at all to me because I travel fast to the fishing spot, then troll with the kicker. For my current use, I would simply try to choose the engine that maximizes MPG at my preferred - or a reasonable - travel speed.

 

[BF]

Re: Outboard Fuel Economy Study

BF -- And you like others, continue to use the word "efficiency" and "economy" erroneously. I also explained those terms earlier.

Hi again,

well... I understood that you were defining efficiency as fuel burnt at a particular rpm... but I just don't believe that rpm should be in that definition.

Efficiency: Definition (english only)
The proportion of energy released by fuel combustion that is converted into useful energy.
Definition source
US EPA. (cut from European Environmental agency web site)

My point is that comparing 2 motors would mean you'd compare the fuel consumed when they are doing the same amount of work (="useful energy"). That would be pushing the same load at the same speed, not necessarily turning the same crank rpm. If your tables had the exact same speeds for all boats (particularly a moderate cruise speed, say 25 mph), it'd appear to me that the new tech engines would be doing a reasonable amount better than the traditional 2 strokes. (~20%)?

I agree completely that their main advantages might be low emissions and low speed quietness, and that they are so expensive that it might be unreasonable to consider the fuel savings as the main reason to switch. But I think your tables DO show that there will be appreciable fuel savings unless you're running in the top 20% or so of the rpm range... I also have a collection of old OMC's and wonder how long the "new tech" engines will live, but that's another issue...

you say,

you will end up doing is looking at the MPH column and saying -- "by golly look how much faster the two stroke engine is pushing the boat at that rpm." Your next comment would be, "well if I cut back the RPMs I would get nearly the same or in some cases better economy than the four stroke."

So, what speed could you throttle back to on your boat, and get better fuel economy than the 4 stroke doing that speed? To me, it looks like it'd be beating you in economy at all lower speeds, but you get better as the speeds go up (?).

At least that how it looks to me.

 

[Silvertip]

Re: Outboard Fuel Economy Study

BF -- Still stuck on MPH eh?? You asked "So, what speed could you throttle back to on your boat, and get better fuel economy than the 4 stroke doing that speed? To me, it looks like it'd be beating you in economy at all lower speeds, but you get better as the speeds go up (?)."

Have I not suggested that? Open your mind and look at all four charts. Don't you see that all four engines have very different sweet spots. As an owner, you don't own all four boats so if fuel economy is an important factor for, then you operate your boat at it's sweet spot to achieve maximum economy. To some (pro fisherpeople), wide open is 90% of the time and fuel economy is a moot point. To others, it something less than that. Now if you want to compare economy do it fairly.

With a fuel flow monitor one does not throttle back to a given "speed" -- you throttle back to the most efficient "RPM" for the conditions and load you have at the time. With one person and no gear in a boat, one given speed will result in one fuel economy figure. Load the boat up and you will not get the same fuel economy at that RPM. The fuel flow monitor tells you instantly where the sweet spot is for the current conditions. MPH does not tell you that. As the chart shows, the sweet spot on my boat for the conditions that day was 4200 RPM and I have seen that as low as 3800. RPM is what dictates speed. And I will say just one more time -- "I do not contend that a carbed two stroke is more economical than a four stroke (carbed or EFI) across the board." I do contend that if every boat was fitted with a fuel flow monitor and people understood what it was telling them, the operators would pay more attention to it than their speedometers and let speed be whatever that engine RPM dictates for the current conditions. As I pointed out early on in this discussion -- speed is the enemy for a four stroke. Even though a carbed two-stroke has a higher burn rate at the same RPM as a four stroke, it is doing more work, pushing the boat faster and therefore time to destination is shorter and as a result fuel economy is nearly the same, the same, or in some cases even better than the four stroke. If you have to drive longer to get where you are going, it still takes fuel so that's the difference. And one last word on "efficiency". Maximum efficiency for an engine (any engine) is at wide open throttle. Whatever energy you put in vs the work output determines % efficiency. That is not the same as "economy".

 

[BF]

Re: Outboard Fuel Economy Study

BF -- Still stuck on MPH eh?? You asked "So, what speed could you throttle back to on your boat, and get better fuel economy than the 4 stroke doing that speed? To me, it looks like it'd be beating you in economy at all lower speeds, but you get better as the speeds go up (?)."

Have I not suggested that?

Actually I thought you suggested...

well if I cut back the RPMs I would get nearly the same or in some cases better economy than the four stroke.

That's why I asked what speed that would be.... and just pointing out that (to me) it appears from your charts that you can get comparable economy to the 4 stroke at fast speeds when throttling UP, not by slowing down.

The reason I'm stuck on MPH is that it then takes distance out of the equation and compares engines doing comparable amounts of work. You say...

Whatever energy you put in vs the work output determines % efficiency

Fixing the boat speed (for test purposes) makes the work output identical. Therefore comparing gph at identical MPH seems more straightforward to me than comparing gph at a given rpm, then looking at boat speed, and then calculating how long it would take to cover some distance, then going back and calculating how many gallons would be burnt in that time. Both ways should give the same result anyway. But identical speeds are not listed in the tables, that was my point. Not that we should run at any particular speed, or that any 1 speed would be the "sweet spot" for all engines.

Anyway, that's my $.02.... I'm done.

 

[Silvertip]

Re: Outboard Fuel Economy Study

Just one more shot! Efficiency and Economy do not track each other. As pointed out, any engine operates at it's peak efficiency at wide open throttle and no internal combustion engine is 100% efficient. I think we agree on that point. Even you said manufacturers play with various parameters to extract best economy under what they think are what the average boater wants. So right off the bat we have two or three variable. 1) what the manufacturer thinks, 2) what average boater thinks they want and 3) what conditions the engine is actually operated under. So -- a real test of economy is for you, me, or anyone to go on a trip to measure the fuel economy of two different engines on two identical boats. If you leave the element of time out of the equation, these charts would show the four stroke and E-tec engines would both get better MPG than either of the carbed two strokes except at WOT where things equal out. Since most of us actually want to get to our destination in a reasonable amount of time, the added speed of the carbed two strokes at the same RPM as the four-stroke results in nearly the same, the same, or less fuel used than the four stroke FOR THE TRIP. That's fuel economy. Saying you get better economy and taking twice as long to get to your destination is hardly anything to brag about. That is not fuel economy. If you really want to win the fuel economy run, make the trip at idle. Just be sure to take a sleeping bag and several lunches along.

 

[thurps]

Re: Outboard Fuel Economy Study

OK I'm confused. Why would I want to buy lets say a Yamaha two stroke 115 @ 360 lbs. rather than a Yamaha two stroke 90 @ 260 lbs. for my 17 ft boat if I'll probably never pull skiers or go much over 30, when I hear all the time that I should use the maximum rated outboard for the boat?

 

[steelespike]

Re: Outboard Fuel Economy Study

Disregarding the economy thing,Generally speaking you don't here a boater say "I wish I had less HP".
Using arbitrary figures a 90 hp at wot uses about 9 gph and gets lets say
35 mph.A 115 at lets say 3/4 throttle cruises close to the 35 figure and is loafing at the same time using about the 9gph of the 90.
If you load the boat up with people the 90 starts to sag a little ,with the 115 you can choose to sag or give it a touch more throttle reaching the planing sweetspot possibly lost by the 90.Yes the 90 will still be at 9gph and the 115 will be higher but the 115 should get there quicker whether it is from here to there or a ride around the lake.

 

[Silvertip]

Re: Outboard Fuel Economy Study

Thurps -- you can buy whatever you want for your boat but there is a point of diminishing returns on going too small. If a small motor needs to work its guts out just to get on plane and maintain a reasonable speed, then the choice was not a good one for performance and economic reasons. The engine must work harder to get the job done and burn more fuel in the process and have a shorter service life. I would not power a boat with anything less than 75% of its rated maximum. If the boat is rated for a 115, 75% of that is 86 HP. 90 HP would be the least amount of power I would install so in that regard, go for it.

 

[thurps]

Re: Outboard Fuel Economy Study

Thanks gentlemen, something definitely to think about.

 

[JRJ]

Re: Outboard Fuel Economy Study

I'm still convinced a 4stroke is best for me.

 

[Silvertip]

Re: Outboard Fuel Economy Study

JRJ has the right idea. Buy what you feel is right for you, the boat you have, and the conditions under which it is operated. It's an expensive purchase and requires more than just a few minutes of thought or taking what the dealer says is best for you.

At this juncture, I will admit I had an ulterior motive in making the arguements I did. That does not mean they are not valid arguements because the data is real. In a nutshell, this was an exercise to get people to think outside the box and to look at the big picture rather than just accepting a number a manufacturer throws at you.

In this series of tests the four stroke delivered better MPG than the two strokes at the same speeds. MPG is so familiar to people primarily because it's the number used by car manufacturers when they tout the fuel economy of their vehicles so it's natural to carry that value over to boats. What I argued was that this is not the only measure of fuel economy and in fact it clouds a performance issue with 4-strokes. Speed and it's relation to RPM with a boat has a very large impact on fuel economy which is born out when one considers the amount of time it takes to make a trip of any given length at the engines sweet spot rather than having all boats operating at the same speed which disregards the engine design. Just as MPH favors a 4-stroke, RPM favors a 2-stroke. Remember also that this is one boat using one HP range of engines. Another combination may produce very different results. To get the best economy out of your boat, I highly recommend adding a fuel flow monitor. It will make you a more economical boater.

Thanks for participating and I sincerely hope nobody has hurt feelings over this. If so, I'll take 40 lashes with a wet noodle. At this point I feel this thread can be locked as further discussion would be pointless.

 

[dingbat]

Re: Outboard Fuel Economy Study

At this juncture, I will admit I had an ulterior motive in making the arguements I did. That does not mean they are not valid arguements because the data is real. In a nutshell, this was an exercise to get people to think outside the box and to look at the big picture rather than just accepting a number a manufacturer throws at you.

Thank God.....I was beginning to wonder if it wasn't my teenage daughter I was trying to reason with.

 

[rickdb1boat]

Re: Outboard Fuel Economy Study

I love this kind of discussion. Everyone benefits from it and it really gets the old brain to tickin'. I agree that results will vary from boat to boat and best to buy what suits your application. For my toon, I felt it best to go 4-stroke. For my runabout, I don't think I would have anything but a 2-stroke. Thanks for the topic Silvertip....

 

[Silvertip]

Re: Outboard Fuel Economy Study

Aren't debates fun??? Especially when there is real data to support both sides of the issue. I will close this (I promise) with a quote from a former CEO of Control Data Corporation -- He said:

Beware of data: answers lie in the data, but the data can also lie!
It?s the questions you ask that determine whether you get answers or lies.

That's pretty profound -- and very true.

 

[thurps]

Re: Outboard Fuel Economy Study

Forgot to deduct the kicker hp.

 

[buckstop108]

Re: Outboard Fuel Economy Study

Just checking to see if I understand this and if I do this would be scary.
If Boat #1 2 Stroke is turning 3000 Rpm's
Boat #2 4 stroke is turning 3000 Rpm's

Boat #1 is traveling at a faster speed then #2 So going from point A to point B Boat #1 would get there sooner then #2 so his MPG would be better. So then if it took boat #2 a full hour at say 2 GPH Boat #1 used less GPH because he got there sooner and wasn't running as long as boat #2??
Am I even close?

 

[Silvertip]

Re: Outboard Fuel Economy Study

You have some of it right and some of it wrong. You said:

Just checking to see if I understand this and if I do this would be scary.
If Boat #1 2 Stroke is turning 3000 Rpm's
Boat #2 4 stroke is turning 3000 Rpm's

At 3000 RPM Boat 1 is burning about 4 gallons of fuel per hour while Boat 2 is only burning 2.5 gallons per hour. But look at the boat speed. Boat 1 is going nearly twice as fast as boat 2 at 3000 RPM (20 MPH vs 11 MPH). So if you measured only gallons per hour and ignored everything else, yes, the 4-stroke uses less fuel. But over a given distance, both boats get nearly the same MPG (4.3 vs 4.4) but Boat one gets to the destination quicker which is what makes the MPG number nearly equal to four stroke. You burn more fuel per hour but a trip that takes an hour with the four stroke would take half an hour with the two stroke (in this example). Conversely, if you ran both both boats at the same speed over any distance at anything less than wide open throttle, the four-stroke would use less fuel. At wide open throttle they are the same regardless of speed or rpm. Economy is all in how you look at the numbers.

 

[dajohnson53]

Re: Outboard Fuel Economy Study

... Economy is all in how you look at the numbers.

Indeed. You really have to figure out what your use will be and look at how the numbers play out with the various options. It doesn't matter if an engine will give you good mileage at, say 20 mph if that is too slow for your purposes. At the risk of beating a dead horse, the only fuel usage data that matters for my use is what the MPG (not GPH) is at around 30 mph because that's the speed I want,need to travel for the trips we do. This is higher in the RPM range, where the mileage factors seem to converge.

As far as I can figure it, using those numbers as an example and given the amount of boating I do per year (300-1000 miles), I'd save maybe $100-200 per year in gas and oil using a 4 stroke or Etech vs. the lowest mileage 2 stroke. Definitely a consideration when it's time to repower, but not worth repowering for. Other factors (weight, torque, reliability, noise, pollution, maintenance required) might indeed be factors, but not mileage ... for my purposes anyway.

I'm not trying to argue any point except that the engine's economy needs to be matched with the user's intentions. The higher tech engines may or may not be more economical in a meaningful way.

 

[v1_0]

Re: Outboard Fuel Economy Study

Well, this looked interesting. The only issue I had with the charts were that they weren't all from the same (original)source - so it's likely that different testing methods were used.

But I ran the numbers (based on the charts given here) and it did look like the 2 strokes really had an advantage - even in fuel economy/efficiency. So the headscratcher is why we run 4 stroke at all - cars, motorcycles, etc. I decided that a bit more research was needed.

So - off to the Yamaha web site. They list the performance data for their engines, so it's just a matter of picking the same boat with a 2 stroke and a 4 stroke of the same HP and comparing the performance data. I grabbed the following:
http://www.yamaha-motor.com/assets/...in_2stroke_hpmidport_al_gls-gx180-115tlrd.pdf
and
http://www.yamaha-motor.com/assets/...rokePerf_MidThrustJetPort_115hp_037-GLS-A.pdf

Same boat - Glastron gx180, same hp 115, same engine manufacturer. For this particular boat, the 4 stroke comes out way ahead. It uses less gas at similar speed, has a higher top end (not by much), and even planes faster then the 2 stroke.

So, I think: maybe that's an exception. Let's look at a few more boats.

I pick a Pontoon boat with 50hp - Aqua Patio 220 Luxury edition. Ok, so here there's much less difference between 2 v 4 strokes. The 4 stroke is slightly more fuel efficient, but is slower at a given RPM. Can't match speed for speed on the two charts, so can't say much more than that.

Then a bass boat: Skeeter SX200. 200 HP engine - now something different: at lower speed the 4 stroke uses less fuel and has more power. But once the RPMs get up to about 3000 - the 4 stroke starts using more gas than the 2 stroke. Even if we match up speeds - the 2 stroke uses less gas at higher speed.

Another bass boat: Skeeter SX70 with 90HP. Here the two engines seem to perform very close to each other, with the 4 stroke using less gas.

Ok, one last boat: Sea Hunt Escape 200 with 150 Hp. Again, close performance with the 4 stroke using less gas. I don't want to spend more time on this.. it's late.

One of the major problems is that the boat companies are basing their charts on RPM - which, in my opinion, is the wrong measurement. It should be MPH - then we could better compare different engine set ups (engine type, gearing, propeller, etc) with each other in terms of fuel economy.

Note that other things - such as look, feel, taste, sound, pollution, maintenance, mixing oil, etc. were not considered in this. I know we each have our own preferences for those things.

So, conclusions: For the most part, looks to be a wash. The 4 stroke, again in most cases, uses less gas at a given RPM - but also goes a bit slower. To get the same speed - the 4 stroke will have to run at a higher RPM, which will cut into the fuel economy - but not eliminate it entirely.
That said, for certain applications there seem to be exceptions where the 2 stroke or the 4 stroke are clearly better in terms of performance and/or fuel economy.
Bottom line: gotta look at the numbers on both for your particular application.

-V