Because I took it easy on the m25 engine in our Cat 30, I wound up glazing the barrels, and had to rebuild the engine for lack of compression. Kabota and volvo engines are well know for this. When our engine lost compression I called the Kabota tech reps in Toronto and this is what I was told.
The worst things you can do to a Universal (Kabota) is to start it up, cruise out of the club at low power, put up the sails and shut down the engine.
Next is to sit at anchor and charge the batteries. Third is to motor sail, and fourth to just start it up for a few moments, then shut it down. If you do this one will glaze the barrels, which results in lost compression, because the rings can not seat properly.
To prevent this one most start the engine and immediately or as soon as possible, put a load on it. It should be run (ideally) at 80%. If charging batteries at anchor, put it in reverse also to put a load on it. The alternater is not enough!. Do not shut the engine down until it has reached normal operating temperature.
I have followed these guidelines from Kabota and I must admit our M35A in our C 36 mk2 is working perfect.
Yanmar engines don't seem to be so prone to this problem!
For what it's worth!
cheers Wilf

Not an expert reply, but what I recollect is:

1. A diesel under low load operates much cooler than it is designed to. Parts don't expand to their optimal sizes for one thing.
2. With low temperatures, there are more unburnt fuel products 'crudding' up the engine.

The most common recommendation I hear/read is that you should run your engine hard for about 10 minutes or more after a period of light loading. It is better than just shutting it down after a light load.

Despite all these cautions, I find it impractical to do that very often. I have to start my engine at the entrance to the channel and motor at light load/low speed for 12 minutes to my dock. What am I going to do then? Tie her up well and run hard in gear for 10 minutes? I never do that, and I just hope that my engine will not suffer too much from the light duty abuse.

Another thought.

Back when I was chartering in the Caribbean, it was common to have boats with engine-driven refrigeration compressors. That was pretty smart in the sense that if you are going to run the engine 1 hour every 12 hours to charge batteries, at least the compressor loaded up the engine a lot more than just the alternator would.

Running your diesel with no load is not the best for it, but please consider the times in the past, when fuel was cheap, long haul truckers would leave their rigs at idle for hours in the rest stops. Also when these same trucks are rolling down the interstate at 55 miles per hour their engine is operating no where near its full power, they are up to operating temprature. These engines are normaly run for 1/2 a million miles before overhaul

Load and temprature are critical, you do need to get your engine up to temprature, make sure the thermostat is doing its job. You also need to keep resonable load on the engine to mantain reasonably high cylinder pressures. On my engine 2200 RPM is about 80% power, 1500 - 1600 I believe will be about 50% power, about the same as those long haul truckers.

Carboning up of the cylinders, you should be aware can happen at full power as well, if your ship is overproped, or you have a very dirty bottom you will note black smoke in your exhaust at full power, this can be worse than idling for long periods of you do not reduce throttle. During normal operation the fuel is scheduled to the engine to maintain a given RPM and you will not have a rich mixture. At full power with a clean bottom and the correct prop if you are seeing black smoke you may need to adjust your max power fuel adjustment, it is on the forward side of the engine just below the injector pump. It has a tall cap nut on it, with that removed you will find a set screw and jam nut. This adjusts the stop on the governor rack to limit fuel flow at full power when the governor is not controlling RPM. This adjusment is made if you are getting black smoke at full power or if you are not getting max RPM at full power with the appropriate Prop.

Below is from Westerbeke:

[B][U]MODEL --[COLOR=red]IDLE RPM's--[/COLOR] [COLOR=blue]Cruise RPM's[/COLOR] ---Maximum RPM's[/U][/B]
M3-20B-- [COLOR=red]1000-12[/COLOR][COLOR=red]00----[/COLOR] [COLOR=blue]2500-3000 -----[/COLOR]3500-3600
M25XPB-- [COLOR=red]1000-1200----[/COLOR] [COLOR=blue]2000-2500[/COLOR] -----2900-3000
[B]M-35B-- [COLOR=red]800-1000----[/COLOR] [COLOR=blue]2000-2500----[/COLOR] 2900-3000[/B]
M-40B-- [COLOR=red]800-1000------[/COLOR] [COLOR=blue]2000-2500-----[/COLOR] 2900-3000
M-50B-- [COLOR=red]800-1000------[/COLOR] [COLOR=blue]2000-2500[/COLOR] -----2700-2800

This information is on Service Bulletin #236 and also listed in the Universal Owner's/Operator's Manual #200550. And is available through any Westerbeke/Universal Dealer.

[B]NOTES:
[/B][COLOR=blue]Attempting to reduce idle speed below the minimum may produce unstable engine operation and stalling.

The propeller used should allow the engine to reach the full or maximum rated speed while underway in forward gear.[/COLOR]

Good info, Allan. Thanks.

And for Steve, those were also some very good remarks. It may well be that the end result of light load operation will depend upon the engine design. I would hope that designers of over-the-road truck diesel engines would consider the 'duty cycle' the engine will experience. Perhaps subtle design changes allow those engines to tolerate periods of light load with less significant effects.

As I stated earlier in different words, I take a moderate approach on this issue. I try to keep my RPMs below 2500 if at all possible, and also try not to 'baby' the engine too much. I would have to say that my average usage is somewhere between light load and high load.

Thanks very much for all the responses, I'll stop "babying" my Kubota V1305 and actually get places a little faster!

Jim