Way Over Propped but it Works, is it Safe?

[DumnMad]

I think some engine data is for trucks with a little air resistance, not for boats with water resistance. Low revs/constant speed in water, say 1000 to 1200, is loaded but low revs/constant speed in trucks is not.

[Ex-Calif]

Originally Posted by djmarchand

Overpropping causes two problems: heat and stress. The heat can burn pistons and drop valves. But if he keeps the EGT under control that won't be a problem. The stress is additional load on the bearings and pistons. But that engine probably has tons of bearing area and can take it.

I am just surprised it is a 2,700 rpm engine. Most of the really heavy iron is slower than that, like the Gardner. But he says that the engine will rev to 2,700 rpm out of gear so unless the governor is screwy, it must make its peak hp at about 2,500 rpm under reasonable load.

David

Yup - A robust 6 cylinder Japanese block. I am not at all surprised at a 2700 RPM recommended running RPM but I don't necessarily subscribe thoat one "must" run at 2500-2700 RPM.

If the engine was overheating it would show up in EGT (a good monitoring to add as djmarchand says) but also there should have been water cooling issues, blow over etc.

As an aside I would also strongly recommend getting a handheld IR scanner and periodically shoot the temps at the injector for each cylinder and keep track of them for consistency and shifts.

I've been on a couple of boats with 6 bangers and the skippers always pop below while motoring and shoot temps. Takes 5 minutes.

We all agree that pushing this engine "beyond" its torque ability to turn this prop would be a bad thing denoted by smoke etc. A gas engine would likely be detonating and certainly result in infant failure.

Someone quoted burning holes in 2 sets of pistons - interesting data point but could be totally non-relative. What engine, what manufacturer, what HP etc., how was it run etc, etc. This thread could take a deep intellectual turn to talk about that engine and at the end of it still maybe not relate to OPs engine.

[jimbunyard]

Has anyone heard of duty ratings? In a nutshell, an engine is rated for power based on its usage, a continuous usage carries a lower power and rpm rating than an intermittent usage. They way they're rated varies by manufacturer, but the principle is the same. From what ti325v says about his usage, it seems that the increased load of the oversized prop is offset by his lower rpms and light to moderate daily usage.

For anyone who is unfamiliar with the concept, links to four manufacturers' explanations of duty ratings are listed below:

https://www.deere.com/common/docs/pr...e_brochure.pdf

http://www.volvopenta.com/SiteCollec...0range/Marine%

20Commercial/Engine_Range_MC_200http://cumminsengines.com/ratings-and-definitions6.pdf

http://www.propellerpages.com/downlo...tion_Guide.pdf

[ti325v]

Gentlemen:
I had a revelation today...
Of course we had the boat out and I had it stopped with the engine hot in neutral, I open the throttle wide and much to my happiness, (and my staff and customers relief no BANG !) but you could clearly hear the turbo spool up and the tach came to a steady stop at 2800 rpm, no more or less just as predicted by yanmar. It was not on the peg at 3000.
That and the correspondence I have had with yanmar, the anecdotal evidence I have from some literature..I am gonna prop it for 2800.
It was not objectionably loud at 2,200 to 2,300 where we will likely be running it. In fact I like hearing the turbo whine. Reminds me of an old nissan 300z turbo I had.
I now believe the engine is supposed to run at 2800 and will prop it accordingly or possibly just under.
Thanks very much for all your help and suggestions...See I did head the words of warning.
Thanks again.
Larry

[Morrissey]

I'm surprised that no-one played around with the Boatdesign.net calculators and posted. Once you get the right inputs they have worked pretty well for me. Here is what I came up with for Larry's situation:

1. existing 1600 rpm and 3.08 gear. Assume output is 90hp. Speed will be 6.5kn and fuel 5 US gph. Prop size should be 91.8 x 63.6. AFAIK this all fits the facts.

2. new engine set to 2800 rpm, 165 HP. Assume you use your other gear, the 3.83 ratio one. At 2200 rpm you should get 6.3kn and use 4.6 gph, with a prop that is 84.5 x 53.1

However, one big plus for going to the gear & prop for 2800rpm is that at WOT you might be able to get as much as 8kn speed, albeit at 10 gph. A more realistic high speed cruise would be at 2600 rpm, using 7.6 gph for 7.4 kn.

But if you do choose to stay with an engine propped for 1600 rpm you will not damage the engine at all PROVIDED you dont try and go above that rpm. The weakness in this configuration is that you can never use the engines full power. If you try it will smoke and if you persist in trying you will damage it.

I dont think your existing set-up was any accident - people either knew what they were doing or experimented a bit to get a pragmatic outcome.

[Morrissey]

Correction.... that should have been the calculators on boatdiesel.com.

Out of curiousity I played around with the calculator some more. If you have room to swing your 103cm prop (and pitch is 74 or thereabouts) then you would need a gear of about 5.35:1 ratio to run your 165 HP engine at 2800rpm. Prob not much chance of finding a gear that ratio!

But the point I'd make is to check both gears you have (eg chip screens/fliters for the oil pump) and look for debris. One of the gearboxes might be in much better condition than the other, and be the logical choice to mate with your 'new' engine. I gave prop size for the 3.83 ratio gear above. But if the 3.08 box is in better condition, then the prop size for it (at 2800 rpm WOT) that I come up with is 73.9 x 42.7.

If you are going to buy a new prop, be confident about the gearbox you are going to use!

[ti325v]

Hello Morrisey:
THANKS...For this information...

But if you do choose to stay with an engine propped for 1600 rpm you will not damage the engine at all PROVIDED you dont try and go above that rpm. The weakness in this configuration is that you can never use the engines full power. If you try it will smoke and if you persist in trying you will damage it.

Onward...
Yes you have hit it exactly and every salient point...
All the numbers work as you say, I have worked them myself, sadly I
cannot access Boat Diesel. com from my address in Vietnam and there
seems to be no way to contact the administrators.
Now a few more points of information and a few questions if I may....
Point 1...The original set up would have worked correctly IF in actual
fact the engine speed was 1600 ROM at WOT. It is not, just this
afternoon I checked again WOT..Stops solid at 2,800 as yanmar says it
should.

Yes, thought went into the initial installation I am sure, just the wrong numbers were used.

Yes there is room for the 1.04 prop.
We swapped the prop down to 94 on the advice of the local prop shop,
a guy with a tin shack and a few props outside.

To give you an idea of what it is like here..
Can you sell me a deep cycle battery I asked at the battery shop... How big you want ?
2X200 AH.
Use this.
But it is a truck starting battery..
That is all we have is you do not like it, find what you want somewhere else....Would you like some tea ?

As my concerns for the age and hours on the engine grew, I found
another engine and dug deeper then before and got the new info from

Yanmar..2,800 WOT 165 BHP.

I posted here for advice...
Got the heck confused out of me, but the consensus was that I was
badly over propped and going to damage the engine.
I wondered about this, as I never run it into a smoking condition.
Question number 1...More for my understanding then anything else..
If the engine is not over loaded as evidenced by.. No smoke, No
Overheat, No burned pistons, valves... How am I damaging it ? This seems to me more like derating it... can you please shed some light on
that question for me ? Forget that question you answered it already...
That was a big question for me ! And we DO NOT run it till it smokes !!! We back it off about 150 rpm.

Okay onward....when I wound it up today to 2,800 in neutral it was noticeably noisier, which was one of the first points of concern I
raised....It may quiet under load at any expected cruising speed of about 2300 but it is noisier then 1700.

Moving along...I went and counted some gear teeth yesterday, and found

the new gear box is 3.83..versus the 3.08 on the old engine...

If I swap gearboxes..(Both look great internally, no chipped teeth and

I have a spare set of clutch plates..also I just replaced all of the bearings in the old box and it is good as new) and leave my current prop....I will up the max revs a bit to (I GUESS) about 2300 before it smokes, still not enough to get 2,800 WOT..

Again everybody is pushing for WOT....
A big issue for me with questions I have posted here and other forums
has been that people try to pilot my boat in their conditions, we are a dive boat..by definition we do not go out in heavy weather....heck
my final anchor choice is fully half what people recommended and we
have never dragged it and never will...

I do not want or need an extra knot, especially at the increase in fuel consumption..We cruise along quite comfortably at 1700 rpm and

11.6 km/h (6.25 knots) burning 5 gallons an hour more or less....

Okay now we address prop pitch..props are not sold here with pitch numbers..
They tell you the diameter, if you like it, buy it, if not find something else you like more. And no I do not know where to get a differant one, would you like have some tea ?

So I have located an 80 Cm prop on the island, which I can swap for my 1.04 and a hundred bucks or so.
I found a copy online last night of
The Propeller Handbook and glanced through it, I believe I can measure it the 80 cm, and I calculate it will need a pitch of about 24 to 25
inches..

Question #2 do you agree with that pitch on an 80cm 3 blade prop ?

Thanks so much for the answer in the first paragaraph, I was under the impresion that I was somehow loading some part of the engine in such a way that was demanding more torque then some critical part could deliver, I could make no sense about how I was going to break it.

Please, your thoughts on the pitch, no rush, engine swap in over a month and I will not have the 80 here to measure for a week or so..
Again my thanks for a sensible answer...At least it fits all of my observations, and what I have been able to learn from reading.

I simply have more engine then I need, in fact I pushed the builder to put in a 4 cylinder, but he was adamant about a 6.

[ti325v]

In my last post I meant to say about the 80 Cm prop...

I can likely measure the pitch of the 80 cm prop I have found on the island, with guidance from the handbook.

(Next time I go to Saigon I will be sure to come back here and post some prop shop pics from the nations biggest city and port. You may then appreciate what I am dealing with.)

I can get a 4.95 gear box for this thing, but I will work with what I have.

Oh and a gear box swap, no problem, engine space is huge, we pulled it to install new bearings and it was four hours out and about the same back in. It would cost me about $10..I have three boat staff to do it for me, oh and it is heavy, according to the book a touch over 300 pounds, just the gear box !...thanks for your help.
Larry

[Morrissey]

Larry
Happy to assist with scenarios through the boatdiesel calculator for you. Glad you got hold of the Propeller Handbook, it’s the ‘bible’ really. Pitch is just as important for the engine loading as diameter, so measure it as best you can. I need both to come up with some answers. I fully understand you have to be flexible and 'make do' where you are, so I can fiddle with the calculator to give you mix ‘n match options with what is available. It wont be 100% accurate as its trial and error to go back from prop dimensions to a guessed engine power curve. I am assuming all your props are 3 blade and not cupped.

But first on overpropping and engine damage. For any given gear ratio and prop size you can plot a propeller HP demand curve. You overlay this onto the engine’s max power curve to see the loading condition. Ideally the curves cross at the engine’s rated max rpm. But if they cross at a lower rpm then you can safely operate the engine up to the point where they cross, because you are not trying to get more out of the engine than it is capable of providing. In practice, for continuous operation (24-7) or for everyday use you come back 10 or 15% from the rpm at the cross-over point. This de-stresses the engine and increases life.

With your boat propped for 1600rpm, you are only in an overpropped or overloading condition when you try and run at a higher rpm than that, or have the throttle wide open trying to get more rpm. Being ‘propped for 1600’ means that 1. You aren’t overloading the engine and 2. You have effectively derated the engine to about 100HP, maybe a bit less. I don’t know how many rpm you need to back off from the smoking (clearly overloaded) point to be safe. I think someone suggested installing a pyrometer, and that would be nice to have. With it you could be confident that overloading (really hot EGR) was not occurring . Installed correctly you would want to stay below 900 F. I suspect your practice of backing off 150 rpm is going to be about right though.

BUT if you are propped for 1600 and you run at 1700 then you are into damage territory and will reduce the engine life. If you are not on the boat all the time then your crew might not be as diligent as you would be about strictly complying with the 1600 limit. To me that would be the main reason to change what already works for you. OK, maybe 1700 would still be kinda OK, but 1800 would definitely be an issue.

From an engine manuf. point of view that throttle discipline would be one of their big concerns. But also, they would take the view that if you want a 100HP engine then buy a 100HP engine, don’t effectively derate a 165HP down to 100HP and expose yourself to operator misuse. If shop-derated, the engine will have different injector and governor setup so that with throttle fully open, no load, 1600 (or a bit above because of no load) is all you can get. And then you end up with a really long life 100HP engine. This is not what you have. Your derating is achieved from operator discipline, and engine life is very dependent on it. Risky.

OK, back to what you’ve got or can get:
1. Gear 3.08 and prop 92 (or is it 94?) – 1600 rpm max. This is subject to pitch being 63.4cm. A diam of 94 with a pitch of 61cm is equivalent, also 1600 rpm max.
2. Gear 3.83 and prop 92 (pitch 63.4) – max 2300 rpm
3. Gear 3.83 and prop 94 (pitch 61) – max 2300 rpm
4. Gear 3.83 and prop 104 (pitch 80) – 1600 rpm max
5. Gear 4.95 and prop 104 (pitch 80) - 2400 rpm max
6. Gear 3.83 and prop 80 (pitch 58.4 ) – 2800 rpm max

So, if your 80 cm prop is 23” pitch you can go to WOT. If its 24.5” pitch then max rpm is 2700. Now, given I am just guessing a conservative engine power curve there is could easily be 10% error in the pitch figures. Particularly as the hull weight is also not that well established.

I would use your new 80cm prop and the 3.83 gear, but if that prop had a pitch greater than 23” I would try and add an EGT gauge so you know for sure if the engine is a bit overloaded. I would still be nervous about unsupervised crew running at WOT. You could tell your crew to run at no more than 2400 anyway (6.6 kn, 5.9gph), even though you are nowhere near continuous duty with the duration of your actual dive trips. Running at 2300 should give about the same speed and fuel use you have been getting.

Your builder was smart to insist on the 6 cyl engine. I don’t know what 4 cyl are readily available there, but the odds are it would have been right at its limit or even underpowered. Your 6 cyl is actually a very good match for you boat I think. It isn’t much more than you need at all. The boat diesel calculator indicates 200 HP would give you 8.6kn, but trying to go above 7.5 kn with your hull would really crank up your fuel use. According to it you should be able to get to 7.5kn with your engine and the 80cm prop, if you ever wanted to. But, another spreadsheet I use indicates that your hull wont do much over 7kn, regardless of power. The relatively high displacement weight kills you. The answer is likely somewhere in the middle. Running low to mid 6's like you do makes a lot of sense. Despite your location, some of the guys there obviously know what they are doing even if they can’t explain it to you in detail.


Let me know what pitch you measure for your props. I'm happy to run some more scenarios if you want.

[ti325v]

Hello Morrissey:

Thanks very much, that was very informative and confirmed again what I have read, with the added knowledge that I do not have too much engine.
All props discussed three blade, no cupping.

I absolutely trust my boat staff to never over speed the boat. They know the very good life that I and the boat provide for them and their families.

The captain and "engineer" were angry at me when I put it in neutral and wound it to the stop. Again rock solid at 2,800.

Also I know there are huge variables we cannot account for, so if you can suggest a pyrometer I will buy one.

I understand with out h.p. and torque curves there is a big fudge factor to consider, and will hold you in no way responsible for my choices.

First some little errors on my part and some clarification.

1) The boat was set originally to rev to 1600 with the 1.04, and the 3.03 (Which I measured today as actually 1.05) It began to smoke at about 1650.

2) The prop was reduced to 96, (again remeasured today sorry for the error) and we cruise at about 1700 and it begins to smoke at about 1850.

3) The second engine came with a 3.83 in what appears to be in excellent condition.

The new prop is sitting on the floor at my feet it is 82 cm. Pitch unknown.
I asked the guy at the prop shop about the pitch and if he could call the manufacturer.
The reply....buy it if you want, if not would you like some tea ?

There is a new twist to the problem now..the 82 is bored for a 50 to 38 taper over 120 cm.
My shaft is tapered 60 to 45 over 120.

So to use this size prop I will have to retaper my shaft, or have a prop made. (The hub of the new prop just does not have enough meat on it to cut it down). Both expensive propositions, and likely unnecessary.
Unless you tell me otherwise. Please consider.

From what you said about theoretical maximum boat speed and that it may never go faster then 7 knots (and I do not need 7 knots), overloading and my knowledge that I can trust my crew to not over rev the engine.(I think I can always reset the throttle stop if I am ever concerned)
I think the smart thing to do is just drop in the 3.83. Max it at 2,300 or so with the current 94 prop.

In essence de rating it without changing the injector or the cam, which I imagine is done at the factory, the point would be to move the peak torque down to the new maximum rpm.
This should cause no harm to the engine.
Please correct me if I am wrong on this point.

We will then see the smoke point and back it off a couple hundred rpm, and gauge it with the pyrometer. If you can advise me on a good product I can actually get one carried over and save me having to fly to Saigon to get it from customs.

From your explanation, as long as my egt is safe, I can pretty much leave things as they are.
You expect no change in fuel consumption or actual load if I gear it to 3.83 and spin it up to about 2,300 the only thing I gain is noise..is that correct ?


You can see the photo of the piston I pulled out a month ago, the thing looks like it has been run underloaded, which it was just before I pulled it out. Incidentally it looks the same as when I first pulled it out over 2000 hours ago.

Can you please consider the above questions and give me your opinions ?
Is it worth making a prop or retapering my shaft considering how we use the boat ?
Again, thanks very much.
Larry
Oh and today I was discussing the fact that rod bearings are no longer available, my mechanic told me not to worry about it, they replate them here for about $5 a piece !

[a64pilot]

900 sounds awful low for a max EGT, not too low for continuous maybe.

Pyrometers are inexpensive and abundant, I've never mounted one in a water cooled manifold though, how do you go about doing that?


Also it's real simple to just limit the throttle to where you hit the stop on the lever, but not at the engine to limit it to about 1600 RPM, that way it can't be hurt.

[ti325v]

Hello a64 pilot...
Thanks, I am in the middle of a very detailed discussion with Morrissey. Your comments are out of place here, we are homing in the correct answers to 5 pages of various replies to a rather simple question....if you would like to comment and it is relevant having read the whole thread, or at least the parts with Morrissey, of course your thoughts are welcome. Please, this is not personal, but I just want to stay on track to get the answers I need.
Thank you
Larry

I know nothing at all about pyrometers and I can say with some certainty that if located downstream of the turbo, 900 is not far out of line.
The reason some people prefer that location is to save their turbo if the tip breaks off....and again..pyrometers are all over the place where you are..I am on a remote island, population 5,000 off the s.e.coast of Vietnam. Nobody knows what they are here.
Wet exhaust manifold may dictate that location.
I hope now you understand this is not your garden variety subject.

[boatpoker]

Originally Posted by ti325v

Hello a64 pilot...
Thanks, I am in the middle of a very detailed discussion with Morrissey. Your comments are out of place here, Please, this is not personal, but I just want to stay on track to get the answers I need.
Thank you
Larry

This is not the place for private conversations, suggest you get Morrisey's phone number if others comments are not welcome.

[Morrissey]

Larry
I dont know much about pyrometer suppliers, the guys in the yard just got new ones for me somewhere without much hassle. Best bet would be to use your local sources to get hold of one, or direct you to a shop in Saigon. You need the pyro itself which srcews into the exhaust manifold at just the right place, and a gauge. There's just a couple of wires between the two. A64pilot might be right about temps, I dont claim to be an expert on that. But my engine surveyor told me that with my old engines not to run much above 900 if possible. IIRC my new John Deere engines dont get above 900 unless I get up close to WOT. I'm never at high rpm now that I have completed the break-in period. High performance engines in aircraft may well run a lot hotter than our diesels.

For props, I think your 96 with the 3.83 gear is the obvious choice. Once you let me know the pitch I will come up with a calculated max rpm, but I think you are pretty close with your number anyway, and watching for smoke then backing off is a good check as well.

It seems the 82 prop will be a lot of time and expense to get sorted, and it might even be the wrong pitch, so I wouldn't go that way. In time you might locate an 82 prop with right shaft size.

Using what you have will be fine given you can trust your crew to not push the throttle too far.

Your fuel use should be about the same. As Jim said earlier, it takes a certain amount of HP to push the boat a given speed. And diesels use only the fuel necessary to generate the HP. More fuel and the rpm would increase, but the governor acts to maintain the rpm you set and ensures there isn't more fuel than needed for 'that load at that rpm'. That's the specific fuel consumption number of 220 g/hp/hr number. Now, with your turbo spinning a bit faster you might even be in a slightly sweeter spot. The 220 figure is an average. Specific fuel consumption for most diesels is above average at both ends of the throttle range. In the mid throttle area its below average, and there can be a really sweet spot. The difference isn't all that great, but is noticeable over enough yours. That's where you want to be, and I suspect that where you will be operating is going to be close it.

And yes, operating the engine as noted above (disciplined throttle use, no smoke, pyro if possible) you will not harm the engine at all. On the contrary, you will have it nicely loaded but not stressed - it should give you long, long service.

[Morrissey]

Larry
a64pilot was on topic, so I think his comment is useful. The beauty of a forum like this is that people with specific knowledge can make valuable contributions. In my view as long as people stay on topic the more contributions the better. I've been glad to assist, but there are lots of people out there who know a lot more than I do - dont scare them away!