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Old 12-05-2014, 10:04 AM   #21
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Originally Posted by Ski in NC View Post
A 3208TA rated at 320hp or whatever is not going to be harmed by running at say 50hp at 1200rpm, even if it can't make rated rpm at full. Just don't try to plane out.
But what if the PO never knew or cared that the vessel was over propped and ran it on the pins frequently. The new owner quite possibly would have then bought a soon to be relic. Is this not a buyer beware issue Ski and why during survey a full load test is done?
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Old 12-05-2014, 10:07 AM   #22
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Ski: True enough but I don't see the benefit of not propping to make full power use an option. Fuel use at 6 knots will probably be the same despite different RPMs.
I agree with that. I would not intentionally overprop a boat.

Where it becomes an issue is when previous owner did it. Then is it worth several grand to prop it correctly? If it is only run at hull speed, it can be run as is.
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Old 12-05-2014, 10:14 AM   #23
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But what if the PO never knew or cared that the vessel was over propped and ran it on the pins frequently. The new owner quite possibly would have then bought a soon to be relic. Is this not a buyer beware issue Ski and why during survey a full load test is done?
That is possible. A good engine guy can tell if engines were run hard, and hints are also available from trip logs and owner interview.

Full power test is crucial to test for cooling system performance, engine performance, blowby, etc. If engine can't make revs, test is cut short.
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Old 12-05-2014, 12:52 PM   #24
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If I were in the market to buy a used boat and the sea trial demonstrated significant over propping I would have to pass. That the owner or broker might tell me they never went above a certain rpm would be little solace and an owner who switched back to a acceptable prop before sale would be fraud if discovered. That owner would be sending a message to me that they think they know more about the engine than the manufacturer, and I would be thinking where else on the boat have they applied that philosophy? The engine manufacturers often specify how they want their engines propped and I for one see no long term practical reason particularly with modern light high out put engines to ignore their advise in the direction of over propping unless the process is well documented and also includes mechanical or electronic safe guards that prevent serious overloading. Even then as Ski points out a motor has to be run out now and then.
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Old 12-05-2014, 01:46 PM   #25
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My $.02 .....
Most of the time most boats are heavy and dirty. Assuming one would want to be correctly propped under these "normal" conditions and considering that frequently a boat is propped just after launch when it would be light and clean under propping 100rpm would seem the best route to being spot on in the future. Then w all tanks full, a slightly fouled bottom, lots of extra "stuff" and several guests aboard the boat run nice and easily for being properly propped.
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Old 12-05-2014, 02:14 PM   #26
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Need to clarify this a bit:

If engine is overloaded at full power, that is, incapable of reaching rated rpm or above, then it will be overloaded also when power is reduced slightly. Say from 100% down to say 70%, or even below if boat is still trying to plane.

But if power is reduced significantly, such as dropping down to hull speed, then the overload condition will disappear.

The above applies to planing or semiplaning type boats with high output engines. Often they are overpropped to optimize hull speed running. No harm in that, but it will make it hard on the engines if they are run at high power.
This is a pretty large generalization. As FF said earlier, anyone contemplating an overprop needs to look at the prop curves (power required) and the engine max power available curve for the engine(s) in question. Our 44 OA will stay nicely on "semi-plane" at about 13 knots. The prop charts say that equates to about 210 HP (105 per engine) power required. The engine power available chart says the engines are capable of producing 420 HP total (210 each) at that same rpm. That's a cushion of 100% with the boat comfortably on plane. Now, the "cushion" gets smaller as the semi-planing speed goes up, but even at rpm for 70% of max rated power (175 HP/engine), the cushion (for each engines) is about 60 HP (34%). That's plenty.


Power required for a given hull speed should be less with a more efficient (larger diameter) prop, but at the same time, the engines could be making the power required at a slightly higher torque than for the smaller prop...again the bigger prop is more efficient and producing thrust required at a slightly lower rpm. The interplay between the various parameters would depend on the engine design and how it makes its power. In any case the changes would be small. There's nothing but goodness in overpropping a boat with slobbering pig engines ...as long as the operator respects the new limits. This is something aircraft operators have done for ages. Surely boat herders are equally as capable.

So overprop, put a new red line on the tachs, and move on. Enjoy the benefits of improved prop efficiency...depending on where in the speed range you elect to operate your slobbering overpowered turbo charged beast...or even your overpowered NA twin Lehman, for that matter.

Regarding a previous remark that it would be dishonest to overprop and then survey with the original rpm rated props...I have to scratch my head. If the boat was operated within the cushion, and the owner explained overprop operation to a prospective buyer...what's the problem. I wouldn't even change out the props unless the prospective buyer couldn't understand the concept and insisted on a demonstration of max rated rpm. In fact, if a prop curve was available for the new bigger prop, the new (lower) max rpm could be demonstrated with the overprop props. By the way, I don't know more than the manufacturer....but I believe I know pretty nearly as much. And as FF says, they recommend propping the boat for max rated power to protect against stupid drivers. Make the boat error proof and you protect against warranty claims. Probably one reason (among others) that variable pitch props don't show up on pleasure boats...if you can't automate them, you're stuck with the least common denominator operator who would rip the guts out of the engines on the first day.
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Old 12-05-2014, 04:41 PM   #27
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If I were in the market to buy a used boat and the sea trial demonstrated significant over propping I would have to pass.
Couldn't agree more. There are plenty of good boats out there that are correctly propped as compared to accepting the risk of a wrong propped vessel.

Having been through the details of what a good manufacturer does for a new build for prop and engine optimizing, I'm confident the knowledge and science is very well utilized in this regard by most if not all reputable builders.
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Old 12-05-2014, 05:25 PM   #28
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If over propping is not such a big deal why is it then that JD would not give me an extended warranty until there technician did a two hour sea trial including verification of correct propping? This was on a one off build. I asked and was told that if over propped no warranty until prop was changed. If people on the used boat market want to buy a over propped boat that a broker or owner tells them the motor was never used above a certain rpm so all is good-good luck to them probably will need it.
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Old 12-05-2014, 07:01 PM   #29
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If over propping is not such a big deal why is it then that JD would not give me an extended warranty until there technician did a two hour sea trial including verification of correct propping? This was on a one off build. I asked and was told that if over propped no warranty until prop was changed. If people on the used boat market want to buy a over propped boat that a broker or owner tells them the motor was never used above a certain rpm so all is good-good luck to them probably will need it.
Because John Deere was covering their warranty butts...precisely as I said earlier. Nothing to do with the technical merit of overpropping. Likewise your last sentence has nothing to with technical viability, engineering logic, or adhering to operating limits. I can't understand the logic of assuming that an owner who carefully repropped using manufacturer data, established safe new operating limits, and understood the consequences of exceeding those limits....would purposely damage an expensive piece of hardware? Why would anyone do that?
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Old 12-05-2014, 08:16 PM   #30
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Because John Deere was covering their warranty butts...precisely as I said earlier. Nothing to do with the technical merit of overpropping. Likewise your last sentence has nothing to with technical viability, engineering logic, or adhering to operating limits. I can't understand the logic of assuming that an owner who carefully repropped using manufacturer data, established safe new operating limits, and understood the consequences of exceeding those limits....would purposely damage an expensive piece of hardware? Why would anyone do that?
Because I am not a expert mechanic or qualified engine specialist I prefer to follow the advise of the engine manufacturer. Your presentation of how over propping affects engines may be right but how would I or other boaters know for sure. I have heard both sides of the argument presented with various technical support. I have lived through a 50 year career that was highly technically based and there were many disagreements on how things work. If one sets up their propping so that it can overload the engine, my question is why? The answer usually is to save fuel. Since most pleasure boating is a low hour per year use situation, fuel is in my opinion a low priority on the total boat cost list. I encourage boaters to follow the engine manufacturers advise on loading. Regarding buying used mechanical equipment it was not I who coined the concept of buyer be ware.
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Old 12-05-2014, 08:27 PM   #31
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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.
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Old 12-05-2014, 09:52 PM   #32
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It appears our conversation on this matter has improved considerably from about 5 years ago.
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Old 12-06-2014, 01:33 AM   #33
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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.
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Old 12-06-2014, 02:23 AM   #34
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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.
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.
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Old 12-06-2014, 09:58 AM   #35
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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.
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Old 12-06-2014, 11:01 AM   #36
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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?
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Old 12-06-2014, 12:08 PM   #37
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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.
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Old 12-06-2014, 02:12 PM   #38
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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.
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Old 12-06-2014, 02:47 PM   #39
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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.
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Old 12-06-2014, 02:49 PM   #40
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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.
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