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Old 02-08-2013, 06:58 AM   #21
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"That is the argument for overpropping a slow speed trawler. But the effect is rather small and has nothing to do with the type of fuel injection."

Not really,

The argument for "overproping" moving the engines actual cruise RMP down considerably so the HP required and the HP produced at that lower RPM are more closely matched.

As noted diesels work best at heavy load , not just at peak RPM but also a lower RPM loaded highly..

In other words if your boat needs 50 HP at the prop, it could be created with a the MFG preferred top speed prop at a higher RPM than a prop selected on experience for your boat.

Creating 50 HP at say 1900RPM (where the engine might male 110HP is noisy ,not fuel efficient and does not load the engine properly.

Select a cruising pitch and diameter prop and at 1500 or 1600 the engine may only be able to create 60-70 HP, a far better load factor with 50 HP required.

The GB folks stock over pitch must have figured their boats would be cruised at modest displacement speeds and set cruise RPM accordingly.And the owners could enjoy low noise , low fuel burn and longer engine life.

I guess they thought the 1nm and 1/2 NM per gallon cruisers would select a different boat to go fast.

Usually about 1 HP to 50 LBS of boat is required for this speedy style cruising.
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Old 02-08-2013, 09:55 AM   #22
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May I suggest readers acquire a copy of:

Internal Combustion Engine in Theory and Practice | The MIT Press

Then revisit the discussion after digesting its contents.
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Old 02-08-2013, 11:46 AM   #23
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Fred:

We have had this debate before on the merits of overproping. Let's do it again but with facts rather than opinions. Take a look at the Cummins spec sheet for the 6BT 210 hp engine at: http://www.sbmar.com/Engines/PDF/6BT...20Nov%2000.pdf

This is an engine used in some of our trawlers and is a real workhorse. The spec sheet is one of the few that has fuel consumption curves for both wot loads and prop loads which makes it easy to compare the two.

So look at the prop curve and you will see that 100 hp is produced at a bit more than 2,000 rpm and it takes about 6 gph or 16.7 hp per gph. That is the place many trawler owners would operate at fast semi displacement speed, say a GB 36 at 8+ kts.

Now overprop the same boat and engine severely so that the 100 hp is produced at about 1,200 rpm which is on the wot output curve. The fuel consumption is about 7 gph.

Now these are extremes and no one should think about overpropping that much, as you will quickly ruin the engine. And I have no doubt that overpropping a little will actually result in fuel consumption below 6 gph. But not by much- maybe to 18 hp per gph or about 5.5 gph.

So I think that a 10% fuel consumption improvement is all you are ever going to see by overpropping. In my mind that just isn't worth the risk of overloading your engine if you ever push the throttles forward.

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Old 02-08-2013, 11:52 AM   #24
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May I suggest readers acquire a copy of:

Internal Combustion Engine in Theory and Practice | The MIT Press

Then revisit the discussion after digesting its contents.
Why confuse an evergrowing thread with the facts
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Old 02-08-2013, 12:15 PM   #25
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Fred:

We have had this debate before on the merits of overproping. Let's do it again but with facts rather than opinions. The spec sheet is one of the few that has fuel consumption curves for both wot loads and prop loads which makes it easy to compare the two. So I think that a 10% fuel consumption improvement is all you are ever going to see by overpropping. In my mind that just isn't worth the risk of overloading your engine if you ever push the throttles forward.

David
David

Further, a surveyor worth his salt will not approve an engine/prop setup that cannot achive rated RPM.
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Old 02-08-2013, 12:21 PM   #26
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What if the rated max rpm is a range? My Volvo Penta manual WOT is 2700-3000. Mine can achieve 2975 and I still think it is a bit over propped as my idle rpm hull speed is too high.
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Old 02-08-2013, 12:25 PM   #27
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I still think it is a bit over propped as my idle rpm hull speed is too high.
What is your idle rpm and speed?
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Old 02-08-2013, 12:27 PM   #28
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David

Further, a surveyor worth his salt will not approve an engine/prop setup that cannot achive rated RPM.
Surveyors don't "approve" anything and it wouldn't show up as a "safety" issue so insurance companies wouldn't care either.

Marin's statements about older GBs coming from the factory kinda dispute your statement as I doubt in the day surveyors were giving thumbs down to brand new GBs.
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Old 02-08-2013, 12:38 PM   #29
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Spy,
I'm going to assume you actually have an appropriate idle rpm. I think that propeller speed v/s boat speed affects the idle boat speed. Lets say you have a direct drive gear and another identical boat has 10-1 reduction. I think the 10-1 reduction boat's idle speed will be considerably slower than the direct drive. This is admittedly a very extreme example but sometimes extremes will make it obvious what small differences fail to show.

Could it be that you have a 1.5-1 gear? My W30 has a 2.57-1 ratio and has an acceptably slow idle boat speed. And I idle at 900. I have the idle set a tad high so I can get underway soon w/o any danger of my engine quitting while shifting gears. The engine idles nicely at 700 but I could back out of my slip and possibly into another boat if I set it at 700.
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Old 02-08-2013, 12:44 PM   #30
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The reason surveyors won't or don't like to survey a boat that isn't propped properly is that a proper evaluation cannot be made of the boat unless rated rpm can be obtained.

Manufacturers never recommend over propping.

Overpropping is not adult behavior.
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Old 02-08-2013, 01:30 PM   #31
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Most surveyors really never say much about the engine and propping as they often leave that up to a separate engine survey....at least not on the East Coast.

Now THAT survey may or may not make a big deal about it.

Adult behavior? Not even worth the effort.
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Old 02-08-2013, 02:23 PM   #32
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Marin's statements about older GBs coming from the factory kinda dispute your statement as I doubt in the day surveyors were giving thumbs down to brand new GBs.
Surveyors who are not worth their salt will approve most things. I'll let you argue the semantics of what surveyors do and don't do.

Today's GBs with pod drives are propped to achieve full load rated RPM, so says both Volvo and GB. I was on a 10 year old GB a few weeks ago with +4000 hours and it too achieved full rated RPM under load. No rocket science here, just boat diesel 101.
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Old 02-08-2013, 02:56 PM   #33
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What if the rated max rpm is a range? My Volvo Penta manual WOT is 2700-3000. Mine can achieve 2975 and I still think it is a bit over propped as my idle rpm hull speed is too high.
Every marine diesel that I have seen specifies the maximum hp and a single rpm that goes with it. Which specific Volvo model do you have? We can look it up and advise.

If you look at the max hp curve (not the prop curve) of a common rail engine, they often are flat for the last several hundred rpm. But there is always a max rpm and a max hp.

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Old 02-08-2013, 04:04 PM   #34
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Parasitic losses are those associated with external "stuff" driven by the engine and not required for its operation. Things like belt driven auxiliaries, alternators, raw water pumps, air compressors, and so on.

The losses you are talking about are internal losses and are the result of friction and pumping losses. They are measured by "motoring" the engine- driving it with an electric motor - and measuring the power required to turn it over at rated speed.
Thats interesting. Do they calculate the friction loss by measuring the diference in the electric motor's no load amperage and engine load amperage? I wonder what the difference between friction loss in a production engine and the same engine after blue printing and balancing?
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Old 02-08-2013, 04:10 PM   #35
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David,

Do you suppose Volvo may be saying WOT rpm is blessed by them over a range of 300rpm? Meaning that 2700 to 3000rpm full load is OK? Never seen anything like that before for diesels but it's very common w OB engines. Personally I like to be as close as possible or up to 50rpm over rated. That way I cannot over load my engine, don't notice any loss of speed (ther'e is some of course) and the engine seems to run easier. The latter being fully subjective obviously.
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Old 02-08-2013, 04:43 PM   #36
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David

Further, a surveyor worth his salt will not approve an engine/prop setup that cannot achive rated RPM.
It is not a toy boat surveyor's job to approve or disapprove anything on your boat. He should keep his opinions to himself unless you ask, otherwise his role is to document condition so far as can be determined by visual examination. Period.

Unless the surveyor is representing a class society and is conducting a survey related to the vessel's classification or statutory status he has no mandate to bless or condemn anything.
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Old 02-08-2013, 05:31 PM   #37
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He should keep his opinions to himself unless you ask, .
I always do ask, shouldn't everybody? Why is a full throttle RPM test an opinion - it is a fact, either it gets there or it doesn't. A full throttle under load RPM test has always been the case with my sea trial surveyor of choice.
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Old 02-08-2013, 05:55 PM   #38
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I always do ask, shouldn't everybody? Why is a full throttle RPM test an opinion - it is a fact, either it gets there or it doesn't. A full throttle under load RPM test has always been the case with my sea trial surveyor of choice.
And does the engine make it to its top rated RPM? I tend to like an over propped boat as i seem to get better fuel economy that way. Trouble is if over proped at light load thyen load her up I believe you can damage your engine by lugging it. The book says light loat you should get max rated RPM
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Old 02-08-2013, 06:02 PM   #39
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I always do ask, shouldn't everybody? Why is a full throttle RPM test an opinion - it is a fact, either it gets there or it doesn't. A full throttle under load RPM test has always been the case with my sea trial surveyor of choice.
And what is the total amount of info that full throttle test by the typical surveyor tells him or you?

Other than overpropped or not, whether the governor is set correctly or not....those two things are nice to know but buying an engine without warranty is a capshoot no matter how you look at it.

If a previous owner told me he was overpropped and had meticulous records and engine room...why is that any less important than a properly RPMed setup that could have been abused it's whle life but just cleaned up with fake logs???

The oil analysis is MAYBE the only real litmus test.

If the rpm test is the OZ of engine tests for your surveyor...than he isn't worth his salt either....[
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Old 02-08-2013, 06:33 PM   #40
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And what is the total amount of info that full throttle test by the typical surveyor tells him or you?

Other than overpropped or not, whether the governor is set correctly or not....those two things are nice to know but buying an engine without warranty is a capshoot no matter how you look at it.

If a previous owner told me he was overpropped and had meticulous records and engine room...why is that any less important than a properly RPMed setup that could have been abused it's whle life but just cleaned up with fake logs???

The oil analysis is MAYBE the only real litmus test.

If the rpm test is the OZ of engine tests for your surveyor...than he isn't worth his salt either....[
Someone told me the best test of an engines remaining life is in the crankshaft end play. If its large it means the engine is nearing the end of its life. Anyone know if this is correct?
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