Cruising rpm's

[lancelot9898]

I don't have the Yanmar but rather a Perkins 4-108 which is rated at 4000rpm intermitent and 3600 continuous. I also have a max prop which allows me to adjust the pitch to achieve various WOT readings. I finally settled on adjusting the pitch to 14 inches( 17 inch dia) and achieved 3000 WOT on my tach. However this year I discovered that the tach was out of calibration and when normally cruising at 2500 rpm, I was actually doing 1900. So with much thought I considered going back and adjusting that pitch smaller to achieve closer to the continuous rating, but now I'm not sure if that is necessary. One of the main factors on engine life is how much fuel flows through it and as long as operating temperatures are achieved then operating at lower rpms should be OK. I've read about over proping an engine resulting in lugging and while I can tell that I'm "lugging" my engine in my jeep when I don't down shift going at a lower speed, I have yet to tell that i'm doing the same with the boat. The only disadvantage that I see operaing at a lower rpm is that the horsepower available is less. My WOT reading is only 2300 rpm which is quite a reduction from the 3600 rating.

[klem]

lancelot,

While I was one of the people arguing for lower rpm, the argument assumed proper gearing/pitch. Glazing will not be an issue for you but there will be other issues like lugging. I would recommend taking out some of the pitch when you get a chance so that your engine can turn over faster for a few reasons. As you mentioned, lugging is bad. Lugging is trying to get too much power out of each combustion cycle by simply forcing more fuel in. What this means is that rather than having the work done by the engine spread out over more combustion events, it is concentrated greatly increasing stress.

You most likely are not realizing the full power potential of the engine because of your gearing. This will be made even worse if you are trying to push into bad weather and you cannot get up into the engine's powerband.

Lugging will not improve fuel economy as many believe. You are trying to force more fuel into the engine than should be there and it runs rich. Diesel engines are fuel throttled not air throttled so improper gearing will result in excess fuel for each combustion event.

Does this make sense as to the difference between gearing and what you choose as a cruising rpm?

[lancelot9898]

Originally Posted by klem

As you mentioned, lugging is bad. Lugging is trying to get too much power out of each combustion cycle by simply forcing more fuel in. What this means is that rather than having the work done by the engine spread out over more combustion events, it is concentrated greatly increasing stress.

I'm not sure about the stress issue considering that I'm backing off about 20% from the WOT setting. Seems like if the WOT was 3600 rpm and I backed off 20% from that number, stresses induced on the engine should be similar? No calculations to back that up. Anyway that point about the jeep is that I can hear the engine "lugging" whereas with the Perkins it sounds "happy" at the lower rpm even pushing into a head wind. I'll probably reduce that pitch back to 12 inches approx. so to get more horsepower rating. At 2000 rpm the shaft hp is 8 whereas at 2500 it's 14. (3000 is 21)

One other thing that I noticed about rpm and WOT. When tied at dock my WOT rpm is about 150 rpm less that when out on the water. This I can understand since less force is needed to move the boat underway vs tied to the pilings. Not sure if I said that right. Anyway expect that I can achieve a higher WOT setting still if I'm going against a stiff current. (flow past the prop allows the prop to work less and I've seen around 2600 rpm on WOT) Hull spreed through the water would still be the limiting factor so there is a limit to how much rpm could be gained going against the current.

[Maine Sail]

Originally Posted by lancelot9898

My WOT reading is only 2300 rpm which is quite a reduction from the 3600 rating.

Most all engine manufacturers suggest the engine be able to hit max rated throttle with a clean prop and bottom. If you can not hit max RPM you are over propped. Being over propped/pitched is not good for the engine and is one more factor that can lead to bore glazing. You should adjust your prop so you can hit the max rated RPM.

You're not "backing off" 20% from max RPM you are forcing and over loading your engine and thus preventing your engine from hitting max RPM by over propping it. There is a big difference from being able to turn max RPM and then backing off by 20% for cruise RPM vs. not allowing your engine to hit max RPM by forcing too much pitch on it for a 20% reduction in available RPM.

[lancelot9898]

Originally Posted by Maine Sail

Most all engine manufacturers suggest the engine be able to hit max rated throttle with a clean prop and bottom.

You're not "backing off" 20% from max RPM you are forcing and over loading your engine and thus preventing your engine from hitting max RPM by over propping it.

From one of my Perkins manuals it states that the max rating of 4000 rpm should not be run for longer than 45 minutes of every hour with the rpm reduced by 20% during the 15 minutes before returning to max rating. However it goes on to state that if the wide open throttle setting is less than the max rating then it too should be backed off by 20% every 15 minutes on the hour. In other words the WOT setting should be treated as the max rating.

I've always heard the term "over proped" and understand that to get the hp rating of the engine that certain rpms should be obtained. However I do not understand if I'm operating the engine at a lesser rpm that I'm over loading it even when I'm not able to achieve the desired rpm rating. Is there some physical sign(temp rise, smoke from exhaust, strange sounds) to show that the engine is being overloaded??

[Maine Sail]

Originally Posted by lancelot9898

From one of my Perkins manuals it states that the max rating of 4000 rpm should not be run for longer than 45 minutes of every hour with the rpm reduced by 20% during the 15 minutes before returning to max rating. However it goes on to state that if the wide open throttle setting is less than the max rating then it too should be backed off by 20% every 15 minutes on the hour. In other words the WOT setting should be treated as the max rating.

I've always heard the term "over proped" and understand that to get the hp rating of the engine that certain rpms should be obtained. However I do not understand if I'm operating the engine at a lesser rpm that I'm over loading it even when I'm not able to achieve the desired rpm rating. Is there some physical sign(temp rise, smoke from exhaust, strange sounds) to show that the engine is being overloaded??

If you can not achieve the max rated RPM of the engine the engine is over propped. I don't know how to be any clearer.

I will use my own Westerbeke a an example as Westerbeke is very clear in their language about prop sizing. I was 350 RPM off this spring with a new prop and was told in no-uncertain terms by both the distributor, Hansen Marine, and Westerbeke them selves in Taunton, MA that I should most definitely have my prop re-pitched with no if's and's or but's about it.

Here's the basic gist. The MAX rated RPM on my four cylinder 44B Four is 3000. Here is exactly what Westerbeke says, in writing.

"Westerbeke recommends a propellor that will allow the engine to turn 3000 RPM under way at full throttle."

Pretty darn clear!!!

My MAX RPM is 3000 and my engine should be able to hit 3000 at WOT. This does not mean that this is where you run the engine but if it can turn max RPM then everything else falls in line behind it and you won't be loading the engine. I cruise at 75-80% of WOT and the 350 RPM difference made a large difference in the tone of the engine. Because I was 350 RPM off, Westerbeke recommended that I also temporarily back my normal cruise RPM off by 350 RPM to "try" and compensate for the over propping and to NOT overload my engine.

By backing my RPM off by 350 it put me at an RPM outside of the recommended cruise RPM range for the engine and one too low for long term use. This was according to the Westerbeke engineers in Taunton that I spoke with. Fortunately the prop maker had me a new prop in about two weeks time and I only had to put about three or four hours on the engine this way..

Many good surveyors also check to make sure an engine can hit MAX rated RPM during sea trials.

[Northeaster]

When I bought my 30'er, which has an older (1982) Yanmar 2GM, 15hp max (3600rpm), 13hp continous (3200rpm), it was overpropped., with a 15x10 fixed 3 blade.
The engine "seemed" happy up to 2000rpm - in fact would push the boat to within a knot or so of hull speed, at these low rpms. But, if I tried to give it more throttle, I would get black smoke, and the engine wouldn't sound happy.

After much reading, and talking to a knowledgable prop guy, I had 1" of diameter and 1" of pitch taken off this winter, to make it a 14"x9". The pitch likely made more of a difference, as there was not alot of surface area in the tips. Regardless, I was told that I might see an increase of 200-300rpms, which would still leave me well short of 3600.

In reality, what happended is I can now rev 3200rpms, (a full 1000 rpms better than before), with no smoke, and hit full hull speed (6.4-6.8 kts) consistantly. I suspect that this is because I only had about 8 hp, to play with at the 2000roms range, whereas now, the prop is able to spin faster, and use more available engine hp as well (approx 13hp). Now, the engine sounds much happier (except for a squeal, that is another issue).

I am still a bit overpropped a bit, as I can hit about 200 higher in neutral, than in gear, with a clean hull / prop. I am not sure if I will have it changed again, as I am very close, and 90% better than it was. If I do, I will send it to the same prop guy, as he had 25 years experience working on props of all sizes (mostly commercial) and you could tell that he knew his stuff.

[lancelot9898]

Originally Posted by Maine Sail

. Because I was 350 RPM off, Westerbeke recommended that I also temporarily back my normal cruise RPM off by 350 RPM to "try" and compensate for the over propping and to NOT overload my engine.

By backing my RPM off by 350 it put me at an RPM outside of the recommended cruise RPM range for the engine and one too low for long term use. .

Appreciate your comments Maine. It seems to me that I am not overloading the engine by the statements above, however apparently my rpm is "outside of the recommended crusie rpm range." (1900 rpm does get me up on the torque curve so not sure if I'm really outside of "cruise range") FWIW...I bought the boat new in 1985 and have been operating the engine in this "out of spec" range ever since. I did not know until I put a photo tach on the engine that my tach readings were so much off however. Good discussion and look forward to any more comments.