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Old 04-12-2014, 09:54 AM   #21
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Rich F wrote:
"I have been told by several mechanics over the years that RPM's will not hurt your engine as long as they fall within the safe operable limits recommended by the manufacturer."

I've been under that assumption all along. But/and "hurt" is a relative word. To properly pickel your engine for 10years will "hurt" it less than to run it in every way conceivable way. I have a VW w a 1.8L engine that turns about 1800rpm at 70mph and I downshift it to 4th gear on hills thinking it's causing less wear and reducing the potential for damage. With the car and w the boats it's a similar consideration.

But the question that many are still asking is "will it hurt to overprop my diesel boat engine"? And I think it's been answered many times here recently and in the past. But I do believe RichF's mechanic is correct.
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Old 04-12-2014, 10:20 AM   #22
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Fred wrote;
"An engine that can make 50 hp at 1600rpm is far better in terms of engine life operating there with a 40 hp prop load than a prop that can give 2500 rpm at WOT and then pulling back to whatever it is that gives 40 hp , and pushes the boat at the same speed."

That is indeed the question but that raises other important questions. It's (usually here) a 120hp engine so what happens when one wants 80hp? What happens when you sell the boat? Or someone else operates your boat for whatever reason?

But the answer to Fred's question as stated w obvious assumptions is probably to possibly yes. Do you know? Not sure really. Does the boat make less noise? Probably. Burn less fuel? Yes. Lot's of variables and many unknowns.

But as psneeld puts it there's not only one way to do it. But there's a best way and there's the safest way and that is to prop to rated rpm. That's why manufacturers recommend it.

Overpropping actually comes under the heading of modifying your boat. On that thread I said it was fine as it's your boat. That applies here too. But suffering the consequences does also.
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Old 04-13-2014, 07:05 AM   #23
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I have a VW w a 1.8L engine that turns about 1800rpm at 70mph and I downshift it to 4th gear on hills thinking it's causing less wear and reducing the potential for damage.

For cars and trucks the usual system is to only downshift IF the engine can not hold the speed on the grade.

Good precaution if your car is towing a heavy trailer .

For folks that worry about a next owner not being briefed/instructed on the use of the engine as fitted with a cruising prop, the simple solution is to reinstall the stock screamer, and let the next guy cruise at 2300 till he wises up.

Lots of noise and high rpm make the boat sound faster so he might be happier than with as near the sound of silence one can cruise at.

For someone borrowing the boat ,> observe the RED line < is hardly too much to ask.
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Old 04-13-2014, 09:52 AM   #24
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As the question pertains to Lehman's, I'd ask Bob Smith at American diesel. He spoke of prop sizing in his 2 day course. Bob will be able to comment on all aspects that relate to the situation about your engine performance.

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Old 04-13-2014, 10:19 AM   #25
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Being over propped is like being overweight, it shortens your lifespan and causes you to over heat when put under stress. But if one is not concerned about the last 1/3 of your life, just loaf along and enjoy a full course meal 3X per day.
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Old 04-13-2014, 10:25 AM   #26
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Being over propped is like being overweight, it shortens your lifespan and causes you to over heat when put under stress. But if one is not concerned about the last 1/3 of your life, just loaf along and enjoy a full course meal 3X per day.
Great analogy!
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Old 04-13-2014, 11:18 AM   #27
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FF says;
"For cars and trucks the usual system is to only downshift IF the engine can not hold the speed on the grade."


Going full throttle up a grade should NEVER be done in a car unless it's for 10 or 20 seconds unless you're at the upper end of engine speed and then only for a minute or less. Just downshift.
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Old 04-13-2014, 12:30 PM   #28
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Great analogy!

Actually not..being overweight isn't necessarily bad...obese is...research has shown some weight is beneficial in recovery from illness, etc. It's just a matter of how much.

Besides unless you talk specifics on overpropping.... it's all just talk..and speculation of wear, tear, longevity etc...all you know is that you aren't following some arbitrary number.
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Old 04-13-2014, 04:47 PM   #29
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Actually not..being overweight isn't necessarily bad...obese is...research has shown some weight is beneficial in recovery from illness, etc. It's just a matter of how much.
Wow... I am going to pitch this one to the Admiral every time she yells at me to put down my beer and get off the settee!

Although she is a touch obsessive.. 26 marathons in the last 8 years and she is out cycling today as she is in training for a Ironman

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Old 04-13-2014, 07:22 PM   #30
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Wow... I am going to pitch this one to the Admiral every time she yells at me to put down my beer and get off the settee!

Although she is a touch obsessive.. 26 marathons in the last 8 years and she is out cycling today as she is in training for a Ironman

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Cruise in the Arctic or Antarctic and you can justify the love handles also as survival gear...
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Old 04-14-2014, 06:09 AM   #31
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Over propped ,a prop not chosen to spin at peak rated RPM , does NOT mean the vessel is overloading the engine.

Each engine has a long term proper rating at most rpm below full tilt .

Most eng mfg provide a graph.

Operating at 40 hp on our 50 hp at that rpm rated engine INCREASES engine life , and lowers fuel burn too.

The best place to look for acceptable low rpm loading is at the graph for the engine when selected for use as a generator.

Usually 1200, 1500 and 1800 will have suggested loading that the eng mfg finds safe 24/7 .

Operating at 1200 or 1500 at rated loading is far kinder on the engine than proping for never used WOT and pulling back to minor hp at high rpm..
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Old 04-14-2014, 10:55 AM   #32
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FF: I'm not doubting what you said. But Bob Smith, who was responsible for marinizing the Lehman engine encourages owners to run their vessel at WOT regularly. If it won't go there there is something wrong...either improper propellor sizing or a perhaps a larger problem with the engine. Could be injectors, or could be something else. This is I why suggested he contact Bob Smith. Bob said that Lehman engines are industrial engines (not originally designed for boats) and are designed to be run wide open for extended periods. The issue isn't whether he can make do with the incorrect prop at a lower RPM. It's whether he has he proper prop in the first place. If he has the proper prop, he should get things looked at if he can't get to WOT. If he doesn't have the correct prop, he should change it.

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Old 04-14-2014, 11:09 AM   #33
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Running an overpropped boat isn't for someone who doesn't mostly understand what's going on with their engines for the full performance curve.

If you don't understand...probably you shouldn't overprop even if you feel there are benefits.

For those that want to anyway...learn the ups and downs of doing it.

For those that do understand...hopefully you know a lot posted truly doesn't affect those that do who take appropriate measures and understand.

Hopefully those who overprop leave the proper info with the next purchaser for them to decide.
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Old 04-14-2014, 11:14 AM   #34
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JDCAVE,
Absolutely correct. ALL OF IT.

But ..... FF is also correct when he says;
"Over propped ,a prop not chosen to spin at peak rated RPM , does NOT mean the vessel is overloading the engine." But he should have said "the skipper is overloading the engine".

Bob Smith is not an engineer but what the guys say here about what he says seems good IMO. And when Mr Smith says "run their vessel at WOT regularly" I'm sure he was referring only to engines propped at rated rpm.
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Old 04-14-2014, 11:26 AM   #35
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JDCAVE, Absolutely correct. ALL OF IT. But ..... FF is also correct when he says; "Over propped ,a prop not chosen to spin at peak rated RPM , does NOT mean the vessel is overloading the engine." But he should have said "the skipper is overloading the engine". Bob Smith is not an engineer but what the guys say here about what he says seems good IMO. And when Mr Smith says "run their vessel at WOT regularly" I'm sure he was referring only to engines propped at rated rpm.
...and for sure he's not an engineer. But he did spend considerable time with the Lehmann guys, during the day and also worked with the vessel manufacturers on the sizing of the props for their boat. And Fred certainly knows more than I do on any of this. If it this boat were mine, I'd be looking into the issue further to determine next steps.

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Old 04-14-2014, 11:28 AM   #36
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Hopefully those who overprop leave the proper info with the next purchaser for them to decide.
Here is the note:

"Dear new prospective owner, I ignored the advice and warranty stipulations as required by Cat, Cummins, JD, Yanmar, Volvo, AD, Perkins Sabre, Lugger etc and propped my vessel so it would not achieve fully rated RPM. I did this because I'm smarter than their hundreds of engine design engineers who are backed by nearly a full century of field experience. I promise you I always operated my vessel at a very low RPM and never damaged the engine in any way.

And that rusty turbo, drip from the RW pump and moldy smell you ask - just normal stuff you'd expect from any Bristol condition yacht!"

Sorry, PS, I couldn't resist it.
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Old 04-14-2014, 11:36 AM   #37
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Here is the note:

"Dear new prospective owner, I ignored the advice and warranty stipulations as required by Cat, Cummins, JD, Yanmar, Volvo, AD, Perkins Sabre, Lugger etc and propped my vessel so it would not achieve fully rated RPM. I did this because I'm smarter than their hundreds of engine design engineers who are backed by nearly a full century of field experience. I promise you I always operated my vessel at a very low RPM and never damaged the engine in any way.

And that rusty turbo, drip from the RW pump and moldy smell you ask - just normal stuff you'd expect from any Bristol condition yacht!"

Sorry, PS, I couldn't resist it.
Can I use that except for the part about the turbo...he'll think I removed it...
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Old 04-14-2014, 11:51 AM   #38
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Tom,
I love your sense of humor.

But that brings this whole issue into sharp focus!!!!
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Old 04-14-2014, 12:17 PM   #39
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Well said

Quote:
Originally Posted by sunchaser View Post
Being over propped is like being overweight, it shortens your lifespan and causes you to over heat when put under stress. But if one is not concerned about the last 1/3 of your life, just loaf along and enjoy a full course meal 3X per day.
I think you are right. The engine is running too hot as well.
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Old 04-14-2014, 12:37 PM   #40
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Alright guys, here's the latest. Again this is with the 3 blade, Michigan, 24x16. The boat weight is ~23,000lbs. She is semi displacement and 36'. This weekend we went out to test the cruising speed and WOT speed. WOT RMP is only 1,900!! This was confirmed with a mechanical SW tach on the flywheel. The boat ran at ~8.3 knots, with some black smoke and steam from the exhaust and the engine temp spiked to 220 degrees F. Way too hot I believe for a Lehman. The sweet spot ranged from 1,600 RPM to 1,650RPM, no black smoke, 7.5 knots cruise speed and the temp dropped to 200 degrees F. The high temp worried me so I went back to the slip and changed out the impeller and wear plate. Temp didn't change but by about 5 degrees F. There are NOT mechanical gauges so I DO worry about the accuracy of the electrical meters for the water temp. I want to install some mechanical gauges for oil pres and water temp in the engine room as a good back up.

Utimately I wanted to know what you guys are running on your Lehmans and how big your boats are. My general understanding is that this prop doesn't seem too large for similar applications elsewhere and other guys get 2,400 RPM at WOT. For some reason I cannot reach the RPM and I can't understand why. Maybe it is an injector issue although the mechanic said not.

I definitely want to contact Bob at American Marine and get his opinion. After I do, I will post what he had to say for everyone info.
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