Overpropping and Engine Strain

[Lakesailor]

I cannot understand how you are all wilfully misunderstanding what I have posted.

I did mention the sweet spot for engines.

I haven't suggested running at low revs, I said keep the revs down.

I haven't suggested that you will use less fuel, power output is a related to the fuel burned. However a diesel will meter more fuel at higher revs even though that amount of power is not required.

What I have suggested is that (and the replies back this up despite seeming to wish to contradict my post) that an engine turning over at higher revs than it needs to push the boat is not under full load and revving an engine needlessly will have a detrimental effect on it's life.

 

[Tranona]

I cannot understand how you are all wilfully misunderstanding what I have posted.

I did mention the sweet spot for engines.

I haven't suggested running at low revs, I said keep the revs down.

I haven't suggested that you will use less fuel, power output is a related to the fuel burned. However a diesel will meter more fuel at higher revs even though that amount of power is not required.

What I have suggested is that (and the replies back this up despite seeming to wish to contradict my post) that an engine turning over at higher revs than it needs to push the boat is not under full load and revving an engine needlessly will have a detrimental effect on it's life.

It will only rev too high if it is underpropped. As I tried to explain, running at higher revs has minimal effect on the life of a diesel engine. Typical yacht engines never even get near half life, let alone wear out!

You are in fact making the case for optimally propping the boat so that you get cruising speed at around 70%+/- max power, with reserve for catching the tide or punching into heavy weather.

 

[Heckler]

Been reading some articles suggesting slight overpropping to acheive hull speed at lower than max engine revs. For example on an engine that will rev up to 3600 rpm you prop it so that it maxs at 3200 and so at 2600rpm you will get the same boat speed as would need 3000 rpm on an engine propped to max at 3600rpm. Lower rpm means lower fuel consumption and quiter cruising with the trade off of not having max hp available in a head sea etc

All fair enough but I thought overpropping meant putting strain on the engine-from what these articles suggest however the engine only starts to suffer if you try to get her to max revs which she can't do and starts throwing out black smoke etc. Provide you run the engine a couple of hundred rpm below this point she isn't under any undue strain so shouldn't affect engine life etc.

Anyone know whether this is right or does carrying too large a prop put a strain on the engine accross the complete rev range? Interested because it seems I may be slightly overpropped-although by accident rather than design!

You need to understand how a diesel works, if you overprop and the engine is powerful enough, then it will use the same amount of fuel at low revs as hi revs. The governor just opens up more, more fuel used at lower revs!
Stu

 

[GrahamM376]

The danger of overproping is that you can't get full power when you need it. With our previous engine/gearbox/prop setup, engine revs were governed by the prop which left us operating at max (prop governed) revs 26/27 HP, not the engine's full potential of 35HP. Opening the throttle more just produced black smoke.

A boat may well reach its theoretical hull speed quite easily at fairly low revs in smooth conditions with a tail wind but, you may well need the max BHP at much higher revs when heading into a chop and strong headwind just to make good progress, never mind max hull speed.

 

[TQA]

It is myth that you reduce fuel consumption by running at lower revs. If you need 20 hp to drive the boat at cruising speed, as in my boat, then you will use exactly the same amount of fuel if you are running a 30 hp motor at 2500 or drawing 20 hp at 2100 if you had the optional 40 hp engine.

If you look at the specific fuel consumption curves of a typical small diesel, you will find the grammes per KW/H is virtually flat over the middle range, meaning that the marginal increase in fuel is linear to the marginal increase in HP (or KW) used. It only starts to climb when you get to the last 20% of the rev range.

It is no myth and the second part of your quote supplies the information that supports the statement that a diesel engine will have improved fuel consumption if run at a lower speed PROVIDING that the speed does not drop below the speed at which max torque is generated. If the torque curve is flattish then the difference may not be great BUT THERE WILL BE A DIFFERENCE.

Running a diesel at lower revs has minimal effect on life - indeed it could do the opposite as running cool and not putting load on the engine can lead to bore glazing. .

I think some one else has posted the explanation but I want to make it clear that a modern diesel has type of governor commonly called a 'constant speed' governor.

The throttle lever controls the "desired " engine rpm. The governor then controls the fuel supply to maintain that engine rpm. If the engine is loaded up somehow then it may require full fuel to maintain that rpm. So it is quite possible to have an engine working hard at low rpm.

Finally there is a great deal of statistical evidence from the records kept by bus and commercial vehicle operators that running a diesel at low rpm say 2000 rpm instead of 2500 rpm will definitely and significantly increase engine life all other factors being equal.

 

[Deleted member 36384]

..... Finally there is a great deal of statistical evidence from the records kept by bus and commercial vehicle operators that running a diesel at low rpm say 2000 rpm instead of 2500 rpm will definitely and significantly increase engine life all other factors being equal.

That's interesting and is the point that Lakesailor was making I believe. My Perkins 4236 is just over 40 years old and is sound with no noticeable oil consumption and instant starting on the key. However, as Tranona states most marine diesels do have a long life.

She does leave a very light fuel sheen on the water at tick over. I believe she has always been over propped. I dont know what are best rpm would be if she was normally propped but I do not need to go above 1500 rpm.

 

[Tranona]

It is no myth and the second part of your quote supplies the information that supports the statement that a diesel engine will have improved fuel consumption if run at a lower speed PROVIDING that the speed does not drop below the speed at which max torque is generated. If the torque curve is flattish then the difference may not be great BUT THERE WILL BE A DIFFERENCE.

Finally there is a great deal of statistical evidence from the records kept by bus and commercial vehicle operators that running a diesel at low rpm say 2000 rpm instead of 2500 rpm will definitely and significantly increase engine life all other factors being equal.

Most of the torque curves (and specific fuel consumption curves) of small diesels are flat between 2 and 3000 revs, which is the operating range for most purposes. Doubt you would notice any difference in efficiency in normal use - for the very reason you say.

As to life, it is a mistake to try and relate experience in one set of operating conditions with another. Bus and commercial vehicle engines have a very different operating cycle from yacht auxilliaries. As I noted earlier, an auxilliary has a potential life of 8000 hours - 80 years of a typical yottie usage. Mechanical wear is rarely an issue. Engines fail because of lack of use, failure of ancilliary components, poor installations and neglect - long before bores and bearings wear out.

 

[GrahamM376]

I think some one else has posted the explanation but I want to make it clear that a modern diesel has type of governor commonly called a 'constant speed' governor.

The throttle lever controls the "desired " engine rpm. The governor then controls the fuel supply to maintain that engine rpm. If the engine is loaded up somehow then it may require full fuel to maintain that rpm. So it is quite possible to have an engine working hard at low rpm.

Finally there is a great deal of statistical evidence from the records kept by bus and commercial vehicle operators that running a diesel at low rpm say 2000 rpm instead of 2500 rpm will definitely and significantly increase engine life all other factors being equal.

Not long after we bought this boat, we had the rings seize in No.2 cylinder and the crankcase compression blew all the oil out. When I contacted Diamond Diesel (the UK importer of Mitsubishi engines) for new rings, the comment was "another one been motor sailing for long periods at low revs?" Obviously, I don't know the boats previous history

The warning received was that these modern marinised engines are mostly industrial plant engines designed for constant running at 3000 rpm and, running at low revs with little load, as in motor sailing, causes excess unburnt fuel to gum the rings up, as does overpropping to the extent of black smoking. Further advice was every now and then put the throttle against the stop for a while and give it a good blow out.

The fuel graph for the Thornycroft T105 (Mitsubishi) I now have shows -

(grams/HP.h) 200 @ 1000rpm, 192 @ 1800rpm (max torque) 200 @ 3000 rpm (max bhp) Obviously, most efficient is at max torque.

Running a car or bus at lower revs without too much labouring does save fuel, they're talking about changing up a gear rather than reving the bol****s off it but a road vehicle engine has different load characteristics. I took part in some car "economy runs" back in the 60's (OK carbs, not fuel injection) and the trick was to keep in as high a gear as reasonably possible and try to maintain a constant speed.

 

[Heckler]

It is no myth and the second part of your quote supplies the information that supports the statement that a diesel engine will have improved fuel consumption if run at a lower speed PROVIDING that the speed does not drop below the speed at which max torque is generated. If the torque curve is flattish then the difference may not be great BUT THERE WILL BE A DIFFERENCE.




I think some one else has posted the explanation but I want to make it clear that a modern diesel has type of governor commonly called a 'constant speed' governor.

The throttle lever controls the "desired " engine rpm. The governor then controls the fuel supply to maintain that engine rpm. If the engine is loaded up somehow then it may require full fuel to maintain that rpm. So it is quite possible to have an engine working hard at low rpm.

Finally there is a great deal of statistical evidence from the records kept by bus and commercial vehicle operators that running a diesel at low rpm say 2000 rpm instead of 2500 rpm will definitely and significantly increase engine life all other factors being equal.

My point exactly
Stu