How often to run engine with full throttle

Hi Guys, I am curious of your opinion. I have a Skagen 53 motorboat with twin Yanmar 6LY-ETP 480 HP engines. The boat is capable to cruise at 20 knots, but as she is built something like a trawler, my typical cruising speed is 8 knots (at 1800 rpm). Except extreme weather conditions I never feel tempted to cruise faster. I read, however, that the engine needs to run full (or at least 80%) throttle regularly (somewhere written 20-30 percent of the time used). Apart from burning much much more fuel (3-4 times per mile), I simply not enjoy fast cruise. Is it really necessary to run the engine with full load or it is only an unfunded advice?

20% is BS

IMO a few minutes every once in a while at full power is all that is advisable while you check performance and temperatures.

Problems often first show up when full power is used.

“Regularly” means 100% for a minute or two several times a year IMO.
Run WOT only for that very short time and then as a regular habbit ... if you can reach rated rpm. Adjust prop pitch or trim dia. To achive rated rpm.

What is rated rpm on your big Yanmars?
I had a little one ... good little engine.

Even a few minutes a year is more than we do.
Being full displacement going at full noise (1800) does nothing but make a lot of noise and burn a lot of fuel
To me it feels like getting a truck, putting it against a wall and putting the boot to the floor
Why?

But we do run her up to 1500 to blow the cobwebs out on occasion.
Usual rpm is 1150 to 1200.

Gyorgy said:

Hi Guys, I am curious of your opinion. I have a Skagen 53 motorboat with twin Yanmar 6LY-ETP 480 HP engines. The boat is capable to cruise at 20 knots, but as she is built something like a trawler, my typical cruising speed is 8 knots (at 1800 rpm). Except extreme weather conditions I never feel tempted to cruise faster. I read, however, that the engine needs to run full (or at least 80%) throttle regularly (somewhere written 20-30 percent of the time used). Apart from burning much much more fuel (3-4 times per mile), I simply not enjoy fast cruise. Is it really necessary to run the engine with full load or it is only an unfunded advice?

Click to expand...

I have been informed via numerous sources that I can slowly troll for hours at a time with my single turbo-charged Yanmar 6LPA 315 HP motor without harm, but that I should run it at 80% (3000 RPM) for 20 minutes or so after 3-4 hours. I do very little of this sort of activity, however. I mostly run up and down the bays here at 2800-3000 RPM. WOT rpm is 3800 RPM in this boat. If I were you, I would adhere to the routine of my first sentence.

Tony Athens, the marine diesel guru formerly of boatdiesel but who now has his own forum at sbmar.com, advised me to run my Yanmar 6LY 370 at 90% of max rpms (which are 3,300, so run at 3,000 or so) for ten minutes at the end of a day of low speed cruising to blow out any accumulated soot and burn out any oil residue in the exhaust manifold and engine ports.

He also recommended running up to wot for just a minute or so every few months to make sure it would hit rated rpm.

Your engine is a bored out version with electronic controls of my Yanmar.

David

It also depends on the engine (setup) to a point. For example, a turbocharged engine will benefit (more) than a non-turbo engine.
From what I have been advised, it is "optimum" to run your engine (with turbo) up to 80% throttle for 5-10 minutes after every 10 hours (or so) of operation at the end of a longer run. For the last minute or so of the "high speed" run, you can go to WOT checking for any developing issues (like leaks, raising temps, black smoke, excessive vibrations, increasing vacuum in the fuel filters, etc.).
All of this assumes that your engine is well maintained, and you are not "over propped". It also assumes you are regularly running at well below what is considered "normal cruise rpm" (80%) for your engine.

EG. I usually run between 12-1400 rpm and my engine is rated at 2800 rpm. I follow the above advise to "blow out" deposits and "spin up" the turbo, running up to 2200 for 10 minutes followed by the 2800 for about 1 minute. I usually notice a very slight temperature rise, more wake and noise, and that is about it. A few times per year, I will also go down into the ER while "running her up" to specifically check engine temps at various locations using my IR thermometer and compare the results against a baseline.

Wifey B: 5 minutes, 10 minutes, 20 minutes, 30 minutes every 3 hours, 4 hours, 6 hours, 10 hours. Pick one from column A and one from column B.

So, depending on engine what is best normal run. With ours it's 70-80% load, not throttle, but then fine to run at lesser speeds. Other engines made for lesss or more % of load. However, the one consistency I see is to at least occasionally open up to 70-80% of load and for just a brief time to WOT as WOT is your best means of checking your engine, prop, and more and seeing if you're getting the RPM you should. I sort of think of opening up once every time used but don't run WOT ever over 5 minutes or so as much as I like speed.

Moderation, but moderation doesn't mean to never open it up.

DavidM said:

Tony Athens, the marine diesel guru formerly of boatdiesel but who now has his own forum at sbmar.com, advised me to run my Yanmar 6LY 370 at 90% of max rpms (which are 3,300, so run at 3,000 or so) for ten minutes at the end of a day of low speed cruising to blow out any accumulated soot and burn out any oil residue in the exhaust manifold and engine ports.

He also recommended running up to wot for just a minute or so every few months to make sure it would hit rated rpm.

Your engine is a bored out version with electronic controls of my Yanmar.

David

Click to expand...

Same engine and similar practices

When full day cruising around 1600-1800 RPM I'll run around 2800-2900 for 30 min every afternoon. When crossing great lakes I'll run 30-60 min when conditions right and want to decrease exposure time.

A couple times / season I will run WOT about 3300 for 5 min just to confirm speed 18-20 MPH.

Basically, just for about 10 minutes at the beginning of each season...just to confirm things are "normal". On very rare occasion in a river or in a (large-ish) canal (illegally, of course) to make a critical bridge or lock schedule.

Doesn't your engine manual tell you how to run your engine? This wot debate is another anchor/oil/antifreeze category where Bob's uncle's best friend's grandfather did it this way for years and therefore it's gospel.

Your manual will tell you your engine specs and ratings and operating instructions and all the rest is conjecture.

Do the required maintenance when you're supposed to.

There are two reasons not to run flat out all day long, your engine rating and fuel burn. The engine rating will tell you how much wot you are allowed and your wallet will tell you how much fuel you can afford; other stuff like noise is peripheral. My Cummins is continuous rating, which means 24 hours at wot. Sucks fuel like a hole in the tank, though.

I am with Xsbank on this.

The manufacturer’s manual for your new engines will tell you what you should, and should not, do with respect to rpm and rated speed. Look closely at the recommended performance and fuel curves and the rating. These will be different for each manufacturer, and for each different capacity engine in a manufacturer’s range.

Cylinder glazing is a real thing (due to chronic underloading), and at the other end of the spectrum overloading (due to continual overpropping) is also a real thing. Both are best avoided. Your engine manual will tell you what is required.

I repowered in 2020 with a set of new QSC8.3 at 550hp, and the engine manual (and the performance and fuel curves and rating contained therein) tell me everything I need to follow about loading /rpm / fuel use. I admit it took me several reads and a little while to be able to properly understand them because they are not simple (and a nod to Tony Athens and his forum which explained how to understand this stuff), but once you know how to interpret them the answers are all there.

Hope this helps!

Hamish.

Xsbank said:

Doesn't your engine manual tell you how to run your engine? This wot debate is another anchor/oil/antifreeze category where Bob's uncle's best friend's grandfather did it this way for years and therefore it's gospel.

Your manual will tell you your engine specs and ratings and operating instructions and all the rest is conjecture.

Do the required maintenance when you're supposed to.

There are two reasons not to run flat out all day long, your engine rating and fuel burn. The engine rating will tell you how much wot you are allowed and your wallet will tell you how much fuel you can afford; other stuff like noise is peripheral. My Cummins is continuous rating, which means 24 hours at wot. Sucks fuel like a hole in the tank, though.

Click to expand...

Absolutely what one should do. Follow the manuals. However, manuals don't always tell everything. For instance, if they say to run at 80% load, they may not say much about running at lower loads. If they are continuous rated like yours, they may not say much about what to do if you normally run slower. How often without WOT? So, should always start with manual, go to manufacturer for information beyond it, and then use judgement based on consensus and experience. Where a lot of TF'ers are seeking advice is that they normally run at hull speed or near it with engines capable of being run at 70-80% or even WOT. Now, they're looking for advice on what the minimum faster time they need.

It's easy on your boat and easy for us on ours as we run a lot at cruise speeds of 70-80% load and vary with other speeds from 30-70% load and don't hesitate to mix in the occasional WOT but it's more difficult for others.

I know of one boat recently that was sea trialed and at high RPM it had a horrible vibration. The buyer thought the seller had lied to them. No, the seller had never run it over 1200-1400 RPM. Turns out with the haulout he had a damaged prop. When repaired it ran fine and hit full RPM perfectly.

Many don't run at the full abilities of their engines and then need to know what they should do, what is the minimum acceptable. But do start with the manual.

It's a bit like load on generators and much of what you hear on forums is so far outdated based on today's generators and owner's manuals. Start with the manual.