Way Over Propped but it Works, is it Safe?

[CarlF]

Every time I have a physical, my doctor tells me that drinking more than one drink a day will ruin my liver. But his fancy tests say that my 60 year old liver is functioning just great. So I'm sticking with what works.

Your personal experience is a whole lot more believable than a bunch of unsupported opinions (notice no one has any evidence of over-propping causing problems).

I've always slightly overpropped (although not that much). I figure that as long as there's no smoke then combustion is fine. And the overpropped engine will certainly not run cold - that's what really hurts a diesel - under-temp causes carbon buildup and a host of other problems.

Maybe there's a little extra wear from the higher load but an over-propped engine is making fewer revolutions for a distance traveled so that's less wear. Maybe the two sources of wear cancel out?

The hand wringers are right that Yanmar may not honor a warranty on an over-propped engine because their lawyers are always looking for ways to get out of warranty claims - but it would be fun to file a warranty claim directly with Yanmar on a 40 year old engine.

Note: I have no doubt that if you tried this on a new Yanmar it would fail quickly. That's progress.....

[ti325v]

Originally Posted by David M

Go ahead and call the manufacturer of your engine and ask if it is okay to over prop the engine. You will get a definitive NO for the reasons I mentioned.

Hello David;
The engine is over 40 years old....the current staff know nothing about it, it was limited production in its time.
Every time I contact them I get different answers, that is why I am here, even their engineers do not know.
As I said, even the manual says 2000 rpm, their current engineer says their books say 2700...and the throttle stop is 2700...
I have put 3000 trouble free hours on it and we do not lug it, NO SMOKE. Documented 30,000 + hours.
With no conclusive opinion coming from the manufacturer, I am looking for info here. However they do tell me to up the revs, I would like to know why, and I am getting some of that feedback here.
Thanks

[Scot McPherson]

Smoke means you have finally pushed it so hard that combustion was not complete when the piston started to push back in to clear the chamber...that's REALLY bad. Throttling to just under than point only means you have reduced pressures to the point that the combustion that's left over can push through the rings, but still complete itself. The hardware is still under a tremendous amount of force that wouldn't be there is the prop were proper or slightly under rated. The life of the engine is just as much a cost as deisel and oil changes are. Burning .05% more fuel to gain and addition 35 years of engine life is a worthy trade off

[roverhi]

Originally Posted by ti325v

Hello Peter;
Thanks very much.
Please note, no prop shops...REMOTE island off the coast of Vietnam.
We have been running the boat in this way for near on four years and 3000 hours, and as I said, dive boat...calm conditions or we do not go out.
in the four years we have run it, we have never encountered a condition where we could not make at least 8.5 km/hr and then for only a short time to get around a headland. (20-30 minutes).
We really do not need the extra H.P. and if we are not damaging the engine running this far out of specification I am tempted to keep it this way.
However one statement you made did interest me, you said with a smaller prop it is likely we will not need the extra revs anyway and we can keep it about where it is and get the same speed/ rpm. that is great as far as quiet is concerned, can you give me more info about how a smaller prop, turning the same revs would give us near the same speeds ?
I did the last set of prop calculations and came up with the 92 cm and worked out to be 150 rpm off. (Pretty good for a first time !)
A new prop using the revised 2,700 rpm would be about 72 cm.... that is a lot smaller ! Please bear in mind I know nothing about this, and I am not debating what you are saying, I would like to know why you believe this to be the case. Can you shed some light on this.
Thanks
Larry

I am not a prop expert but I do play one on the internet. Have owned two boats that were over propped when I got them. When I eventually put the proper sized prop on, there wasn't a big increase in engine rpm for the same boat speed. Wasn't keeping exact track of the difference, however. I was looking to have the engines produce their advertised hp when needed. Both engines ran for years over propped without issues but I didn't push them above their comfort range. Could get about 5k cruise, about 80% of theoretical hull speed, without laboring the engine. Pushing the throttle forward gave little increase in speed and a big increase in black smoke, however, so just didn't do it. I'm a sailor and only use the engine to get in and out of the marina and those occasional times when there wasn't enough wind to sail. Engines to me are strictly auxiliaries. You are in a different category but have had no issues doing it AND with way more engine hours than i'll ever get.

Like others analogy that it's like driving a car in 6th gear at 40mph on flat ground. Works great until you come to a hill or need to accelerate quickly. You have to be especially careful with a boosted engine as lugging those engines melts pistons real quick. If you had the capability to install an Exhaust Gas Temperature (EGT) gauge, you could monitor the engine so as not to over boost it. Had a little time in super charged. variable pitch propped recip. planes in the Navy. One thing they made real sure you learned is don't over boost the engines if you want them to keep running.

[ti325v]

Hello Carl:
Thanks for a well reasoned opinion, and the only dissenting reason I have seen so far is that I am not getting anything out of the turbo at these low revs, any thoughts on this ?
Thank you
Larry

I am not worried about it failing, if you saw the bearing sizes,
it would impress you, heck even the piston skirts are hardly scuffed. Ring grooves still not at wear limits, and before anybody mentions that I have the engine apart..no I do not...you can pull each head, open an access door in the crankcase, swap out a rod and a cylinder sleeve along with piston in an hour and have the head back on in 45 minutes.
some reasons why I love this engine.

Originally Posted by CarlF

Every time I have a physical, my doctor tells me that drinking more than one drink a day will ruin my liver. But his fancy tests say that my 60 year old liver is functioning just great. So I'm sticking with what works.

Your personal experience is a whole lot more believable than a bunch of unsupported opinions (notice no one has any evidence of over-propping causing problems).

I've always slightly overpropped (although not that much). I figure that as long as there's no smoke then combustion is fine. And the overpropped engine will certainly not run cold - that's what really hurts a diesel - under-temp causes carbon buildup and a host of other problems.

Maybe there's a little extra wear from the higher load but an over-propped engine is making fewer revolutions for a distance traveled so that's less wear. Maybe the two wears cancel out?

The hand wringers are right that Yanmar may not honor a warranty on an over-propped engine because their lawyers are always looking for ways to get out of warranty claims - but it would be fun to file a warranty claim on a 40 year old engine.

Note: I have no doubt that if you tried this on a new Yanmar it would fail quickly. That's progress.....

[ti325v]

Originally Posted by Pete7

You said earlier it was turbo charged, but the photo doesn't show a turbo?

http://www.cruisersforum.com/forums/...ase-49854.html

thats another story...turbo and intercooler were swiped by the guy who was supplying the engine, for resale.. with the first question in the old post from Laidback, I made claim to my turbo and intercooler, raised a hell of a stink, slapped the kid supplying the engine across the face..it turned up 30 minutes later, matching paint with aligning scratches..Gotta watch your back here, I have thanked laidback many times for the info !

[ti325v]

Hello David:
Here is a photo of the engine installed.
Don`t worry I was kidding about hacking off the prop, I would get a new one.
Here is a photo of it installed with the factory turbo and intercooler. All these parts are shown in the parts book.

https://www.dropbox.com/s/adlpoxkhe2...IMG_0018.1.JPG

And the boat..
https://www.dropbox.com/sh/t42bbkbuf...P_MB6LXxMujzda

Now remember this beast is 40 plus years old and the output cannot be compared to a more recent cummins design.

I tend to believe the figures I am getting from yanmar, 2700 rpm as that is what it revs to in neutral.
I would expect at least 150 h.p. from a 6 liter turbo of any vintage.

I hope the photo provides you with some reliable reference point to make further judgements, you have so far been a voice of reason asking the 'why' questions that lead me to believe you know something of value, not just dropping an opinion and calling it a day. Please do have a look at the photo and let me know what you think. You can clearly see the turbo, fresh water intercooler.
Thanks
Larry


[QUOTE=djmarchand;1602876]Can you post a pic of the exhaust manifold and where the exhaust goes. On your previous identification thread I could not see a turbocharger, much less a fresh water (jacket water) cooled intercooler. Engines with that kind of turbo charging of about 6 liters make about 250 hp at 2,600-2,800 rpm (the Cummins 6BTA 250).

All of which leads me to believe it is naturally aspirated and produces about 150 hp at 2,500 or so rpm like the 5.8 liter Perkins 6.354.

So, frankly with so much unreliable data it is impossible to advise what to do. I would keep the old prop as is. And don't chop off the blades 4". That will result in a horribly inefficient prop.

[ti325v]

This makes some sense as well ! Thanks crazy.

[darylat8750]

There is a difference between running your engine "loaded" and " overloaded". I think you are running enough cylinder pressures to be OK without beating your engine up. If you are nervous about it buy an exhaust temp gauge. Install the temp. gauge about 6 inches down stream of the turbo will give you good information. I have a friend who works in the "let's see what we can break department" at Case International. He has seen 1250F for several hours on a big tractor. He said the whole exhaust system was glowing red. I added a turbo from a wrecked end loader to a tractor I have. It does not have sodium filled valves that the better turboed engines have. He said if I don't run continuously over 900 degrees F I should be OK. I don't run my tractors over 850 degrees F and have not lost an exhaust valve yet. Manifold pressure gauges are not expensive either and would be interesting but not critical. I would bet a cold beer that that engine burning 5 gallons an hour turning that prop the turbo is putting out at least some boost.

Banks Power | Why EGT is Important
Isspro gauges

I'm just going to say "Run what ya brung " (old street racers credo) and go to an anchor thread.

[DumnMad]

1st post says 3ft prop and 2.5ft draft. Looks over-propped.

[StuM]

Originally Posted by DumnMad

1st post says 3ft prop and 2.5ft draft. Looks over-propped.

WTF? That must mean that at least 6 inches of the prop is above the waterline?

[Ex-Calif]

There is no doubt you are over-propped. The question is should you do anything about it.

I would definitely not be buying a new smaller diameter prop.

This thing has been working fine for a long time. However that isn't to say it can't be improved.

I would look at the cost of repitching the current prop slightly. The only reason is to maybe improve acceleration.

It's a dive boat for Pete's sake and no on wants to hear that engine turning 2500RPM. Better to be somewhat overpropped and not push the engine too hard.

You wanna get to a from the dive site at a decent rate of speed, burning minimal fuel (you are a business) and with as much peace and quiet on board as possible.

All the discussion about smoke, getting the turbo spinning and impact on injectors is interesting but in a long period of operation OP is not complaining about coked up turbo, bad injectors or smoke.

Bottom line is that if the engine is not smoking then combustion is completing and there is really no issue. When the throttle is "pushed" this engine will smoke as it is being lugged and the engine can't push this prop any faster - if he does not operate in the smoke range there is no issue.

Decreasing pitch a bit aiming for a few hundred RPM increase may give the engine more flexibility and a bit better acceleration.

[Jim Cate]

I have to wonder: how many of you who are saying this engine is being abused have ever gotten 30,000+ hours on your own, properly loaded engines?

It may not be optimal usage, but by golly, it seems to have worked pretty damn well, and I doubt if the OP will live long enough to put another 30K hours on the new mill.

And for you experts: as I understand things, it will always take the same amount of HP to drive the boat at his given cruising speed. The engine will always burn about the same number of grams of fuel per HP hour. Thus, his fuel economy shouldn't change much if he should repitch the prop for more revs at cruise speed. Is this not right?
BEtter acceleration... perhaps, better ability to push into wind and sea... perhaps, longer engine life... doubtful, better fuel economy... doubtful. If he is satisfied with performance as is, why in the world should he change anything?

Jim

[boden36]

Don't worry yourself too much. Just open the throttle until it won't rev any harder, and then drop back a couple of hundred revs and all will be fine. From a cost/benefit point of view unless you are desperate for more thrust, and are sure the boat will travel significantly faster, leaving it alone is a no brainer.

Regards,
Richard.

[Scot McPherson]

Originally Posted by Jim Cate

I have to wonder: how many of you who are saying this engine is being abused have ever gotten 30,000+ hours on your own, properly loaded engines?

It may not be optimal usage, but by golly, it seems to have worked pretty damn well, and I doubt if the OP will live long enough to put another 30K hours on the new mill.

And for you experts: as I understand things, it will always take the same amount of HP to drive the boat at his given cruising speed. The engine will always burn about the same number of grams of fuel per HP hour. Thus, his fuel economy shouldn't change much if he should repitch the prop for more revs at cruise speed. Is this not right?
BEtter acceleration... perhaps, better ability to push into wind and sea... perhaps, longer engine life... doubtful, better fuel economy... doubtful. If he is satisfied with performance as is, why in the world should he change anything?

Jim

Jim,
He didnt put 30k hours on it. He bought one with those hours.

Scot