RPMs, turbos, and mpg

[Burnt Norton]

I have a 29ft, semi displacement hull with 315 hp turbocharged Yanmar diesel (6LYA-ute). WOT is about 3450rpm, but for both comfort and fuel efficiency I mostly run at hull speed, which is about 1300 rpm, at which rpm I wonder if the turbo is even spinning. Will this damage the engine or the turbo in the long haul? This is a mechanical injector, 1998 engine. I imagine many forum members do the same. What do you think?

 

[rslifkin]

If it gets up to normal operating temperature and you're not gaining oil from fuel dilution it's likely fine. Just run it harder for a few minutes periodically to get everything good and hot.

 

[DavidM]

I had a Mainship Pilot 34 with the higher hp version of that engine, the 6LY STP 370, but low rpm performance should be about the same as your engine.

I required about 1,600 rpm to get to 7.2 or so knots which was hull speed on that boat. That rpm kept the engine up to temp- coolant and oil so I was fine with that. Your boat is a little smaller so probably runs at a little lower hull speed, so it needs less power so that is why you can do it with 1,300 rpm.

I think that 1,300 rpm is a little low for that engine to keep it up to full oil and coolant temp. You could do two things, or maybe both:

Run it up to 2,800 for ten minutes every 8 hours of slow cruising to blow out any accumulated carbon.

Cruise it at a little more than hull speed at 1,500 rpm which I suspect will keep things at the right temp.

You can check the oil temp with an IR gun. Shoot the incoming oil port on the oil cooler. You should get it up to 200 degrees.

David

 

[tiltrider1]

The manufacturer will have minimum operation standards. Try a Google search for your engine.

 

[Comodave]

We run our engines at slow speeds usually. At the beginning of each day I putthe boat up on plane for 15 minutes or soto get the engines warmed up. Then I slow down. By running it up first and getting the engines heated up I am not running them cold all day.

 

[Burnt Norton]

I had a Mainship Pilot 34 with the higher hp version of that engine, the 6LY STP 370, but low rpm performance should be about the same as your engine.

I required about 1,600 rpm to get to 7.2 or so knots which was hull speed on that boat. That rpm kept the engine up to temp- coolant and oil so I was fine with that. Your boat is a little smaller so probably runs at a little lower hull speed, so it needs less power so that is why you can do it with 1,300 rpm.

I think that 1,300 rpm is a little low for that engine to keep it up to full oil and coolant temp. You could do two things, or maybe both:

Run it up to 2,800 for ten minutes every 8 hours of slow cruising to blow out any accumulated carbon.

Cruise it at a little more than hull speed at 1,500 rpm which I suspect will keep things at the right temp.

You can check the oil temp with an IR gun. Shoot the incoming oil port on the oil cooler. You should get it up to 200 degrees.

David

I’m puzzled. Why does the oil temp have to be over 200? Isn’t cooler better when it comes to oil?

 

[Comodave]

You want the oil hot enough to get any water out of the oil. If it runs too cool the water can stay in the oil and that isn’t good.

 

[rslifkin]

With oil, 200 - 220 is pretty much ideal for most engines and oils. But I'd generally be happy with 180 or hotter. As long as the engine is run long enough after it gets up to temp, 180 is hot enough to generally avoid fuel and water buildup in the oil.



If the oil isn't getting hot enough under light load, follow the oil cooler lines. If it's got full flow through the cooler constantly, you can either re-plumb for a cooler in the coolant loop or you can add a thermostat to the oil cooler. If everything is sized well, the oil will still cool just fine under heavy load, but it'll come up to temp better under light load.

 

[jleonard]

I have a 29ft, semi displacement hull with 315 hp turbocharged Yanmar diesel (6LYA-ute). WOT is about 3450rpm, but for both comfort and fuel efficiency I mostly run at hull speed, which is about 1300 rpm, at which rpm I wonder if the turbo is even spinning. Will this damage the engine or the turbo in the long haul? This is a mechanical injector, 1998 engine. I imagine many forum members do the same. What do you think?

To answer one of your questions yes the turbocharger is spinning. It spins at an idle but not enough to compress the air hence no “boost”. “Generally speaking” you’ll need to get to 1500 to 1800 rpm to start generating some boost.

 

[firehoser75]

We ran our Cummins 6BTA (WOT 2800) at between 1200-1400 rpm most of the time. I regularly used an IR gun to track temps and we always reached good operating temps.

Like DavidM suggests, we would run periodically at WOT to "burn off" any carbon and to check for any potential developing issues. I found a very small coolant leak that way and repaired same.
Another NT 37 owner I know has operated his Tug at approx. 50% throttle (hull speed) for years (he goes from Wash. to Alaska every year) and now has 12,000 hours on his 6BTA. He is meticulous with his maintenance, and the engine is still running well. He is (this year) rebuilding his turbo as it has finally developed a "seal" leak.
Engine life can be measured by fuel burned (at least in a way), therefore, a lower rpm (as long as good engine temps are reached) can be a good thing.

 

[kapnd]

You might look into removing some prop pitch on next haulout, a few more rpms at cruise might be good for the motor.

 

[DavidM]

You might look into removing some prop pitch on next haulout, a few more rpms at cruise might be good for the motor.

He says wot is 3450 rpm. The rated rpm is 3300. 3450 sounds like plenty.

David

 

[kapnd]

He says wot is 3450 rpm. The rated rpm is 3300. 3450 sounds like plenty.

David

The discussion is centered on cruise rpms, so maybe a look at the torque curve would be in order here.
My reasoning is that the OP has a high revving motor, that may be overloaded at such meager revs.

 

[Bonedoc]

I run a similar 28 ft DE with a Yanmar 6-LPA STP 315. It is generally said that Yanmars like to run hot and are recommended to run at or near about 80% load.
You can run at lower speeds and “ blow it out” periodically but it is suboptimal for long term health of that motor IMO.

A more thorough and elegant discussion concerning running diesels at proper load can be found, I believe, in a recent article in Passagemaker magazine.

For me its 3000 RPM at 17 knts.(max rated 3800 RPM)

 

[Nomad Willy]

he says wot is 3450 rpm. The rated rpm is 3300. 3450 sounds like plenty.

David

PERFECT

Now he can run 20-30 minutes at 2800 - 3000. And continuous at at 2800 or less.
IF that align’s w the operators manual.

 

[Nomad Willy]

Bonedoc,
Yes 7-800 down is a good rule of thumb. IMO