In search of the 80% power setting

I started a thread on the Trawlers and Trawlering email list asking about what folks there thought of the concept that most diesels operate well with reasonable longevity when loaded to 80% of their full power rating. While nobody disputed the idea, I got several ideas about how to figure out to what RPM that equates. Some thought it was referenced to the max RPM at idle (around 4200 for this 6LPA-STD of mine). Some thought it was 80% of the rated 3800 RPM at wide open throttle (which calculates to 3040). Another said you should use 80% of the fuel flow of 15.5 GPH at WOT, which ended up being 12-ish GPH are equated to 3500 RPM which is 92% of the WOT RPM because of the rate of change of power absorption by the prop. Lastly, Tony of boatdiesel.com was quoted as saying that running at 2.2 GPH per liter of engine displacement (4.164 liters) is a relatively happy place for a diesel, and that also lands you on the curve at about 3000 RPM using 9 GPH.

The engine manual says the engine is designed to run at 2800 to 3600 RPM 90%of the operating time.

My boat achieves the rated 3800 WOT, and I have normally run at 2800 which gets my 30 Pilot II over the hump plus a little.

I would not feel comfortable running at 80% of max fuel flow, 3500 RPM (92% of WOT RPM), and feel more confident in using the 80% of WOT RPM/2.2 GPH/liter RPM of around 3000 RPM as a cruising power setting for this single engine installation.

How about what the engine manufacturer says? Starting with what the ideal % is and of what?

What Yanmar says is in the body of the original message. Just a range.

It’s not an rpm question.

It’s a fuel consumption question.

If your boat burns 10gph at WOT and you want to operate at 80% output just monitor your consumption and when find the rpm where you burn 8gph you’re at 80% output. That’s it.

I’m not sure one needs to be propped to rated rpm or if the above applies to any boat but I suspect being close to propper engine loading at WOT may be needed for accurate results.

The above is my opinion.

As I first wrote, I don't disagree with the 80% fuel burn methodology, I do not agree with the resulting 92% WOT RPM that brings with my engine/prop combo. It's too fast.

Too fast?
Get a smaller engine or slow down.

What’s too fast?

Quote:
It’s not an rpm question.

It’s a fuel consumption question.

If your boat burns 10gph at WOT and you want to operate at 80% output just monitor your consumption and when find the rpm where you burn 8gph you’re at 80% output. That’s it.

I’m not sure one needs to be propped to rated rpm or if the above applies to any boat but I suspect being close to propper engine loading at WOT may be needed for accurate results.

The above is my opinion.
__________________
Eric

Nomad Willy said:

Too fast?
Get a smaller engine or slow down.

What’s too fast?

Click to expand...

Eric, you are on a roll with this thread. Well stated. Agree.
Had my mechanic onboard testing out the pilot he installed. We ran the boat at 1650 RPM out of 2100 WOT in gear.according to the OP, I am just shy of 80% of his analogy. Had bits and pieces of 7 knots. Asked what he thought of the engine performance. "Perfect" This setting gives me a 1.6 gph fuel burn.
All it well Erick, good post.

Al-Ketchikan

Sometimes you just have to feel the boat to see what's the right power. Each boat has its sweet spot. Step away from the numbers and listen to your boat. You'll find the sweet spot.

I really think you are splitting hairs that don't matter. Don't run the engine wide open all the time, but other than that, just get out and use it. It's there to serve you, not the other way around. You aren't going to break it.

Practicality "just go out and have fun" aside, this is an interesting thread if trying to truly calculate the 80% threshold. I believe that you have to go back to your manufacturer's prop power and specific fuel consumption curves. A fuel map if they are willing to release one.

You will see that the BSFC is not the same throughout the range. Many engines peak at efficiency in the 50%-70% sweet range, but BSFC can grow by 10-20% more at WOT. So just using "80% of WOT fuel consumption" won't be accurate enough.

There appears to be misunderstanding here. I have this boat as a downsize from the Grand Banks 42 I owned for 29 years. That boat's engines were always run at an RPM which got me just at hull speed of 8.6 knots, and that was nearly 80%. Never slower unless required by circumstances. This tuned out the be the perfect RPM for the engines as the boat was well designed with the proper power plant. Same goes for this single engine boat with a turbo powered engine almost three times as powerful as either of my twins were, but this boat has a larger range of acceptable RPM once on plane than my trawler did at around hull speed. I got tired of that slow speed trawler and maintaining a lot bigger boat than I needed anymore. I intend to run this boat at its most advantageous RPM on plane. Economy is not my goal - the highest speed while not killing the engine is what I am after. Thus the search for a good answer to my question. This Pilot is perfect for me, and at its designed use, up on plane, runs anywhere from around 14-23 MPH within the range specified by the maker of the engine, I have been happy to find that I can now add 2-300 RPM to what I have been using for cruising RPM to get more speed. Doing it by the numbers and avoiding unnecessary repair bills because of ignorance of those number makes me enjoy the boat all the more - many thousands of miles under my keel with never a mechanic aboard in 29 years is proof ny way works for me. I am not a "just jump in the boat and run the throttle to the firewall" type of person. Now pardon me while I jump in the boat and zoom off at 80% full power.

twistedtree said:

I really think you are splitting hairs that don't matter. Don't run the engine wide open all the time, but other than that, just get out and use it. It's there to serve you, not the other way around. You aren't going to break it.

Click to expand...

FlyWright said:

Sometimes you just have to feel the boat to see what's the right power. Each boat has its sweet spot. Step away from the numbers and listen to your boat. You'll find the sweet spot.

Click to expand...

__________________
Al

Custom Google Trawler Forum Search


You both nailed it

I see 80% of the power, that your yanmar 6LPA-STD has up to 232kw / 3800rmp and 80% power is 186kw / 2600rmp about.


Your yanmar max crankshaft torque is700 Nm 2200-2600 rmp


This informations Yanmar performance curves.


NBs

twistedtree said:

I really think you are splitting hairs that don't matter. Don't run the engine wide open all the time, but other than that, just get out and use it. It's there to serve you, not the other way around. You aren't going to break it.

Click to expand...

Like he said. Sometimes it is too easy to over-think these things.

My sweet spot is a knot below hull speed achieved at 40% fuel-load (efficient). Hull speed is achieved at 72% (inefficient unless facing a strong current.) Different boats will vary.

Nothing sacred about 80%. And that 80% can be calculated in a few different ways: a percentage of peak rpm, rated rpm, peak gph, even peak torque at a given rpm.

No way would I want to run a high output Yanmar continuously at 80% of rated power. Can it do it? Sure. Best thing for it? No.

Others posted the best way to approach this: Find where the boat feels the best speed wise. And hopefully that puts the engine in a happy spot too. Hopefully well below 80% on a high output engine.

Every commercial diesel boat I have run (at least a couple dozen), the suggested cruise was 100 to 200 rpm of WOT. Thus around that elusive 80% power some guess at.

These were 300 to 1000 hp Cats, Deeres, Volvos, and Detroits.....no Yanmars.

rgano said:

The engine manual says the engine is designed to run at 2800 to 3600 RPM 90%of the operating time.

My boat achieves the rated 3800 WOT, and I have normally run at 2800 which gets my 30 Pilot II over the hump plus a little.

Click to expand...

At what speed is the hull most comfortable? At what speed are YOU most comfortable? "Over the hump plus a little" isn't always what the boat likes best; you might see a rising curve in speed as the hull planes better at slightly faster RPMs...

What RPMs does it take to get there? Is that within Yanmar's 2800-3600/90% range? Is it near 80% of anything?
Are you happy with fuel burn at that rate?

FlyWright said:

Sometimes you just have to feel the boat to see what's the right power. Each boat has its sweet spot. Step away from the numbers and listen to your boat. You'll find the sweet spot.

Click to expand...

twistedtree said:

I really think you are splitting hairs that don't matter. Don't run the engine wide open all the time, but other than that, just get out and use it. It's there to serve you, not the other way around. You aren't going to break it.

Click to expand...

Ski in NC said:

Nothing sacred about 80%. And that 80% can be calculated in a few different ways: a percentage of peak rpm, rated rpm, peak gph, even peak torque at a given rpm.

No way would I want to run a high output Yanmar continuously at 80% of rated power. Can it do it? Sure. Best thing for it? No.

Others posted the best way to approach this: Find where the boat feels the best speed wise. And hopefully that puts the engine in a happy spot too. Hopefully well below 80% on a high output engine.

Click to expand...

Yep.

-Chris

John Deere has a much simpler approach.

First, they feel that percentage of maximum is based partly on rpm. Simply, there is a maximum load for each rpm setting. So the computer monitors fuel consumption and rpm. It then displays the percentage of load for rpm and fuel consumption on a guage. For my engine there is a time limit and percentage of time per hour it can run at 100% and above 80%. The computer starts warning you when you exceed the limit.



Ted

North Baltic sea said:

I see 80% of the power, that your yanmar 6LPA-STD has up to 232kw / 3800rmp and 80% power is 186kw / 2600rmp about.


Your yanmar max crankshaft torque is700 Nm 2200-2600 rmp


This informations Yanmar performance curves.


NBs

Click to expand...

I have no idea what curves you are looking at, but they surely are not the ones from my engine's manual. Look back at the PROPELLER power curve numbers I quote, and look again at the curves of the 6LPA-STD from the engines maintenance manual. At 3000 RPM I am taking a mere 120 KW of the available 220 KW. This engine is loafing at this 54% power setting. Come on guys, it's not rocket surgery. It's just fun with numbers in a fun hobby.

rgano said:

I have no idea what curves you are looking at, but they surely are not the ones from my engine's manual. Look back at the PROPELLER power curve numbers I quote, and look again at the curves of the 6LPA-STD from the engines maintenance manual. At 3000 RPM I am taking a mere 120 KW of the available 220 KW. This engine is loafing at this 54% power setting. Come on guys, it's not rocket surgery. It's just fun with numbers in a fun hobby.

Click to expand...

This yanmar page, i thel cranksaft Kw and you write tehority propel out put and i dot no what is your gearbox and propel, therefore crankshaft readig ...


If you have these facts known, why are you asking for clear things?

https://www.yanmar.com/media/global...pulsion/catalog/Yanmar-6LPA-STP2datasheet.pdf

NBs

FlyWright said:

Sometimes you just have to feel the boat to see what's the right power. Each boat has its sweet spot. Step away from the numbers and listen to your boat. You'll find the sweet spot.

Click to expand...

“Sweet spot”
What can that possibly be other than a low vibration rpm.
And engines could care less about vibration.

Maximum power output should have to do w pressures and heat and other related things. All about engineering and nothing to do w feelings and sweet spots. IMO

So, you don't want to run at 80% load, then don't do it. Modern set ups actually give you load information. Otherwise you just go by % of fuel flow for an estimate. Your engine manual says 2800 to 3600 RPM but they don't say you have to run it at that or have to run at 3600 RPM. 3600 RPM is probably about 80% load. Our boats have best cruising speeds between 65% and 78% of load, based on actual performance charts we've made. I would not run constantly at 3600 RPM but would be comfortable running within the recommended range with occasional 3600 RPM for a few minutes. I'd on rare occasions run at 3800 RPM to insure I'm still getting peak RPM.

I’m quite sure when they say run at 2800 to 3600 they are giving a lot of slop to the prop loading issue. 2800 to 3600 could mean 65% to 85% load. Lots of leeway so it means little.

Here is my tug and cummins qsb 5.9 380hp, max rmp 3000 and about 2700 rmp 80% load. Load% varies slightly throughout time, due to waves.


I can not say where that 80% load my Cummin means, enybody knows?


Link to video
https://vimeo.com/240820129

If you run at 80% of hp your engine probably will go double or even triple the hours between overhaul compared to wot. The whole point is keeping the exhaust gas temperatures down. In most makes and models of diesels, EGTs should remain below 1200°F. Basic aluminum melts at about that temp. While steel and cast iron melt at about double that temp, metal degradation and wear start to happen at 1200° to 1250°F in long runs. Oil doesn't lube as well at extreme temps. That's based on heavy duty engines, not recreational grade engines that use smaller castings and cheaper alloys. Turbos at max hp can turn 100,000 rpm and burn the oil in the bearings.
A pyrometer is the best indicator. But you shouldn't need a pyrometer unless you're running a commercial engine at near rated hp continuously. Like a tug on a long tow.

North:

Here is the calculation of your prop curve using a 2.5 exponent. I am curious how well it fits at other RPMs


(2710/3000)^2.5=0.77556837258

Yanmar 6LHPA has a continuous rating at 3680 rpm. Find the NMPG peak rpm under that and you're good IMHO. Fastest speed for the least fuel/load. Every boat is different and the engines don't know what boat they are in. If there is no peak, then your favorite sweet spot. There is no magic number where say 3400 is bad but 3399 will run forever. Every moving part in an engine wears at a different rate. No doubt they all wear on a exponential curve. Obviously more power equals more wear.
I remember that 80% thumbrule from the 70's USCGAux Boating Skills and Seamanship class. 70% for gas. Broad guidance for those that would otherwise run WOT all day until their engines blew then need a tow.

I reach maximum (hull) speed at 2200 RPM (with 80 HP JD) at 72% power (actual vs. maximum fuel consumption). Going to 2400 RPM (engine's maximum: 4 GPM) does nothing but increase fuel consumption. Can run at max. boat speed all day, but that increases fuel consumption by 100% for only one knot increase of speed compared to an efficient 1800 RPM.

As the proud new owner of a JD 6068T that has 18,000 hrs without a rebuild, cold starts instantly and has zero blowby, I'm cured of the "80%" and "400 off the top" myths. This engine was run at about 50% wot or 33% continuous duty rated load for 21 years by the same owner. Keel cooler flow was restricted to maintain engine temps.

I'm of the opinion now that running lower rpm hurts nothing as long as you're in the recommended operating temp range.