Way Over Propped but it Works, is it Safe?

[darylat8750]

Jim,
Lower rpm at a given hp is more efficient until one gets so low that other things like cam design and intake and exhaust velocity come into play. Take your 5 speed car and run it in 5th gear you get "x" miles per gallon. Run at the same speed in 3rd gear and you will burn more fuel per mile. Guaranteed. A fine pitched prop is like a car geared for acceleration or top speed. Coarse pitch is like an "over drive".

[Jim Cate]

Originally Posted by Scot McPherson

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

Scot

Yeah, Scott, but AFAIK the engine was in that boat with that prop the whole time. I'm not sure if that was specifically said, but it was the impression that I got.

Jim

[Jim Cate]

Originally Posted by darylat8750

Jim,
Lower rpm at a given hp is more efficient until one gets so low that other things like cam design and intake and exhaust velocity come into play. Take your 5 speed car and run it in 5th gear you get "x" miles per gallon. Run at the same speed in 3rd gear and you will burn more fuel per mile. Guaranteed. A fine pitched prop is like a car geared for acceleration or top speed. Coarse pitch is like an "over drive".

I don't think that this is a useful analogy.

There is a rule of thumb for diesel engines that says you burn something like 220 grams of fuel per horsepower-hour... minor variations between engine designs. It does not matter very much what rpm the engine is turning as long as you avoid extremes. So, because it will always take the same HP to drive the boat at the same speed, changing the prop to vary the RPM at the cruise speed will not change the fuel consumption very much. Changing the propeller design may alter the efficiency with which the prop turns HP into useful thrust, but we were not discussing that aspect of his situation.

Cheers,

Jim

[ti325v]

Hello Gentlemen:
Wow, who would have though a question about an extreme over prop would turn into a "which anchor is better thread ?"
Oh and the anchor I brought works fine thanks ! despite it being half the weight some folks recommended.

It seems we are arriving at a consensus, with ex-calif, Boden36, jim cate and some others seeming to have given me reasonable answers that match my real world experience.
I await more info from djmarchand, he did raise some interesting points.

Things may change a bit when we drop in the new engine..could be much more powerful, though I doubt it.
We do not do hole shots with the boat, and it comes right up to speed smartly, no throttle histrionics to keep it from smoking, just an easy opening of the throttle.
Yes I do want to keep it quiet, we are on nearly deserted islands and are often the only boat in sight....the louder it is the more it encroaches on peoples enjoyment of being on it, and if we up the revs, I expect more noise.
We have no need to go any faster, even beating into the strongest headwinds we have ever faced. ( We are only out in recreational conditions but have been caught in a couples of serious blows.)
Fuel economy will not increase AFAIK.
I only became concerned about this issue as the install date of the new engine approaches and I started thinking about piston speed after reading a bunch of horror stories about over propped boats here on cruisers forum.
Currently mine is about 6 meters a minute and engines of this size and date seem to be about 9 meters a minute.

I kinda look at is as a boat propped correctly for an 1800 rpm engine, which it is. Giving me all power I need.

I thought there would be an easy answer to this one and again I am left with you pays your money and you takes your chances. I think have some good advice here, and since I do have another old prop, original to the boat..(1.03 meters) HUGE ! I may just for giggles trade that in for a 72 Cm or so that will give me 2700 rpm, just to see it run. The prop swap might cost me all of $250 installed.

Thanks guys, dj please do update me. I await word from you.

To those that have not figured it out, the prop is about 3 feet in diameter, and the rough draft of the boat averages about 2 feet....the keel extends past the prop to support the bottom of the rudder at a depth of nearly five feet.
My propping calculations called for an average draft. In fact the top of the prop is more then 24 inches underwater. We do not use it to chop vegetables from the back of the boat.

[roverhi]

Originally Posted by ti325v

To those that have not figured it out, the prop is about 3 feet in diameter, and the rough draft of the boat averages about 2 feet....the keel extends past the prop to support the bottom of the rudder at a depth of nearly five feet.
My propping calculations called for an average draft. In fact the top of the prop is more then 24 inches underwater. We do not use it to chop vegetables from the back of the boat.

Think I figured out why you are resisting buying a new prop. Scrap value on that large a chunk of bronze would discourage most people. When you talk about taking a little off the diameter you may be talking feet, not inches.

[CSY Man]

Some guys in the know says a Diesel engine is most efficient at 70% of max continuos power and the same engine should be able to reach max rated RPM under full power with a properly sized prop while underway. The same engine/prop combo should be able to turn 90% of max rated RPM while tied to a dock.

[ti325v]

Hi CSY:
For the vast majority of users I would agree with you, just as in the case of my anchor. I have a very set and specific set of conditions in which I anchor, a maximum depth of 25 meters, a clean hard sand bottom, in moderate conditions. Despite choosing an anchor 50% lighter then many told me to get, and only 5 meters of chain despite being told I should have 100% chain I have never dragged. I never expect to and if I do, I do not care as I have a dedicated 3 man crew. We simply are not out in the conditions that dictate the huge anchor I was told to get.
So we have rules, like do not put gas in a diesel engine. Guaranteed to cause problems, everybody except the complete nutters agree.
Then we have guidelines, applicable to the vast majority of the population, and in the case of boating often not applicable to me. My boat never goes more then 15 miles from her moorings. (Well once in the past 4 years, when we were chartered to go out looking for that downed Malaysia airlines plane)
We obviously have many cases of people way underpropping their boats with no problem. I have been doing it for years and over 3000 hours.
So, do you feel this is a rule or a guideline ?
I feel it may be leaning towards the side of a rule, that is why I posted the question and do want the best answer for the life of my engine.
At the same time it works, and the boat runs great, so am I the exception to the rule as I was with the anchor ?
No sarcasm intended, I sincerely want to know and learn.
Thanks
Larry

Does

[ti325v]

Oops, make that meters per second...

Originally Posted by ti325v

Hello Gentlemen:
Wow, who would have though a question about an extreme over prop would turn into a "which anchor is better thread ?"
Oh and the anchor I brought works fine thanks ! despite it being half the weight some folks recommended.

It seems we are arriving at a consensus, with ex-calif, Boden36, jim cate and some others seeming to have given me reasonable answers that match my real world experience.
I await more info from djmarchand, he did raise some interesting points.

Things may change a bit when we drop in the new engine..could be much more powerful, though I doubt it.
We do not do hole shots with the boat, and it comes right up to speed smartly, no throttle histrionics to keep it from smoking, just an easy opening of the throttle.
Yes I do want to keep it quiet, we are on nearly deserted islands and are often the only boat in sight....the louder it is the more it encroaches on peoples enjoyment of being on it, and if we up the revs, I expect more noise.
We have no need to go any faster, even beating into the strongest headwinds we have ever faced. ( We are only out in recreational conditions but have been caught in a couples of serious blows.)
Fuel economy will not increase AFAIK.
I only became concerned about this issue as the install date of the new engine approaches and I started thinking about piston speed after reading a bunch of horror stories about over propped boats here on cruisers forum.
Currently mine is about 6 meters a minute and engines of this size and date seem to be about 9 meters a minute.

I kinda look at is as a boat propped correctly for an 1800 rpm engine, which it is. Giving me all power I need.

I thought there would be an easy answer to this one and again I am left with you pays your money and you takes your chances. I think have some good advice here, and since I do have another old prop, original to the boat..(1.03 meters) HUGE ! I may just for giggles trade that in for a 72 Cm or so that will give me 2700 rpm, just to see it run. The prop swap might cost me all of $250 installed.

Thanks guys, dj please do update me. I await word from you.

To those that have not figured it out, the prop is about 3 feet in diameter, and the rough draft of the boat averages about 2 feet....the keel extends past the prop to support the bottom of the rudder at a depth of nearly five feet.
My propping calculations called for an average draft. In fact the top of the prop is more then 24 inches underwater. We do not use it to chop vegetables from the back of the boat.

[djmarchand]

Ex-Calif expressed my views exactly. That and maybe an EGT gauge to keep an eye on things. As someone else said, if you can run below 900 deg F, you should be fine.

David

[captain58sailin]

I have had one experience with an over propped vessel and on 2 different occasions it melted a hole in one of the pistons. This was running the engines at a reduced rpm. After the last time the vessel was dry docked and they cut the props down to the recommended size. For my money, I wouldn't do it.

[CSY Man]

So, do you feel this is a rule or a guideline?

Not sure the power and efficiency curves for Diesel engine and propeller sizes are either rules or guide lines, just measurable facts.
Certainly you can over-prop a boat and get away with it, but the engine will work harder than it is designed to do and it could cause problems in the long run. Burned pistons was just mentioned above.
No free lunch.



Sent from my iPad using Cruisers Sailing Forum

[Scot McPherson]

It doesn't matter, enough people gave the OP enough of what he wanted to hear. You can see he's counting the number of supporters and ignoring the number of people who say it's not a good idea. I don't think anyone will dissuade him now. I hope his engine lasts as long as it has so far. One thing going for him is it's an old engine, so it's probably more up to the task than one built today.

[djmarchand]

Originally Posted by Scot McPherson

One thing going for him is it's an old engine, so it's probably more up to the task than one built today.

Ain't that the truth. I am one of those who "told him what he wanted to hear", but only after he described his engine. Individual heads bolted to the crankcase and removable ports to change out individual rods and maybe main bearings easily. That beast probably weighs double what a modern engine weighs.

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

[ti325v]

Hello Scott:

Oh and a new post from David, yes I can get at the mains as well, they are very nice..
and this thing is HUGE and weighs about 2.500 pounds. (another guess)
(And for all I know, David, you may be right and what I am getting from Yanmar Japan may be wrong and indeed it is a 2000 rpm engine as the shop manual says it is!, and somebody could have played with the fuel stop for all I know, but this is direct from Yanmar...)

Hello Larry,

Actually, nobody know what rating this engine is being built to.
Because correct engine name plate is already not available.

Propeller matching is very difficult. The engine is rated at 2700rpm or 2800rpm.
With correct propeller matching, engine rpm would reach to the above.
If propeller matching is light, rpm would increase to approx 100 to 200 more ( depending ).
If propeller matching is heavy, rpm would decrease and cannot achieve to rated.

Anyway, we should set the rpm to 2700 or 2800

Thanks

Best Regards
Gary Wong
Dear Larry,
6DH-HT , L rating : 165ps@2800rpm ( Low )
6DH-HT , M rating : 155ps@2700rpm ( Medium )
These are maximum rpm of the engine.
Continuous is approx 85% of maximum rpm
Thanks
Best Regards
Gary Wong

"YANMAR GROUP COMPANY - CONFIDENTIALITY NOTICE:


I think you read me wrong. I am not looking for opinions that match mine, This discussion has directed me to swap an extra prop I have for a 72 cm, which should be just about right to get it to 2800 rpm. It will cost me near nothing and I will know something about the engine performance.
BTW I had a piston out last week, and it looked great...after 3000 hours of this type of use. and about 33000 running hours total.
Photos here...
https://www.dropbox.com/s/jtshciesskil4uu/P1150358.JPG
https://www.dropbox.com/s/xblhzus1nr4pv8q/P1150359.JPG

I am asking others for their experience with this sort of thing, relative to my own experience.
So far I have had reported one case of a melted piston, but in that case, was the engine overloaded and smoking ?
Mine does not smoke and there seems to be a of a lot of people that are doing the same thing.
The only thing I hope to gain by this is noise reduction, there is no fuel savings. I do not need the power, and a new prop will cost near nothing.
If I put the smaller prop on and I do not gain a heap of noise I would be stupid not to keep it, on the other hand if the thing makes a huge racket and it is indeed safe with the big prop why not ?
You seem to be firmly of the opinion that it is not safe. I need this kind of information to help me make the decision.
It has been this way for 4 years, have a look at that piston and you tell me. Yes it needs some carbon blown off it, it was run very easy for a while before I pulled it out.
No the engine is not apart, the thing has six heads and access doors in the crank case to remove the rods, pistons and sleeves, and mains, it took less then 2 hours to pull the piston and have the engine running again.
I will report back when I have swapped the prop in a few months, and let you know how it runs with both.
Thanks for your input.
Larry

[Scot McPherson]

Well I don't know what you mean by safe...safe to me means no one will come to harm....so is your setup safe? Probably

Does it have a higher chance to fail, probably.


Carbon on your pistons indicates incomplete combustion. The heat has dried the oil to the pistons like a seasoning on cast iron cookware...that reduces the volume of your chambers, and makes the engine work even harder in addition to what you are doing to it since the engine is tuned for a specific chamber volume.

Since it didnt burn off with easy running as you said, it means it's been there a long time and got really hard.

I am glad you are getting smaller prop. Just don't over rev it the engine and I think it'll be quiet. An engine under load is quieter than an engine in nuetral.