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Old 05-31-2016, 09:23 AM   #21
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You're so right. I'll trust what Perkins says the most.
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Old 05-31-2016, 10:59 AM   #22
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RB

Props move boats. Keep in mind Perkins did not prop the vessel, they built the motive power. Re-read Ski's post #16 for sea trial RPMs to keep in mind.

Did you get the manuals and check maintenance recommended vs performed? Don't forget the after coolers are due for servicing, not cheap and if done by the wrong guy could prove fateful.
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Old 05-31-2016, 11:07 AM   #23
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Be careful about what the engine manufacturer's say. They are motivated to satisfy customers by telling them to run the engine all day at 200 rpm off of top. This is often too much for best life. And they are very unlikely to pay a warranty claim during the first two years of warranty. It is only after a 1,000 hours or more that you see the effects of high load.


If that engine is rated for its maximum hp at 2,500 rpm, then I would cruise it at 2,100 rpm or less. But also think about what your sea trial tells you. If the boat is still struggling to get over the hump at 2,100 rpm then you can't plane. You have to run less. Hp requrements at the hump is very high and hard on engines.


I would want to be on a clean plane at 1,900 so I can cruise at 200 rpm more. If you have to cruise too close to the hump then a head sea will drop you off and it will take lots of hp to get back up.


All of the foregoing assumes that this is a planning hull.


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Old 05-31-2016, 11:38 AM   #24
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RickyB,
Note the continous rated rpm is predicated on the engine being propped to rated max power rpm. Engine load is what we need not to exceed ... not rpm. Within limits of course. It is assumed you are propped right to make use of the continous rpm rating. If you're overpropped even 100rpm your assumed continous rpm will probably be more than 100 rpm less than the manufacturer's specs.
So rated continous rpm is only valid if you can reach max rated rpm or slightly more.
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Old 05-31-2016, 11:38 AM   #25
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Originally Posted by djmarchand View Post
Be careful about what the engine manufacturer's say. They are motivated to satisfy customers by telling them to run the engine all day at 200 rpm off of top. This is often too much for best life. And they are very unlikely to pay a warranty claim during the first two years of warranty. It is only after a 1,000 hours or more that you see the effects of high load.
The support group of engine manufacturers is motivated to have the engines last as long as possible, as trouble free as possible. I don't understand your feeling that they have some motivation to give bad information. I've also found engine manuals to sometimes have recommendations spelled out. If one wants to be extra cautious, the way to be so is to take their break in recommendations (those after the first very few hours but before the next threshold) and use those always.

Engine manufacturers have no incentive to cause damage to their own engines.
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Old 05-31-2016, 12:19 PM   #26
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There is nothing complicated on this issue. First, have the engines been maintained properly. Second, is the vessel propped correctly. Third, will the vessel remain comfortably on plane at 2050 to 2200 RPM.
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Old 05-31-2016, 12:39 PM   #27
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The support group of engine manufacturers is motivated to have the engines last as long as possible, as trouble free as possible. I don't understand your feeling that they have some motivation to give bad information. I've also found engine manuals to sometimes have recommendations spelled out. If one wants to be extra cautious, the way to be so is to take their break in recommendations (those after the first very few hours but before the next threshold) and use those always.

Engine manufacturers have no incentive to cause damage to their own engines.
Don't put too much faith into manufacturer's tech manuals. I used to write them and there was a strong incentive to just copy stuff from earlier versions even though the system changed. Make changes and you get all sorts of extra scrutiny.

I see examples of this in some engine manuals: Manual covers a certain engine with many hp ratings. Same "200rpm off the top" is there for a low rated natural or mild turbo engine. New super hot engine comes out and manual has zero changes. Same "200 off the top", or whatever. Even though engine guys cringe at that.
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Old 05-31-2016, 01:16 PM   #28
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Don't put too much faith into manufacturer's tech manuals. I used to write them and there was a strong incentive to just copy stuff from earlier versions even though the system changed. Make changes and you get all sorts of extra scrutiny.

I see examples of this in some engine manuals: Manual covers a certain engine with many hp ratings. Same "200rpm off the top" is there for a low rated natural or mild turbo engine. New super hot engine comes out and manual has zero changes. Same "200 off the top", or whatever. Even though engine guys cringe at that.
I've never run across the 200 of the top routine. I also have manuals specific to the engine, not the range. And if I'm not comfortable I'd talk to the service side of the manufacturer, not the sales side. I'm sure some are just as you say, just not the ones I've encountered.
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Old 05-31-2016, 02:46 PM   #29
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Further to Ski's point above, Yanmar rates ALL of their engines for continuous duty at 200 rpm off of top. Everything from a two cylinder sailboat engine, my 6LY 370 hp turbocharged and after cooled engine and especially the notorious 6LY2 440 hp engine that will quickly self destruct at that rpm are all rated continuously at 200 rpm off of top.

That continuous rating for my engine is 290 hp at 3,100 rpm (3,300 is rated). 290 hp is 55 hp per liter which is about 20 hp higher than what most pros (Tony Athens) recommend for decent engine life. I say decent, because it is a compromise. Best life is probably half of that. OTOH, sportfishermen often run the snot out of their DD TI engines and are happy to get 2,000 hours between rebuilds. But I suspect that we trawler owners expect more engine life than that.

But Yanmar finally got some religion when they issued their propping specs some years ago. Even though my engine is rated at 3,300 rpm they recommend propping it to a minimum of 3,425 rpm. That gives them a 325 rpm hedge against their 3,100 rpm continuous rating. Not enough, but better than 200 rpm.

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Old 05-31-2016, 03:33 PM   #30
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Further to Ski's point above, Yanmar rates ALL of their engines for continuous duty at 200 rpm off of top. Everything from a two cylinder sailboat engine, my 6LY 370 hp turbocharged and after cooled engine and especially the notorious 6LY2 440 hp engine that will quickly self destruct at that rpm are all rated continuously at 200 rpm off of top.

That continuous rating for my engine is 290 hp at 3,100 rpm (3,300 is rated). 290 hp is 55 hp per liter which is about 20 hp higher than what most pros (Tony Athens) recommend for decent engine life. I say decent, because it is a compromise. Best life is probably half of that. OTOH, sportfishermen often run the snot out of their DD TI engines and are happy to get 2,000 hours between rebuilds. But I suspect that we trawler owners expect more engine life than that.

But Yanmar finally got some religion when they issued their propping specs some years ago. Even though my engine is rated at 3,300 rpm they recommend propping it to a minimum of 3,425 rpm. That gives them a 325 rpm hedge against their 3,100 rpm continuous rating. Not enough, but better than 200 rpm.

David
Rating the engine and saying to run it that way constantly are sometimes two different things. Our only Yanmar is on RIB's and while rated at 180 hp is not run at -200 or anything close.

I agree with them grouping engines into manuals the way they do being bad.

Our manual says nothing about running after break-in. It has two break in periods. During the second of those periods (10 to 50 hours), it recommends against extended cruising at low speed, recommends running it primarily at -400 RPM or about 70% load, with 10 minute runs at -200 RPM and 4 to 5 minutes at WOT every 30 minutes.

Now we talked with them as well as others. On a RIB it's certainly not practical to run at 70% load all the time as that's around 30 knots and not the purpose for which we use it. The advise we ended up with which made sense to us (only for our application though) was that after running it 30 minutes at low speed to run it 4 or 5 minutes at 70-80% load. We typically run around at 15 knots or so, some at around 25 knots but then do our 30 knot runs. If we use it for 6 hours or so in a day, we'll typically open it up to WOT for about 2 minutes.

I admit to their guidelines, or lack thereof, not being very useful. They also tend to not even realize that some of their engines are not in trawlers but in boats that only weigh 2400 pounds including the engine.

Regardless, the most hours we have on a Yanmar diesel at this point is about 120 hours so we can't speak as to what kind of durability we might have running as we do.

We don't have props and they will turn at WOT full RPM of 4000. We have no continuous rating on them as they're really not designed for continuous running in our situation.

That said, on every other diesel engine we have whether propulsion or generator we have far better information in the manual plus available from them. That includes MAN, MTU, Cummins (Onan), John Deere (Northern Lights), and Kohler. I can't speak as to anyone beyond that.
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Old 05-31-2016, 04:42 PM   #31
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Cutting through all of the BS, mine included, this is what I would recommend as a max continuous cruising rpm for that engine:


The OP's engine produces 300 hp from 6 liters at 2,500 rpm. Using the 35 hp per liter rule of thumb for decent engine life, that is 210 hp. That engine should be making about 210 hp at 2,200 rpm if propped to reach 2,500 at wot.


So, 2,200 would be my recommended max cruising rpm. But since it is a planning hull be aware of my comments about cruising near the hump.


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Old 05-31-2016, 06:54 PM   #32
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Sea,trials seemed to go well. The engines hit the 2500 rpm & kept up on plane at 2200 rpm. The boat weighs 34,000 lbs. The Floscans showed about 1 mpg at 20 mph. They have been maintained by the yard since new in 2008. The man had deep pockets & was very fussy about all mechanical maint.
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