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1982 Skye 51
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Discussion Starter #1
There’s not much wind here in Indonesia and I have been compelled to do more motoring than I am accustomed to, sometimes several days at a time. My engine is a Perkins 4.236M with a Borg Warner Transmission and V-drive. To conserve fuel, I am often running running between 1000 and 1200rpm. This keeps the engine temp at 155deg (it only goes up to 160 when I run her up to 1800rpm) and goves me 4.5-5.5kts. A diesel likes to be run under a load but would these low RPMs and temps enough to prevent sooty, gunky buildup? Periodically, I do run her up just to blow out all the carbon.

So, what constitutes a load? I’m sure it varies from engine to engine but is there a general rule of thumb? When the engine is in gear? Above idle-forward rpm?
Thanks.
 

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I think you answered your own question: if your engine heats up to 160 at 1800rpm, then 1800 is your engine operating under load that is enough to "get it's juices flowing." I would operate it at that level to avoid excess carbon build up. ;)

Tod
 

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Disclaimer: not a diesel mechanic

I had the same questions, and have found guidance that varies.

Load is more than just RPM. One major consideration is prop pitch. I believe that your engine's max intermittent rpm is 2500 rpm. ( low speed, high torque) The max continuous rpm is 2250. It's my understanding that if a prop is pitched correctly you should be able to achieve or come very close to design criteria at WOT. (including temps) If you can't make max rpm, then that could be an indication of prop pitch or other issues.

Your operating temp from what I found ( closed system) is between 168 and 197 degrees.
Your hull speed is somewhere around 8.8 kn. based on your waterline.

For cruising, I've seen numerous recommendations to operate at between 75-80 % of design criteria. However, there's also plenty of anecdotal information of diesel owners who regularly operate at lower percentages to conserve fuel with no loss of engine life.

My engine's max rpm is 3,000. I typically cruise at 1800 rpm (60%) which seems to be a sweet spot. (@ 180 degrees temp) With a clean bottom and smooth sea, that will produce 4.8- 5 kn. However, I'll run at 2,000 if bucking a current, and I've occasionally had it up to 2500 rpm for a few hours at a time to load it up. ( with throttle left over). My engine is also 30 years old.

At between 1000 and 1200 rpm, you'd be operating at between 40 and 48 %, providing your prop is pitched correctly. It seems to be running cool (155 deg.) at that rpm. (if the gauge is accurate). 1800 rpm @ 160 degrees is 72%.

I believe it's more important for its' health to run an engine at proper operating temperature. Once that's achieved, I don't think you hurt the engine running at lower RPM at temp and conserving fuel. You could do a series of runs at various speeds and fill the tank to record the difference in consumption.

http://www.cheoyleeassociation.com/RestorationIssues/Manuals/Perkins Marine Engine Handbook.pdf


See my disclaimer above.
 
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