Archhibb
Veteran Member
That could be so. It’s definitely true light loading on diesels can coke them up.
"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.
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, 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...
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.
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.
That is actually your conclusion, since the expert you reference makes no such statement.Summary: The “Propeller Demand Curve” you see in engine manufacturer’s docs is a fiction. It is completely useless when discussing propeller selection.
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.As Mr. McPherson states, PDC’s are >>only<< useful for estimating GPH at RPM.
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.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.
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".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’.
"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.
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.
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.
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.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."
"...contention that ANY excess pitch results in harmful over-loading of an engine, regardless of the boat it is in."
Of course you said it. Right here: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.
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.
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.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.
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.
...
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!
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.
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.
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.