Fuel Mixture and Load

manyboats said:

It would seem to me the distance from the closest exhaust valve to the "elbow" would vary considerably and give various results. Dosn't varying the distance from the "fire in the hole" to the pyrometer vary the results? I would think the position of the thermocouple would effect the readings. I put a custom aftermarket SS high rise exhaust on Willy that may have moved the "elbow" 8 or so inches "down the pipe" (up) from where it was.

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The temperature away from the exhaust valve is hotter than where the manifold connects to the head, particularly on turbocharged engines.

It doesn't matter what the exact temperature is, you just want to know what it is where you are measuring it when the engine is working at maximum load. Once you know what that temperature is you will be able to tell when you have exceeded normal maximum load. You are looking for relative temperatures, changes to the normal, unexpected deviations. Most of us here are not thermodynamicists, you might be but I am not.

RickB said:

Just pulling a figure out of my butt, I would guess your EGT would be between 600 and 700F at maximum load and down to around 200 at no load. Close?

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I would say so. The "normal" mark from a previous owner is at about the 400-500 degree point and this is where the pointers are at our cruise rpm of approx 1650.

RickB said:

"And once you read the results there would be little reason to keep the instrument unless one was over propped."

Rick never wrote than and never will!I agree with you that they are very valuable diagnostic and monitoring tools.

It doesn't really matter where the EGT probe is located for our purposes. We are looking for trends, not absolute temperatures. We don't operate within a few degrees of material failure so we don't really care if it is off by a few degrees. Considering most engines don't even offer EGT probe fittings, anything we can get is a plus.

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My bad - I poorly edited Eric's remarks, and he didn't accuse you of saying that. I just deleted part, but not all of his remarks, so my apologies for the confusion....

I'm used to using EGT in a plane where it is used to set the mixture. But I use the EGT readings on our boat along with the other instruments to confirm that all is normal with the engines. Should we get something wrapped on a shaft, for exampe, this could put an additional load on the engine and this would be reflected in an increase in EGT.

FF said:

But if you measured the thrust at a reduced rpm like 1500 to 1700rpm I think the larger prop/lower geared boat would loose more (as a percentage) of thrust than the smaller/faster prop boat. The low geared big prop unit at half speed should have less thrust.

The thrust required to move a boat at a set speed does not change because of prop diameter.

Producing the required thrust at slow (under 10K) is always more efficient with the largest diameter that will fit under the boat that it can be geared for..

This assumes the number of blades and blade width is also optimized.

As an example a ride on a tug (HUGE PROP and frequently 6-1 gearing) will show the engine is slowed very little to cut speed while free running .

Going from 10K to 5K does not require half RPM, just far less thrust.

Run a bunch of props thry Skenes to see how Diameter is king of thrust efficiency.

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that makes sense. If you were to increase the prop diameter then engine load would increase with a resulting loss of rpm

markpierce said:

About 850 RPM is my JD's idle speed.

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Checked the JD 4045's manual today. Listed idle speed is 750 RPM, but the engine seems more comfortable with 800.

RickB said:

It doesn't really matter where the EGT probe is located for our purposes. We are looking for trends, not absolute temperatures. We don't operate within a few degrees of material failure so we don't really care if it is off by a few degrees. Considering most engines don't even offer EGT probe fittings, anything we can get is a plus.

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I was following this thread as it drifted into EGTs while I crossed Georgia Strait today. Back in my hotrod Powersmoke days we used to argue ad infinitum about whether pyrometers should be located pre or post-turbo. In our toyboat situation we might be able to generate a meaningful "danger" number for a pre-turbo installation but the vagaries of exhaust routing and plumbing would make a post turbo installation meaningless. And while pre-turbo may generate more repeatable numbers, if the pyrometer goes south it leaves the engine through the turbo.

As Rick points out we don't need an absolute number anyway. Nobody here is pushing the engineering limits for their engines. In the Powersmoke world we were dealing with chipped engines in pickup bodies pulling way bigger loads than Ford ever intended them to pull. In that situation we needed a number where we could back out of the throttle to avoid burning valves. In our business we ran a couple of IHC 466s that were set at 350 HP - that's way too many ponies to take out of that wonderful little engine and our concern there was when to back out of the throttle to avoid melting pistons. I'd hazard to say that - other than idle curiosity - EGTs are meaningless to most users of this forum.

Northern Spy said:

I do right on spec at 850. I choose a 2.27:1. In hindsight, I should have chosen the 2.74:1. I was trying to hit a sweet spot at the 1800 rpm mark.

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Spy I've thought about it more and concluded that your problem is that of too much power. NT originally had 36hp in the NT26. Then the customers or potential customers took over the designing of the boat and doubled the power. The 30' Willard is a considerably heavier and larger boat w half as much power as your NT so w that information one can't help but think it over powered to go 2 knots. With the 36hp original NT 26 there would be (I suspect) no problem. Since you probably never use the upper speed range you could under prop the boat. You would be at a lower risk of damaging your engine from mis-propping it than the many members here that over prop and the increase of fuel consumption would be very small fly stuff. Yup that would solve your problem w almost no undesirable results. In the process I'd check on where the governor limits the rpm. Should be about 3200. You could even change that to 3000 and operate very close to 3000 w/o danger of overloading. Something to consider.

Some NT 26s have 150hp

Don't blame Nordic Tug, blame me. Spy originally had a 42hp BMW (really a 3 cylinder marinized Hatz). After 5000 mostly trolling hours, she was tired. I like having a few extra usable knots and the ability to lift my bow and dig a hole in the stern just like the Bayliners that cruise by the house on the way to and from Desolation all summer long (cheap jab, I know, but still fairly accurate).

I spent oodles of time creating comparative spreadsheets, picking my engine, calculating pitch (thanks Dave Gerr), and installing it. Almost as much fun as using the boat. Next time Spy is on the hard, I'll pitch it down an inch or so (More spread sheets! ). Until then I'll jig.

I actually posed the recommended Volvo Penta 2700-3000 range question to start a discussion about the theoretical WOT rpm, and make some people (like you) a bit uncomfortable.

Thanks for trying to help solve my "problem" though. Cheers.

Sounds like a good plan Spy.

Yes I really enjoyed very much spending time going over and over the variables re choosing my re-power engine. Wish I could do it all over again even if I choose the same engine. According to the great psychiatrist Carl Jung we are "P" type personalities. We like things undecided giving us options.

Bottom fish are the best. Yellow Eye Rockfish, Ling Cod and of course Halibut.
In that order.

manyboats said:

Bottom fish are the best. Yellow Eye Rockfish, Ling Cod and of course Halibut.
In that order.

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OMG! Again, I agree with Eric! (I must get a check up PDQ.

Northern Spy said:

I spent oodles of time creating comparative spreadsheets, picking my engine, calculating pitch (thanks Dave Gerr), and installing it. Almost as much fun as using the boat.

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I've been toying with the idea of cruise propping our hugely overpowered, semi-planing boat. I'm confident that the engine overload issue is easily addressed with some simple operating limits. The idea is to redefine the operating envelope toward the lower speed end. Anyway, I looked at Gerr's books and didn't see comparative data showing thrust/rpm/power curves for various prop sizes and configurations. Anyone know of a source for generic propeller thrust/efficiency data in chart or table format? I have a prop curve for the boat, but of course it's based on the rated power for the engine.

Thanks

skidgear said:

I've been toying with the idea of cruise propping our hugely overpowered, semi-planing boat.
Thanks

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• What engines?
• Post question with boat, engine and current prop details on boatdiesel.com - they also have a prop calculator

sunchaser said:

• Post question with boat, engine and current prop details on boatdiesel.com - they also have a prop calculator

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Thanks. I'm familiar with the boatdiesel calculator. It's designed for selecting the "correct" prop. Don't believe it will be of much use for an over-propping analysis across a range of boat/engine speeds. I'm really looking for comparative prop efficiency data. Thought I'd ask the pros here first.

By the way, there's a half decent prop calculator at the address below. I ran it for our boat and it was on the money for diameter, but about an inch too shallow for pitch. (They have a note about using 90% engine power for the calculation, so that probably accounts for the difference). Another great tool is Dave Gerr's Propeller - Diameter/Power/RPM chart....allows you to play around with gear ratios, engine power, and prop diameter. Combine that with a decent engine curve and a prop curve, and the possibilities regarding an over prop begin to emerge. Problem is it doesn't clearly define potential fuel savings. Anyway...

www.vicprop.com/calculator.htm

skidgear,
I't's been stated here on the forum by many that ther'e is savings from over propping but it's very small.

manyboats said:

skidgear,
I't's been stated here on the forum by many that ther'e is savings from over propping but it's very small.

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Yes, I know what owners of low powered, primarily hull speed boats have said. But there's virtually no data. And like you, I prefer documentation, or at least a solid analysis. And since my boat configuration is in another power/prop category, I'm taking a look for myself.

The extent of "very small" must vary with the extent of the over prop. What, exactly, is that relationship. That's a question which is very similar to those you posed in your prop efficiency thread...I'm just relating it directly to the perennially popular overprop proposition.

So, have you run across any useful data in your search?

So, have you run across any useful data in your search?


Sorta hard to get hard data on the extent of engine life extension by properly loading an engine.

There are many tales of woe from the fish guys (bigger engines and noisemakers) about the disaster of underloading.

A 25% decrease in fuel burn is of no consiquences on a 200 hour a year 3GPH boat.

How long between engine rebuilds IS a concern.

FF said:

So, have you run across any useful data in your search?

Sorta hard to get hard data on the extent of engine life extension by properly loading an engine.

There are many tales of woe from the fish guys (bigger engines and noisemakers) about the disaster of underloading.

A 25% decrease in fuel burn is of no consiquences on a 200 hour a year 3GPH boat.

How long between engine rebuilds IS a concern.

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More conjecture. I'll see if the Skenes tables you mentioned in another thread have any useful information.

"More conjecture"

National Fisherman has done a number of articles on the price of under loading .

And given a number of techniques to help service life.

Perhaps a search of their archives , although most pro fish guys do have larger engines 400-700hp being common.

FF said:

"More conjecture"

National Fisherman has done a number of articles on the price of under loading .

And given a number of techniques to help service life.

Perhaps a search of their archives , although most pro fish guys do have larger engines 400-700hp being common.

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I was referring to your 25% fuel savings statement, not engine service life, which for a recreational boat I'd consider more of a byproduct from a cruise prop. Quantifying relative fuel savings with a sound analysis is my objective. While I do have a spare set of larger props, I want to understand what's possible before going through the trouble of swapping them out....it's this winter's mental gymnastics exercise.

Again, my question is in regard to finding data in chart or tabular format which I can use to quantify through simple analysis, the relative benefits of one propeller over another. Stating that the savings is very small, or stating that it's 25% (fairly large) is conjecture.

I just googled for tables from Skeene's that might be on the net and found nothing specific, so my next stop is the library. But from references that I did read, I don't expect to find much more than what I have from Gerr's books. I might be able to back into some comparative data by messing around with horsepower and gear ratios in the prop calculator that I mentioned earlier. I'd prefer to avoid making my own charts via tedious calculations, but it appears that's probably the next stop.

Regards

skidgear said:

Again, my question is in regard to finding data in chart or tabular format which I can use to quantify through simple analysis, the relative benefits of one propeller over another. Regards

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Skidgear

The way the pros (commercial Post type charter fishermen, offshore racers etc) do this is to first understand that each boat is different. Then they do as you are, dial in on paper their best prop setup and have a prop on either side of the diameter and pitch equation to test as well to see if any benefits arise by going bigger or smaller. I had this exact test done years ago on a high speed boat. Many choose to go for quiet and smoot props which are not necessarily the most fuel efficient. I've a friend who tinkered with this for years after his boat was initially - "perfectly" propped. Your quest can be frustrating and expensive.

Accurate fuel flow and speed measurement is a must. But, and a very big but, at what boat load conditions do you plan ot run the majoriity of the time because the prop selection goes to hell as you add or subtract tons of water, gear and fuel or hit a small stick that bends a prop.

What is your boat, desired perfect prop speed range and engine(s) combination?

sunchaser said:

Skidgear

The way the pros (commercial Post type charter fishermen, offshore racers etc) do this is to first understand that each boat is different. Then they do as you are, dial in on paper their best prop setup and have a prop on either side of the diameter and pitch equation to test as well to see if any benefits arise by going bigger or smaller. I had this exact test done years ago on a high speed boat. Many choose to go for quiet and smoot props which are not necessarily the most fuel efficient. I've a friend who tinkered with this for years after his boat was initially - "perfectly" propped. Your quest can be frustrating and expensive.

Accurate fuel flow and speed measurement is a must. But, and a very big but, at what boat load conditions do you plan ot run the majoriity of the time because the prop selection goes to hell as you add or subtract tons of water, gear and fuel or hit a small stick that bends a prop.

What is your boat, desired perfect prop speed range and engine(s) combination?

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For discussion purposes say the boat is 30,000 pounds, semi-planing, twin turbo 250's, with 2:1 transmissions. The target "efficiency" speed for discussion purposes is roughly 8.5 knots. Prop speed is currently 750 rpm at that hull speed. It should change incrementally with larger, more efficient props.

But keep in mind that I'm not thinking in terms of optimizing the "perfect/correct" prop as seems to be the case in your charter boat example. The cruise prop scenario at its most basic, installs a bigger "wrong" prop while keeping the rest of the drive train unchanged...same engines...same gear ratio, etc. So, start with a perfect prop and increase the diameter an inch at a time. Then plot the fuel specifics across the speed range for each new prop size. I understand that efficiency gains will be small (and boat specific)...but that's not the point of the exercise. Documenting an examplle of what we're really talking about when it comes to cruise props is what I'm trying to quantify.....curiousity if nothing else.

Regards

Marin said:

GB slightly overpropped all their boats back when the typical powerplants were one or two FL120s or FL135s. As they began putting larger and larger engines in their boats starting in the mid-80s i don't know if they continued the overpropping practice or not.

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They still do. My 2009 47' EU was over-propped by about 2".

...just for grins I ran some numbers through Gerr's "bollard pull" thrust formula using one half of the total horsepower (58 SHP) required to move the hull at its calculated hullspeed as a constant. That horsepower number was "derived" by jacking around with the vicprop calculator.

Per the Gerr formula, thrust for one 24" generic prop (absorbing 29 SHP) came out to about 938 pounds. Thrust for one 26" generic prop @ 29 SHP is about 1005 pounds. And thrust for one 28" generic prop @ 29 SHP is about 1056 pounds. That's about 7% thrust improvement going up to a 26", and about 12.5% jumping from a 24" to a 28". Of course, thrust is what moves the boat. My boat has room for 26" props....14% total improvement in thrust for a given horsepower....

A point of interest...perhaps...

Sorry I'm coming in a little late here...

Yes, with diesels you really need to ditch the whole concept of mixture. All that matters is that there is enough air to fully burn the injected fuel. It's fine to have too much air, but if there isn't enough, you start to get black smoke and all you are doing by injecting more fuel is making more black smoke. Throughout a diesel's whole power range, there is always more air than needed right up until it reaches max power. That's the upper curve in the charts that David posted. If you try to push above that line there is no longer enough air in the cylinders to completely burn the fuel.

The whole concept behind a turbo on a diesel is to jamb more air down it's throat so you can successfully more fuel. If you can jamb in more fuel, then all of the sudden that power line moves up 'cause you can burn more fuel.

twistedtree said:

They still do. My 2009 47' EU was over-propped by about 2".

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No way!!!!

Go back to post #30 where it states "Overpropping is not adult behavior."....surely you are mistaken....

I also think I read from some zealous TF poster...that no way would a manufacturer ever overprop a boat...

twistedtree said:

Sorry I'm coming in a little late here...

Yes, with diesels you really need to ditch the whole concept of mixture. All that matters is that there is enough air to fully burn the injected fuel. It's fine to have too much air, but if there isn't enough, you start to get black smoke and all you are doing by injecting more fuel is making more black smoke. Throughout a diesel's whole power range, there is always more air than needed right up until it reaches max power. That's the upper curve in the charts that David posted. If you try to push above that line there is no longer enough air in the cylinders to completely burn the fuel.

The whole concept behind a turbo on a diesel is to jamb more air down it's throat so you can successfully more fuel. If you can jamb in more fuel, then all of the sudden that power line moves up 'cause you can burn more fuel.

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Aren't there other factors involved here like cylinder temperature that determine combustion eficiency? Dosen't the compression, temp, ratio of reactants also determine the type and amount of pollutants and of course if to rich raw fuel emitted into the air and or water.

I resemble that person psneeld.

GB may do what ever they want to do and because they do X,Y or Z dosn't mean it's good practice. I'm very surprised to hear they do over prop though.

Engine manufacturers don't recommend it.

By the way I'd rather you called me by name than "zealous". It's rude.

manyboats said:

GB may do what ever they want to do and because they do X,Y or Z dosn't mean it's good practice. I'm very surprised to hear they do over prop though.

Engine manufacturers don't recommend it.

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I wish Ocean Alexander would take my boat back and over prop it. I'll even tell them where to place the "throttle" stop...or the red line on the tach if they insist on being cheesy about it.

skidgear said:

I wish Ocean Alexander would take my boat back and over prop it. I'll even tell them where to place the throttle stop...or the red line if they want to be cheesy about it.

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are you guys sure about this? I think its common for the yard to prop boats more for the shipping weight than the loaded outfitted weight. Would make sense since they have no control over how the customer will outfit or load the vessel unless he stipulates during the build. If this were true then when loaded the boat would appear to be over propped wouldnt it.