'Cruising RPM' - What Does this Mean ?

[Mark Johnson]

Originally Posted by Chipg

Caterpillar says for the larger 3208 you can run it for 5 hours at low rpm as long as you run it at cruise speed and operating temp for 30 min before shutdown.

Comparing the recommendations from the manufacturers of larger, heavier, low RPM engines, to those from the manufacturers of smaller, lighter, engines (like Yanmars), that were "designed" to run at higher RPMs, is "apples to oranges". One can not be compared to the other. They're too different.

With small Yanmars, that have a max of 3,600 RPM and a "continuous" max of 3,400 RPMs, the goal is to run it at between 2,600 and 2,900 RPMs for engine longevity.

If this lower 2,600 RPM, is over the hull speed and just makes the hull plow, (wasting fuel), then you likely have the wrong transmission, (of the available three).

Since changing the transmission out is impractical, you could go down a notch on the pitch of the prop.

This gets the hull up to speed @ something just over 2,600 RPM, the engine will last longer, you get good fuel economy, and when bucking a strong headwind, you have an almost 50% HP reserve left over, if you don't want to let the wind slow you down.

At least that's my take on it...

Mark

[Chipg]

Does that mean cruise is still 80% on either engine and they will both glaze at the same percentage lower throttle setting?

[Dockhead]

Originally Posted by Mark Johnson

Comparing the recommendations from the manufacturers of larger, heavier, low RPM engines, to those from the manufacturers of smaller, lighter, engines (like Yanmars), that were "designed" to run at higher RPMs, is "apples to oranges". One can not be compared to the other. They're too different.

With small Yanmars, that have a max of 3,600 RPM and a "continuous" max of 3,400 RPMs, the goal is to run it at between 2,600 and 2,900 RPMs for engine longevity.

If this lower 2,600 RPM, is over the hull speed and just makes the hull plow, (wasting fuel), then you likely have the wrong transmission, (of the available three).

Since changing the transmission out is impractical, you could go down a notch on the pitch of the prop.

This gets the hull up to speed @ something just over 2,600 RPM, the engine will last longer, you get good fuel economy, and when bucking a strong headwind, you have an almost 50% HP reserve left over, if you don't want to let the wind slow you down.

At least that's my take on it...

Mark

I think that the load is more important than the RPM. The article linked in one of the previous posts in this thread says that you won't get glazing or bore polishing in a properly broken in diesel engine at any RPM as long as you have at least a little load on.

My father's boat had 10,000 hours on the Perkins 4-108 and a broken engine hour meter when he bought it 14 years ago. Since then he has put about 50,000 miles on the boat so God knows how many hours now -- probably another 10,000 (he motors 80% of the time since his boat, with its long keel, doesn't go upwind at all). That engine has 4,000 RPM maximum and he cruises at 1,900 RPM and practically never exceeds that RPM. Furthermore, he idles the engine for a few hours every day when he is out cruising to charge batteries since he's got no genset or wind generator. When I asked him if he was afraid of glazing the bores, he said "what's that?".

That engine, which is now about 30 years old, still runs like a top, doesn't burn any oil (leaks it like crazy), doesn't smoke, starts if you even look sideways at the starter button, never had any major service. I don't think he's even adjusted the valve clearances.

[Mark Johnson]

Once again... trying to talk about diesel engines as if one type is like the other. I was trying to clarify about small marine Yanmars only.

Not running them "long term" at under 2,600 RPM, is what they (Yanmar),suggest... Regarding the larger low RPM engines I don't know, but in trucks & buses, it appears that they can idle all night long without damage. (Different type of engine!)

Both types on engines can not be generalized about in the same discussion. APPLES TO ORANGES... Mark

[Juniper]

I also wonder about this as I had a 4L 6cylinder 100HP Mitsubishi in a 1979 Dodge pick-up, top governed speed 4000, max loaded speed 3600, run mostly around 1800-2200, average load was about 30-40HP, running at half rpm and load still running strong after 250,000 mi. But it was never running one rpm for very long, would it be better to vary the rpm up and down all the time? say cruise control, set a speed then let the engine adjust to it as needed?

[smurphny]

As mentioned above, it seems that large diesels designed for trucks and construction equipment regularly idle for hours without any glazing problem. They must be designed with different alloys in sleeves and rings than the little Yanmars. The typical large Cat, Cummins, and GM diesels, it seems, get tired with fewer hours than the small Yanmar, Kubota diesels. Probably only a diesel design engineer can answer those questions.

[barnakiel]

A little side-track:

- Is there any slow-rev small diesel engine still made today? I am asking because I noticed that the newer VP have lower rpms than their earlier series.

b.