Way Over Propped but it Works, is it Safe?

[ti325v]

Gentlemen:
I come back to the drink from the fountain of knowledge.
You guys have been very helpful in the past, and I hope I can get some good advice in the future.
The situation.
I have an absolutely (40 + years) ancient yanmar straight 6 turbo..there are some threads about the boat build and engine already on this board.
Originally the boat propped for a maximum engine speed of 1600 rpm, (Stated on the engine data plate) it will run to 1,700 then smokes.
current prop is 92 cm, (About 3 feet) 3 blades. gear eduction is 3.08.
I have located and refreshen another one of these (IMHO) amazing 40 year old yanmars.
Currently I have 32000 hours on the old lady and she runs great, but clearly she is tired, rod bearings are showing copper...bores are polished.
The plan is to replace to old with the much lower hour engine of the exact same type, bolt in installation.
It would take a month or more to replace the engine with a different make or model with the resources I have where I am. This is the better route for me both in time and economics and the limited skills of the mechanics here on the island.

Here is the problem....Prop size.
I have finally gotten additional information form yanmar japan, (This was a domestic model, little information still exists and what there is, is in Japanese) and I have since learned that the engine plate is incorrect and the actual maximum continuous speed for this engine is 2,700 r.p.m. and 150 H.P. (No, I sadly do not have access to torque curves)

Boat is about 41 feet lwl and 100,000 pounds,(according to the builder) 12 ft beam, 2.5 feet draft, unladen..we add a couple of tons when we run it.

So, when the "New" engine goes in should I reprop it for 2,700 and 150 H.P ?
Currently it is not objectionably noisy and does not vibrate....
uses about 5 gph at 12 km/h, or 8 smph.
Sounds about right that it would need about 100 hp to do this....
this is plenty fast for us, and we run it just below the 150 rpm below the smoking starts.


Negatives I see in a much smaller prop...much higher rpms, more noise.

Positives...
Engine will be operating as designed...

To those who can give me some advice
PLEASE keep in mind, this is a dive boat...fair weather only.
Short run times of 1 +/- hour at 1,600 r.p.m. 3x a day. 3 days a week.

re prop or not ?
I look forward and am grateful for all thoughts and advice.

[roverhi]

If you are overpropped, the motor will not give it's advertised power when asked. If you try and push the engine beyond its comfortable rpm, it will start smoking and will undoubtedly shorten the life if you try and run it lug the engine continuously. In aircraft terms, you will be over boosting the engine and that could have disastrous results like burned pistons If you are content to run the engine at its comfortable rpm, then it should be fine. Unfortunately, you will probably be giving up 1/2 or more of its available hp potential. Get yourself into a strong head wind/ head sea situation and you could be unnecessarily SOL for engine power.

If it were me, I'd try and get the proper sized prop so the engine can be run to just beyond redline in steady state calm water conditions. You probably will still be able to cruise at your current comfort speed with only a slight boost in rpm. You may be able to do this cutting down the diameter and/or decreasing the pitch. Go to a prop shop. They will be able size a proper prop and tell you if your current prop can be modified.

[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

[Boracay]

When I put a new John Deere 4045 (85hp at 2500rpm, 4.5 litres) into Boracay (15 tonne and 36' waterline length) it struggled to get near 1900 rpm with a 22" dia and about 15" pitch.

So when the boat came out of the water I had the prop depitched a couple of inches which got up to about 2200rpm and better than 7 knots and then the next time I came out I had an inch cut from the diameter which got me up to about 2400 rpm and 8 odd knots on a good day.

Coating the prop with Propspeed gave the last 100rpm so now I can get full revs flat out.

Last time I came out I had 15 odd years of antifouling and poor quality paint abrasive blasted off and now seem to be getting over 8 knots but the boat feels overpowered so I rarely run over 1800rpm.

If you're buying a new prop why not take your figures to the prop maker? I'm sure there will be a few in Vietnam. They'll probably know the engine and boat type.

Your rebuilt engine may have a lot more power so I'd try the new engine before fiddling with the prop.

Then I'd be thinking of taking an inch or so off the diameter and then a couple of inches off the pitch. You can always take off more later but its hard to put it back on.

I'd be hoping for 8 knots or so with everything clean which is about double what you're getting at the moment.

[Pete7]

That turbo isn't working at 1700 revs which is why it is smoking because the FIP is over fueling the air/fuel ratio. Once the turbo kicks in then the smoke goes as the ratio is corrected. However, sounds like you have too big a prop to allow the turbo to spool up.

This is a pic of my rib with a 4.2 litre Ford Dover turbo engine. The photo was probably taken just as the throttles are opened and the engine is accelerating but still under the 1800 revs when the turbo can be felt to kick in. Once up to 2600 revs the exhaust always cleared.

Fit the other engine and see if it will run up to full speed, after all someone worked out the size for the prop originally. If you can't pull full revs then change the prop, but I wouldn't run it long term with an oversized prop as you are over fueling the engine, creating heat and carbon plus wasting fuel.

Pete

[ti325v]

Gentlemen, prior to posting, please read what my original post, it will save me all manner of confusion.

Hello Pete;
Perhaps I missed it in my original post, we have no running or starting problems, no smoke.

Boracay, I know how to get there, and I am pretty sure of my numbers if I change the prop, the question is....should I ? Currently I am getting a satisfactory 6 knots. 74 Cm with the current pitch will get me very close, to 2,700 and yes just under 8 knots but at a massive increase in fuel consumption,as far as I understand.

Do I actually need to ? and if I do, it seems appartent to me I will be moving up in the rev range, adding noise and possibly wear on the engine.

I understand it is desirable that the engine can just over rev, but does it need to ? I understand that in how diesels operate there will be no real increase in fuel used, I will about 100 H.P. in any case weather at 1,600 where we are now are closer to 2,000 which is where we will be with a smaller prop.

Boat does not smoke now, only when I rev it to 1,800 I set it up for 1,700.
THX

[djmarchand]

Something in your data doesn't add up. You currently burn 5 gph (which is maybe 80-90 hp on an old engine) to go 8 mph at 1,700-150=1,550 rpm. But you also say that the engine is rated at 150 hp at 2,700 rpm. A typical turbocharged but non intercooled engine that produces 150 hp at 2,700 will not produce 80-90 hp at 1,550. More like 50-60 hp.

What is your wot rpm not in gear? That will tell you what the governor is set to and the engine's maximum power point will typically be 200 rpm less than that. Also what is the displacement of that beast?

I suspect that the rated wot hp of that engine is something less than 2,700. Maybe 2,400 rpm like the old Lehmans or Perkins.

You may be overpropped but I don't see the harm. That old beast ran that way for 32,000 hours, right? What is wrong with that?

David

[djmarchand]

Let me modify/edit my last post. Look at the attached curves for the Perkins 165 hp engine at http://goldcoastpower.com/brochures/6.3544-165new.pdf. It produces 165 hp at 2,800 and about 80 hp at 1,600, all at wot.

That is pretty close to what you reported for your engine and yes it will produce enough power to drive a very over propped prop to 1,600 rpm but it will have to be with the throttle wide open.

I still think that your numbers are suspect but not as suspect as I stated in the previous post.

Check the no load wot rpm and post the displacement and let's go from there. If it is like the Perkins, I would reprop it. But I suspect it is more like 8 liters than the 5.8 of the Perkins.

David

[boatpoker]

41' LOA, 12' beam, 2.5' ...... 100,000lbs. Highly unlikely .... what is she made of ?

[darylat8750]

I would leave the current prop on it. If you are burning 5 gal. an hour you are taking between 55 and 65 horse power out of the engine. (Most likely closer to 55.) That is around 40% of rated power. It should be happy doing that. Just like driving a standard transmission car at 40 miles an hour in top gear on a flat road. It is just loafing along. You don't have any steep hills to deal with and that big prop geared as low as it is should push head seas and wind with no issues and very little additional effort. (This isn't a sail boat with a wimpy little prop and he is not trying to get up to a high speed/length ratio.) As long as you have the self discipline to not push the throttle up into the smoky range I think you will be fine. Like someone else said "This engine lasted 30,00 hours." If you do re-pitch the prop you will burn a little more fuel on each trip.

[ti325v]

Originally Posted by boatpoker

41' LOA, 12' beam, 2.5' ...... 100,000lbs. Highly unlikely .... what is she made of ?

1.5 to 2 inch thick mahogany of some sort, comes from Laos....photos of the build in this old post..
When I was just starting on this adventure 4 years ago..Engine photos on first page.

http://www.cruisersforum.com/forums/...e-49854-2.html

LWL is about 41, loa is about 45..Beam 12 +..
Builder reported weight, I have no idea if it is true, but the timber it is made of is kept on the bottom of a river, it does not float. Maybe when dried, but certainly not freshly cut !
I have no idea if the weight is accurate....I have no idea if anything I am posting is accurate, i have a manual for the engine that says 95 h.p. at 2000 r.p.m. Max, Yanmar japan reports 2700 and 150 h.p. Nobody knows, that`s why this is so hard.

[ti325v]

Hello David;
Thanks for your interest in this ongoing drama...
I just wrote you the most detailed post in reply to all you mentioned and the internet ate it.
I will try to recreate as best I can, and thanks for staying with me on this one.
Okay, first have a look at this post, engine and build photos from 4 years ago when I was trying at that time to get information...


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

That is when I propped it for 1600 rpm and you can see why...
6 cylinder Bore 100 mm stroke 110 mm I make that just under 6 liters, unless my math is far far off.
freshwater after cooled turbo.
Yanmar spec 220, 220 grams /h.p./h.r
That works out to and we use just over 5.5 gal per hour....
rule of thumb 20 h.p. per gal/hr..I come up with about 100 h.p. at 1600-1700 rpm..based on fuel consumption.
No load rpm in neutral WOT is 2700.

Throttle is not wide open, when run at 1700. I do not know hope far it is from that as I have never pushed it hard into smoking, but for all I know it may be close and is one of those things I have just learned from you, and it another bit of information and something to look at.
No h.p. or torque curves available...40+ years....nobody knows.

According to my dubious calculations, (Which got me me within 150 rpm or so with the first prop at 92 cm), I will need to cut the prop down to about 70 Cm, that is 4 inches off the 3 blades to run it to 2700...lots of brass to whack off with a hacksaw, no there are no sawzalls here.

Again the point..it SEEMS to work, but there have been a lot of recent threads about overpropping by just a few hundred rpm`s and here i am 1000 rpm over, the boat is not going a heck of a lot faster with 150 h.p. available, will consume more fuel and likely be noisier.
Oh and one of the reasons I have learned that the engine [label was swapped from the original to one that says 55 PS is taxes. Bigger engines, more tax. (There are photos in the linked post).
So, you said in your last post with more info you might consider a reprop, does this info help ?
Should I reprop it ? We do not need to go faster, we do not make smoke.
Thanks very much for your interest and support.
Larry

Originally Posted by djmarchand

Let me modify/edit my last post. Look at the attached curves for the Perkins 165 hp engine at http://goldcoastpower.com/brochures/6.3544-165new.pdf. It produces 165 hp at 2,800 and about 80 hp at 1,600, all at wot.

That is pretty close to what you reported for your engine and yes it will produce enough power to drive a very over propped prop to 1,600 rpm but it will have to be with the throttle wide open.

I still think that your numbers are suspect but not as suspect as I stated in the previous post.

Check the no load wot rpm and post the displacement and let's go from there. If it is like the Perkins, I would reprop it. But I suspect it is more like 8 liters than the 5.8 of the Perkins.

David

[lancelot9898]

I too suspect some of your numbers are off( ie., 100000lbs dispalcement and 32000 hours on the engine), but regardless as long as you operate that engine at 80 % of the actual WOT rpm then I would think everything would be fine. While it is sometimes desirable to get the actual WOT setting to equal the specified WOT rpm, as long as you are getting the operating temperature up to specs and running no higher than 80% you should be fine realizing that you are not getting all the available hp out of the engine.

[David M]

The formulas are just estimates and don't necessarily accurately calculate what is the correct prop size. The CORRECT prop size is where your engine can reach its maximum governed RPM. The bottom line is that if your engine cannot reach its maximum governed RPM with the prop that it has, then you are over propped.

There are three MAJOR problems with being over propped. One is that you are lugging the engine which will cause a much shorter engine life. The next problem is that you will never be able to reach the speeds where you can blow the carbon out of the cylinder heads. The last problem is with Yanmar. They may not warrant the engine if there is a problem because it cannot reach maximum RPM. I know with Cummins that if you over prop an engine that the warranty is invalid. I had to have a Cummins tech onboard on my sea trials for my new engines to confirm that the engines could reach maximum RPM. Only after that did they validate the warranty. I don't know what Yanmar's policy is.

[Pete7]

Originally Posted by ti325v

Gentlemen, prior to posting, please read what my original post, it will save me all manner of confusion.

Hello Pete;
Perhaps I missed it in my original post, we have no running or starting problems, no smoke. THX

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

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