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Old 10-08-2016, 07:28 AM   #1
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Ford Lehman 135 low power operation

I like to cruise at a low to moderate power level. Unfortunately that creates some buildup in the cylinders (guessing carbon buildup). I typically run at ~ 900 and 1200 rpm. I am (at least to me) drastically over propped at 24"X24" 4 blade (factory was 24"X20" 4 blade) PO changed the prop and the WOT rpm is ~2100 with a clean bottom.

Looking for some suggestions on how often, how long and at what load I should run the engine to keep it clean. IE every 10 hours run at 3/4 load for 15 minutes, or every 20 hours run at WOT for 15 minutes. What is reasonable and will help keep the engine clean. As I was getting it ready to pull for the winter I ran it for about 15 minutes at WOT (after warming it up for over a hour) and it smoked for the first 5 minutes. Quite a bit for the first couple of minutes. The last 5 minutes no smoke was seen coming out of the exhaust.

I would appreciate any feed back on this as I don't want to ruin the engine.

I am new to diesel engines and also would like feed back on being over propped. I am thinking of switching back to a 24"X20" prop as I feel the factory would supply the optimum prop for the boat (GB32 1985)

Thanks in advance for the help Don
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Old 10-08-2016, 07:38 AM   #2
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No downside to being correctly propped. But, if you choose to install smaller props be sure to scan and balance to be sure old are tuned up and match.

IMHO operating RPM are way too low. Have you checked tachs?
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Old 10-08-2016, 09:05 AM   #3
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You may be able to get the existing prop repitched for considerably less money than buying a new prop. The prop shop I deal with says they can routinely repitch by 3" and more can be done depending on the prop.
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Old 10-08-2016, 09:39 AM   #4
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Well, the issue of over propping has been debated extensively on this forum. Do a search and you will find at least 100 opinions on the subject- for and against.


But there is one thing that all 100 would agree on- running an over propped engine at wot is very bad for it. You are putting much more stress on the engine than it was ever designed for.


Also 900 rpm is very low for routine cruising and may not be enough to put sufficient load on the engine to get it up to full operating temps. That is one reason why you are seeing accumulated soot after running that low.


I think the best thing that you can do (short of propping the engine right) is to run at no lower than 1,200 rpm and then run it up to say 1,800 every 10 hours or so to blow out the soot. That should do the job without putting too much stress on the engine.


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Old 10-08-2016, 09:39 AM   #5
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it is mostly myth that you use less fuel being over propped.
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Old 10-08-2016, 12:04 PM   #6
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Quote:
Originally Posted by sunchaser View Post
No downside to being correctly propped. But, if you choose to install smaller props be sure to scan and balance to be sure old are tuned up and match.

IMHO operating RPM are way too low. Have you checked tachs?
Sunchaser - - Yup I checked the tachs they are within 50 rpm. I mostly go out just to be on the water not going anywhere in particular. For me it's about the journey not the destination. So I just poke along and watch the scenery and traffic (freighters) go by.

I'll most likely go back to the original prop I'm not a big believer in over propping especially by 20%. Not sure why the PO went that much. Even if you gain 15% in fuel economy (which I doubt you do) with this engine at just below hull speed you burn < 1 gal per hr so your not saving much.

Thanks for you comments DonF
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Old 10-08-2016, 12:05 PM   #7
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Quote:
Originally Posted by bayview View Post
it is mostly myth that you use less fuel being over propped.
Bayview - - My sediments exactly
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Old 10-08-2016, 12:25 PM   #8
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Quote:
Originally Posted by djmarchand View Post
Well, the issue of over propping has been debated extensively on this forum. Do a search and you will find at least 100 opinions on the subject- for and against.


But there is one thing that all 100 would agree on- running an over propped engine at wot is very bad for it. You are putting much more stress on the engine than it was ever designed for.


Also 900 rpm is very low for routine cruising and may not be enough to put sufficient load on the engine to get it up to full operating temps. That is one reason why you are seeing accumulated soot after running that low.


I think the best thing that you can do (short of propping the engine right) is to run at no lower than 1,200 rpm and then run it up to say 1,800 every 10 hours or so to blow out the soot. That should do the job without putting too much stress on the engine.


David
Yup I already did a search on over propping and tend to agree with the ones that say it's not worth it. And also with the ones that say you don't really improve your fuel economy.

I have the original prop so I think I'll switch back to it before I launch it next spring. It looks like new and I'll have it checked out at a prop shop over the winter.

Then run it at 70% load every 10 hours or so like you suggested. (That would be around 2000 rpm with the factory prop) Although I did run it at 1500 rpm for 10 minutes about every 30 hours and it never smoked at that speed and load but maybe that wasn't high enough to loosen things up. It should have been high enough to get it to operating temperature. Even at 900 the water temp reaches 180F. But I know that doesn't mean the combustion chamber temp is up to what it should be.

I have some experience with diesels having worked on developing the 6.7 liter DuraMax at GM R&D for about 10 years before I retired but didn't do any durability work just emissions testing. And you don't want to know what we put those engine's through. EGR levels as high as you could go and still get the fuel to burn. LOL. Soot levels in the exhaust so high the opacity meter couldn't see through it and the Bosch Smoke meter paper was black even with a short sample time. But for older marine engines I don't have any experience.

Thanks for your comments they are much appreciated! DonF
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Old 10-08-2016, 12:34 PM   #9
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Quote:
Originally Posted by djmarchand View Post
Well, the issue of over propping has been debated extensively on this forum. Do a search and you will find at least 100 opinions on the subject- for and against.


But there is one thing that all 100 would agree on- running an over propped engine at wot is very bad for it. You are putting much more stress on the engine than it was ever designed for.


Also 900 rpm is very low for routine cruising and may not be enough to put sufficient load on the engine to get it up to full operating temps. That is one reason why you are seeing accumulated soot after running that low.


I think the best thing that you can do (short of propping the engine right) is to run at no lower than 1,200 rpm and then run it up to say 1,800 every 10 hours or so to blow out the soot. That should do the job without putting too much stress on the engine.


David
I agree.

Dauntless appears to be abit over-propped. WOT is about 2400 to 2500 rpm.

I find that anything below 1400 is too low and the engine does not maintain enough heat. Occasionally I will run 1800. Once in a blue moon higher than 2000.
Normal running is between 1500 and 1700.
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Old 10-08-2016, 09:49 PM   #10
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Agree with the others. Below 1400 seems to generate very little heat. I sometimes cruise in the river and even at 1100 it can take 20 minutes just to come up somewhere near normal operating temp. I normally cruise at about 1800 and never get any soot.

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Old 10-09-2016, 07:07 AM   #11
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Originally Posted by Wxx3 View Post
I agree.

.

I find that anything below 1400 is too low and the engine does not maintain enough heat. Occasionally I will run 1800. Once in a blue moon higher than 2000.
Normal running is between 1500 and 1700.
Identical usage with Bay Pelican, same engine, sister boat.
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