CAT 3116 Transom Soot

[Archhibb]

That could be so. It’s definitely true light loading on diesels can coke them up.

 

[Riverguy]

Definitely...overpropped (according to Caterpillar)

Hi Avalon,

You are definitely over-propped


At wide open throttle with a 3116TA you need to be able to get above 2,800 RPM when the boat is fully loaded with fuel/supplies etc. and in "worst case" sea conditions.

The attached is from Caterpillar's Marine Applications & Installation Manual -- and they put it right on page 1.

Sadly...there is much misinformation out there on this topic, and I have gotten into (and won) arguments with otherwise knowledgable surveyors and even diesel mechanics on this. The idea that you only need to be "close to" the max HP RPM number is pervasive...and wrong.

As long as you are overpropped, you will be making soot, even if everything else is perfect. I would take off at least another inch of pitch, and if you expect to be running offshore or in heavy seas, and/or more heavily loaded -- take off two more inches.



Another thought...is your propeller 'cupped'? For example, depending on RPM, a cupped propeller with a 22" pitch will behave like a 23-24" pitch (from your engine's perspective).

 

[Avalon36]

Thanks for the insight and clarification on this. Lots of folks have suggested that I need to be able to run over 2800 so I either need to confirm that the engine is right or look at a new prop.

 

[FF]

"Do not reject that you may be over propped. Until a fully loaded 2850 can be achieved, a question remains."

Operating at 2000 to 2200 the boat will hardly be "overproped" or overloaded while underway.

I would look at intake temperature after the engine is operating for an hour or two.

Hot air is thin air , perhaps the problem with combustion results on transom.

 

[Archhibb]

"Operating at 2000 to 2200 the boat will hardly be "overproped" or overloaded while underway.

Until I see data showing otherwise, the resources I trust say overpropped is overloaded throughout the entire poweband.

Also, two 3116 owners on BoatDiesel have documented that their EGT’s are higher at 2200 than at 2400: the load on their engines is higher at the lower rpm. Operating below rated cruise won’t make up for being overpropped.

 

[twistedtree]

Until I see data showing otherwise, the resources I trust say overpropped is overloaded throughout the entire poweband.

Also, two 3116 owners on BoatDiesel have documented that their EGT’s are higher at 2200 than at 2400: the load on their engines is higher at the lower rpm. Operating below rated cruise won’t make up for being overpropped.

That’s not correct. If you look at the allowed load curves for the engine, it will show two lines. The top line is the engines max power output across the rpm range. The lower line is the prop load which is the load presented by a propeller.

You load is obviously a prop, so you load curve will always look like the prop curve. Changing the pitch of the prop to increase load shifts the prop load curve up. Decreasing pitch shifts the curve down.

Overloading happens when your prop curve runs above the max engine load curve, and the only time that’s going to happen, given the shapes of the curves, it up around max rpm, which is also max power.

The engine manufacturers requirement is to ensure the prop load never exceed max allowed engine load. If the engine can turn faster than rated rpm, that’s true, hence the rule.

However at lower rpms it’s nearly impossible to overload the engine, purely by virtue of how the prop load drops as rpms are decreased. Some people choose to overprop, and just never run the engine above an rpm that exceeds allowed load. Manufacturers don’t want that because if an installation can be overloaded, it will, eventually, by someone.

Your “rich mixture” comment is worth a response as well. There really is no such thing with a diesel. Gas only ignites, or ignites well, at a particular air/fuel mixture. Not so with diesel. Diesel burns, and burns clean, as long as there is enough oxygen to complete combustion. Too much isn’t a problem. It’s like you breathing in a room. The amount of air in the room doesn’t matter, right up until there isn’t enough.

When a diesel operates, the design goal is for there to always be an excess of air to ensure complete combustion. This is why a turbo helps so much because it can really pack in the air, quite literally.

When you see a diesel putting out black smoke, it’s because there is more fuel being injected than there is air to allow it to burn completely. Is some ways this is a mixture, but it’s very different from a gas engine, and diagnosing and fixing problems like you have can be very different. So it’s important to understand the differences.

Most common black smoke on a boat is at full throttle trying to get going, especially trying to get up on a plane. Full fuelnis being injected, but there aren’t enough rpms and turbo to provide full air. At a steady load,!there shouldn’t be excess smoke because there should be ample air. If there is black smoke, like when over propped at full throttle, it’s because the engine is dumping max fuel into the cylinders, but the engine isn’t turning fast enough to provide the air.

You have soot at less than full power, so the question is whether it’s normal, because all engines soot to some degree, or is it abnormal. If abnormal, it’s probably an air deficiency of some sort. That’s why earlier I suggested checking the air filter, and checking the turbo boost. You could also have an engine room air intake restriction of some sort.

 

[Archhibb]

Twistedtree -

Thanks for the explanation. It fills in some gaps or misunderstandings that I have had while also explaining why overloading is viewed by some (like me) as bad across the powerband.

 

[Riverguy]

Overpropped is Overloaded

Archhibb, you are exactly right. A boat that is overpropped will be overloading the engine throuout the entire powerband. Overpropped is overloaded, regardless of speed.

Caterpillar's highly technical discussion on this topic should end any further uncertainty here: http://www.propellerpages.com/downl...ngines_Application_and_Installation_Guide.pdf


Note specifically their warnings >>against<< changing the fuel rack settings:






That said, the extent of 'overloadedness' will vary depending on the hull design and speed through the water, because the engine's power curve is relatively linear, while the power needed to move the boat above displacement speed is exponential. So...if you are running at displacement speed (not making any appreciable wake), the overloading will still be there, but it will be relatively minor.


One of the logical fallacies that leads people to want to 'overprop' is the belief that reducing RPMs (by increasing pitch) will reduce fuel consumption. This is wrong. Engines produce horsepower (and consume fuel) depending on the load they are seeing, and >>not<< on RPM. To prove this, hook up a flow-scan and (with the transmission in neutral) run the engine up to 2000 rpm and look at GPH. With no load on the engine you will see that it is using almost zero extra fuel to run at 2000 rpm vs. running at idle! Now, at that same 2,000 RPM, an overpropped 3116TA will be burning ~6gph (and spewing soot). If underpropped it will be burning ~4gph, and when correctly propped it will be ~5 gph. The engine will be running most efficiently (and will not be making any soot) when it is correctly propped, but the key take-away here is that GPH will vary with load, even at constant RPM!



A related question is -- is there a downside (or any benefits) to being underpropped?



The downside is you will lose some speed at the top end. The upside is...by 'unloading' the entire drive-train (including engine bearings and the transmission), stuff should last longer. Also, should you ever wind up running with a fouled bottom or running gear, or if you need to tow someone (or even a dinghy) you are much less likely to damage your engine if you are overpropped.



For the boats we call 'trawlers' (and the lifestyle that goes with them), I always lean toward the underpropped side. Among other things it helps me sleep better at night knowing that I am not wrecking my engines...

 

[Riverguy]

Twistedtree, what you are saying is true for a pure (or a 'true') full-displacement hull, like your Nordhavn(s).



For boats that are designed to run above displacement speeds (i.e. semi-displacement hulls), overpropped is overloaded.


James Hamilton (M/V Dirona) wrote an excellent article on this when he used to own his Bayliner...

 

[Delfin]

Until I see data showing otherwise, the resources I trust say overpropped is overloaded throughout the entire poweband.

Also, two 3116 owners on BoatDiesel have documented that their EGT’s are higher at 2200 than at 2400: the load on their engines is higher at the lower rpm. Operating below rated cruise won’t make up for being overpropped.

You already have the data but you're not looking at it. Per TT's post, you can't overload a marine diesel until the prop demand curve goes above the maximum power curve. It's printed in the operators manual. You can tell when you've hit the cross over point because fuel consumption eguals or exceeds the maximum power fuel consumption for a given rpm.

If you look at this curve you'll see the power demand lines come together at max rpm, as does fuel consumption. Before that, within reason it is very hard to over prop an engine and do the slightest harm. And yes, EGT will go down at max or near max rpm if you flatten the prop for the obvious reason that the power required to spin the prop at high rpm goes down. If reducing that power was itself a virtue, then cut the size of your prop in half and let it spin away at much reduced power. But reduced power consumption isn't necessarily a virtue, so the argument about prop pitch is a bit more complicated than some here assert.

As mentioned before, Delfin was signed off by CAT for activation of the warranty based on her being able to reach WOT rpms running around Long Beach harbor without a stitch of furniture, plumbing, walls, doors, etc., but she couldn't reach that max rpm after I filled her up. At that point she was overpropped, and at max rpm I could get to - around 200 off design - the EGT would hit 850. Now that the prop has been flattened a bit - I get within 75 rpm - the EGT at WOT is 750 because less power is required to spin the prop. And 750 indicates the engine isn't working overly hard at that point, so when someone tells me this is bad for the engine I ignore them because the empirical evidence says otherwise, as would any knowledgeable prop shop. And critically for understanding this, the EGT and fuel consumption of my engine now is EXACTLY the same at cruise speed of 7.75 knots - as it was before I flattened the prop, even though I have to spin the engine a bit faster now to achieve that speed. This can only mean that the power loading on the engine at that speed was identical with an extra 1.5" of pitch or without. Meaning the concept of over-propping at that loading is horse pucky.

If the OP has transom soot, there are lots of possible explanations. The wrong oil is being used, which CAT says will produce sooting, so I'd start there. Perhaps the air flow into the ER is the culprit. Perhaps a few other things. He might be able to eliminate the prop question by securely taping a mesh over the exhaust port and run the engine for an hour or so in neutral at cruising rpms. If soot is picked up, look someplace other than prop pitch for the culprit.

 

[Delfin]

Twistedtree, what you are saying is true for a pure (or a 'true') full-displacement hull, like your Nordhavn(s).



For boats that are designed to run above displacement speeds (i.e. semi-displacement hulls), overpropped is overloaded.


James Hamilton (M/V Dirona) wrote an excellent article on this when he used to own his Bayliner...

It's true for all engines, regardless of what boat they're in.

Over propping within reason only has an effect at high rpms. If the rpm you run at is below WOT, and even for semi-displacement hulls like the OPs isn't going to run at WOT, it is going to be largely irrelevant. That isn't an argument against propping the boat so you can reach max rpm. It is just the argument against blanket statements that over propping occurs throughout the power curve, which it doesn't.

 

[Avalon36]

3116 Sooty Transom

When it comes to marine diesels, overpropping is black & white; either you are or you’re not, and even a little is very bad for engine longevity. Cat specs 2800, but they base that on the best case scenario and just assume you’ll follow it to the letter. And if you don’t, they’re happy to sell you a new engine...

Putting two and two together, I realize you posted earlier on the downeast forum. It’s a safe bet that your custom Nauset cruiser is made from the Royal Lowell 35ft. sport cruiser mold that Nauset acquired when Bruno Stillman went out of business. They made a number of 35’s but if I’m correct, yours @ 22,000# might be the heaviest I’ve heard of.

That is a lot of boat for 300hp. For comparison, my 15,000# Five Islands 35 (same Royal Lowell hull design) with the same 300hp 3116 gets 11 kts @ 2000.

Getting the fuel rail and injectors synchronized is a good idea. But if you want to truly know if you’re overpropped or not, spend less money and have a pyrometer installed. The EGT’s will tell you whether the engine is operating within acceptable load tolerance.

Thank you everyone for all of your input and insight. There are more variables than I could have ever imagined to ensure that everything is running right and I have learned a lot from all the posts!
I spoke with David Deschamps, the production manager at Nauset who was in built my boat and confirmed that this boat is actually a 36. Nauset bought the 35' mold from Bruno Stillman and added a foot to the transom. It is a heavier boat than the 35, especially when configured for cruising and 22K lbs. (+/-) fully loaded is the normal/expected displacement. They laid up ~30 of these hulls, the majority of which were configured for Lobstering and powered with a 450hp Cummins. The others were built for cruising and powered with the the 300hp 3116 TA. Mine is the only hull finished by Shannon.
We discussed all the data I have collected over the first 100 hours (WOT rpm, cruise rpm, speed, burn rate, reduction gear, prop configuration, sooty transom, etc. and David believes that everything is within the expected design range(s). While he would prefer to see a more "square" prop where the diameter and pitch are closer together (i.e. 20x18 cvs. 22x15) he suggested that before I mess around with the prop, I should first confirm that the engine is operating at 98% (or better) of spec. Confirming that the rail and injectors are properly set/sync'd, boost pressure is correct, engine oil is corrected etc. will establish a baseline to work from. Until I have a solid baseline, any other adjustment will be a crap shoot.
One other consideration he mentioned was that the exhaust on this boat is on the center-line and creates the station wagon effect of sucking exhaust back to the transom and the cockpit. While I may not see black smoke (which I don't), there are particulates in the exhaust. He suggested adding an extension to the exhaust under the swim platform or even adding an external elbow and extension to get the exhaust port over to the port or starboard side of the boat to help mitigate this effect.
That is not to say that an extension (of any kind) is the end all/be all solution. I still need to ensure that the engine is operating properly. Once I am confident in that, I can start looking at the prop to see what, if any adjustments need to be made to bring everything up to spec.

 

[Riverguy]

Ok…let’s take a step back here.



The statement “overpropped is overloaded across the entire powerband” needs to be re-written as “overpropped at W.O.T. means the engine is potentially overloaded at any RPM.”


To explain, we first need to dispel a huge myth about the “Propeller Demand Curve”.



Expert analysis by Don MacPherson of Hydrocomp is here:
https://www.boatdesign.net/threads/prop-power-curve.9726/

MacPherson’s supporting white paper is here:


http://hydrocompinc.com/wp-content/uploads/documents/PerfWithECEngines.pdf


Summary: The “Propeller Demand Curve” you see in engine manufacturer’s docs is a fiction. It is completely useless when discussing propeller selection. PDC a very rough, theoretical approximation of horsepower required to maintain steady-state RPMs in perfect conditions (calm seas, no headwind, clean bottom, etc.), or more accurately, in a laboratory dyno-tank. As Mr. McPherson states, PDC’s are >>only<< useful for estimating GPH at RPM. They serve no other useful purpose because this curve does not exist anywhere in the real world.



The large gap (noted by twistedtree) between the max power curve and the theoretical, steady-state demand curve that exists at lower RPMs is absolutely necessary! The size of this gap >>must<< be maintained. This is because the instant you try to accelerate, you exit ‘steady state’ mode and propeller demand will quickly approach max power. Without that extra horsepower, a semi-displacement hull (like the Shannon 36 in this example) will never climb out of the hole it is digging as it tries to get up on its ‘semi-plane’. This will be exacerbated in rough seas, headwinds and heavily loaded conditions, or with a fouled hull or prop. If you have ever tried to out-run a following sea in a boat with flo-scans and watched GPH increase wildly as you climb the waves and drop quickly going down the waves, you have seen the fallacy of the ‘propeller demand curve’.


So, any over-propping will reduce the amount of extra HP available to deal with the above dynamic and real world situations. Even a modest amount of overpropping virtually guarantees that (in the real world) you will be frequently overloading your engine at low or mid-range RPMs. How often you will be overloading your engine depends on the typical conditions you are running in.


Finally…hull design does matter. A true full-displacement hull, one that never needs to climb out of a hole because it is impossible to do so anyway, will present a load that looks more like (but will never match) the theoretical propeller demand curve. As Mr. MacPherson notes, PDC’s are somewhat useful for “…slow speed displacement hulls with conventional propellers” but are “completely unsuitable” for anything else. So, one could argue (as I do) that a full-displacement hull can tolerate some overpropping because it’s real-world demand curve looks more like steady-state. That is, until you are running headlong into gale force winds...


In the end, I don’t understand the widely held view that ‘some overpropping is ok’ or worse, even desirable. Overpropping never meaningfully reduces fuel consumption. Overpropping always increases the load on the engine, which in turn increases wear, decreases oil life, increases exhaust temperature, maintenance costs and the chances of early-life engine failure. Overpropping always robs you of additional mid-range power you might need in an emergency. Over-propping is also the most common cause of ‘sooting’.


So, if one accepts that real-world 'propeller demand' is a dynamic number, and that the additional 'headroom' needed above the PDC is critical, especially at low RPMs, then an overpropped engine can be overloaded at any RPM, and murphy's law suggests it will happen at the worst possible time.

 

[Riverguy]

Perfect example...

One of my boats I kept for 21 years had twin 3116 (300 hp). A friend who helped develop the 3116 for Cat always told me they were designed for 300 hp and I never had a problem other than aftermarket exhaust which was fixed. I always use the Cat fixed weight oil. They smoked very little but only when towing a 25 ft open fishermen.

Re: "smoked...but only when towing". Your experience illustrates how a small additional load (even at low RPMs) can overload the engine and cause smoking and sooting. Do you recall if you may also have been slightly overpropped? If the boat was correctly propped, you should have had (between the two engines) an extra 100hp or so to use before getting into the overload zone. This would have been more than enough to tow a 25' fishing boat at any reasonable speed.

 

[Delfin]

Summary: The “Propeller Demand Curve” you see in engine manufacturer’s docs is a fiction. It is completely useless when discussing propeller selection.

That is actually your conclusion, since the expert you reference makes no such statement.

As Mr. McPherson states, PDC’s are >>only<< useful for estimating GPH at RPM.

Bingo. In other words, if the GPH at a given RPM is below the max power curve, as TT posted, then the engine is not being overloaded because it is consuming no more than the maximum amount of fuel to produce no more than the maximum power.

The large gap (noted by twistedtree) between the max power curve and the theoretical, steady-state demand curve that exists at lower RPMs is absolutely necessary! The size of this gap >>must<< be maintained.

Well, no, that's silly. What is necessary is to keep the power demand below max power. Going up and down within that range is simply the real world of a boat on a non flat ocean and this increasing/decreasing power demand will have zero adverse effect on a diesel engine. As long as you are within that range, you aren't, by definition, overloading the engine.

If you have ever tried to out-run a following sea in a boat with flo-scans and watched GPH increase wildly as you climb the waves and drop quickly going down the waves, you have seen the fallacy of the ‘propeller demand curve’.

There is no fallacy. The prop power curve is what you said it was - a representation of the power consumed in a flat sea to propel the boat at a given rpm. It would be a fallacy if it didn't do that, but it does. And the fact that an accelerating boat will need more power hardly makes the measure of the power it consumes without acceleration a "fallacy".

What the expert does say is that the prop curve doesn't apply for non displacement hulls during the frequent transition from one power demand to another, no more, no less, and I doubt this is a revelation to anyone. He says absolutely nothing about prop pitch, over-propping, so other than demonstrating the obvious, I'm not sure how this expert opinion on how to read a prop curve supports your contention that ANY excess pitch results in harmful over-loading of an engine, regardless of the boat it is in. What it does support is that non displacement hulls are going to operate within a range of power demand somewhere between the minimum of the prop curve and what current conditions require. Yawn. But that tells us nothing about whether the soot on the OP's boat is caused by over-propping; something the builder of the boat puts somewhere down the list of the possible answers.

 

[FF]

"But that tells us nothing about whether the soot on the OP's boat is caused by over-propping; something the builder of the boat puts somewhere down the list of the possible answers."

If one examines the theoretical prop curve it cam be seen that a slight 10% reduction in RPM will lessen the prop load a good deal.

So the simple answer to the OP might be to operate at full throttle , see if there is black smoke from overload and if not , pull back 10% or more to cruise..

Then you would be out of the overload zone, so any smoke would not be from prop overloading.

As the OP runs 600-800 rpm below max RPM , an overload caused by the prop is not possible. The transom smoke is engine related.

Weather the engine is in the bet BMEP zone for max efficiency would be a totally different question.

 

[sunchaser]

Back to the OP's question, he has three issues that confuse the story. First, an engine rebuilder that did unknown, to us anyway, work on the engine. Second, incorrect oil, third the vessel is propped such that it can't reach full RPM +

A Cat rebuilt engine carries with it a long list of specs specific to the rebuild whether re-using old parts or adding new. The new parts will have numbers that verify the engine CPL, serial number and parts all match. These can be rechecked.

The oil can be changed easily enough as can the props. The tough nut in this discussion is what did the rebuild consist of and was it done to spec. Many will say the 3116 is a throw away and can't be "completely" rebuilt, including Cat.
Hopefully the OP will chime back in once the boat is launched and issues dealt with.

As an aside, my transom shows zero soot, really. Our vessel is not uncommon in this regard. It is possible.

 

[Avalon36]

Thanks, Sunchaser! Your summary is spot on.
The engine was rebuilt by Jacobs Diesel https://www.jacobsdiesel.com/. Per the website: "The engines are delivered with new pistons, reconditioned connecting rods, new main bearings, new cam bearings, new rod bearings, reconditioned cylinder block, cylinder head (with new valves and injector cups), crankshaft, injectors, and turbo. All gaskets and seals are new. The engines are delivered with a one (1) year unlimited miles/hours parts warranty."
I am aware of two other 3116s that was rebuilt by Jacobs in the area. There were some relatively minor issues after install but my understanding is that the engines are performing well.
There is typically some white exhaust carbon deposits on the water at start up, which quickly dissipate and it sounds like one cylinder is "hitting a bit hard" (Cat mechanic's term) but it smooths out as engine gets up to temp. It runs up to 3200 rpm with no load. Burn rate is in line with specs. When running at idle and cruising RPM, there is no visible exhaust.
Based on this discussion, I am somewhat concerned that the engine has never gotten above 2800 RPM under load and generally runs at 2750 at WOT, which I only do for short periods of time. Typical cruise RPM is 2000 (+/-).
My game plan (today) is to do the following in the Spring:
* Change the Oil to CAT SAEO 30w
* Add an exhaust extension under the swim platform.
* Pull and inspect the aftercooler
* Inspect the turbo for carbon buildup
* Run the rack and sync the injectors
* Evaluate prop pitch once I have a confirmed engine operating baseline

My question for everyone is: Does that sound like a reasonable and logical plan?

I can't thank everyone enough for their input and comments. The discussion on prop curve and over/under propped has been incredibly helpful and educational.

Happy Holidays!

 

[twistedtree]

"But that tells us nothing about whether the soot on the OP's boat is caused by over-propping; something the builder of the boat puts somewhere down the list of the possible answers."

If one examines the theoretical prop curve it cam be seen that a slight 10% reduction in RPM will lessen the prop load a good deal.

So the simple answer to the OP might be to operate at full throttle , see if there is black smoke from overload and if not , pull back 10% or more to cruise..

Then you would be out of the overload zone, so any smoke would not be from prop overloading.

As the OP runs 600-800 rpm below max RPM , an overload caused by the prop is not possible. The transom smoke is engine related.

Weather the engine is in the bet BMEP zone for max efficiency would be a totally different question.

[emoji106]

He’s cruising well below full power. So soot is either normal for that engine, or something with the engine. Since not using the special Cat oil is supposedly a source of soot, that seems like a really good place to start, along with verifying boost pressure and confirming no intake air restrictions.

 

[Riverguy]

Cubic Demand Curve...

Quote:
Originally Posted by Riverguy
Summary: The “Propeller Demand Curve” you see in engine manufacturer’s docs is a fiction. It is completely useless when discussing propeller selection.

That is actually your conclusion, since the expert you reference makes no such statement.


He certainly did say exactly that, and more. Specifically, he said "the prop curve is a completely fictional, idealized curve that does not represent any particular boat, but is intended to generically represent all boats."


He also said that the prop curve is not useful for >>any<< purpose except for estimating GPH at RPM. So, being that the prop curve is (a) 'completely fictional' from the start, and (b) it is only useful for estimating GPH at RPM, the obvious inference is that it is useless for determining the load that any given combination of boat/propeller/transmission/load and sea conditions will present to the engine.



Finally, he also said (correctly) that this ficticious curve should not even be referred to as a "propeller demand curve", exactly because of the confusion it creates.


Quoting: "I have been trying to get engine manufacturers to change their naming of this "prop curve" for almost 15 years - and I've had some success with the companies that we work with. This curve is a simple function of RPM-cubed (a pump law relationship), and is found from a dyno test in the plant. Some companies are now calling it what is really is, a "cubic demand curve". It has very little real usefulness, with one exception - it does give a pretty close measure of fuel consumption for a particular operating power."

Re: "...contention that ANY excess pitch results in harmful over-loading of an engine, regardless of the boat it is in."


I didn't say that. In fact, in the case of a true full-displacement hull, I said exactly the opposite. I'll leave it to you and others to go back and read what I actually did say.

I would ask that, before continuing to ridicule or characterize other opinions as 'silly', please read a little more carefully. In fact, I hope you will refrain from ridiculing other viewpoints altogether. Reasonable minds can disagree without being disagreeable.

 

[Cigatoo]

For comparison purposes let’s say that 3116 was in my Mack Dump truck loaded with 40,000 lbs of gravel. If I run along at 2000 RPM in 1st gear or 4th gear when is there more load on that engine? Obviously 4th. If you down shift from 4th to 3rd and so on you progressively lower the engine load. Think of the prop as a single speed transmission. Your only option to reduce load is to change the pitch on the prop.

 

[Delfin]

Summary: The “Propeller Demand Curve” you see in engine manufacturer’s docs is a fiction. It is completely useless when discussing propeller selection.

That is actually your conclusion, since the expert you reference makes no such statement.

He certainly did say exactly that, and more. Specifically, he said "the prop curve is a completely fictional, idealized curve that does not represent any particular boat, but is intended to generically represent all boats."

Since he makes no mention of propping whatsoever in his commentary, taking his thoughts on how to read a prop curve and what it means as support for your position on propping is an unwarranted stretch. Bit like me quoting relativity theory in support of mandatory spaying of cats. The man says zero about overloading, the effect of prop pitch on over loading, etc.

"...contention that ANY excess pitch results in harmful over-loading of an engine, regardless of the boat it is in."

I didn't say that. In fact, in the case of a true full-displacement hull, I said exactly the opposite. I'll leave it to you and others to go back and read what I actually did say.

Of course you said it. Right here:

Even a modest amount of overpropping virtually guarantees that (in the real world) you will be frequently overloading your engine at low or mid-range RPMs.

You go on to say that this blanket statement doesn't apply to displacement hulls until they are dealing with "gale force winds", which is also wrong, but in any case doesn't unsay what you have repeated more than once - over-propping by any amount is a really big problem. Which it is not.

I would ask that, before continuing to ridicule or characterize other opinions as 'silly', please read a little more carefully. In fact, I hope you will refrain from ridiculing other viewpoints altogether. Reasonable minds can disagree without being disagreeable.

When you make categorical statements that are unsupported by physics; when you quote someone in support of your categorical statements who doesn't actually support what you say; and when you deny writing what you clearly wrote, you have to expect to be challenged, and even to have some of your emphatic conclusions characterized as silly.

 

[Ski in NC]

To the OP- Your list seems reasonable, but add to it to check charge air pressure (AKA turbo boost) at intake manifold ports. Make sure it is in spec for WOT. Also record the number for your cruise rpm. Low boost will increase soot when under load.

Note boost will be a psi or two low because you are not quite at 2800, that is normal.

Also check rpm via phototach, can't remember if you already covered that.

 

[Riverguy]

Until I see data showing otherwise, the resources I trust say overpropped is overloaded throughout the entire poweband.

Also, two 3116 owners on BoatDiesel have documented that their EGT’s are higher at 2200 than at 2400: the load on their engines is higher at the lower rpm. Operating below rated cruise won’t make up for being overpropped.

Re: "EGTs are higher at 2,200 than 2,400". Yes, this is very common when running a semi-displacement (or planing) hull near it's transition speed, and for most such hulls, this means 2,000-2,200 RPM. For both of my diesel powered boats (one semi-displacement, one planing) this is the speed where running angle is highest and my wake is biggest. This is also where I occasionally see some soot in the exhaust. Once I either get fully up on plane, or back down throttles till running angle drops, I never see any soot.


Semi-displacement and planing hulls present their highest loads to the engine when they are 'plowing water'. This is also the speed at which overpropping is likely to do the most damage to an engine. Not many of us run at W.O.T., but every weekend on the ICW I see boats with bows pointing skyward, plowing water, making huge wakes and (frequently) leaving a smoke trail that looks like this:

https://www.sbmar.com/articles/understanding-low-power-troubleshooting/



...thanks to Tony Athens (SBMAR) and James Hamilton (MV Dirona)

 

[Riverguy]

Add one to the list?

...
My game plan (today) is to do the following in the Spring:
* Change the Oil to CAT SAEO 30w
* Add an exhaust extension under the swim platform.
* Pull and inspect the aftercooler
* Inspect the turbo for carbon buildup
* Run the rack and sync the injectors
* Evaluate prop pitch once I have a confirmed engine operating baseline

My question for everyone is: Does that sound like a reasonable and logical plan?

I can't thank everyone enough for their input and comments. The discussion on prop curve and over/under propped has been incredibly helpful and educational.

Happy Holidays!

Avalon, I don't think you are at risk of ruining your new engine in the short term, so probably do the cheapest things first and see what happens.

That said, the first thing I would do (and probably the cheapest) is to install an EGT sender in your exhaust and put a gauge at the helm. This way you will be seeing any engine overloading in real-time, as it happens.



With EGT monitor you can immediately see the impact of every subsequent change you make.



And...since you will be doing an oil-change anyway, maybe get an oil analysis -- also very cheap. One of the compounds they look for is 'soot' in the oil. If your engine is producing soot because of overloading, then your blowby gasses are going to be putting that same soot into your crankase. This will show up in the oil analysis.


We'll be standing by waiting for your report on what works and what doesn't!

 

[Cigatoo]

An EGT gauge will tell the story. Simple as that. Get readings before and after re-pitching and you will see.

 

[Delfin]

For comparison purposes let’s say that 3116 was in my Mack Dump truck loaded with 40,000 lbs of gravel. If I run along at 2000 RPM in 1st gear or 4th gear when is there more load on that engine? Obviously 4th. If you down shift from 4th to 3rd and so on you progressively lower the engine load. Think of the prop as a single speed transmission. Your only option to reduce load is to change the pitch on the prop.

Perhaps a better comparison is a constant speed prop on a plane. When I would take off in mine, I'd firewall the throttle, and flatten the prop. This gives you max power to get off the ground in the shortest runway - low gear, so to speak. However, as soon as you are off the ground, the prop pitch is increased - shifting to a higher gear - with the result that with the same throttle setting, rpms go down. At that point, one could ask if the propeller was over-pitched, which it would be for the 30 seconds of take off, but certainly not for the next 6 hours of flight time. For modest rates of climb or maintaining altitude it is ideal and you know it's ideal because you are watching the EGT per cylinder, which climbs as you flatten the prop showing an increase in engine loading, but still within the envelop of safe conditions for the engine. Saying that a boat is "over-pitched" as an absolute and that it is always harmful would be like saying that an airplane with a constant speed prop in cruise configuration with the propeller pitch increased is doing harm to the engine, which is certainly is not.

 

[Delfin]

An EGT gauge will tell the story. Simple as that. Get readings before and after re-pitching and you will see.

If the prop is flattened, my guess is he will see a drop in EGT at WOT but zero difference as lesser power settings, which was was my experience.

 

[Cigatoo]

If the prop is flattened, my guess is he will see a drop in EGT at WOT but zero difference as lesser power settings, which was was my experience.

Wasn’t my experience. I ran a Yanmar propped to 3300 WOT loaded. Ran at 2800 RPM (cruise speed) EGT was running about 900 at cruise.
Yanmar came out with a bulitan and wanted to see these engines turning 3400 plus WOT loaded. I took 2 inches out of the pitch to get me to 3450 plus WOT. EGT dropped to 700 at 2800 cruise. And of course I lost a couple of knots of speed at 2800 but could cruise to 3200 if I wanted. At 3200 EGT was still below the original 900. EGT ran much cooler through the whole RPM spectrum.

Keep in mind that we are not talking about a full displacement hull.

EGT on a spread sheet with associated RPM and speed through the water is your diesels best friend.

 

[Delfin]

Wasn’t my experience. I ran a Yanmar propped to 3300 WOT loaded. Ran at 2800 RPM (cruise speed) EGT was running about 900 at cruise.
Yanmar came out with a bulitan and wanted to see these engines turning 3400 plus WOT loaded. I took 2 inches out of the pitch to get me to 3450 plus WOT. EGT dropped to 700 at 2800 cruise. And of course I lost a couple of knots of speed at 2800 but could cruise to 3200 if I wanted. At 3200 EGT was still below the original 900. EGT ran much cooler through the whole RPM spectrum.

Keep in mind that we are not talking about a full displacement hull.

EGT on a spread sheet with associated RPM and speed through the water is your diesels best friend.

Does Yanmar advise on what EGT to rpm values you should see?