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Old 01-28-2019, 07:00 PM   #21
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Originally Posted by CptnPete View Post
Just so i understand load correctly, it is the HP for a given RPM as a % of total HP according to the power curve such as one below?

And for a trawler would you use the displacement one vs the second one?

For example, at 2200 RPM would it be 102/210 or 49% load?

Or would it be 184/210 or 87% load?

Makes a big difference, not sure i understand why there are two versions...

The PROP curve is an approximation of a real world boat load scenario. It's just math.

The other curve is MAX POWER, ie the maximum power output at WOT at a given RPM. The only way you get there is to overload the engine so it can't reach max rated rpm, then measure the output.
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Old 01-28-2019, 07:19 PM   #22
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As usual, lots of useful nuance here. However, the often unspoken conclusion our personal biases want to leap toward is that longevity equates to wear. If we simply maintain our engines and donít run them above or below a certain point that they will last forever. Ive seen many engines replaced over the years, some of them mine. I can only think of maybe one that was worn out. Lots of failures donít particularly relate to the wear or run hard equation. Time, corrosion, cost of parts, etc become bigger factors. You should follow much of the advice given, but that does not mean you wonít be spending on engines as a result. At least, thatís my opinion from my own observations. Just another variant of trying to estimate engine life by the hour meter, assuming that since Iím running easy hours Iím in the clear.
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Old 01-28-2019, 07:27 PM   #23
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The PROP curve is an approximation of a real world boat load scenario. It's just math.

The other curve is MAX POWER, ie the maximum power output at WOT at a given RPM. The only way you get there is to overload the engine so it can't reach max rated rpm, then measure the output.
That's right. And engine manufacturers normally publish a specific prop curve for each of the various hull shapes in which the engine might be operated. While it's a mathematical approximation, the one for our hull/engine combination is very close to real world.
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Old 01-28-2019, 07:36 PM   #24
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As usual, lots of useful nuance here. However, the often unspoken conclusion our personal biases want to leap toward is that longevity equates to wear. If we simply maintain our engines and donít run them above or below a certain point that they will last forever. Ive seen many engines replaced over the years, some of them mine. I can only think of maybe one that was worn out. Lots of failures donít particularly relate to the wear or run hard equation. Time, corrosion, cost of parts, etc become bigger factors. You should follow much of the advice given, but that does not mean you wonít be spending on engines as a result. At least, thatís my opinion from my own observations. Just another variant of trying to estimate engine life by the hour meter, assuming that since Iím running easy hours Iím in the clear.
So true. You can do everything right and out of nowhere an engine grenades. However, the norm is that care and maintenance and how you run them improves your odds. Most of the time when they've had key parts go or sudden failures it's been an accumulation over the years. There are no guarantees, but proper care sure helps most.

There are so many variables and in spite of all those we can control, there's still the one called "luck."
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Old 01-28-2019, 07:36 PM   #25
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Heavy duty purpose built engines are a different animal with different economics. If the application requires a certain constant power output, for longer than the lifetime of an engine, then total power output x time between replacement / cost is the equation. Therefore it may make sense to run at 90% power for a shorter time at less replacement cost, than 30% power on a much larger engine for longer, but with higher replacement cost.

Recreational boat engines, especially trawlers have a different mission and different economics. 1st, most diesels in this service are killed due to mistreatment, long before they would have worn out. Unless you acquire it late in it's life cycle, it will likely last longer than you own it, whether run at 80% or 30%.

I'd agree with Tony, the less fuel you burn the longer it will last - but I suspect it will outlast my ownership regardless.
I believe many diesels are rated for longevity based on total fuel consumed over the projected life span. So yes, if two identical engines are run side by side, with one at 4 gph and one at 8 gph, they will both burn the same amount of fuel between rebuilds, but the 4 gph will run twice as long.

I had the opportunity to speak with a gentleman at CAT marine who said he was on the design team for the CAT 3306 I have. It was designed largely with the north slope Alaskan oil project in mind as a motor that had to run without much power loading much of the time since everything was kept idling when not in use so it didn't freeze. He said it was a 50,000 hour engine under those load conditions, so I guess that means I can leave it to my grandchildren if I take care of it.
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Old 01-28-2019, 09:25 PM   #26
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Over powered

Let me see if I got this right. Our new to us 32' Nordic Tug just happens to have a 3208 Cat, although it is a turbo unit with 300 hp. If I'm not mistaken a 32 Nordic tug has a hull speed of 6/7 knots, and according to the surveyors sea trial and a log that the P.O. kept, this vessel reaches hull speed at right around 1500 RPMs depending on weather, current. Now realizing that I'm pretty cheap and will want to get the best fuel mileage I can, I'm going to stay at hull speed as much as possible even though at WOT she will do 15/19 knots. So if I'm doing the math correctly that would be around 17% of full power.
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Old 01-28-2019, 09:31 PM   #27
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As usual, lots of useful nuance here. However, the often unspoken conclusion our personal biases want to leap toward is that longevity equates to wear. If we simply maintain our engines and donít run them above or below a certain point that they will last forever. Ive seen many engines replaced over the years, some of them mine. I can only think of maybe one that was worn out. Lots of failures donít particularly relate to the wear or run hard equation. Time, corrosion, cost of parts, etc become bigger factors. You should follow much of the advice given, but that does not mean you wonít be spending on engines as a result. At least, thatís my opinion from my own observations. Just another variant of trying to estimate engine life by the hour meter, assuming that since Iím running easy hours Iím in the clear.
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Old 01-28-2019, 10:23 PM   #28
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Evidence please

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Maybe for recreational or light duty rated engines. While I don't think running at 30% load will harm a diesel, I know running at 80% of hp will double the life of most engines, compared to running flat out.

Heat and dirt are the real life killers of engines. Clean oil causes very little wear. But heavy loads of soot and other contaminates will start wearing bearings, rings and cylinder walls or sleeves. As the bearings wear the clearances open and let oil escape causing a lowering of oil pressure. Cross hatching on the cylinder or sleeve wall will wear away faster with dirty oil, eventually leaving smooth walls. Once the cross hatching is gone, ring and wall wear happens very fast. Then the engine becomes hard to start, burns and passes oil in the exhaust. Cross hatching is the tiny scratches purposely made in the walls during the honing process. It carries oil to the upper cylinder, lubing the rings. See pic of Detroit sleeves. Most full flow oil filters bypass oil once the filter becomes clogged with dirt. Then oil circulates w/o any filtering. Full flow filters only catch dirt to about 30 microns when new. 30 micron dirt will cause wear. Adding a bypass filter or centrifuge will capture debris down to 1 micron, maybe smaller. Stopping almost all wear.
Too much heat, measured in exhaust gas temperatures, especially turbo engines, will break down the metal alloys in cylinder components - valves, rings, pistons, cylinder walls or sleeves. Running a turbo engine at 80% of hp will greatly extend the engine life by lowering temps inside the cylinder. In my experience, it will more than double the engine life (along with clean oil).
Heavy duty engines with continuous ratings probably get the best hp/fuel usage when fully loaded, Like ship, tug and other commercial use engines. But they're made differently. Most continuously centrifuge the engine oil. And the oil is heated before starting. Some have hydraulic accumulators that store oil at pressure that is released before the engine is started to pressurize the bearing surfaces. Most wear occurs at startup. HD engines have heaver castings, thicker rings, special expensive alloys requiring special machining that typical recreational engines don't have. And heavy duty engines are designed to be rebuilt in place many times.
You want to extend your engine life? Add a bypass filter or centrifuge and keep the rpm down to 80% of HP.
Do you have any evidence of your 80 percent claim, other than anecdotal and opinion? This kind of thing is like religion. I have had drivers tell me it costs more in terms of diesel to start an engine than to left it run. Crazy stuff.

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Old 01-29-2019, 12:31 AM   #29
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"I have long questioned the "run it hard" mythology. The history of it seems to come from large, lightly loaded mechanically injected generators, which suffer various maladies from old injection technology, like after dribble and leakage etc."

Certainly this discussion varies with the model engine. Mechanical versus electronic. And also the owner's personal preferences.

I ran my 1940's twin NA jimmies thousands of miles at 30% power. Never varied except to dock or anchor. My mechanic told me several times to run those engines hard and now that I think about it I think he was right. Even though water temperature and oil pressure always read correctly, that big blower must have been cooling down the block and components in an unhealthy way. Perhaps one reason for the white smoke.

Also I think it is a waste (purchase cost, maintenance cost, time, fuel, engine room space) to pay for a big engine and only run it low. If I could re-do my old boat, I would rip out the twins and just install a single, running at 60%.
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Old 01-29-2019, 10:57 AM   #30
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Does anyone have an engine manual which recommends running the engine at 80% or any other loading to preserve the life of the engine?.

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Old 01-29-2019, 10:57 AM   #31
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I think there are too many nuances to live with many of the dumbed down rules of thumb. On my Volvoís I was able to observe that increasing the cruise rpm on a heavy boat loaded with dive gear actually saw an almost immediate drop in exhaust gas temps. We would tend to say that the engine was run ďharderĒ because more throttle was applied, but empirically we see that the engine was actually run easier. Rpm by itself is not a bad thing, itís all about load.

Think about pulling a weight up a hill with a rope directly or with a block and tackle. You will do the same work. But with the block and tackle, you will pull the rope that much further. If you use an engine and increase the rpm with the block and tackle, you will do the same work over the same time, but with much less load per revolution. Clearly rpm is not the whole story.

In my case, my engine would lug more at lower rpm, than higher, just as when you put your car in 2nd gear it lugs more than just before you shift into third.

So when we prop a boat, we often like to claim that getting more rpm out of the higher rpms is better. Because this is setting a very low load at high rpm, ensuring the load at the lower rpm we actually run at is done without lugging. At least, as far as it goes.

Surely these nuances presented themselves are incomplete. Even so, trying to make a blanket statement with just these few is pretty challenging. What if we throw turbocharging into the mix and achieve Intake pressures capable of a much more efficient fuel burn and overall engine efficiency?

Lots of the advice, even the dumbed down variety, carries some valuable insights. But it only goes so far and when people try to turn it into religious style dogma, much of the value vanishes. There is a lot more going on than simply running at 30% or 80% of throttle, sometimes, maybe, probably.
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Old 01-29-2019, 11:07 AM   #32
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Does anyone have an engine manual which recommends running the engine at 80% or any other loading to preserve the life of the engine?.

Gordon
I have one that recommends running at 80% but does not say that is to preserve the life of the engine. It's simply recommending a cruise speed and considering the boat and engines aimed more toward not running at WOT than anything.

What I've seen too is most recommendations of how to run engines are the recommendations for break in. We choose to follow those long term. Typically, not run at WOT over X minutes at a time, vary speeds, run at 80% load or less. While some might consider following break in procedures babying engines, I've done it for 35 years and never had an engine problem so not likely to change now.
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Old 01-29-2019, 11:40 AM   #33
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My old 3208 manual recommends running at 400 RPM below WOT, which would be 2400RPM on a 2800RPM motor. That is 86% of WOT and just about 60% load for a FD hull propped appropriately where load as I now understand is calculated as fuel consumption at 2400/fuel consumption at WOT of 2800 (7GPH/11.9GPH).
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Old 01-29-2019, 12:13 PM   #34
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If someone would just fund the purchase of 100 engines and dynos, we could do a test to determine the longest life. First we would need to define what that is: Longest number of hours run? Most rotations before failure? Most power produced over the life? Most miles done in a particular application? These are all likely different answers, and the latter highly dependent on the specific installation. The engine manufacturers probably have this data, but hold it closely.

An additional problem in a boat (unless you are designing it from scratch) is that load is not an independent variable. Most owners do not set the throttle by load without regard for fuel consumption, sea state, noise, current, wind, handling, etc. If you knew the answer to the above question you might not act on it. On my own trawler, running at 80% load is simply impractical. Perhaps the builder fitted too large an engine, but I'm not going to change it out just so I can run 80%.

Taken to extremes, I'm quite sure that Tony's advice is correct: If you burn no fuel at all in an engine, it will last indefinitely.

When I said above that I was skeptical of the "run it hard or it'll die" mythology, it is because I am aware that a mountain of empirical evidence exists against that claim, and that is truck engines which are run easy, and last a long time. Would they last 5% longer or shorter if run at 80%? We know that all of the manufacturers derate the engines substantially in the commercial versions of light truck diesels compared to the consumer versions, which suggests lower power = longer lasting. But 5% isn't going to make a difference to most people one way or another.
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Old 01-29-2019, 12:19 PM   #35
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The prop on your boat will be the determining factor on when and how much load is on your engine. Exhaust Gas Temp will tell the story. Throttle speed is pretty much irrelevant.

“Should have stated the prop combined with gear ratio will determine load”
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Old 01-29-2019, 12:24 PM   #36
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I have one that recommends running at 80% but does not say that is to preserve the life of the engine. It's simply recommending a cruise speed and considering the boat and engines aimed more toward not running at WOT than anything.

What I've seen too is most recommendations of how to run engines are the recommendations for break in. We choose to follow those long term. Typically, not run at WOT over X minutes at a time, vary speeds, run at 80% load or less. While some might consider following break in procedures babying engines, I've done it for 35 years and never had an engine problem so not likely to change now.
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Questions:
- what engines, model and HP are on your larger vessel
- what is the measured max RPM under load
- have you spent time with Steve Z talking engines
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Old 01-29-2019, 02:15 PM   #37
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BB
Questions:
- what engines, model and HP are on your larger vessel
- what is the measured max RPM under load
- have you spent time with Steve Z talking engines
We have various so I'll just give two examples:

44' Riva with twin 800 hp MAN's, i6-800's. Max RPM is 2300 and it runs 42 knots at that. Fast cruise for us is 1950 RPM, 36 knots, 75% load. 80% load would be just a little over 2000 RPM and about 37.2 knots or so.

85' Pacific Mariner with twin 1500 MTU's, 10V2000's. Max RPM is 2450 and 27 knots. Normally we cruise at 1800 to 2000 RPM, 20-23 knots, 50-65% load. 80% load is 2170 RPM and speed is just over 24 knots. Very little gained between 65% load and 80%.

These engines do have electronics reporting load which comes very close to calculations based on fuel consumption.

No, I've never spoken to Steve Z. Most of what little I gather about engines comes from talking to Captains and Engineers we employ and people I've talked to over decades as well as manufacturers recommendations.

Steve Z's 35% load on the above two boats would be about 1275 RPM and 17 knots on the Riva and 1580 RPM and 16 knots on the Pacific Mariner. Interestingly, on the Riva 35% gives about 40% of the speed of WOT, whereas on the PM it gives 59%. We do sometimes run the PM at 35% but never the Riva.

Where it becomes really interesting is looking at jet RIB's, both gas and diesel. I might have to do that later, but back to a meeting for now.
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Old 01-29-2019, 03:00 PM   #38
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DDW: "I have long questioned the "run it hard" mythology."

I agree and this topic has been batted around on boatdiesel.com many times over the years, with many opinions questioning the need for hard running to achieve engine health. With the thousands of work and fish boats that spend most of their lives at idle and with only small PTO hydraulic loads and that go 20,000 hours+ BMOH, it tells the tale in my opinion. I do goose mine up to 80% power once in a while which seems to reduce smoke at startup, but that's it. Otherwise I'm at about 50% load or trolling. No problems.
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Old 01-29-2019, 11:12 PM   #39
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Can someone define "% under load" please since the main (only?) control we have on a diesel is throttle/RPM?

I understand "% of RPM" as a concept - it's easy to know that 80% of WOT RPM is "x" and set the throttle at that.
I understand a general "% of HP", as one can read the generic manufacturer prop curves to determine the RPM as a % of the horsepower/kilowatt.

But what is "% of load"?!

If I'm in neutral and idling at 850rpm, is there "0% of load"? Well I'm in neutral so I assume so!
If I'm at 1000rpm and in forward, and have a WOT of 2400rpm, what "% of load" am I at? Surely I'm more at a "% of RPM" rather than "load"?
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Old 01-30-2019, 12:56 AM   #40
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Can someone define "% under load" please since the main (only?) control we have on a diesel is throttle/RPM?

I understand "% of RPM" as a concept - it's easy to know that 80% of WOT RPM is "x" and set the throttle at that.
I understand a general "% of HP", as one can read the generic manufacturer prop curves to determine the RPM as a % of the horsepower/kilowatt.

But what is "% of load"?!

If I'm in neutral and idling at 850rpm, is there "0% of load"? Well I'm in neutral so I assume so!
If I'm at 1000rpm and in forward, and have a WOT of 2400rpm, what "% of load" am I at? Surely I'm more at a "% of RPM" rather than "load"?
There is equipment that will actually tell you. There are various formulas considering peak torque and air flow or fuel flow and even taking barometric pressure into consideration. However, a very good approximation is Fuel consumption divided by fuel consumption at WOT. So, if a boat consumes 20 gph at WOT then 60% load would be that speed at which it consumes 12 gph. Most of the time load % will be less than the % RPM. Often 50% load will be at around 70% RPM.

You're trying to determine who percentage of the engine's capability are you using.

Some more mechanically minded might be able to actually explain some of the more complex calculations used by electronics and technically more accurate than this rough method.
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