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Old 12-21-2018, 03:43 PM   #61
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Does Yanmar advise on what EGT to rpm values you should see?
11 or 1200 max.....From my memory
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Old 12-21-2018, 03:47 PM   #62
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11 or 1200 max.....From my memory
Then whether at 900 or 700, you'd be hard pressed to say the engine was over loaded and being harmed at cruise....

I remember from managing a turbo Lycoming 540 that 1650 was max before the turbo went plastic, so 1200 seems reasonable.
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Old 12-21-2018, 04:06 PM   #63
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I am not an engineer. In my overly simplistic mind the more RPM that you turn to produce a given amount of HP the easier it is on your engine.
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Old 12-21-2018, 04:10 PM   #64
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I am not an engineer. In my overly simplistic mind the more RPM that you turn to produce a given amount of HP the easier it is on your engine.
If that were true, then fast turning Yanmars would last longer than slow turning Luggers, but that doesn't appear to be the case.
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Old 12-21-2018, 04:15 PM   #65
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Remember we are talking aluminum pistons in said Yanmar. So I didn’t want to go anywhere near the rated max temp. And I was probably 1/2 to an inch under propped.
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Old 12-21-2018, 04:20 PM   #66
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If that were true, then fast turning Yanmars would last longer than slow turning Luggers, but that doesn't appear to be the case.
Don’t sell the Yanmar’s short. They will go many hours if propped correctly. (Most are not) it’s not RPM that kills diesels. It’s heat and therefore load that does them in.
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Old 12-21-2018, 04:36 PM   #67
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EGT vs RPM?

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Originally Posted by Delfin View Post
Does Yanmar advise on what EGT to rpm values you should see?
EGT doesn't vary (much) as a function of RPM. At any RPM, EGT is overwhelmingly a function of the % of max HP the engine is producing -- for that RPM.



Although the differences can be small, the highest EGTs are often seen at the lower RPMs.


That's why EGT is the surest indicator of engine load.


This is also why any overloading at low RPM is particularly bad...
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Old 12-21-2018, 04:37 PM   #68
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I am not an engineer. In my overly simplistic mind the more RPM that you turn to produce a given amount of HP the easier it is on your engine.
Bingo. Which is why we always want to err on the side of under-propping!
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Old 12-21-2018, 05:07 PM   #69
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Originally Posted by Easting View Post
11 or 1200 max.....From my memory

The 'zone limits' are not expressed as only a temperature, rather they are expressed as the heat content of the exhaust gasses running through the system -- which is a combination of temperature and volume/mass. So if you look at the zone limits, you'll see it's always specified as EGT (temp) at CFM (volume), and the two must be specified together.


Together these two tell you how much heat is being transferred to the exhaust system, and the spec is based on the limits of how much exhaust heat engine/turbo can absorb and dissipate. Most of us have seen a 'cooked' turbocharger, where the paint has been severely discolored and in some cases totally burned off. This is a classic sign of a badly overloaded engine. Of course, when you talk to the owner, you hear "How can I have cooked my turbo?!?!? I never run over 2,200 RPM!" Generally the next thing you hear is "But I was told overpropping is only bad at high RPMs."


In the example below (Caterpillar 3406) the maximums are 836 degrees at 307 CFM (1,100 RPM) or 648 degrees at 1,446 CFM (2,100 RPM).



Heat vs. Temperature -- it's only physics, baby!
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Old 12-21-2018, 05:41 PM   #70
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Note that some Yanmars have the EGT port upstream of the turbo turbine, so due to expansion and pressure drop across the turbine those will read higher than other engines, where the probe is post turbine.

Lots of good stuff in this thread. And some not so good.

Of course the exponential prop curve is an approximation. It will not fit any vessel exactly. It certainly fails on planing or semi-planing vessels as there is a discontinuity at the "hump" where it climbs the bow wave. But prior to and after the hump speed, the curve still works (somewhat) but the two exponents tend to be different.

I have a lot of sea trial data with computer engines that read gph in real time. You can make a really good curve from that. With those data I can extrapolate using an exponential prop curve and the conclusions from such have proven to be fairly accurate.

Each boat has a hp required vs. speed curve. Each engine has a BSFC map, or lacking that, a hp produced vs. rpm curve. The engine is happily loaded in a zone (on either) of the map or curve. The Caterpillar doc really showed that well. The goal is to match the boat curve to the engine curve(s) so that no matter what vessel speed, engine stays in the happy spot.

Without having all the data for a particular boat, more often than not if you get WOT rpm to the desired spot, the engine will be in the happy spot at a reasonable cruise.

I have been on boats where what the owner thought was a happy cruise rpm actually had the engine heavily loaded, and we ended up "fixing" it by going up like 100 rpm. Too close to the "hump".

I too would rather have and engine making hp at a higher rpm than lower, if lower meant high specific load. Lots of bad can come from too high of a load, not much bad can come from higher rpm.
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Old 12-21-2018, 06:03 PM   #71
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"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.

Re: "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."


Not true, because the 'theoretical' propeller demand curve is meaningless here, because OP is not running a full displacement hull.


Consider this example. I run my semi-displacement hull up on 'plane'. It takes a LOT of throttle to get me over the hump and up on top of the water. Once I'm on plane, even a very slight reduction in throttle can cause me to 'fall off' of plane, and now I'm plowing water!


In this very common case, propeller demand (as a percentage of max HP @ RPM) does not decrease when I reduce RPMs, rather it increases dramatically -- because now by bow is pointed into the sky and I am digging a huge hole and plowing water like crazy.


Elsewhere in this thread I point out why the "propeller demand curve" is a fictional construct, and that is almost totally useless except for predicting GPH at RPM in >>full displacement<< hulls only.


Everyone who ever looked at GPH vs. RPM while putting a semi-displacement hull or a planing hull up on plane knows that there is a HUGH hump at the 'transition' speed. All you have to do to understand how meaningless the 'propeller demand' curve is is notice the total lack of a 'hump' at the transition speed.
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Old 12-21-2018, 06:29 PM   #72
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Hydrodynamics vs. Aerodynamics

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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....
Obviously, the first and most glaring problem with this analogy is that air is a >>highly<< compressible medium and water is a totally >>incompressible<< medium. Accordingly, aircraft propellers and marine propellers are completely different from physics, design and engineering perspectives.
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Old 12-21-2018, 06:55 PM   #73
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So I get that a sport fishing boat needs to dig itself out of the hole to get up on a plane, and that the engine will be heavily loaded at less than full RPM while doing so. So during that transition time I think we can all accept that the engine will be loaded somewhere above the nominal prop curve.


But the real questions is how much, and more specifically, does it exceed the max power rating, which would constitute overloading.


The nominal prop load curve is well publicized and well understood. What does the get-on-plane load curve look like? There must be good data on it. Anyone seen anything credible? Inquiring minds want to know....
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Old 12-21-2018, 07:23 PM   #74
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Inquiring minds want to know...

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So I get that a sport fishing boat needs to dig itself out of the hole to get up on a plane, and that the engine will be heavily loaded at less than full RPM while doing so. So during that transition time I think we can all accept that the engine will be loaded somewhere above the nominal prop curve.


But the real questions is how much, and more specifically, does it exceed the max power rating, which would constitute overloading.


The nominal prop load curve is well publicized and well understood. What does the get-on-plane load curve look like? There must be good data on it. Anyone seen anything credible? Inquiring minds want to know....

Twistedtree,

The idea that these load curves are >>impossible<< to predict (unless you are a PURE displacement hull) is exactly the point I have been trying to make.

It is not just applicable to a sport fishing boat, it is ANY boat that is designed to operate above displacement speed!


Re: "There must be good data on it."


Sadly, there is not! Every non-displacement hull is different, and that is EXACTLY the problem. The only solution (for non-displacement hulls) is to make sure you can >>easily<< get above the minimum WOT RPM number, and even this does not guarantee a good result at transition speeds!



Below is from Caterpillar's Marine propulsion engineering manual. Note THEY choose the word "impossible".


The only solution is to keep reducing pitch until the semi-displacement or planing hull can reliably climb out of it's hole under the worst conditions that can be expected.


Yes...this is an entirely unsatisfying answer to most of us, but it is reality.
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Old 12-21-2018, 07:31 PM   #75
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Don’t sell the Yanmar’s short. They will go many hours if propped correctly. (Most are not) it’s not RPM that kills diesels. It’s heat and therefore load that does them in.
Yanmar's can be great engines. I've owned a couple. My only point was that a faster turning diesel at a given horsepower generally will, in equal operating conditions, run for fewer hours between overhauls than its slower turning cousin simply because they turn faster, which generates wear.

Manufacturers have striven for reduced weight and higher horsepower in smaller packages. The way you meet that design brief is to go with smaller displacement, lighter metals, and higher revving engines. The Yanmar 6LPA 299 hp marine diesel weighs 800 pounds, has a displacement of just over 4 liters and turns at 3800 rpm. My CAT 3306, 270 hp, weighs 2,469 pounds and has 10.5 liters of displacement and runs at 2200 rpm. The Kelvin diesel I looked at in a Malahide that developed 240 hp, weighs 7,300 pounds, has a 23 liter displacement and turns at 1,000 rpm.

The Kelvin will never wear out, my CAT will wear out for my grandchildren, and your Yanmar will give you great service probably for as long as you want to own the boat.
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Old 12-21-2018, 07:57 PM   #76
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Obviously, the first and most glaring problem with this analogy is that air is a >>highly<< compressible medium and water is a totally >>incompressible<< medium. Accordingly, aircraft propellers and marine propellers are completely different from physics, design and engineering perspectives.
Totally false. The different densities of the medium dictate different physical design of propellers, but both media are fluids, with aerodynamics being a branch of fluid dynamics. The point is that if your statement were true that

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A boat that is overpropped will be overloading the engine throuout the entire powerband. Overpropped is overloaded, regardless of speed.
then an airplane engine with a trimmed prop for cruise must be overloaded since it is incorrectly pitched for take off. Since that clearly is not the case, over-propping within reason will not, as you suggest, take an adverse toll on an engine at all speeds.
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Old 12-21-2018, 08:10 PM   #77
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Twistedtree,

The idea that these load curves are >>impossible<< to predict (unless you are a PURE displacement hull) is exactly the point I have been trying to make.

It is not just applicable to a sport fishing boat, it is ANY boat that is designed to operate above displacement speed!


Re: "There must be good data on it."


Sadly, there is not! Every non-displacement hull is different, and that is EXACTLY the problem. The only solution (for non-displacement hulls) is to make sure you can >>easily<< get above the minimum WOT RPM number, and even this does not guarantee a good result at transition speeds!



Below is from Caterpillar's Marine propulsion engineering manual. Note THEY choose the word "impossible".


The only solution is to keep reducing pitch until the semi-displacement or planing hull can reliably climb out of it's hole under the worst conditions that can be expected.


Yes...this is an entirely unsatisfying answer to most of us, but it is reality.
So, when I read CAT's statement, I read that where a planing or semi-displacement hull will land on a power curve at a given rpm is dependent on the hull. Big whoop. You read this as vindication to your repeated statement that any over-propping will result in overloading the engine at all speeds, which is nonsense. As TT and I have pointed out to you, unless you make a power demand that is greater than the design max power output of the engine as shown by the power curve, the engine is not being over loaded. It is operating within the power band it was designed for. The fact that displacement hulls operate at the low end of the band is a function of hull design. The fact that other designs operate above that is a function of their hull design, but neither is over loading the engine as long as it is somewhere between the power and prop curves.
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Old 12-21-2018, 08:52 PM   #78
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Fortunately the over load, if any, on a diesel can be fairly easily determined. No guess work or interpretation is needed. Boost and EGT measurement provides key data on older engines with newer electronic engines providing spot on data. I've seen larger diesels where data for each cylinder is monitored to control fuel flow and cylinder head temperatures.

Amazing stuff is going on with tugs and ships utilizing constant speed Diesel engines and variable pitch props, not unlike the airplane engine mentioned by Delfin. Hate to see us get too hung up on the nuances of predicting over loading with so many direct measurement or newer engine algorithms available.
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Old 12-21-2018, 09:26 PM   #79
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Fortunately the over load, if any, on a diesel can be fairly easily determined. No guess work or interpretation is needed. Boost and EGT measurement provides key data on older engines with newer electronic engines providing spot on data. I've seen larger diesels where data for each cylinder is monitored to control fuel flow and cylinder head temperatures.

Amazing stuff is going on with tugs and ships utilizing constant speed Diesel engines and variable pitch props, not unlike the airplane engine mentioned by Delfin. Hate to see us get too hung up on the nuances of predicting over loading with so many direct measurement or newer engine algorithms available.
Constant speed with a variable pitch prop seems like it would be ideal. With a fixed pitch prop on a boat, it has to be pitched for some power state, and the most conservative is Max rpm. If you operate at Max rpm never, or for the few seconds it takes to get up on plane or never if you get up on plane or cruising speed at less than full power, getting worried about being 100 rpm this side of Max rpm at WOT seems like a waste of worry beads.
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Old 12-21-2018, 09:45 PM   #80
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I too would rather have and engine making hp at a higher rpm than lower, if lower meant high specific load. Lots of bad can come from too high of a load, not much bad can come from higher rpm.
That seems like the critical qualification. Just going off my own experience, I took 1.5" of pitch out, and now cruise 100 rpm higher, but at the same EGT, speed, and fuel consumption as before. It doesn't seem like there are any markers of higher specific load before detuning, but am I missing something?
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