[Kniffin]

With the cost of fuel going through the roof, many boaters are slowing down to get better economy. Some argue that by purchasing a semi-displacement hull, that they get the best of two worlds - to operate at displacement speeds with some economy and yet have the horsepower and speed to save time if needed, (which a displacement hull and lower horsepower does not offer). The question is - Does this adversely affect the modern electronic diesel engine which is rated at higher loads to operate at lower rpms?

[JHA]

In short - yes, if you run engines at reduced RPMs for an extended period of time you can do some real damage.
http://www.ntnoa.org/enginebreakin.htm
More about break-in but still applies...

[goplay]

There has been a lot of discussion about this topic on this and other boards. The view from individuals who know something about diesel engines is no, it does not harm the motor to operate at lower RPMs for extended periods (when was the last time you heard about a diesel needing to be rebuilt because low RPM use). It is helpful to get the motor to termperature first.

That said, I am no diesel engine expert, and I certainly would like to hear a definitive answer from the manufacturers!

[Pascal]

this is a common topic on many forums.

Usually, most agree that two ways to avoid any damage or wear is to make sure the engines are operating within the correct temp range (hot enough) and that low speed runs are followed by a run at normal cruise speed when possible.

if you them too slow, therefore too cold, yes damage is possible. On most boats, hull speed will be enough to keep them warm.

[Kniffin]

Well, the responses so far bring up some interesting issues. I think that we all agree that operating temperature is critical to healthy engine life, as is a proper break-in. I have heard that synthetic oil usage in diesel engines is desirable as well (after break-in). By the way, the article about 4 stroke gas engine break-in is quite interesting. What do the manufacturers concur on proper diesel break-in? All I have heard about this subject is to vary throttle without too much high end operation - with proper temps and loading, then a prompt oil change at recommended interval.

We all know that proper loading is critical to diesel engine health - but typically on larger yachts, props need to be "sized" to fit the load and rpm requirements of each boat, thus often subjecting the boat to undue break-in stress. And even though I claim no background expertise in prop design, my observation is that there is some "black magic" involved in attaining the best design (often at owner expense) for acheiving decent performance throughout the low, mid and upper ranges of cruising - with the measuring stick often being able to obtain 100% load at full recommended rpm. Even so, I have seen dramatic improvements (and the other way around) by switching prop styles and blade counts with the same rpm's attained.

Some, if not most yachts also have different operating characteristics at different weight loads, with the props sized to slightly "overrev" when light in order to compensate for heavier loads when the props get loaded. So, if a boat with higher power goes slower, should it carry two sets of props - one used for slower operation with heavy fuel loads. would variable pitch props or composite props (with a pitch adjustment) be a good idea?

And what about the turbo?- I have heard that operating a boat without the turbo boost can sometimes foul things up. Would the concept of going WOT at the end of the cruising day be applicable and desirable for engine health?

Finally, we sometimes figure that single screw operating on twin screw boats can help with range goals, provided that transmission cooling is achieved - but often the prop drag and the need to compensate the rudders counteracts the intended fuel savings.

I am interested to hear input on these issues.

[JHA]

At the end of the day the answer is still - Yes, operating your diesel engine below the prescribed RPM can result in damage. So trying to save a buck on fuel here may cost you the difference if not more replacing liners etc... when you manage to glaze the cylinders.

[goplay]

Originally Posted by JHA

At the end of the day the answer is still - Yes, operating your diesel engine below the prescribed RPM can result in damage. So trying to save a buck on fuel here may cost you the difference if not more replacing liners etc... when you manage to glaze the cylinders.

Ahh... I disagree. You can run at low RPM without damage. It is done all the time. Some captains even run a single motor. Also, if you are making a 400 mile run, the difference in fuel consumption is more than a buck, it can be 4-5x.

The thing to consider on low speed runs is the rolling of the planing vessel. If you don't have stabilizers, it can be quite uncomfortable and low speeds and in beam seas.

[C4ENG]

I was at a Cat training seminar where the topic of low RPM running came up, mostly in sport fishing yachts trolling. They went in depth of the issues they were facing in trying to produce an engine that can troll for a long time and still be powerfull enough for high speed travel. They showed pictures of the damage from long term trolling and running at low load. The answer is definetely YES.. running at low load can hurt your engine.

[goplay]

Hearing from CAT directly certainly is definitive.

What kind of issues did they describe?

Was damage still present with sport fishers that trolled but had high speed runs before and after trolling?

Thanks.

[Kniffin]

I would be also interested to know what kind of damage is caused by slowing down on a high power engine.

JHA mentioned glazing on the rings and cylinder wall, which sounds like it could reduce compression and lead to other problems caused by blowby.

What is CAT specifically saying about the trolling issue and what damage is caused?

I have noted a few manufacturers touting the ability of their boats to go slow for range or perform at speed. It would be nice to hear input from some Naval engineers on the subject.

[C4ENG]

Here is straight from Cat..
At low load you have low cylinder temps (not to be confused with water jacket temps) which then causes there to be un burned fuel. Un burned fuel will then enter the oil through blow by which every engine has and even more so at low load becuase the metals or not at there expanded rate for best seal. Fuel in oil now.. besides thining out the oil.. fuel has sulfur in it. When sulfur meets water it creates sulfuric acid (there will always be some condensation taking place in your engine for there to be water inside) Now you are throwing sulfuric acid all over the inside of your engine to create corrision and pitting. That's when they showed pics of pitted cylinder linings and bearings of low load running engines.
But then there is the additive in oil to combat and nuturalize the acid which is called TBN (total base number). Cat sells kits to test your oil TBN becuase when that aditive is used up it is no longer combating the acid. You may need oil change sooner than recomended according to usage but who ever wants to change sooner? That's when the problems arrive.
At rated suggested loads the cylinders are up to temp and almost completly burning the all the fuel.

[Kniffin]

Good information indeed.

So do engine manufacturers have a formula for operating at certain lower parameters, just as they do for higher parameters?

[JHA]

My experience has been with larger motor yachts. In one instance running Deutz 604BV12s and more recently 604BV16s it can be tremendously damaging to run at low RPMs. I had to replace (not personally but by the fine men at MSHS) 26 of 32 cylinder kits on the BV16s at a HUGE expense to the boss to avoid catastrophic failure... all because the previous captain ran the engines below the RPMs that were prescribed by the manufacturer. By the way I haven't been in the day-tripper / bahama cruiser mindset on this - You can run low RPMs but you oughta (inmho) blow them out with some high load runs at the start and end of a trip. Eg; departing from Panama headed for Galapagos (5 day trip give or take) get the engines hot for 20 mins each day to avoid the glazing issue and give them a good run on the last couple of hours before arriving.

[C4ENG]

Originally Posted by Kniffin

Good information indeed.

So do engine manufacturers have a formula for operating at certain lower parameters, just as they do for higher parameters?

They do some what. For Cat you can have a E rated engine which is like suggested full load at 80% of the time, oppose to a A rated engine that is suggested for full load at 20% of the time (don't quote me on these numbers, example only!) So then you need the proper rated engine for vessel usage.

[goplay]

I wonder then, if the issue is incomplete burning of fuel, an oil cleaner system can be created.

Then again, it might just be simpler to change the oil after a long (many 100 mile) low speed run.

That raises the question of how much it takes to cause the pitting. Is it 100's of engine hours at low rpm or a dozen hours, and over what time period. Is it the engine sitting around for days afterwards?