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Old 11-23-2013, 12:27 AM   #1
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Yanmar 170 Not Reaching Max RPMs

We did a sea trial today on a Mainship 350/390 with Yanmar 170s (4LH-DTE-93) with about 750 hours on the engines. It is propped with 22-19 wheels. Our mechanic and the Yanmar manual say that WOT should turn about 3300 RPMs but we were only able to achieve 2700. In speaking with the owner and broker they say that 2700 RPMs has always been the max that they obtain since it was purchased 6 years ago.

The mechanic thought the engines were well maintained, ran fine and he did not find any issues with them. From my research it seems like the cause of this
could simply be the high speed screw setting that limits the throttle or it
could be over propped however the engine showed no sign of overloading.

We are looking to run efficiently and are not overly concerned with speed.

I would appreciate any thoughts on what we are seeing and any potential issues that we should be concerned about given what we are seeing.

Thank you in advance for any thoughts and/or advice you can provide.
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Old 11-23-2013, 05:34 AM   #2
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With the engine warmed up , put in neutral and open the throttle to the pin..

That is the governed engine speed , if its only 2700 , there would be no problem , tho if you really like rapid defuling the gov could be readjusted to 3300.

The key is IF 2700 is all she will pull in gear with 3300seen at no load , be sure to operate at 2500 or less.
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Old 11-23-2013, 11:04 AM   #3
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Sounds like it could be the prop. I have a Yanmar LHA -HTP 160 with a 22.8 - 16 (.7 pitch ratio) prop and at WOT I get 3300 RPM. You say your prop is 22 - 19 which would be .83 pitch ratio and only get WOT 2700 RPM. This would concern me as it may be overloading the engine.

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Old 11-23-2013, 05:27 PM   #4
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I recommend you spend $25 for a year's membership on Boatdiesel.com and post your question there. The regular contributors are very knowledgeable about all things Yanmar.

FWIW: I have a pair of 4LH-DTE's and I consider Boatdiesel as valuable a resource as the workshop manual.
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Old 11-23-2013, 07:11 PM   #5
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Steve's observations are correct. You can post to boatdiesel, but as a long time member, I know what the answer will be.

But something is a little screwy. Mainship doesn't consciously overpitch their props. The two Mainships I have owned, a 34T and now a 34P were pitched spot on.

The fact that the engine will apparently only rev to 2,700 rpm in gear at wot is a huge amount below the rated rpm of 3,300. Is the bottom clean, prop clean, the tachometer reading correctly? Check the tack with a phototach.

Check those things and go from there. If all are ok then maybe you need new props.

And don't just run at 2,500 rpms as Fred suggests. With those props and at that rpm the engine will be asked to put out much more torque than it was designed to produce continuously. More torque means more stress (higher pressures, piston side forces, exhaust gas temps, etc).

But since this is a purchase and the seller is probably not interested in all of the above, you should probably find another boat.

David
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Old 11-23-2013, 10:54 PM   #6
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Thanks. You all may have just saved me big time.

As recommended I did a membership to boat diesel and researched further. On the RPM issue my plan was to have the bottom and props cleaned, sea trial the boat again, warm it up then check no load max RPMs and load RPMs. If it is not developing much more than 2700 RPMs under no load then I believe the most likely cause is that the engine has been governed for less. If it is developing the full 3300 then the boat is clearly way over propped. When I checked the prop calculator it looks like the 22 x 19 wheels that are on it should be about right. Please let me know if my logic is flawed in any way?

Now for the other issue I found during my boatdiesel research. The experts on boatdiesel say that the normal green antifreeze is a major no no as it causes corrosion in the aluminum exhaust manifolds. They recommend the orange stuff that contains no silicates or phosphates. These engines have the green antifreeze and in looking at the owner's maintenance logs it looks like it has been in there for at least 5 years. This causes me serious concerns.

This boat has been very well maintained but I am not moving forward with the purchase until we get to the bottom of both the RPM question and whether the exhaust manifolds have been damaged by the green antifreeze.
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Old 11-24-2013, 01:14 AM   #7
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I can but echo the need to achieve full rated RPM. D Marchand as a Mainship owner and very prop/engine combination savvy is correct in my book. A base line check as to engine health is the ability to reach max rated RPM without overheating, until you can ascertain this fact go slow and keep looking.

Your finding on the antifreeze is a very good catch. I think you will figure the purchase question out pretty quick.
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Old 11-24-2013, 05:47 AM   #8
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Most times the pink anti-freez is required to stop the exposed to coolant cylinder walls from dissolving.

As the cylinder rings ( like a bell) it moves quickly enough to cause a bubble next to it.

As the bubble collapses some external cylinder wall is removed , eventually the cylinder leaks .

SCA in the diesel pink anti-freez stops the cavitation erosion.
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Old 11-24-2013, 07:34 AM   #9
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Originally Posted by djmarchand View Post
With those props and at that rpm the engine will be asked to put out much more torque than it was designed to produce continuously. More torque means more stress (higher pressures, piston side forces, exhaust gas temps, etc)
That engine is only capable of producing X amount of torque at Y rpm. You can't ask it to produce more torque than it can make. If the load requires more torque than is available at a given rpm then the engine will slow down, more fuel will be added by the governor as it tries to maintain setpoint rpm and at some point, the engine is defined as overloaded.

Overload in a diesel means it is receiving more fuel than it can burn which causes other problems. Unless you add a lot of aftermarket hardware to increase the amount of charge air and fuel, you will not produce a mechanical overload. You may overload the cooling system capacity with predictable results but you won't produce too much torque.
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Old 11-24-2013, 08:57 AM   #10
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RickB:

I think you misinterpreted my comment. I said that an overpropped engine will be asked to put out more torque than it was designed to do CONTINUOUSLY.

Marine engines are expected to operate on or near their prop curve. That is one reason that all marine engine manufacturers produce an rpm vs hp curve while the engine is turning a prop. Yes I know it is a mathematical curve, but it is what manufacturers expect the engine to operate at.

If the engine is overpropped then it will operate above the prop curve. If this engine will only rev to 2,700 rpm and the rated rpm is 3,300 then at lower rpms, say the 2,500 rpm that Fred thinks is ok then it will pull much more than the prop curve hp at 2,500 and therefore more torque and torque is stress and too much stress equals early failure.

The data sheet for that engine shows a prop hp of 95 at 2,500 rpm and 115 at 2,700. If it is propped to only reach 2,700 rpm at wot then it is pulling about 190 hp at that rpm (read from the upper wot curve), not the 115 hp that the prop curve would suggest . Then if you were to construct a prop curve from that point then the prop hp at 2,500 would be about 150. That is 55 more hp than the manufacturer's prop curve of 95 at 2,500 rpm. Your engine won't last very long if you run it there.

Sorry for the bunch of numbers. The bottom line is if that engine really is propped to only reach 2,700, it is way, way overpropped and won't last long in that condition.

The green antifreeze is also a deal breaker in my opinion. You have a very good chance of having an exhaust manifold leak at some point in time.

David
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Old 11-24-2013, 09:02 AM   #11
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Horsepower and torque are mathematically related by rpm.

Horsepower=Torque (ft-lbs) x rpm/5252
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Old 11-24-2013, 09:10 AM   #12
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Old 11-24-2013, 10:52 AM   #13
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I have a Klassen/Mitsubishi engine w a steel exhaust manifold and I have green antifreeze. The only thing aluminum on my engine is the oil filter bracket/holder and the valve cover. The steel manifold is one of the main reasons I chose the Klassen/Mitsubishi engine. Could have had a Vetus or Westerbeke but they both had aluminum manifolds. Klassen has been using these steel manifolds in fish boats since the early 70s and only had to replace one. That stat is 8 years old though. Perhaps thev'e replaced others by now. The steel manifold is welded together and very nicely made.
And that riser/exhaust pipe is SS.
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Old 11-24-2013, 03:22 PM   #14
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If the engine is overpropped then it will operate above the prop curve.
You are reading way more into this than exists, and apparently don't understand the relationship between torque and power.

If an engine is "overpropped" it will operate at the torque required to turn that prop at the rpm it happens to be turning. Nothing more, nothing less.

The engine is capable of producing the torque described by the output curve.

The prop curve just shows how much HP the prop will absorb if turned at a certain rpm. A "well matched" prop, engine, and hull will absorb the maximum rated horsepower at the design speed of the hull. If you really want to know what the torque is, you can calculate it or measure it but it really doesn't matter because unless you have a variable pitch prop you can't change what it is.
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Old 11-24-2013, 11:22 PM   #15
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RickB:

I use torque in my analysis of overpropping to make the point that higher rpms (which can mean more hp) does nothing bad to an engine. It is torque which is related to loads on the bearings, side forces on the piston and the extra fuel required to make that torquethat leads to overheating of engine components. All of which, if they are more than the manufacturer intended, leads to premature engine wear.

BTW, I just read Tony Athens' response to this same question asked on boatdiesel. He said to service the complete raw water system, prop the boat to reach 3,450 in gear and replace the antifreeze with Yanmar approved coolant.

That is a whopping 750 rpm over the reported wot rpm in gear. Something is very wrong with the data or the engine/prop.

David
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Old 11-25-2013, 11:09 AM   #16
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RickB:

I use torque in my analysis of overpropping to make the point that higher rpms (which can mean more hp) does nothing bad to an engine. It is torque which is related to loads on the bearings, side forces on the piston and the extra fuel required to make that torquethat leads to overheating of engine components. All of which, if they are more than the manufacturer intended, leads to premature engine wear.
You keep missing the point that unless you increase the weight of charge air and fuel delivered to the cylinders, there is nothing you can put behind that engine that will cause it to exceed the amount of torque shown in the performance curve generated by the manufacturer.

"Overpropping" will prevent the engine from reaching the rpm at which full power is developed. It may also lead to overfueling and overheating but the torque developed to turn that prop at whatever rpm the engine is capable of turning it will never exceed what the engine has been shown to develop at that rpm.

As you were reminded, there is a carved in stone relationship between horspower and torque. If the prop demands more horsepower than the engine can develop at a given rpm, the rpm will decrease until the torque produced matches the power required to turn it. If the torque required increases further (a CP wheel for example) then the rpm will drop further. Unless you have a torque limiting governor you will overfuel and overheat because of an overload condition ... not because the engine magically produces more torque than it can handle.

An electric motor or a reciprocating steam engine develop their maximum torque at 0 rpm, they also produce 0 horsepower at that rpm. If we were to follow your reasoning to the limit, any electric motor or steam engine would twist its shaft off if you prevented it from rotating.

If that Yanmar was capable of producing more than ~480Nm of torque at 2800rpm, the manufacturer would be tickled pink to advertise that fact.





























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Old 11-25-2013, 11:34 AM   #17
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RickB:

Let's stop this silly back and forth. I am talking past you and you are talking past me.

David
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Old 11-25-2013, 01:28 PM   #18
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Kite,
I responded to your posting over at the Mainship site, for the benefit of the discussion here, this is what I posted:

I own the identical boat, 98, 350, twin Yanmar 4LH-DTEs with 22 x 19 props. It's
at 3300 RPM WOT. If the sync is on it does about 3100 RPM.

So, you are not over-propped. I think your initial thought on set screws and
cable adjustment issues at the throttles, sync and engine might be more
realistic. If you were using the sync when trying to check WOT, try running
without it. I am sure your mechanic would be able to determine if it is an air
or fuel problem.

Also, Yanmar issued a bulletin concerning the antifreeze issue about 7 years ago. It definitely needs to be changed out.

Good Luck
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Old 11-25-2013, 03:43 PM   #19
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Kite,

So, you are not over-propped. Good Luck
David
There is one very salient issue here that is being overlooked. Kitelog does not own the boat. It is up to the current owner to make things right so the vessel can achieve rated RPM without overheating as well as other unacceptable issues (if any) found during the survey..

A full rated RPM test has not been successful which indicates a problem, minor or major who knows. I can think of some things that could be wrong, but like every one else only guessing. Just recently I walked from a vessel purchase over something less onerous. It happens.

Kitlog, how do you know the props are 22X19?
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Old 11-25-2013, 04:49 PM   #20
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My observation based on being a Mainship group member for many years.
At northeast group rendezvous and raftups I would usually offer to check and calibrate tachometers for members as I had a photo tach and like to do those kind of things.
Every Mainship I ever checked that had Yanmar engines (there were maybe 8 or so) the tach (or tachs) were off to the photo tach. Usually the twin Yanmar boats had one that was good and one that was off. And they were all fairly new boats, I would say 3 years old or less.
It was never said that the original mechanic checked the tachs so perhaps that is the issue. I know the prospective owner said he was going to do that, I am just reinforcing the becessity of making sure it gets done.
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