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Old 01-19-2015, 08:25 PM   #21
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Why does a couple hundred rpm seem like such a mystery on relatively low performance engines?

I don't think I have ever heard anyone suggest over propping high performance diesels in a go fast sportfish.

If the big deal is always about some specific engine being overpropped...then be specific....most people did using over propping as far as I can tell are usually discussing old iron .

I knew when every lawnmower I ever owned was cutting well or not...yet I never balanced the blade, cleaned the deck and performed a WOT test on any Briggs and Stratton.

Yet I always had nice lawns....
I hear ya. But my engines are 330 HP common rail full electronic and they may need different handling. As for cutting grass on my three previous horse farms that was done with mainly diesel tractors and Zero turn radius diesel cutters All needing considerable maintenance. Yes on my sailboats with diesels from 10 to 75HP I pretty much handled the way you say. The newer motors are the bigger issue with over propping than the old iron. If you have a twenty or thirty year old boat with old iron and you run slow with over propping you are probably in there with most sailboats and the motor may outlive you. Try that with a 440 Hp Yanmar and bang.
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Old 01-19-2015, 08:33 PM   #22
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I hear ya. But my engines are 330 HP common rail full electronic and they may need different handling. As for cutting grass on my three previous horse farms that was done with mainly diesel tractors and Zero turn radius diesel cutters All needing considerable maintenance. Yes on my sailboats with diesels from 10 to 75HP I pretty much handled the way you say. The newer motors are the bigger issue with over propping than the old iron. If you have a twenty or thirty year old boat with old iron and you run slow with over propping you are probably in there with most sailboats and the motor may outlive you. Try that with a 440 Hp Yanmar and bang.
don't disagree...but like thruster threads....it seems like people get rabid about 2 different philosophies when in reality good boaters use'em if they have 'em and learn to drive safely without'em if they don't have one.

seems like people who have a reasonable understanding of engines would only do it in certain situations with certain engines...not every boat they might own....

but like so many TF discussions....one size doesn't fit all and saying that something is impossible or shouldn't be ever done....well...whatever....but certainly will get some feedback from someone who knows that those two circumstances rarely exist in the real world.
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Old 01-19-2015, 09:16 PM   #23
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Overpropping or underpropping, pretty generic terms.

What's really getting discussed is matching load to the engine. Lots of info needed to do this right. Engine details, boat details, duty cycle, existing sea trial data, customer preferences.

As posted above, high speed boats with high specific output engines it is a BIG DEAL to get rated rpm+ at full power and full vessel load. If that boat can't make turns, it is hard on the engines even when cruised 200rpm off top.

But let's take an example closer to typical for this site: An old school NA diesel in a trawler, where cruise is 7kts and full power is 10 at 2500. At 10, boat is plowing badly and eating fuel and making noise. But it is prop'd right according to the full load rpm criterion.

Now go back to the 7kt cruise. Say it is at 1900rpm and burning 3gph. That's probably about 50hp. Go look at full load engine curve and at 1900 say the engine is developing 100hp. So at cruise engine is at 50% load. 1900 is well above torque peak of 1600, which is usually where engine is most efficient.

So let's prop it to 7kts at 1600. Same 50hp. At 1600 at full load from engine chart, it makes 75hp. So prop'd this way, load is 66%, a happy place. Boat is quieter and probably a touch more efficient. Quieter is a big deal to some, and the primary basis for doing this.

Now engine will be overloaded if you go to full power, or even around 2000. Is this wrong? Depends on the owner and what they want.

Note those numbers came out of my head from a recent inspection/calculation, not exact and just for demonstration.

I do lots of engine inspections and many trawlers or other low speed craft are "overpropped" using the full power criterion. But at cruise load is better matched. As long as the operator understands that trying to climb the "hump" is pointless and puts the engine in the red zone, no big deal.

If a NA trawler engine can't make full rpm on survey, that is not an automatic flunk. There are enough ways to assess the engine and determine its health even if it can't make turns. It will be noted and discussed, though.

Nothing wrong with load matching where the engine is going to operate. If you are going to operate it there!!
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Old 01-19-2015, 10:13 PM   #24
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Very very good way of putting it Ski. Best definition of over propping I've heard. And like FF says "Cruise Prop".
Everything about it is good as in real good. I understand it and have for years but your last sentence is where the monkey wrench falls into the spinning gears. "If you are going to operate it there". And never stray into the redline where overloading begins. Sounds easy enough to do but most don't know where on the rpm scale the overloading begins. EGT is said to identify that spot but how dependable is the EGT meter reading? What if you're off in locating the probe? Will the temperature be off 10 degrees .. 100 degrees? I've never used an EGT gauge. When I flew ultralight aircraft I used the cylinder head temp gauge w sensor under the spark plug seat.

If over propping is done skillfully and the limitations rigidly adhered to the cruise prop loading is indeed optimized. And to be really successful one would benifit greatly from an overpowered boat application. In noise reduction. Beyond that no significant advantage exists .... IMO.

But I assumed most would eventially stray when the current in the narrows was greater than expected and another 150rpm was needed to get through in an acceptable length of time. What do you think Ski? Perhaps I've been too pessimistic thinking most would eventially over load the engine. What's your guess on that one?
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Old 01-19-2015, 11:27 PM   #25
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True, last sentence is key. But these engines are not going to blow up as soon as you get to overload. Many run that way for years and do fine. Not good for them, sure, but not fatal, mostly!!

I put this into a category of other boating precautions:

Don't run engine in overload
Don't overheat or run out of lube oil
Don't go offshore into a hurricane or nor'easter
Don't run into channel markers
Don't run your honda gennie in lazarette
Don't come into an inlet at night with a whitewall of breakers
Don't run out of fuel
Don't nav with your plotter in big scale in the Indian Ocean
Etc, etc

Just another rule to follow, the overload thing. Not as bad as dropping a wrench into a reduction gear. Not a big deal compared to other considerations.
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Old 01-19-2015, 11:45 PM   #26
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I suppose one could put some kind of mechanical limitation on a over propped engine so the throttle can not go into the red zone.
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Old 01-19-2015, 11:47 PM   #27
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Isn't that's what governors are for?
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Old 01-20-2015, 12:37 AM   #28
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Isn't that's what governors are for?
Yes if the DIY can adjust same it is the mechanical stop, but I think most over propped boats don't go to that trouble and many probably don't use a EGT to monitor depending mainly on RPM setting and the judgment of the hand on the throttle. Its sort of a fly by the seat of your pants thing. Fortunately most old iron is very tolerant of abuse and only a few abused engines will completely fail same as smoking Many will have some degree of lung damage but only a few will get cancer.
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Old 01-20-2015, 01:08 AM   #29
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Another reason I wouldn't want to spend the money to overprop is because the idle speed is quite fast enough.
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Old 01-20-2015, 09:30 AM   #30
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The concept of overproping is purely for the folks that will be OPERATING at that RPM.

Matching the load to the most efficient range the engine is built for is the key to cruising.As SKI noted.

Everyone Balks at the use of an EGT gauge , which is only about $100 or so.

1/10 a boat buck to have the engine perhaps last 2x as long due to proper loading.

The sad part is the engine MFG have fuel maps (but wont give them out) that would allow the engine to be properly sized and operating range matched to the boat.

With the current , set it up as a ski boat or sport fish,(operate at full throttle RPM) however inefficient, the engine will get past the warantee period , the Mfgs only concern.

If its sucking 50% more fuel and is 500-800 RPM over what the boat requires , thats not the Mfg concern.
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Old 01-20-2015, 10:20 AM   #31
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Discussing any aspect of engine/drivetrain performance needs to be limited to like kind equipment. An engine originally produced to drag a bus loaded with apacked passenger compartment and pigs,chickens and a side of beef tied to the roof over dirt roads in a mountainous 3rd World country will give significantly different performance and sensitivity to operating ranges than an engine designed with an absurd focus on emissions and light weight. The first design will last for years lugging at full throttle up a grade, overloaded, blowing black smoke from the exhaust. The latter will have to be tended to by a geek in an immaculate jumpsuit with secret software tools to get the mixture just right for optimized fuel burn/ emission ratios. Any operation outside of the factory specs voids the warranty and burns off the tops of the super light pistons. YMMV/IMHO/ETC
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Old 01-20-2015, 10:46 AM   #32
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The more expensive boats I sell almost always have an engine surveyor and they always make a big deal about reaching top design RPM. They say how do you know that the engine is working properly if it will not get to the correct RPM. Is it the prop(s) or is it turbo charger, dirty fuel, bad injectors?
If you decide to over prop your boat, save the old prop to use when you put the boat on the market in the future.

Last week I did an inspection of a very large yacht with 3 propulsion engines. The Captain and engineer were anal about being able to assess overall engine performance at full rated RPM. Very detailed check lists were in place as to what to look for at various RPM when doing full load tests.

Fuel burn data was available for runnning 1, 2 or 3 engines on long passages. Guess what - best economy was achieved with all 3 running at low RPM and around 1.1 hull speed.
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Old 01-20-2015, 10:59 AM   #33
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If its sucking 50% more fuel and is 500-800 RPM over what the boat requires , thats not the Mfg concern.
Not my experience. Some years ago I bought a new SeaRay. The dealer gave me spare props to try out to insure the vessel would achieve full rated RPM at the 4000' lake altitude. It was SOP for this very large dealer.

Usually wrong propped new boats are as much the fault of the buyer as the builder. One reason is the self perpetuating myth of the virtues (yes, what are they?) without any downside noted of over propping.
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Old 01-20-2015, 11:58 AM   #34
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Another reason I wouldn't want to spend the money to overprop is because the idle speed is quite fast enough.
From experience I have discovered in boats with large engines that slow speed maneuvering is much enhanced by UNDER propping so that at idle speeds the boat does not lurch when put in gear. This was a real problem on my previous boat also keeping within marina speed limits was a problem had to go in and out of gear. My present boat is under propped and behaves very well around docks and in restricted speed areas. For this boon in low speed management I am more than willing to give up 2 Knots at the top end and a possible small amount of fuel efficiency. the cost of fixing hull dings was not insignificant.
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Old 01-20-2015, 12:00 PM   #35
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Mark,
Yes .... Once the point of over load rpm is established the governor could be adjusted to limit engine speed/load to safe limits. You and I could live w that but most others here would want extra power and the option to use it.

Re the post above I liked the boat much better at low speeds around harbors and marinas. Much less in and out of gear motion and the clutches even have an easier job of it. Ideally we'd have a diesel/electric boat that can run in gear at any rpm ... 75rpm if you want.
When making a landing one must stop the boat w reverse gear and now I'm very optimized for that. I'm propped to rated rpm (3000) and I have a prop w a symmetrical shape that has very good reverse thrust. For an eight ton boat w only 40hp stopping w reverse gear is not a high performance event. But what I like most is that the engine runs much more freely or effortlessly. That's a subjective statement but the fact is that there's less load on the engine. Going at a given speed the prop sees the same load over propped or under propped but the engine sees less load per piston stroke. Less force per stroke but the same torque and power is sent to the gears and shaft because there's more strokes. So the engine is doing it's job more easily and it can be felt ... by the operator and passengers and the engine itself.
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Old 01-21-2015, 08:30 AM   #36
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The confusion is the word OVER,

An OVER loaded engine is much ungood .

Propped to match the engine to the load is good! it is not an OVER load

To not OVERLOAD an engine is very simple , run full throttle , note the RPM and pull back 10%.

At that setting the exhaust should be clean.
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Old 01-21-2015, 10:16 AM   #37
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It must be more complicated FF. You say pull back 10%. Ten percent of what? Ten percent of max load it would seem. But then you'd need very good fuel burn numbers and if it was 10% of throttle shaft rotation how would one measure it? Baaaah ......

EGT temp and rpm observation seems far simpler and much more accurate.
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Old 01-21-2015, 10:21 AM   #38
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Last week I did an inspection of a very large yacht with 3 propulsion engines. The Captain and engineer were anal about being able to assess overall engine performance at full rated RPM. Very detailed check lists were in place as to what to look for at various RPM when doing full load tests.

Fuel burn data was available for runnning 1, 2 or 3 engines on long passages. Guess what - best economy was achieved with all 3 running at low RPM and around 1.1 hull speed.

Did you mean 1.1 *sq rt of water line?? Hull speed term means so many different (wrong) thins to people.
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Old 01-21-2015, 10:28 AM   #39
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Given that a slow turning prop has more slippage and is less efficient many elect to increase pitch in order to gain back some of the loss. Since diesels are most efficient under load and that only happens during acceleration or at WOT we clearly have oodles of horsepower headroom available. We generally pitch the prop to the intersection of two curves namely the engine load curve which is linear and somewhat convex and the propeller load curve which is exponential and concave. All the area between these curves represents safe combinations of pitch/load and rpm/hp/fuel burn.If we had controllable pitch props we could optimize and fly closer to the flame in order to gain peak efficiency. If you decide to play around with this a pyrometer can give you an indication of how hard your making the engine work. One other reason for propping to WOT is that any performance issues the engine may be experiencing will be revealed at WOT.


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Old 01-21-2015, 10:59 AM   #40
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Did you mean 1.1 *sq rt of water line?? Hull speed term means so many different (wrong) thins to people.
Yes. 1.1 by my less than clear meaning would be about 20% below sq rt 2.
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