Over Propping Questions - Page 2

kev_rm · 10-07-2017, 03:29 PM

just fyi valve timing overlap doesn't have anything to do with forced induction. many normally aspirated high performance engines have loads of overlap.

Nomad Willy · 10-07-2017, 06:15 PM

Quote:

Originally Posted by kev_rm

just fyi valve timing overlap doesn't have anything to do with forced induction. many normally aspirated high performance engines have loads of overlap.

And an open valve or port may be far enough from an injector nozzel so as to not let fuel escape. Also flame travel and air movement inside the cylinder probably plays a part. If air travel was clockwise the injector may be placed so the unburned fuel didn't escape the combustion chamber.

So a fact that a port or valve was open may not mean fuel will escape.

FF · 10-08-2017, 07:43 AM

The key to understanding the "overpropping" discussion is weather the boat is a planning boat or a displacement boat.

A planning boat should get rated RPM at WOT to be safe when operating pulled back a bit.

The displacement boat is matching cruise power and RPM to properly load the engine at a more efficient fuel flow and noise level.

The usual mistake is calling a properly done cruiser "overpropped" when it is just using knowledge and engineering to aid the vessel efficiency.

eyschulman · 10-08-2017, 12:18 PM

Quote:

Originally Posted by FF

The key to understanding the "overpropping" discussion is weather the boat is a planning boat or a displacement boat.

A planning boat should get rated RPM at WOT to be safe when operating pulled back a bit.

The displacement boat is matching cruise power and RPM to properly load the engine at a more efficient fuel flow and noise level.

The usual mistake is calling a properly done cruiser "overpropped" when it is just using knowledge and engineering to aid the vessel efficiency.

understood, but the boat in question is a planning boat with planning power and the OP has no idea how the boat was used by previous owners. He can know how it is presently propped with a proper sea trial. And if it is seriously overloaded good cause to walk or get a discount for engine rebuilds. gambling on rebuild can be a PITA particularly if engine goes in middle of a trip in the middle of nowhere as happened to me in 2007. Best to use caution and good judgment in this instance. As to the theoretical argument for over-propping a FD boat it holds little water in real life where the fuel saved is on most boats such a small fraction of upkeep that its purpose is very questionable as is its efficacy regarding possible engine damage. Engine manufacturers usually specify how to prop a boat those who believe they know better right or wrong are playing with voiding their warranty. My power boat has a 5 year extended warranty which was not granted before a full day of testing by a JD tech including on the water load measurements at every 100 RPM. Actually on high power boats I am a proponent of under propping. This allows for better manners in marinas and docking and is a safety margin for dirty bottom or prop. An under-propped motor can rev up spread more good juices to the engine with less likelihood of overheating or damage. On common rail engines under-propping is like down rating to a lower M rating without paying for the expensive chip to do the same thing.

Nomad Willy · 10-08-2017, 02:34 PM

Very well put eyeshulman,
I like to be propped about 100rpm under so my 3000rpm engine turns 3100 at WOT. On a long grade a truck driver will, upon finding he can maintain speed in 7th gear at WOT, downshift to 6th gear and let the engine sing a bit and back off on the throttle some .. at the same speed.

But FF is right a small amount of fuel will be saved and the engine will make a bit less noise. Some noise comming from torsional vibration may be greater on an overpropped boat whereas valve, chain and belt noise will be higher on the underpropped boat. But generally speaking an overpropped boat will make less noise and burn less fuel. No engine manufacturer recomends it though.

FF · 10-08-2017, 03:14 PM

One measure of long term engine wear is piston miles.

A boat cruising lightly loaded at 2000 RPM will create considerably more piston miles than a boat happy with the same power at 1500 RPM.

Yes a 1/2 or 1 GPH lower fuel burn is minor at a few hundred hours a year ,

and with a modern engine the high RPM vibration might similar , or not too bad ,

the noise level should be way lower on the low RPM boat.

The mfg. want every engine to get past the warranty period and propping a displacement boat like a sport fish works for them.

Weather its best for a displacement boat ?????????????/

Nomad Willy · 10-08-2017, 05:47 PM

FF,
I haven't forgotten my promise to you but I get pulled in.
Piston strokes are one thing but probably more important is piston to cyl wall load .. or side load. The combustion pushes the piston down but it's connected to the crankshaft off center so it has a strong side load depending on combustion pressure that is dependant on engine load. The more the load the more the side load on the piston.

So yes fewer strokes overpropped but higher piston to cyl wall pressure. Of course I don't know what causes the greatest wear but I'm betting on lower rpm and greater piston to cyl wall force or pressure. That's why they say downshift going up steep hills. And "don't lug the engine" is a well established saying older that us.

FF · 10-09-2017, 06:34 AM

Setting an engine to run lower RPM in the BMEP sweet spot should hardly create excessive side wall loads , its where gen sets and big pumps operate to get the biggest service life.

The flip side , a high operating speed with very low load may not create enough combustion pressure to pressurize the cylinder well.

The rings only seat because of combustion pressure behind them , if its too low cylinder wall burnishing occurs , lots of blow by , short oil life , oil hone marks departs and the engine gets really old rapidly.

The cylinder operating temperature will be lower causing carbon deposits to collect behind rings , and probably on valve stems

The longest engine lives seem to come from a well engineered system approach.

Simply screaming the engine with a water ski boat propeller , and pulling back to cruise may not be the best long term setup.

Volvo has noticed this and most of their sailboat engines are "overproped" when using the factory tables.

Cigatoo · 10-09-2017, 09:11 AM

“Volvo has noticed this and most of their sailboat engines are "overproped" when using the factory tables.”

By the same token Yanmar sent out a bulletin, pertaining to certain engines, years ago that they wanted the loaded WOT RPM to be increased to 3450+ not 3300 RPM as originally published. That is with a no load RPM of 3700. Best to get the info from each individual mfg as they should have data based on original specifications and long term field findings for their products.

smitty477 · 10-09-2017, 09:27 AM

General statements about all diesel engines are often loaded with errors as the engines can and do vary greatly.
if you can secure a chart that shows your engines hp produced v s fuel use you may be surprised that it uses less fuel to produce a unit of hp at various rpm.
Knowing that relationship and that all engines are not the same in that area can also help choose the best rpm to operate within.
If I had a fixed speed that I would not want to exceed (as in over propped) It would be prudent to 'pin' the throttle so that the new 'safe' WOT rpm (fuel load) would not be exceeded.
I have witnessed many folks who say they are not going to travel at more than XXX knots get tied up in a current , time constraint or weather issue only to push the throttle(s) much further then their claimed upper limit.

Nomad Willy · 10-09-2017, 11:30 AM

We talk about this frequently but this is becoming one of the better discussions on this issue IMO.
And we're not even off topic.

River Cruiser · 10-09-2017, 09:43 PM

IMHO over propping is never a good thing. The amount of fuel being injected isn't determined by the throttle position. The governor senses the load delivers the amount of fuel necessary to maintain the rpm at that load. As the load increases the governor opens the rack to increase fuel, the governor on boat in question is delivering full fuel at a much reduced rpm. It is always over fueling all the way through its rpm range and it's been that way as long as the props have been on it.

sunchaser · 10-09-2017, 11:06 PM

RC

I may be misreading what you said, but I'm not sure there is a direct correlation between RPM and fuel delivery. Even with a perfectly propped vessel. RPM and fuel consumption will independently vary dependent upon boat load, sea state and wind velocity.

stanfromhell · 10-09-2017, 11:34 PM

Thanks eveyone for your well thought out and very informative comments, I truly appreciate your responses.
I contacted Carver and the props on the boat are the same diameter and pitch as when the boat left the factory.
So......Props are correct, hull is clean, injectors have recently been rebuilt, no black smoke when running at 2100 RPM, engine hours are low, both exhibit identical sympoms and neither has excessive blowby.

Perhaps the wrong nozzles were installed when the injectors were rebuilt, but I'm not going to take a chance where the wrong decision could cost many, many thousands of dollars somewhere down the road.

My search for the perfect boat continues.

CaptSteve53 · 10-10-2017, 12:13 AM

Moving backwards to my original mundane post-Have you checked the throttle is moving to the full throttle stops on the injection pumps?

Is there enough air getting to the engines?

Just saying!!

Cigatoo · 10-10-2017, 06:28 AM

“I contacted Carver and the props on the boat are the same diameter and pitch as when the boat left the factory.“

There lies your problem. That boat was propped to perform correctly as it left the factory. Now you have thousands of pounds added to the weight of the boat and therefore the props have to be adjusted to compensate for the extra weight / load

Go back to basics as Capt Steve mentioned above. Or get a qualified diesel guy to look at it. YOU DO NOT WANT TO BE OVERPROPPED.

psneeld · 10-10-2017, 07:20 AM

Some tips from a marine engineer who works with diesels every day.....

"The governor senses the load delivers the amount of fuel necessary to maintain the rpm at that load."

Unless his recreational trawler has been retrofitted with a torque sensitive governor (in which case this whole discussion is moot) his governor only senses and responds to rpm changes either side of the setpoint determined by the "throttle" lever.

"As the load increases the governor opens the rack to increase fuel ..."

No, as the rpm decreases below setpoint the governor increases fuel flow.

" ...the governor on boat in question is delivering full fuel at a much reduced rpm."

The only time the governor would deliver full fuel flow is when it can no longer increase rpm to the setpoint. At that time the governor should limit fuel as the internal delivery control reaches the "fuel stop." It would take a seriously "over propped" condition to have this occur at any rpm setting below maximum.

"It is always over fueling all the way through its rpm range and it's been that way as long as the props have been on it."

The only time the governor would "over fuel" is when the load on the engine exceeds that which the engine can produce at a given rpm. This can be determined by the engine power curve. The propeller can be grossly over pitched or oversized but until the torque required to turn it exceeds what the engine can produce at a given rpm it will not present an overload condition.

Cigatoo · 10-10-2017, 08:02 AM

Quote:

Originally Posted by psneeld

Some tips from a marine engineer who works with diesels every day.....

"The governor senses the load delivers the amount of fuel necessary to maintain the rpm at that load."

Unless his recreational trawler has been retrofitted with a torque sensitive governor (in which case this whole discussion is moot) his governor only senses and responds to rpm changes either side of the setpoint determined by the "throttle" lever.

"As the load increases the governor opens the rack to increase fuel ..."

No, as the rpm decreases below setpoint the governor increases fuel flow.

" ...the governor on boat in question is delivering full fuel at a much reduced rpm."

The only time the governor would deliver full fuel flow is when it can no longer increase rpm to the setpoint. At that time the governor should limit fuel as the internal delivery control reaches the "fuel stop." It would take a seriously "over propped" condition to have this occur at any rpm setting below maximum.

"It is always over fueling all the way through its rpm range and it's been that way as long as the props have been on it."

The only time the governor would "over fuel" is when the load on the engine exceeds that which the engine can produce at a given rpm. This can be determined by the engine power curve. The propeller can be grossly over pitched or oversized but until the torque required to turn it exceeds what the engine can produce at a given rpm it will not present an overload condition.

Exactly correct. The boat is overpropped.

Cigatoo · 10-10-2017, 08:35 AM

I would also be willing to bet that if you were to add EGT gauges you would see that the EGT is much higher then what the mfg is calling for. The injection system is dumping fuel into the engine, trying its best to regain the requested RPM. All that extra fuel is just creating heat that does damage to the engine.

Those props were set at the factory to help sell boats. They want to highlight maximum speed and fuel efficiency. As soon as you start adding weight you are asking for more HP and torque than the power plant can produce. You have to either decrease the load (remove weight) or change the torque requirements via decreasing the pitch on the props.

sunchaser · 10-10-2017, 10:15 AM

PS
Thanks for post 37 and the imbued knowledge from a solid marine engineer on how diesel governors work.

When cruising offshore and climbing large swells, the momentary increased fuel consumption and harder working engine at the same RPM, are clearly evident.