Specific Fuel Consumption- different engine types

[DavidM]

When the OB mfg decide there is enough demand they will start making units with enough reduction gearing to spin a good sized prop at displacement speeds.

This will rival diesel in fuel burn and be perhaps half the engine costs over the engines useful life.

The last sentence is not true. Gasoline engines inherently burn 1/3 more gallons of fuel to make the same horsepower as diesels, irrespective of prop efficiency. But yes, poor prop efficiency at low speeds does make the gasser's Nm/gal a little worse.

But that brings up an interesting point: the majority of outboard powered and trailerable trawlers are designed to be cruised at planing speeds and from what I have read on C-Brats most owners do run them that way- 12 to 15 kts is typical.

Why? Easy, you get where you want to cruise with your tow vehicle not your propulsion engine. So fuel mileage is much less important once you get there.

David

 

[Donsan]

Hey David,

A little off topic but what do you think of the lack of oil changes and the service interval on the E-TECs? One poster has said his E-TEC 300 had a 500 hour service interval. I looked at the E-TEC 60 and it claims a 300 hour service interval.

 

[High Wire]

David I agree with your original post numbers and are within 10 percent of estimates I've done for my own boats:

25' Hydra-Sports W/A T/140 carb 2 stroke 8 hp/gal/hr. 9 not on its best day lol. Performance curve based. Repowered with S/225 hp eTec - 11 hp/gal/hr. Performance curve based. Actual fuel used on a known trip dropped 40% and WOT speed increased 2 kts.

29' Phoenix FBC t/270 hp carb straight inboard with Floscan. 10 hp/gal/hr.

21' Sailfish CC S/250 hp 4 Stroke fuel injected with fuel flow monitor. 11 hp/gal/hr.

36' Monk S/135 hp NA 4 stroke diesel. 16-17 hp/gal/hr. Performance curve based.

Unless you have lab calibrated equipment, we're all just educated guessing.

 

[bayview]

BSFC data is usually at full load. I don't often see BSFC data at less than full load. Boats run on a prop curve, not full load. I dont see how peak efficiency point from a full load curve can be assumed to apply to a midpoint on a prop curve.

 

[Ski in NC]

BSFC data is usually at full load. I don't often see BSFC data at less than full load. Boats run on a prop curve, not full load. I dont see how peak efficiency point from a full load curve can be assumed to apply to a midpoint on a prop curve.

Cummins and Cat publish burn rate data along the extrapolated prop curve, not just full load. BSFC data can readily be calculated from what is published.

Gasser data is real hard to find. About the only data point you can work with is full power where gph is displayed and you assume it is making its rated power. Nothing published where you can relate gph at cruise to some actual shaft hp other than full.

In defense of the gassers, many of them run rich mixture at full load to keep cylinder temps lower. Then lean out the mix at cruise. So BSFC at full may look worse than it would be at cruise. But without data, who knows.

I have no idea why the outboard MFR's are so stingy with the data. They absolutely have it, and someone is going to better than the others. So in that case they would have a marketing advantage.

Something like "our 300hp motor when running at 4000rpm and making 200hp burns xx gph, which is 2gph less than competitor Y making the same hp". People buying motors get it on that level.

Head scratcher.

 

[Nomad Willy]

High Wire wrote;
“Unless you have lab calibrated equipment, we're all just educated guessing.”

Yes,
I have for years taken w a grain of salt all fuel consumption statements here on TF. Way way too many variables to do any reasonable comparisons or postulate what anothers fuel burn will be or my own. And many claims are just unbeliveable. Most are interesting to read though.

Is my fuel burn number correct? No. But I know from experience it works for me re calculating fuel burn for long trips. And I,ve compared it w the credible stats and seen things add up nicely. But often one can guess a better number than what’s presented here on the forum.

This dosn’t mean fuel consumption talk is not worth reading. That’s what wer’e here for .. to share opinions. But if anyone thinks we can talk and argue for a week or two and come up w a magic insight into fuel consumption they are mistaken. But we all learn a bit as we go. We’ve talked so much about fuel consumption for so long one can’t help to have some good takaway from time to time.

Unfortunate that this post follows an excellent post by Ski. He and David and probably 5-6 others post information that is very much worth reading .. and even taken to the bank.

 

[sunchaser]

Cummins and Cat publish burn rate data along the extrapolated prop curve, not just full load. BSFC data can readily be calculated from what is published.

Ditto my Perkins Sabre 225TIs. These engines are identical to the Cat 3056 except for color. I suspect any engine used for major genset or industrial service is with plentiful BSFC data. That verifiable data is a decided selling factor for high hour use industrial intended engines.

 

[DavidM]

Hey David,
A little off topic but what do you think of the lack of oil changes and the service interval on the E-TECs? One poster has said his E-TEC 300 had a 500 hour service interval. I looked at the E-TEC 60 and it claims a 300 hour service interval.

Not at all off topic. I started this thread to mostly discuss outboard gassers and their suitability for powering a trailerable trawler. The Evinrude E-Tec has some interesting features and capabilities that make it very suitable for small trawler power.

Usually we take our 4 stroke gasoline cars in for service and the only real thing that is done is to change the oil and filter. This is necessary to remove oxidized oils and replace the oil additives which deteriorate over time.

The E-Tec doesn't need to do this because its oil is consumed as it is injected into the cylinder walls and bearings. The 300 hour service interval is probably what all auto manufacturers would do if they could accept synthetic oil. We bought a new Mercedes eight years ago and they extended the service intervals to 20,000 Km or about 12,000 miles which is roughly 300 hours because they specified synthetic oil which deteriorates much less.

I think that E-Tec is on to something good in the outboard world. I have watched over the years as Evinrude steadily extended the product down to about 25 hp now. My back of the envelop research has shown that the E-Tec is comparable in fuel efficiency with the new 4 stroke outboards. The only open question is longevity and my money is on the E-Tec.

David

 

[High Wire]

Hey David,

A little off topic but what do you think of the lack of oil changes and the service interval on the E-TECs? One poster has said his E-TEC 300 had a 500 hour service interval. I looked at the E-TEC 60 and it claims a 300 hour service interval.

Excellent. Had the service performed (2006 225 hp) 2 times in 5 years at about $600 each. The etec winterizes itself except for the grease fittings and stabilizer in the fuel tank. My Yam costs that every season.

 

[Ski in NC]

The ETEC has some real advantages with regard to part load fuel economy. Probably better than four strokes. But of course I don't have any friggin' data to back that up.

The ETEC effectively runs unthrottled in the cylinders, so compression ratio (effective) is much higher at part load than a standard efi 4stroke.

If the four strokes go to direct injected, then that would put them in a similar category as the ETEC.

 

[DavidM]

The ETEC has some real advantages with regard to part load fuel economy. Probably better than four strokes. But of course I don't have any friggin' data to back that up.

Ahh, but you have a good theory. Sometimes that is enough. Who said that with one data point and a good theory, you can draw a curve?

I didn't realize that the E-Tec was not throttled. As you note that might make it better than current 4 strokes at partial load.

David

 

[OldDan1943]

A well equipped trailerable trawler with one or 2 outboards, a perfect match, IMO.
It is a lot easier to re-power this trawler, when a better OB comes out or the OB craps out.

 

[twistedtree]

BSFC data is usually at full load. I don't often see BSFC data at less than full load. Boats run on a prop curve, not full load. I dont see how peak efficiency point from a full load curve can be assumed to apply to a midpoint on a prop curve.

They are harder to come by, but I have seen full BSFC "maps" for select engines. It's a 3-d thing which of course is a bit hard to draw on paper, but is done like a topo map. The axis are RPM and Load, and the topo contour lines (Z- dimension) show BSFC at any combination of RPM and Load. For best BSFC, you want to operate at or near the top of the "hill" in the map.

 

[twistedtree]

While talking about BSFC, it's interesting to see how good new engines are. The ideal engine would have the same BSFC at any and all power levels, i.e. a flat BSFC curve.

Attached is the data sheet for the Scania engine I am using in a boat build, and on the second page are the BSFC curves. The solid line is the full power line, and it is a near flat as I have ever seen. The total variation is about 10%, and other than a blip up at 1300 RPM, it's nearly flat through any realistic operating range. Now it has been pointed out that the full power curve isn't very interesting for a boat, so look at the dashed curve which is the prop curve. The units are different (l/hr), so it curves up as power load increases, but it's an almost perfect sweep, with just a slight bump at the previously mentioned 1300 rpm point.

 

[FF]

"so look at the dashed curve which is the prop curve. The units are different (l/hr), so it curves up as power load increases, but it's an almost perfect sweep,"

Prop curves are theoretical , perfect sweeps that change fr every prop combination.

The fuel map (if you can obtain one) is the best for specking a vessels inshore cruise and LRC speeds .

 

[twistedtree]

"so look at the dashed curve which is the prop curve. The units are different (l/hr), so it curves up as power load increases, but it's an almost perfect sweep,"

Prop curves are theoretical , perfect sweeps that change fr every prop combination.

The fuel map (if you can obtain one) is the best for specking a vessels inshore cruise and LRC speeds .

The load is a perfect sweep, but the engine's response to that in terms of fuel burn is engine-specific. Hence the bump around 1300 rpm.

I don't disagree with using the full fuel map, if available, however you are still using it with the same theoretical prop load curves. So I guess I'm not understanding your point.

 

[FF]

A fuel map allows one to attempt to select a prop setup that will give normal cruise power at the low RPM ,) which is a better load for the engine) , as well as see at what speed the the engine/vessel will be able to maintain with good economy.

The map will allow one to work out Long Range Cruise , speed and ballpark endurance and distance.

Some engines are required to operate at higher speeds , big engines frequently can handle 1200RPM , but smaller may require 1500RPM.

Staying in the most efficient bullseye always assures no over or under loading.

With a fuel map and a copy of Skeens the reduction gear and prop can be selected with enough accuracy that the pitch might need to be adjusted , but the diameter should be OK.

Even then on a new boat 3rd time lucky is common.

 

[bayview]

I am missing something because on those curves th SPFC curve is stated as full load. The bump is at 1300 RPM, well off full load as shown on the dashed prop curve unless of course the engine were propped to that as a limit.

 

[Ski in NC]

I am missing something because on those curves th SPFC curve is stated as full load. The bump is at 1300 RPM, well off full load as shown on the dashed prop curve unless of course the engine were propped to that as a limit.

Yea, confusing data. The BSFC trace is solid line which per legend is full power. But that is nonsense for a boat as you will only be at full power near rated rpm. Anything below full power you will be at a lower rpm and lower hp depending on your boats load curve. A decent representation would be to show BSFC along the 2.5 exp curve, then it would be a dashed line like the LPH data.

Each mfr makes somewhat of a hash out of these data which is frustrating as heck. Not sure if it is intentional obfuscation or they gave the task to a newbie engineer, who knows. But the numbers I see for that Scania look really good so why hide it!!

 

[bayview]

I agree and restate my conclusion that using the peak on that curve to predict best operating point on the prop curve is baloney.

 

[twistedtree]

I agree and restate my conclusion that using the peak on that curve to predict best operating point on the prop curve is baloney.

We may end up going in circles here, but hey, why not....

There are generally two approaches to propping a boat.

1) Prop to extract max HP out of the engine. To do this you select a prop such that the prop load curve intersects the engines max power curve right at max allowed RPM. This assumes that max RPM is where the engine makes max HP, and is nearly always the case. This lets you run the engine up to full RPM, with max load at that RPM, and max HP. This is the way that all engine manufacturers require you to prop the boat if you want the warranty.

2) Prop for max efficiency at some desired cruise speed. This is what FF was talking about. The idea is to ignore any goal of utilizing max HP, and instead pick a prop where the prop curve passes through the region on the BSFC map where max fuel efficiency is achieved. Then run the boat at that most efficient operating point. When you do this, you are essentially cutting off the top end of the engines HP capability. This is because the new prop curve will load the engine to full power somewhere before it can reach full RPM. Anything above that will be overloading the engine, and it's up to you to be sure you don't do that. You should only prop this way if you either don't care about your engine warranty, or your engine is out of warranty. And only if you feel you can realistically keep the engine our of the overload zone.

It should be noted that in #2, you are maximizing the engines efficiency, but that's not the same as the boat's efficiency. The boat's efficiency would be measured in (nautical) miles per gallon, not HP produced per liter or gallon or gram of fuel burned. Then engine efficiency feeds into the boat's efficiency, but it's frankly a minor factor. By far the dominant factor in boat fuel economy is speed. It really begins and ends there. The slower you go, the better the fuel economy. The faster you go, the worse the fuel economy. There is no "sweet spot" where fuel economy reaches a maximum, other than parked at the dock. It's all down hill, all the time, the faster you move the boat. So unless you are tuning a tanker for optimal fuel burn at a particular speed, I would argue that fussing with non-standard prop tuning isn't going to buy you much, especially compared to slowing down by 1/4 kt.

 

[Nomad Willy]

twistedtree,
That was a great summary!

 

[Art]

This was passed along to me.....

"One thing to keep in mind when comparing BSFC is that the energy content of a gallon of diesel is about 15% higher than then energy content of gasoline."

That is one of the reasons why specific fuel consumption is based on weight, not volume like they are all doing. Their contortions just confuse the situation.

BSFC, more accurately Brake Specific Fuel Consumption is measured in pounds per brake horsepower hour, the weight of fuel burned to produce one horsepower for one hour.

Also..... since I was a kid..... taking the hp of a 2 stroke outboard and dividing by 10 gave you the fuel burn at WOT.

I cant believe todays outboards arent a bit better on fuel consumption.

Can we then reason that with o/b used at half power the formula would become: Hp of o/b divided by 2 and then that number divided by 10 gives approximate fuel burn at half throttle? Considering that boat is either remaining comfortably on top of water at full plane or traveling easily through the water just below hull speed.

Examples, regarding 50 hp, 2 cyl o/b...

At WOT - 50 hp divided by 10 = 5 gph fuel burn

A 1/2 Throttle - 25 hp divided by 10 = 2.5 gph fuel burn

I don't know rate of fuel burn at WOT with our 1975 50 hp Johnson pushing our 14'8" Crestliner runabout - because I seldom hit WOT but for seconds at a time.

I do know that it takes one of the six gallon gas tanks about 3 hours to empty at a steady 1/2 throttle.

Seeing as we are doing around 25 mph at 1/2 throttle, and six gallons is burned in 3 +/- hours, that means - 75 miles :- by 6 gallons = 12.5 miles per gallon.

I've also learned by other ways of calculation that when I reduce throttle well below 1/2 and cruise [sill on full plane] at 18 o 20 mph [which we often gently cruise at] that mpg increases way over the 12.5 mpg mentioned above at 1/2 throttle.

 

[FF]

No question that at displacement trawler speeds the difference in fuel burn will be not huge.

But with a better load on the engine its service life should be extended and cruising with 2-400 lower RPM is always easier.

The rare ocean voyager might need to operate with best efficiency to obtain range, with out bladders or 50G drums in the cockpit.

For most inshore folks a gallon or two an hour lower fuel burn , at 200 hours a year is no big deal.

 

[bayview]

TT I agree that propping to max out at the peak BSC point is the only way to use that data even though it may overload the engine.
Unfortunately people look at that data and assume if X RPM is the peak efficiency point at full load it must be the peak efficiency RPM at other loads. There is no date to support this assumption.
Within middle operating range IMO this is much ado about nothing as there probably is not much change over the range and as pointed out above minor changes in boat speed caan have a greater effect on ful use than small changes in engine efficiency.

 

[DavidM]

I worked up another boat and engine using data from boattest and projected hp at lower rpms from boatdiesel. This case was a Mako planing skiff of 21' with a Mercury 150 hp 4 stroke outboard. Given that this boat was a flat bottom skiff I used boatdiesel's fast planing hull type for their power required calculator.

The boatdiesel calculator gave me exactly the wot speed reported on boattest so I felt good about this case. Then I asked the calculator to figure the power required to hit 27 kts at 4,000 rpm. It gave me 69 hp and boattest said it used 6.1 gph at that speed for 11.3 hp per gph, about what I would expect.

So I took a chance on running the case of 9.1 kts at 2,000 rpm and 2.4 gph. Boatdiesel said it would take 8 hp or 3.1 hp per gph. That is pretty bad and probably not that bad due to errors in boatdisel's calculator being that far removed from the calibration point. But I am pretty sure it is bad, maybe 6 at that low power output.

And FWIW that engine produced 10.7 hp per gph at wot.

That same issue of boatdiesel had an Andros center console 26' with a single 300 hp E-Tec engine, so I ran the numbers on that boat. I had to drop down a step to average planing for it to calculate 300 hp at 45 kts. Then at 4,000 rpm and 30 kts it required an estimated 134 hp and 10.4 gph resulting in 12.9 hp per gph. It seems that the E-Tec does beat 4 stroke engines at mid range rpms and load. It even made 12.4 hp per gph at wot better than the other 4 strokes I checked by 10+%.

So that is the end of my playing with O/B fuel consumption numbers. I think I can say with fair certainty that modern 4 cycle or E-Tec outboards produce about 12 hp per gph at mid range rpms and load and that E-Tecs do slightly better than the 4 strokes.

David

 

[Ski in NC]

Nice analysis, Dave.

Supports the guestimate that 4-stroke gassers run about 12-14 hp/gph at normal (fast) cruise and 10-12 hp/gph at WOT.

Slow cruise like displacement speed is really hard to crunch the numbers as you did and have confidence in the accuracy. Most gasser stuff I have seen is that BSFC goes real high when you are down around 10-20% power on a gasser.

Makes me want to set up a dyno lab!

 

[twistedtree]

I worked up another boat and engine using data from boattest and projected hp at lower rpms from boatdiesel. This case was a Mako planing skiff of 21' with a Mercury 150 hp 4 stroke outboard. Given that this boat was a flat bottom skiff I used boatdiesel's fast planing hull type for their power required calculator.

The boatdiesel calculator gave me exactly the wot speed reported on boattest so I felt good about this case. Then I asked the calculator to figure the power required to hit 27 kts at 4,000 rpm. It gave me 69 hp and boattest said it used 6.1 gph at that speed for 11.3 hp per gph, about what I would expect.

So I took a chance on running the case of 9.1 kts at 2,000 rpm and 2.4 gph. Boatdiesel said it would take 8 hp or 3.1 hp per gph. That is pretty bad and probably not that bad due to errors in boatdisel's calculator being that far removed from the calibration point. But I am pretty sure it is bad, maybe 6 at that low power output.

And FWIW that engine produced 10.7 hp per gph at wot.

That same issue of boatdiesel had an Andros center console 26' with a single 300 hp E-Tec engine, so I ran the numbers on that boat. I had to drop down a step to average planing for it to calculate 300 hp at 45 kts. Then at 4,000 rpm and 30 kts it required an estimated 134 hp and 10.4 gph resulting in 12.9 hp per gph. It seems that the E-Tec does beat 4 stroke engines at mid range rpms and load. It even made 12.4 hp per gph at wot better than the other 4 strokes I checked by 10+%.

So that is the end of my playing with O/B fuel consumption numbers. I think I can say with fair certainty that modern 4 cycle or E-Tec outboards produce about 12 hp per gph at mid range rpms and load and that E-Tecs do slightly better than the 4 strokes.

David

Cool stuff. At least for a geek like me....

Was the Merc 150 fuel injected or carbs?

 

[sunchaser]

An excellent source of fuel consumption data for boats and motors is "Boating" magazine. Every month they have boat and motor tests, literally dozens.

The data presented shows in tabular form RPM, GPH, MPG, noise and running angle. If one is a subscriber their vast digital library is a 20+ year treasure trove of testing. The articles show data and comparisons for every boat, drive system and motor imaginable, say under 40 feet.

 

[DavidM]

Was the Merc 150 fuel injected or carbs?

It was multi-port electronic fuel injection. I think all but the smallest outboards are EFI today and I would expect those will also be EFI before too long. EFI eliminates many ethanol fuel problems since the entire fuel system is sealed. Carbs are vented which means that moist air can get in and screw with the fuel.

David