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Old 02-23-2017, 01:42 PM   #1
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over powering your vessel

I have heard various things about using a larger than necessary diesel engine to power a vessel. specifically i'm referring to a full displacement trawler (21 ton) operating on a single screw at low RPM utilizing a less than a 100 HP engine (as the designer recommends).

setting aside the extra cost of a larger engine than needed and the lost fuel economy, is it bad for a modern diesel engine to be run under loaded?

i have heard it both ways. yes- the engine needs to be properly loaded to run efficiently and maintain its well being and avoid internal build up.

I have heard no- modern engines, particularly turbos, suffer no ill effects being run continually at low RPM.

As I shop for used engines, i find very few for sale in the 100hp range. however, there seem to be many 6bt's and comparable lehmans in the 200+ range at good prices.

what say you?
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Old 02-23-2017, 02:12 PM   #2
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The 6b has models from low 200 HP to above 400 HP so ask your self if the 400 HP is harmed by running at 200. Early engines especially 2 stroke might over fuel at low loads but modern electronic controlled injection systems are designed to provide only the fuel required for the load because of environmental regulations, better efficiency and less smoke and longer life. Quieter too.

IMO running a modern 200 HP at half load will only result in it outlasting several owners grand kids provided it is set up to run at proper operating temperature.
The diesel boating world never seems to forget old stories.
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Old 02-23-2017, 02:17 PM   #3
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Folks like straight sixes for marine engines because of their inherent balance and smoothness. That often means a bigger engine than necessary.

Lots of boats out there running Lehman 120's and Cummins B210 at like 40hp down the shaft and hold up fine.

I've run my Cummins 8.3 450hp for probably half its life at 950rpm and about 35hp. No ill effects. I do run it up to 1950rpm and 200hp to clear it out periodically.
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Old 02-23-2017, 02:50 PM   #4
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Light loads at low rpms won't harm a diesel. Light loads at high rpm can glaze the cylinders like happens in some generator engines that run long hours with light electrical loads.
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Old 02-23-2017, 02:59 PM   #5
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scott,
It sounds like you need to shop more.
At least w a FD boat figuring out how much power is needed is easy.
3.5 to 5hp per ton of displacement.

Overpowering w a SD hull at least has some possible benefit whereas there is none re FD. However if you find the perfect engine but it's too big for $100 ??
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Old 02-23-2017, 04:25 PM   #6
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scott,
It sounds like you need to shop more.
At least w a FD boat figuring out how much power is needed is easy.
3.5 to 5hp per ton of displacement.
Thanks eric but i know how much HP i need, the question is,is under powering bad for the engine. Not sure about what the "need to shop More" means. im not buying yet, im researching.
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Old 02-23-2017, 04:36 PM   #7
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The 6b has models from low 200 HP to above 400 HP so ask your self if the 400 HP is harmed by running at 200. .
The Cummins 6B5.9 in genset form can be as low as 80KW or so at 1500 RPM. I've had 6B fire pumps rated at around 100 HP at 1800 RPM. Same can be said for other brand 6 cylinder diesel gensets.

In marine versions of similar block, so long as lube and water temps are in safe range, operating at low HP, is as Ski, Bayview and others have said, commonly done. Works for me.
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Old 02-23-2017, 06:59 PM   #8
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Thanks eric but i know how much HP i need, the question is,is under powering bad for the engine. Not sure about what the "need to shop More" means. im not buying yet, im researching.
Re the question it gets talked about a great deal. Ask yourself if it was'nt better to run a diesel almost hot or darn warm why would TF need to be talking about it so often? It's almost like a religious chant.
I personally think the lube oil needs to be run 170 degrees or more for the deposits like varnish and sludge to be carried away and not burned into ring lands ect. A 50% load (again my opinion) is about the minimum load in most engines to keep the lube oil up to 170 degrees.

A huge number of skippers on this forum are overpowered or just putzing along trying to minimize the fuel burn. They are trying to justify running light w repeated statements like "I've never heard of an engine wearing out from lack of load". They actually do but most of the glaring examples are gen sets. Running a diesel engine on a boat is best run at 55 to 65% load. Load is determined by the percentage of maximum WOT power that the engine can produce. An FL can burn 6gph correctly propped wide open so if you have one and are burning 3hph you're at a 50% load.

It's a compromise of engine heat and noise IMO.

"Need to shop more" = keep looking .. you'll find engines at the power rating you want. Shopping is fun.
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Old 02-23-2017, 08:00 PM   #9
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Wow...this has been eye opening for me....I would have thought that easier you are on your engine, the longer it will last..but I guess that's not true. In about 15 minutes of web searching, its pretty clear that under loading a diesel is very detrimental to its health. The exact definition of under loading isn't so clear, but its definitely bad. From what I gather the litmus test is exhaust gas temperature to ensure proper operating temperatures.
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Old 02-23-2017, 09:42 PM   #10
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I would humbly suggest actual research and facts may be lacking on this topic and its doubtful manufactures who probably have that information will share it. Outside of the manufacturers no one is motivated to spend the likely millions of $$ to test and do the math.

In absence of that we'd all like to think how we treat one of the most expensive parts on the boat just right, so clearly randomly slowing down in the ICW from 80% to 20% a few times a day when it gets shallow is the best approach.
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Old 02-23-2017, 09:48 PM   #11
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When I have looked at used Yanmar engines, there are many in the 75-125 hp available, usually from sailboats.
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Old 02-23-2017, 10:00 PM   #12
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Running a diesel engine on a boat is best run at 55 to 65% load. Load is determined by the percentage of maximum WOT power that the engine can produce.
There is another commonly used definition of load. It is power produced at a given rpm divided by max power available at that rpm. My engines, for example (QSM11's) produce 660hp at 2300 rpm. I run them at 1000 rpm. Let's say that at that rpm it takes 120 hp, total (ie, 60 hp/engine), to turn my props at that rpm. That would equate to less than a 10% load if measured as a percentage of max power at max rpm. But at 1,000 rpm, the engines probably can't produce more than 150 hp (just a guess), but under the alternative definition, that would be a 40% load. Similarly, if my boat was propped to require 300 hp (total) at that rpm, the boat would be, in a very real sense, fully loaded. Reporting the load, under those circumstances as less than 25% (300 hp / 1320 hp) would, IMO, be misleading.
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Old 02-23-2017, 10:37 PM   #13
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MYTraveler,
Never heard of that and find it very interesting. Kinda muddies the water a bit though.

Somehow I don't think it's valid but I'm not sure how. I do'nt think it works that way. An alternative may be to use the graphs that hp is usually presented. Like in a brochure. That is .. a WOT power rating with the engine braked to a specific rpm. Lets say 1800rpm. Observe the fuel burn. Then run the engine at 1000rpm and observe the fuel burn. Then I think you would have a true % of load comparrison. But it would not be practical as you'd need a truckload of propellers and spend a great deal of time swapping props.

Any thoughts?

I don't think that's valid either.
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Old 02-23-2017, 10:42 PM   #14
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A 50% load (again my opinion) is about the minimum load in most engines to keep the lube oil up to 170 degrees.
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A well designed marine engine has a coolant cooled lube oil heat exchanger. The oil will then remain about 10 degrees above engine temperatures. In my case this is true even at very low loads of about 25%. Once warmed up I can cruise for hours at low loads with engine temperatures remaining in the thermostat controlled operating range of 172 to 180F.
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Old 02-23-2017, 11:01 PM   #15
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Eric

A well designed marine engine has a coolant cooled lube oil heat exchanger. The oil will then remain about 10 degrees above engine temperatures. In my case this is true even at very low loads of about 25%. Once warmed up I can cruise for hours at low loads with engine temperatures remaining in the thermostat controlled operating range of 172 to 180F.
Like he said. I'm firmly convinced this under-loading thing, as long as not taken to silly extremes, is the boating equivalent of "the bogey man will get you". Once the engine is up to working temp, both water and oil, I suspect working it hard will wear it just a wee bit more than working it light. And that's what the majority do, and our engines last for ages. Take your pick, Pat or Mick..? There really is so much more to worry about.
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Old 02-23-2017, 11:23 PM   #16
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Most of what I came across was related to generators, but there is not shortage of reliable information online.

Here's a web page from a generator company that seemed to explain it pretty well, and I would think they would: a) know there stuff, and b) not have any motivation to spread disinformation or falsehoods.

https://www.auroragenerators.com/blo...fficient-loads
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Old 02-24-2017, 12:14 AM   #17
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Most of what I came across was related to generators, but there is not shortage of reliable information online.

Here's a web page from a generator company that seemed to explain it pretty well, and I would think they would: a) know there stuff, and b) not have any motivation to spread disinformation or falsehoods.

https://www.auroragenerators.com/blo...fficient-loads
Well, the page you linked to is also an old wives tale only applicable to certain vintage generators. I hear comments related to load all the time, but they are not in agreement with what the manufacturers are saying in their manuals and through their engineering groups. The engines used in diesel generators have evolved as well. We have generators from three manufacturers and none of them adhere to the "must be run at minimum of 50% load" theory. They vary but generally recommend 30-70% or 25-75% and limited time under or over those ranges.

Also, the site you linked to has nothing to do with marine generators. They handle residential and commercial. They don't indicate size or brand or age they're talking about. The issues of glazing and carbon build up at lower loads have been greatly reduced. Now, I'm not suggesting running a generator for long periods with no load or maximum power. Just that some of the older statements don't apply the same to current generators.

Now to propulsion engines they apply even less. A single engine may be set up and modified slightly for many different uses. In one set up it might be fit for continuous run at heavy load and in another 70-80% load but not continuous. The same base engine might be used in heavy, slow commercial vessels or in very light fast boats. When all else fails, listen to the manufacturer.
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Old 02-24-2017, 12:25 AM   #18
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In my own ignorance, I can only parrot what I have heard and read from folks that work in the field.

Modern diesels are different than legacy engines. They can be run at lower load levels for long periods of time with no damage to the engine. It is a good idea to run them up to a higher rpm occasionally (I seem to recall about 15 minutes per 8 hours of run time) but as long as the coolant and oil temps are in range, you don't have to worry.

Having said that, it seems to me that there are engines available in the lower hp range that might work just great. These have been installed in large sail boats (ie displacement hulls) for a while. You should be able to find something.
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Old 02-24-2017, 12:46 AM   #19
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I guess the issue isn't as clear cut at it seems..... I didn't think it would matter if the application was marine related or not.....and since the OP was looking at used engines, it seemed safe to assume he would not be getting the benefit of more recent advancements.

Here's another article that refers to marine propulsion diesels...but I suspect this is one of those issues that people have their own beliefs on and aren't about to change them unless they are visited by the ghost of Rudolph Diesel himself.

Why You Shouldn't Go Easy on a Diesel - Professional BoatBuilder Magazine
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Old 02-24-2017, 01:23 AM   #20
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Geezzz - By reading thread title I thought you meant mentally overpowering "your vessel". A near impossibility... for sure!
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