Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 10-03-2011, 15:01   #61
Commercial Member
 
Mark Johnson's Avatar

Cruisers Forum Supporter

Join Date: Sep 2009
Location: New Bern NC
Boat: Searunner 34 Trimaran
Posts: 1,565
Re: 'Cruising RPM' - What Does this Mean ?

Quote:
Originally Posted by Chipg View Post
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
__________________

__________________
Mark Johnson is offline   Reply With Quote
Old 11-03-2011, 02:20   #62
Registered User

Join Date: Oct 2009
Posts: 323
Re: 'Cruising RPM' - What Does this Mean ?

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

__________________
Chipg is offline   Reply With Quote
Old 11-03-2011, 03:49   #63
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: 'Cruising RPM' - What Does this Mean ?

Quote:
Originally Posted by Mark Johnson View Post
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.
__________________
Dockhead is offline   Reply With Quote
Old 11-03-2011, 05:13   #64
Commercial Member
 
Mark Johnson's Avatar

Cruisers Forum Supporter

Join Date: Sep 2009
Location: New Bern NC
Boat: Searunner 34 Trimaran
Posts: 1,565
Re: 'Cruising RPM' - What Does this Mean ?

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
__________________
Mark Johnson is offline   Reply With Quote
Old 11-03-2011, 05:51   #65
Registered User

Join Date: Oct 2010
Boat: 1975 Pearson 35
Posts: 145
Re: 'Cruising RPM' - What Does this Mean ?

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?
__________________
Juniper is offline   Reply With Quote
Old 11-03-2011, 06:14   #66
Registered User

Join Date: Feb 2011
Posts: 586
Images: 3
Re: 'Cruising RPM' - What Does this Mean ?

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.
__________________
smurphny is offline   Reply With Quote
Old 11-03-2011, 19:19   #67
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
Re: 'Cruising RPM' - What Does this Mean ?

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.
__________________

__________________
barnakiel is offline   Reply With Quote
Reply

Tags
rpm

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Yanmar 3GMD Cruising RPM ? sailstoo Monohull Sailboats 2 05-08-2010 13:37
Erratic RPM Amwasp Engines and Propulsion Systems 8 03-12-2009 21:45
Yanmar RPM Woes PaulM Engines and Propulsion Systems 24 12-11-2008 14:40
Cruising RPM? Jack Long Engines and Propulsion Systems 20 22-07-2008 15:11
Max RPM PaulM Engines and Propulsion Systems 14 26-04-2008 01:10



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 18:27.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.