Way Over Propped but it Works, is it Safe?

[crazyoldboatguy]

(OP) I have located and refreshen another one of these (IMHO) amazing 40 year old Yanmars.

(Response) The last problem is with Yanmar. They may not warrant the engine if there is a problem because it cannot reach maximum RPM.

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

I didn't know they warranteed 40 y/o engines over @ Yanmar.

I didn't even review this post as it didn't seem relevant but was interested nonetheless. I, too , have a boat with an over-propped engine (Pearson Triton with an Atomic 4). Been that way for 50 years. The deadwood is so wide that a too large prop is needed to get the tips out into clear water. My engine doesn't reach more than 50% of rated RMP or HP. Been that way for 50 years. Lugging is supposed to damage the motor. I don't lug mine and it doesn't sound like you lug yours. That's the key, no? You get decent speed and fuel usage even tho you aren't using the rated power. Drop in motor with same bolt patterns - what's not to like? Drop it in, use the same prop. If/when you get back to civilization you can check out some propage, but you may just keep the arrangement since it's working, right.

[David M]

Originally Posted by crazyoldboatguy

(OP) I have located and refreshen another one of these (IMHO) amazing 40 year old Yanmars.

(Response) The last problem is with Yanmar. They may not warrant the engine if there is a problem because it cannot reach maximum RPM.

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

I didn't know they warranteed 40 y/o engines over @ Yanmar.

I didn't even review this post as it didn't seem relevant but was interested nonetheless. I, too , have a boat with an over-propped engine (Pearson Triton with an Atomic 4). Been that way for 50 years. The deadwood is so wide that a too large prop is needed to get the tips out into clear water. My engine doesn't reach more than 50% of rated RMP or HP. Been that way for 50 years. Lugging is supposed to damage the motor. I don't lug mine and it doesn't sound like you lug yours. That's the key, no? You get decent speed and fuel usage even tho you aren't using the rated power. Drop in motor with same bolt patterns - what's not to like? Drop it in, use the same prop. If/when you get back to civilization you can check out some propage, but you may just keep the arrangement since it's working, right.

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.

[djmarchand]

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.

David

[darylat8750]

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.

If he was expecting to extract 75% or more from the engine you would be correct. But he is not. Of course the manufacturer would not advise this. They want to protect themselves from people with no common sense. Would you put your 5 speed manual transmission car in 5th gear and go up a hill that was steep enough that full throttle would be limited to 1600 rpm for two hours? No, even the best car would most likely have a problem if you did that. Could you drive same car on a level road at 45 mph for days on end with no issues? Yes. If the OP was operating at full throttle with the black smoke pouring out his engine would not last long. He is not. He is taking well under 75% of the available power out of the engine at between 60% - 80% rpm (depending on which manual is applicable) and has the common sense and self control to not push the throttle up to the point of smoking.

Also, carbon build up is linked to low cylinder temps, not specific rpm or even specific power output. Moderate load at moderate or even low rpm can keep the cylinder temeratures up enough to avoid carbon build up issues.

[darylat8750]

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.

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

I think your gut reaction to the operation is fine. As long as it does not smoke and you do not burn more than 5 gallons an hour I don't think you will hurt your engine. I do think that if you decide that you want your engine to turn higher rpms you should not just cut off the ends of the blades. Decreasing the pitch, even though problematic for your location would be WAY more desirable.

[Cheechako]

I'd be tempted to de pitch some, or go slightly smaller. But I wouldnt go all the way so you can redline the engine. The phrase .."if it aint broke don't fix it" comes to mind.
You got 32000 hours on the original engine! You have "derated" the engine unknowingly and it lasted a long time probably because you did! That's a trick engine makers do to rate the same engine for commercial use vs pleasure use and make them last a long time.

[zeehag]

over-propped is just as inefficient as under-propped.

[Scot McPherson]

Yes but under propped will put less strain on the engine if you don't try to push it hard. Over propped will put constant strain on your drive train parts.

[David M]

Originally Posted by zeehag

over-propped is just as inefficient as under-propped.

Over propping causes a shorter engine life and has the potential to damage an engine Slowing down how fast you go through the water increases efficiency. These are two different subjects. Don't think you can over prop and increase efficiency. It does not work that way. It has to do with an engines specific fuel consumption which changes at different RPM's. You don't radically change how much energy you extract from a unit of petroleum (change its efficiency) by overloading an engine.

By slowing down an engine it is burning less fuel... but that is not efficiency. Slow down and you will burn less fuel per mile....that is called efficiency. Lugging your engine does not increase efficiency....(the amount of fuel burned per amount of horsepower produced.)

http://en.wikipedia.org/wiki/Brake_s...el_consumption
"Any engine will have different BSFC values at different speeds and loads. For example, a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak." You never reach the torque peak of an engine when you are lugging it.

[crazyoldboatguy]

But over-propping the engine doesn't automatically mean it's being harmed. If there is room left on the throttle to raise the RPM you aren't lugging it - the absence of smoke is proof of that. If you are in 5th gear in your car and you can accelerate you aren't lugging the engine.

It is in the operation of the engine that you can tell if you are at risk for causing a problem, not the advice from the company. If the engine is under warrantee I would be concerned but this is a 40 y/o engine.

It has been established that the engine isn't being used to the best efficiency and power available. I like the concept that the engine has been de-tuned. If the old engine lasted that long it could be safe to assume the "new" one would last just as long with that honking huge prop. Once you get back to civilization and can have her pulled out take the opportunity to get the prop adjusted a bit to perhaps gain a bit of power. Maybe you could take some folks skiing.

[Scot McPherson]

Crazy,
That's just not true...just because an engine can accelerate doesn't mean you should. A car moving 10 miles and hour and going down hill can be put in 5th gear and you can hit the gas and it will accelerate, but it's still get beat AS HARD as a car trying to go up hill in 5th gear in the same speed range, just because you _can_ get out of overloading your engine doesn't mean you aren't overloading it. Thing is, on the water there is no down hill, it's all uphill so there is no relief from the overloading. It's always best to slightly under load and then slightly Under power your power than it is to overload...an overloaded prop is NEVER under loaded when it's under load.

[ti325v]

Originally Posted by 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.

Hi Lance;
With what you are saying, now I am seeing the kind of answers that my intuition was telling me. Now if there are no dissenting opinions, such as more info from david, I am holding on for his comparison, but the consensus seem to be if I am not lugging it, which we do not and we keep the revs a bit under where it smokes, we are good, that might save me a $600 prop ! Thanks


50 tons was the weight put to it by the builder and that is suspect, hours are not in doubt, I have seen a couple of other examples of this engine with over 25,000.

[crazyoldboatguy]

I didn't say you should attempt to accelerate, I said that if you can it means, generally, that you are not lugging the motor. It's always better to not lug the motor. No one is disputing that. What folks seem to not be getting is that the OP ran this over-propped motor for years putting on thousands of hours of operation and the motor kept going. Why? because he didn't push the motor to far. My 50 y/o motor has NEVER operated at the proper RPM while it, too, was over-propped. I purposely do NOT attempt to push the motor for that reason and she runs like a watch. Has never been opened. If doing what I have been doing for all these years is SO bad for the motor, why is she still running?

[ti325v]

Originally Posted by crazyoldboatguy

(OP) I have located and refreshen another one of these (IMHO) amazing 40 year old Yanmars.

(Response) The last problem is with Yanmar. They may not warrant the engine if there is a problem because it cannot reach maximum RPM.

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

I didn't know they warranteed 40 y/o engines over @ Yanmar.

I didn't even review this post as it didn't seem relevant but was interested nonetheless. I, too , have a boat with an over-propped engine (Pearson Triton with an Atomic 4). Been that way for 50 years. The deadwood is so wide that a too large prop is needed to get the tips out into clear water. My engine doesn't reach more than 50% of rated RMP or HP. Been that way for 50 years. Lugging is supposed to damage the motor. I don't lug mine and it doesn't sound like you lug yours. That's the key, no? You get decent speed and fuel usage even tho you aren't using the rated power. Drop in motor with same bolt patterns - what's not to like? Drop it in, use the same prop. If/when you get back to civilization you can check out some propage, but you may just keep the arrangement since it's working, right.

Sorry to push you off, but if you had read the thread, your comment would likely be less trollish and I I am sure I would not feel like banging my head into the wall again...To set the record straight...the RESELLER of the second hand engine was offering the warrantee. The fact that this was not self evident, possibly you need to get out on your boat for a while.
The boat is here to stay, on a remote island as a dive boat.
After you got done venting the useless, you did provide me with useful information from your experience, so thanks for that. I will get over the bruises on my forehead.

[Scot McPherson]

With the shape of your hull I don't doubt your numbers anymore. It's a barge or at best a scow and those numbers can be true for those types of boats.