Engine RPM

I think most newbies will follow the manufacturers recommendations...and it's good that they do.

As people age and gain knowledge and wisdom....they stray from many of their "early on" ways.

No one expects a newbie to gather enough info to believe what they read in a forum like this, especially with tens of thousands of dollars worth of engines...but that doesn't mean at some point, some people can't gather enough to surpass manufacturer generic guidelines.

Obviously NASCAR teams (if that's too extreme than the local dirt track/drag guy) can do more than what is suggested for the "daily driver.

It appears our conversation on this matter has improved considerably from about 5 years ago.

This is an interesting thread.
Engine manufacturers are interested in the following:
1/ selling engines
2/ having as few of those engines come back under warranty as possible
3/ spending the least amount of resources to make and sell the engine

For a given hull each engine could be adjusted to run a range of propellers with changes to injectors, governor settings, and compression ratios. Adjusting the engine to the requirements of the prop hull combination.
No engine manufacturer wishes to spend the time to do this. No money in it.
So they build an engine and they call it x hp. Every engine they make of x hp in that model is built and set exactly the same. And as a result you change the propeller to match the hull engine combination.
The engine manufacturer knows that most times the guy running the engine is not going to take the time to "drive" the engine watching all the gauges, checking and mapping EGT and boost (if turbo equipped) constantly watching oil temperature and pressure water temperature etc.
They also know that if the prop puts a light enough load on the engine that you can make WOT produce the max RPM the governor is set to you are not pushing the engine to or over the edge of the envelope. That there is no way you can overload or overwork the engine even with the least aware or inexperienced operator. No overload, no overwork, no warranty claims. Happy engine manufacturer and healthy bottom line.

BryanF said:

This is an interesting thread.
Engine manufacturers are interested in the following:
1/ selling engines
2/ having as few of those engines come back under warranty as possible
3/ spending the least amount of resources to make and sell the engine

For a given hull each engine could be adjusted to run a range of propellers with changes to injectors, governor settings, and compression ratios. Adjusting the engine to the requirements of the prop hull combination.
No engine manufacturer wishes to spend the time to do this. No money in it.
So they build an engine and they call it x hp. Every engine they make of x hp in that model is built and set exactly the same. And as a result you change the propeller to match the hull engine combination.
The engine manufacturer knows that most times the guy running the engine is not going to take the time to "drive" the engine watching all the gauges, checking and mapping EGT and boost (if turbo equipped) constantly watching oil temperature and pressure water temperature etc.
They also know that if the prop puts a light enough load on the engine that you can make WOT produce the max RPM the governor is set to you are not pushing the engine to or over the edge of the envelope. That there is no way you can overload or overwork the engine even with the least aware or inexperienced operator. No overload, no overwork, no warranty claims. Happy engine manufacturer and healthy bottom line.

Click to expand...

Regarding the last sentence I wholly agree and add a happy bottom line for the engine manufacturer in this case coincides with the best interest of the average boat owner who will be living with that motor long after the warranty expires with the full weight of the potential expense of failure on the boat owners shoulders. I also think that the newer high output diesel engines are much more prone to overload damage than older heavy iron. People who have lived with older heavy iron that was over propped and got away with it may well be surprised at how different the newer motors are in this regard. A look at the HP output per displacement is a good rough indicator of the level of sensitivity to overloading. Three of the most common causes of early engine demise quoted frequently are overloading-bad exhaust geometry-poor maintenance.

I think BrianF pretty well summed it up and agree some engines have performance characteristics that probably shouldn't be overpropped.

Like many discussions here on TF, there are so many "maybes" that absolute comments are going to stir up disagreements.

And usually most people aren't right or wrong unless you dig in and try to defend an absolute comment without being much more specific.

With but rare exception, diesel engines are not designed for the marine environment. Probably 99.999% of them are destined for non marine uses where over fueling (similar to over propping) is not only a ticket to shorter engine life but illegal. Over fueling results in all sorts of bad things such as burned valves, scorched pistons, overheating, head cracking and on and on. Sound familiar?

Worldwide environmental regulations require certain emissions to be achieved without the engine cratering within a very short span. Not only do emissions have to be legally met (yes fines and factory model line shutdowns have occurred over this) when the diesel leaves the factory but for many hours thereafter. So diesel over fueling in the real world leads to very bad outcomes. In a very gross sense bye bye DD 2 strokes, Deutz air cooled and other non compliant designs.

In Ed's case, JD has a engineering, business and legal responsibility to insure the engine is performing up to design criteria. It has nothing to do with Brian F's notion that a conservative design will guarantee no hassle back to JD. But rather than a conspiracy, this is the way diesel engine business is done in 21st century. Whether Cummins, JD, Cat or MTU the highest HP, smallest, lightest and longest lasting warranty is what industry demands - and this is a very daunting task.

For over 4 decades I've been involved with all sizes of diesels with 4000 HP the largest. Never once did I hear from any of the major manufacturers that we want you to baby your engine or we'll revoke your warranty. Quite the contrary. the engines are designed for a maximum fueling at a desired RPM and the builder will guarantee they operate at that criteria for X hours without undue incident.

In the non marine world there are caveats such as use specified oils, coolants, filters, CPU service and check intervals, critical part rebuilds (think engine hang ons) , EGT and engine history tracking and on and on. Do this and you will be granted a very long warranty, in some cases guaranteed opex and full factory support if anything goes wrong.

It is not uncommon for a base block and innards to be warrantied for 15,000 or more hours. This may sound like a lot to us pleasure boaters but may be no more than 3 years in an industrial environment. Pleasure boat diesel warranties are hard to come by because we knowingly severely abuse our engines and even try to talk others into it. Ever seen the question, is it necessary that I change my oil, filters, coolant, impellers, heat exchangers (pick one) after only X years of use?

So say JD is wanting Ed to baby his engine by setting it up to achieve the correct fuel burn without overloading it all you want, this is the real world or diesels and how it is done. Now we are clamoring for small, light, efficient and properly fuel mapped (not over loaded) diesels for our cars. Do you think the car builder will allow you to maintain warranty while you aftermarket change out CPUs or shift points? No, and why not?

Much better summary Tom.

But I think propping so one can't overload is very important for the trawler owner. Rated rpm .. or 100 over is just about all one needs to know.

Skidgear: While there could be a terminology issue here Operating on semiplane would worry me as that is exactly where the load is high and airflow low. IMO well below plane or on full plane, semi plane is to be avoided.

As another side line to this issue of over propping I submit the following thoughts directed at those who support the practice. The reason usually given for over propping is greater efficiency and this supposedly comes from higher loading with larger props at lower rpm producing the higher heat needed for efficient fuel burn. I ask the question are the specific engines intended for this deviation of use engineered so that the cooling system and the lube system can handle the heat at the lower rpms? The engines if propped according to spec. are meant to handle that kind of heat level at higher rpms . Do the people who want to tamper with this know for a fact that there is ample reserve in the lube and coolant systems to handle this issue? I have come across the statement many times that a engine over propped is over loaded all through the power curve, and the thoughts I present above may be where that issue plays out.

I think you missed my point.
Why does JD do a sea trial? To confirm that the engine will reach specified max RPM at WOT. Why? Because that tells them that the engine will be able to operate within the designed envelope. In other words too specification. Why? Warranty. To the end user. To the EPA. JD warrants that their engine will meet the criteria that they say it will in terms of emissions performance long life what ever. It is all warranty related. If it could not come back to bite them they would not care. In the end it is about preventing come backs- from what ever source. Could the engine in question be set up to run inside the emission /hp requirements with a different prop? Say it was "over propped" and could not reach full RPM. The answer is that yes the engine could be adjusted to meet the requirements and turn a different prop at different RPM and fuel delivery rates but every 6068 JD built would be different.
Now they do,in fact, do that - use one engine at different ratings in different pieces of equipment.
But it is not economically sensible to do that one engine at a time . There are only 3 variables. A given hull, a given engine and a given prop. Which one makes the most sense to change?
Can all of them be changed. Of course.

bayview said:

Skidgear: While there could be a terminology issue here Operating on semiplane would worry me as that is exactly where the load is high and airflow low. IMO well below plane or on full plane, semi plane is to be avoided.

Click to expand...

Yes, it's a terminology thing. The hull is semi-displacement, so I use the word semi-plane. There is a definable "hump", but the hull never achieves a classic plane, even at full power...even with very big engines. It lifts to reduce the wetted surface, but it won't get up and sit on top. The deep trawler type forefoot is always in the water. Maybe with a couple thousand horsepower...

Back to original post by IMG. Those 3208 engines have probably got some years on them

BryanF said:

I think you missed my point.
Why does JD do a sea trial? To confirm that the engine will reach specified max RPM at WOT. Why? Because that tells them that the engine will be able to operate within the designed envelope. In other words too specification. Why? Warranty. To the end user. To the EPA. JD warrants that their engine will meet the criteria that they say it will in terms of emissions performance long life what ever. It is all warranty related. If it could not come back to bite them they would not care. In the end it is about preventing come backs- from what ever source. Could the engine in question be set up to run inside the emission /hp requirements with a different prop? Say it was "over propped" and could not reach full RPM. The answer is that yes the engine could be adjusted to meet the requirements and turn a different prop at different RPM and fuel delivery rates but every 6068 JD built would be different.
Now they do,in fact, do that - use one engine at different ratings in different pieces of equipment.
But it is not economically sensible to do that one engine at a time . There are only 3 variables. A given hull, a given engine and a given prop. Which one makes the most sense to change?
Can all of them be changed. Of course.

Click to expand...

Absolutely spot on. And to your earlier post, the current practice of propping to maximum power does indeed result in the engine having a very easy time of it in the lower parts of the operating envelope. Very convenient for the engine OEM in terms of warranty.


By the way, aircraft engines are routinely derated (with a red line on the pertinent gage), FAA certified, and warrantied by the engine manufacturer.

With that in mind, the reasons for not reaching rated RPM could be many. If the prop is clean and hasn't hit anything, i would be looking for engine issues. The basics all mentioned above, are cheap. Internal stuff not so cheap. Nozzles for those engines are fairly reasonable price wise, and not a lot of trouble to change. There are adjustments in the pump to increase fuel flow, frowned on by CAT. there is normally a sealed safety wire on the cover to keep you from tinkering in there. No way to tell on these older engines what flow is w/o hooking up guages. They are probably just getting tired. I worked around a bunch of trailers that were powered with 3208's, and their life span was short. These were not turboed. Good luck, hope it is just barnacles!

BryanF said:

It is all warranty related. If it could not come back to bite them they would not care. In the end it is about preventing come backs- from what ever source. .

Click to expand...

Nope, it is about making a product that fulfills if not surpasses requirements and expectations so the business and company can thrive and earn profits. Preventing come-backs does nothing for generating future business if the product is not reliable and price competitive in terms of initial purchase and operating costs.

Cat, Cummins, JD and a few other smaller size diesel manufacturers have long established track records by being far more than preventers of comebacks.

eyschulman wrote;
"I have come across the statement many times that a engine over propped is over loaded all through the power curve"
Many times?? Hard to believe very many people believe that. Even an overpropped trawler is only overloaded at the upper end of the power curve or rpm range. If you're over propped 300rpm on an FL I can't imagine being overloaded at 1600. If I've got this wrong please enlighten me.

manyboats said:

eyschulman wrote;
"I have come across the statement many times that a engine over propped is over loaded all through the power curve"
Many times?? Hard to believe very many people believe that. Even an overpropped trawler is only overloaded at the upper end of the power curve or rpm range. If you're over propped 300rpm on an FL I can't imagine being overloaded at 1600. If I've got this wrong please enlighten me.

Click to expand...

Look at my previous post. If the motor is over propped it will be generating more heat at each rpm than it would by intended design. Now the question is does the motor which was designed for dealing with that higher heat level at higher rpms where a turbo kicks in and where the lube pump and the coolant pumps work harder to do their jobs also be able to do a good job at the lower rpm with more heat than is expected by the original design? Therefor a over propped motor is challenged by overloading at multiple rpms not just WOT. That is my best understanding and explanation. Obviously some engines have the reserve to handle the problem and some may not. Maybe another way to look at it is that all motors put into gear are loaded and there is a designed load curve for that motor. When you challenge that motor to do more work anywhere along its curve than intended it is overloaded. Thus if you over prop at every rpm you are overloading relative to the design load curve no mater 1600 rpm or 3000 rpm. That more damage would likely result at the higher rpm is intuitive but that does not mean that over a long term the lesser effect of overloading at 1600 rpm will not be there. That is my best understanding of the situation and I am not a expert in this field.

A friend has a Kings Craft with 6.354T 135 hp Perkins. The no load high idle is 2700, Kings Craft propped them so that at wot the rpm was 1800, IMHO extremely over propped. The owner was convinced it was propped correctly because that's how the builder manufactured the boat when new. I finally convinced him to give Marcus at TAD a call and to ask what Perkins would recommend for a wot rpm. Marcus told him 2600 rpm at wot. The owner talked with a guy that was Vice President of Kings Craft and when they built a 44' with diesels they propped it to run 22 mph and he said they would do that at approximately 1800 rpm. So it not only owners who believe that over propping isn't a problem but also some builder also. The owner bought another set of props that'll get installed next spring.

eyschulman said:

Look at my previous post. If the motor is over propped it will be generating more heat at each rpm than it would by intended design. Now the question is does the motor which was designed for dealing with that higher heat level at higher rpms where a turbo kicks in and where the lube pump and the coolant pumps work harder to do their jobs also be able to do a good job at the lower rpm with more heat than is expected by the original design? Therefor a over propped motor is challenged by overloading at multiple rpms not just WOT. That is my best understanding and explanation. Obviously some engines have the reserve to handle the problem and some may not. Maybe another way to look at it is that all motors put into gear are loaded and there is a designed load curve for that motor. When you challenge that motor to do more work anywhere along its curve than intended it is overloaded. Thus if you over prop at every rpm you are overloading relative to the design load curve no mater 1600 rpm or 3000 rpm. That more damage would likely result at the higher rpm is intuitive but that does not mean that over a long term the lesser effect of overloading at 1600 rpm will not be there. That is my best understanding of the situation and I am not a expert in this field.

Click to expand...

The manufacturer establishes the maximum rated power curve (your "design load curve") for the engine with the marine gear installed. The maximum rated power curve is far above the typical prop curve...100% cushion for most of the rpm range for our engines, and probably yours. That means that for most of the operating range the cooling system was adequate at twice the power demanded from the prop. That's a huge buffer. You need to look at a prop curve and an engine max rated power curve. This becomes very clear when you compare the two.
There are some in the archives.

Also, it's best to think in terms of overall hull performance, as the hull reacts to prop thrust, not horsepower. A bigger prop should produce a given thrust (hull speed) more efficiently than the smaller prop. That efficiency gain feeds back to the engine in terms of reduced power for a given thrust. Reduced power/fuel flow equals less heat on the whole.

skidgear said:

Also, it's best to think in terms of overall hull performance, as the hull reacts to prop thrust, not horsepower. A bigger prop should produce a given thrust (hull speed) more efficiently than the smaller prop. That efficiency gain feeds back to the engine in terms of reduced power for a given thrust. Reduced power/fuel flow equals less heat on the whole.

Click to expand...

So at what prop size increase does one finally say "big enough" if engine loading/HP/fuel burn is no longer used as a criteria for judging efficiency?

And what current reputable manufacturers of boats and diesel engines subscribe to this new concept?

I have to say I have been reading about this stuff since 2007 when I lost an engine with 275 hours. I went back east to Mack Boring for several days ripping apart the same version of the engine I lost and questioned the resident experts. I corresponded with many other engine owners who lost engines with low hours. I corresponded with mechanics who where responsible for these motors. I have been an avid reader of the posts on Boatdiesel.com and articles related to this issue since 2007. One of the things I have learned is that somewhat like religion there are different camps and some strong opposing views regarding propping and overloading, and of course every side seems to be certain of the truth. I am not certain of the truth but I have formed some opinions, but I still look for answers.

manyboats said:

eyschulman wrote;
"I have come across the statement many times that a engine over propped is over loaded all through the power curve"
Many times?? Hard to believe very many people believe that. Even an overpropped trawler is only overloaded at the upper end of the power curve or rpm range. If you're over propped 300rpm on an FL I can't imagine being overloaded at 1600. If I've got this wrong please enlighten me.

Click to expand...

Actually Eric, many people believe that. But not us with trawlers especially of a vintage that are long ago gone in the build cycle. Most of us on TF have go slows that could never reach the hump and start planing. For the most of us with way too big engines where max HP or RPM will never be seen or dreamed of, an over prop situation will do little damage for the first few thousand hours.

As we titillate over boats made 20, 30 or 40 years ago about prop sizes, 90% (or more) of the plus 30' vessels made in the last 20 years are "go fasts" where design RPM is critical or the engine will suffer from early demise as laboriously can be read on boatdiesel threads. As example, NT, AT, Fleming, GB, OA, Horizon, Offshore, Azimut, Searay, Meridian etc - or better said the majority of vessels you see for sale in the back of the boating mags.

When you get into special build boats like Ed's Devlin, MJM or many other down east designs where 80% load at speed is quite normal, prop sizing is done so max design engine RPM and optimum fuel burn can be achieved without the engine overheating. I suspect Ski's special build is the same, but best he comment on that.

For the go slow new build trawlers like Nordhavn where 10,000 + hours on an engine are expected, prop sizing is done to match the engine. Again, for the first few thousand hours a too big prop on a Nordhavn may be attractive to a few new owners, but with the smaller engine they use and operate well up the HP curve over propping will eventually show up as a negative. Especially on a dry stack where high EGTs resulting in elevated ER temps are an issue

On TF - Willy and Coot seem to rightly balanced with props and small engines. They are in the minority. The rest of us lug around our ballast AKA too big an engine because we have them. Some, like mine, are too big by far but at low load and right props will never wear out from use, only time or lackluster maintenance.

I've no issue with what Skidgear and others are saying about the virtues of over size props on our go slows.. But only because on low hour vessels the engine will give out for other reasons before the over loading issue arises.

sunchaser said:

So at what prop size increase does one finally say "big enough" if engine loading/HP/fuel burn is no longer used as a criteria for judging efficiency?

And what current reputable manufacturers of boats and diesel engines subscribe to this new concept?

Click to expand...

I didn't say fuel burn isn't a criteria for judging overall efficiency. It obviously is. I was just pointing out that prop efficiency is a factor in establishing power/fuel burn. So if you want to discuss what happens to the boat when a larger prop is installed, it has to be rolled into the matrix.

Unless one is willing to change gear ratios to take full advantage of a bigger prop, only modest size increases make any sense. And then the discussion of is it worth the expense gets into the mix. I got into this discussion when I said I'd keep the OPs 25x17 props even if they were slightly oversized. Why? Because I'd like to keep a semiplaning speed capability, but am also willing to lop off a couple knots at the high end for some efficiency at low and mid range. Would I go out and drop a couple of grand to buy bigger props....no. But if I came across a good deal, I'd jump at it. By the way, "big enough" for our boat would be dictated by hull clearance and hull speed with the engines at idle.

Current manufacturers aren't going to change their warranty recommendations because as Bryan pointed out earlier, the current propping practice nets that huge cushion and a very lazy life for the engine unless it's operated on the pins all the time. It would be interesting to see what they'd say if a boat company came along with a requirement for variable pitch props...overpropping to the extreme. Would they stand behind their maximum rated power curve when the engine could be operated up against that curve at virtually every rpm from idle to maximum. I suspect not....and there goes your point about engine OEMs encouraging hard use of their products.

As I mentioned in an earlier post, this stuff is commonplace in the propeller airplane and helicopter world where a modifier wants to reengine with a different brand that happens to have more horsepower than the original. Since the original gearbox/transmission was likely approved for the power rating of the original engine, the easy way out (without recertifying the gear train for more power) is to limit the output (typically torque) of the new engine with red lines. Pilots are used to respecting operating limitations and it works fine. Nobody in their right mind would risk damaging the equipment because it's expensive and dangerous. Another example is a twin engine helicopter. Main transmissions are typically sized to handle the reduced power of two engines operating at the same time. In the event of one engine failing, the remaining engine accelerates and its reserve power (still less than the limit for two engine operation) gives a decent engine out capability without exploding the gear train. So, there are two sets of redlines on the torque gages. Some of this is being automated with limiting devices, but pilots don't like them...if they are in extremis, they want the option to pull the guts out of the transmission/remaining engine. Yes, different reasons for limiting engine output, but the same fundamentals. Boat and engine manufacturers stick to the current propping philosophy to protect against the least common denominator operator. That's OK, but it doesn't make overpropping or variable pitch props a mystery. or mortal sin.

sunchaser said:

Actually Eric, many people believe that. But not us with trawlers especially of a vintage that are long ago gone in the build cycle. Most of us on TF have go slows that could never reach the hump and start planing. For the most of us with way too big engines where max HP or RPM will never be seen or dreamed of, an over prop situation will do little damage for the first few thousand hours.

As we titillate over boats made 20, 30 or 40 years ago about prop sizes, 90% (or more) of the plus 30' vessels made in the last 20 years are "go fasts" where design RPM is critical or the engine will suffer from early demise as laboriously can be read on boatdiesel threads. As example, NT, AT, Fleming, GB, OA, Horizon, Offshore, Azimut, Searay, Meridian etc - or better said the majority of vessels you see for sale in the back of the boating mags.

When you get into special build boats like Ed's Devlin, MJM or many other down east designs where 80% load at speed is quite normal, prop sizing is done so max design engine RPM and optimum fuel burn can be achieved without the engine overheating. I suspect Ski's special build is the same, but best he comment on that.

For the go slow new build trawlers like Nordhavn where 10,000 + hours on an engine are expected, prop sizing is done to match the engine. Again, for the first few thousand hours a too big prop on a Nordhavn may be attractive to a few new owners, but with the smaller engine they use and operate well up the HP curve over propping will eventually show up as a negative. Especially on a dry stack where high EGTs resulting in elevated ER temps are an issue

On TF - Willy and Coot seem to rightly balanced with props and small engines. They are in the minority. The rest of us lug around our ballast AKA too big an engine because we have them. Some, like mine, are too big by far but at low load and right props will never wear out from use, only time or lackluster maintenance.

I've no issue with what Skidgear and others are saying about the virtues of over size props on our go slows.. But only because on low hour vessels the engine will give out for other reasons before the over loading issue arises.

Click to expand...

Most of the boats on this forum are not really "go-slows", but semi-displacement hulls that were supplied with power plants that are way over powered, but not overpowered enough to get them over the hump and into a "semi-planning" range . Arguably the most ridiculous design spec of them all. Why did they do it? Because Lehmans were cheap, fuel was cheap, and people wanted twin engine safety. (I also wonder how well behaved those old Taiwan hulls would be if pushed harder toward the 13-15 knot range). Anyway, at least the Grand Banks and OAs and tugs with semi displacement hulls can get up and go. That said, boats like the twin 375 Grand Banks, for example, got a little stupid with huge excess installed power to gain an extra knot or two at the top end. Anyway, even the Lehman/Perkins boats...and particularly the twins...would benefit from a reasonably larger prop (I'm going to stop calling it overprop because of the negative connotations of the term. If done properly, there are only positives).

The engines in hulls you mention that suffer an early demise are typically those where the engine OEM stuffed 50 pounds in a 10 pound bag by turbo charging and intercooling a basic design that was never up to it. Different issue from this.


If I had the dough to purchase a long range cruiser like a Nordhavn, I'd think hard about a variable pitch prop to maximize efficiency.

Your last few paragraphs appear to be at odds with an earlier remark that engine manufacturers encourage hard use of their engines. If the engine is "good to go" anywhere under the manufacturer's maximum rated power available curve, the engine should do just fine with a larger prop (and appropriate limitations). Once again, I'm not speaking of an "overprop" where the operator has been operating in an over fuel condition without knowing it. I'm speaking of a properly thought out size increase with attendant limits.

Tom,
Re "a engine over propped is over loaded all through the power curve" .. I don't believe it. That an over propped boat is over propped throughout it's rpm range. I have no idea how I came to believe what I believe and could probably express it handily w graphics and load curves. Can't present that so I'll need to do it in text.

With a boat that has the correct propeller there is only one engine speed where the engine in loaded to the prop correctly or ideally. WOT. At all engine speeds below rated power rpm the prop presents a load less than ideal. That's why FFs "Cruise Prop" delivers slightly lower fuel burn to move his boat at a given speed. It works (no doubt) but the gain is very small .. I think 5% or less has been posted on the forum at least several times.

1. Starting w a correctly propped boat/engine/prop combination WOT gives a correctly loaded engine. Let's say 2500rpm.

2. Now lets over prop 100rpm. There's still only one engine speed where the engine is loaded correctly .. 2400rpm is now max power and max engine speed. But at WOT and 2400rpm the engine is overloaded. At some point in time as we decrease rpm the engine is loaded correctly. What that rpm is unknown unless it's determined by fuel consumption. As an example let's say it's 2300 (but I think it would be lower). At any engine speed below 2300 the engine would be at less than full load. For example let's say the engine manufacturer lists a safe continuous load for the engine at 10% less than full power and the engine burned 10gph at WOT propped correctly to rated rpm. As one decreases the rpm of the 100rpm over propped engine one will reach a point where it will burn 9gph. Let's say that's 2300rpm. Lets establish an ideal engine load below max continuous power (2300rpm) that will not prematurely wear out the engine and will create enough heat to prevent cold running problems. Lets say 55 to 75% power. To run our engine ideally we'd need to burn 5.5 to 7.5gph. Anything below 5.5gph would be underloaded. We can establish what rpms this best load range is by fuel burn monitoring via objective observations.

3. Now if we over prop the engine another 100rpm (2300rpm at WOT instead of 2400) the maximum continuous power (10% down or 9gph) would be lower of course but as we decreased rpm (as before) the minimum rpm (of the "best load range") also drops ... but nowhere near idle speed. So at the rpm where the engine burns 5.5gph all engine speeds below that point will be underloaded. So even an over propped engine will have much of it's lower rpm range as in an underloaded state. So an over propped engine is not always over propped. Basically because the load curve of the prop and the engine are not straight or the same otherwise. As the engine speed goes down at a linear rate (flat line curve) the prop load curve is very curved so the load at half speed is much less than half load.

I've made some slight errors but but in the interest of post length left them in. I hope my point is there to see.

I include this because I think it's related.
I think the prop load curve for props w less pitch and more diameter is more curved (more convex w the curve down) so the prop load is less at the half way point. The prop load drops off faster than w a prop w more pitch and less dia. But if you are applying enough power a larger dia prop will give more thrust and efficiency. So a trawler could increase dia and decrease pitch gaining speed and thrust at WOT but loosing efficiency at lower speeds .... where most trawlers run.

manyboats said:

Tom,
Re "a engine over propped is over loaded all through the power curve" .. I don't believe it. That an over propped boat is over propped throughout it's rpm range.

Click to expand...

This came up when eyshulman tied engine cooling to the "overload" term. Cooling is unrelated to the power "overload" issue in the first place. Further, application of the term "over prop" to a correctly installed larger prop (cruise prop in FF's terminology) greatly confuses the subject. A correctly installed larger prop in a boat that was originally propped for max rated power is by definition not over propped...nor is it over loaded if operated within defined parameters.

By the way, I've been searching for prop curves for various prop sizes as installed in various sized/shaped hulls for years. The best I can find is comparative data for different engine/prop combinations in a given hull. If you have a source that defines specific propeller performance curves and shapes, I'd appreciate the reference.

I believe the term over propped refers to the condition of placing too much load on a engine. It is not the size of the prop or the pitch or the transmission ratio but a combination of all three that counts. One can increase the size of a prop to gain efficiency provided the room to swing it and at the same time decrease the load on the engine if the pitch is reduced or the transmission ratio changed. What counts is the amount of rotational force(work)the motor has to do to turn the system. I look upon over propping and over loading as any combination of transmission and prop that causes the motor to work harder at a given rpm then the builders specifications call for. The results of this extra load are no doubt variable by motor and degree of abuse. A high output modern light diesel run over propped-overloaded at high rpm may well have a catastrophic event with low hours while a over stressed motor of a lesser degree run at 1600 rpm may not fail at all during its recreational boat life..

When you'all say "larger prop" I assume you mean over propping .. that is running a boat that cannot reach rated rpm. Fred's cruise prop.

Skid,
At this time I have no load or power curves to present that would help or I would have included them in post #55.
If you "correctly" install a "larger" prop on a boat it will attain rated rpm at WOT as if necessary you will change prop pitch or/and diameter or the gear ratio in the Tx.

eyeschulman wrote "over propped refers to the condition of placing too much load on a engine." Kind of .. IMO if a boat is over propped it can't make rated rpm and is over propped while not even running. It's over loaded of course when it is being over fueled but if it was propped correctly it simply could'nt run in that state .. because at any throttle setting the engine load from the prop is not so much that the engine will not be able to turn the prop (and hence the engine) fast enough to burn all the fuel injected.
And again over propping can be done safely if the engine is not run in an overloaded state or without sufficient load to heat the piston, valves, combustion chamber and oil to a high enough temperature to prevent cool running problems. I can't remember what causes cylinder glazing but I think it involves piston side loading and of course engines need to have mechanical forces high enough to prevent this as well. I think cool running problems start to occur at at higher engine loads than glazing so if one runs hard enough to prevent cool running problems glazing will probably not not occur. How high a load one needs to prevent cool running problems is open to much debate but IMO it starts below 50% load. So if your engine can burn more than twice as much fuel as you burn normally you could be under loading .. IMO.
Remember the PMM tech editor said that 75% load or greater needs to be applied usually while cruising to attain safe operating temps. As you can see my personal red line is far below that. Very few skippers normally run at or over 75% load. It's probably just 2 or 300rpm below max rpm on most to many engines. Think of a FL120 at 2200rpm. And of course coolant temps should not be considered re loading issues because coolant is artificially controlled by the thermostat. Oil temp of about 175 degree should be a good benchmark.

Coolant temp can remain normal while there are significant hot spots in engine and is not a good indicator of over loading. An exhaust pyrometer is the better way to evaluate how hard your motor is working. Heat is related to the work level demanded. When a motor is over propped it causes greater work and heat at any rpm than the designers planed for at that rpm. There is a safety cushion built in but it is there for a reason. The cooling Sx and lube system become very much involved because the pumps and distribution are tied to rpm. If one tampers with the load so it is greater along with the linked increase of heat at lower rpms the safety margins will be encroached and depending on how far the encroachment goes there may be apparent damage. My opinion in this mater is that the amount of fuel saved by the typical recreational boater by tinkering with propping above the recommended settings is insignificant when taken into total boat costs. Some people including myself believe in under propping particularly with common rail electronic motors, and that can start another whole bag of worms discussion.

manyboats said:

If you "correctly" install a "larger" prop on a boat it will attain rated rpm at WOT as if necessary you will change prop pitch or/and diameter or the gear ratio in the Tx.

eyeschulman wrote "over propped refers to the condition of placing too much load on a engine." Kind of .. IMO if a boat is over propped it can't make rated rpm and is over propped while not even running. It's over loaded of course when it is being over fueled but if it was propped correctly it simply could'nt run in that state .. because at any throttle setting the engine load from the prop is not so much that the engine will not be able to turn the prop (and hence the engine) fast enough to burn all the fuel injected.
And again over propping can be done safely if the engine is not run in an overloaded state or without sufficient load to heat the piston, valves, combustion chamber and oil to a high enough temperature to prevent cool running problems.

Click to expand...

We're into a definitions clash. I'm suggesting that the term "over propped" is universally identified with the over load condition at maximum rated power, and is universally associated with the classic propping methodology. When I say "correctly retrofitted with a larger (cruise) prop" I mean that the engine is correctly "derated" (in aircraft parlance). Appropriate gage markings are established, and if necessary, throttle stops and EGT gages are added to the installation. It meets the new maximum rated power and is therefore propped correctly.

Until everyone starts differentiating terminologies, these discussions will always result in misunderstandings and confusion. Maybe there's a better catch phrase..perhaps "cruise prop with de-rated engine". Any suggestions?